European Researchers Propose Quantum Network Between Earth and ISS

New kalalau_kane writes with this tidbit from Extreme Tech: "A group of European researchers has proposed the largest quantum network yet: Between Earth and the International Space Station. Such a network would see entangled photons transmitted over a distance of 250 miles — two or three times greater than previous quantum communication experiments. Not only will this be the first quantum experiment in space, but it will allow the scientists to see if entanglement really is instantaneous over long distances, and whether it's affected by gravity." The proposal (licensed CC BY).

Comments

Re:"Quantum Entanglements in Spaaaaaaaace"

[CodeHxr]

And Miss Piggy?

10,000 times faster than the speed of light?

[schneidafunk]

From the article: "As we recently reported, another research group recently showed this quantum channel to be at least 10,000 times faster than the speed of light."

I don't get it. I thought it was instantaneous and that number is just a crap number based on distance.

Re:10,000 times faster than the speed of light?

[Anon, Not Coward D]

actually it comes from this article:
http://www.extremetech.com/extreme/150207-chinese-physicists-measure-speed-of-einsteins-spooky-action-at-a-distance-at-least-10000-times-faster-than-light

Re:10,000 times faster than the speed of light?

[UnHolier than ever]

It is instantaneous, but you can't measure zero - all you can measure is "it took less than x picoseconds" where x depends on your timer's precision, and from this infer "it went at least this fast".

Re:10,000 times faster than the speed of light?

[MouseTheLuckyDog]

Given that according to SR there is no such thing as simultaneity at two different locations in space, how can you tell it was instantaneous?

Re:10,000 times faster than the speed of light?

[Immerman]

>It is instantaneous
Correction: current theory postulates that it is instantaneous, but theory can only be known to be accurate to within the limit of our measuring devices, so "at least 10,000 times faster than light" is the more accurate statement. The whole point of the ISS experiment will be to see if the addition of more potentially confounding factors causes the measured reality to depart from the predicted behavior.

Re:10,000 times faster than the speed of light?

[slashmydots]

While that statement does sound comically stupid and would basically be a "divide by zero" problem, I think they're talking about time to transmit including translation and error correction + processing time inside the NIC divided by just processing time inside the NIC. Honestly, I would think that's closer to 3x faster over that short of a distance but whatever, lol.

Re:10,000 times faster than the speed of light?

[TheDarkMaster]

Exactly.

Re:10,000 times faster than the speed of light?

[Anonymous Coward]

Exactly what does instantaneously mean? We don't line in a Newtonian universe!

Re:10,000 times faster than the speed of light?

[ananamouse]

And just how fast do they need to be able to download pr0n?

Re:10,000 times faster than the speed of light?

[MyLongNickName]

Basically it is impossible to prove instantaneous travel because our most sensitive instruments will only go down to a certain value. Our most sensitive measurements demonstrate a minimum of 10k times speed of light. If our instruments become 10 times more accurate then they will likely be able to say that it is a minimum of 100k. Translation, error correcting and processing time have nothing to do with it.

Re:10,000 times faster than the speed of light?

[V!NCENT]

Yeah they probably mean this paper, lol:
http://www.wired.com/wiredenterprise/wp-content/uploads//2012/06/Dyckovsky-Publication.pdf

Also "Spooky action at a distance" == Einstein fail...

But nothing can go faster than light, because time is a measurement of change. Something can't possibly go faster, unless it doesn't change but folds dimensionaly. (E8, Lisi, standard model and information theory).

Re:10,000 times faster than the speed of light?

[Immerman]

Well, not quite with only two points, or so I understand. But according to special relativity with three it's possible to create a signal loop in which the signal returns to the original location before being sent which is why, assuming Special Relativity is accurate, FTL information transfer implies time travel. The Lorentz transformation implies that in another relativistic reference frame some spatial directions actually lead backwards in what our reference frame considers to be time. So long as everything is limited by light speed causality is protected, but go any faster...

Only 250 miles to the ISS

[pellik]

While the ISS may be only 250 miles above the ground, I can't imagine they only intend to do tests when the ISS is directly above the transmitter. I suspect the journalist completely failed at understanding the spatial relationship between a space station and a spot on the ground.

Re:Only 250 miles to the ISS

[jabuzz]

I doubt the extra 400km you get from doing it on the ISS is the point, because picking two points on the earth's surface that are opposite each other would be 12740km apart so the extra 3% is hardly significant.

Probably of more interest is that the ISS is doing an average speed of around 27,800 km/h which is sufficient for relativistic effects to noticeably come into play. In addition the ISS is in a different frame of reference to anything on the ground. These factors are much more interesting than the extra distance.

Re:Only 250 miles to the ISS

[buchner.johannes]

Also, different curvature.

Re:Only 250 miles to the ISS

[camperdave]

I suspect that if you want to see if something is affected by gravity, it's helpful to make observations at different points in the gravity well.

Re:Only 250 miles to the ISS

[saveferrousoxide]

average speed of around 27,800 km/h which is sufficient for relativistic effects to noticeably come into play

not really...
27,724 kph = 7.7 kps = 2.5688 e-5 c = fast, but not relativistic fast which is more like 0.1 c.

Re:Only 250 miles to the ISS

[leonardluen]

it is fast enough that they can notice the difference in very accurate clocks...thus it is relativistic

for example GPS Satellites loses about 7 microseconds a day due to relativistic effects.

Re:Only 250 miles to the ISS

[matfud]

Still more than enough to be detected. I have no idea if that is relevant for this. So I must sort of agree.
It is tiny but yet it is probably the best that can be done now.

Oooh gravity experiment

[sandytaru]

Quantum entanglement is one of those more sci-fi than actual science, and yet it's a real thing we can't quite explain yet. Testing whether it's affected by gravity is a very cool method of poking the phenomenon a bit more. Maybe one day we'll get an answer besides "It's a quantum thing! You wouldn't understand!"

Re:Oooh gravity experiment

[iggymanz]

wrong, it is actual science and the way things behave, and the equations are complete (outside of realm of heavy space-time curvature such as near black hole). It is just different from the mental model most humans have. Nothing stopping anyone from taking prerequisite basic calculus and then basic quantum mechanics course.

Re:Oooh gravity experiment

[khallow]

It took more than a basic QM course before I had a real inkling of what was or might be going on. Even now, I have to handle this stuff gingerly with some rather complicated math tools, my intuition sucks.

Re:Oooh gravity experiment

[jabuzz]

Well saying they are complete is wrong. Both General Relativity and the Standard Model are correct within experimental error within the tested limits, which does leave very little wiggle room. However they are not complete and very unlikely to be complete even in regions of normal space-time curvature.

It would be like saying Newtonian Mechanics was complete at the beginning of the 20th Century. Sure within experimental error at the time it appeared complete. However we now know that it was not. A combination of experiments with smaller uncertainties and moving to a faster speeds showed that for certain.

The problem is getting to regions of high space-time curvature or reducing the uncertainties in our experiments is very hard and very expensive and nobody has managed to do it yet. When they do it will hopefully provide the insight necessary to come up with a way of unifying gravity and quantum. mechanics.

Re:Oooh gravity experiment

[Anonymous Coward]

Quantum entanglement ("spooky action at a distance") is only a real thing if you accept the idea that observing something really does cause it to change state, and that a whole swathe of reality suddenly and magically disappears. In any other part of physics, the idea of throwing away half your (energy/momentum/whatever) would be anathema; here, it's canon. I can't help feeling that nothing, other than our perception, is actually changing, and that the established view is simply wrong - in actuality, we make two measurements, we find they're correlated (fine), then we infer spurious cause and effect based on a faulty interpretation and make a fallacious deduction (not fine). I predict that, in this and all such experiments, no limits will be found on the distances, speeds, time-ordering, immunity to environment or indeed anything else of the "effect" being observed - because there's nothing actually happening. Sadly, "you won't find anything, ever" is only capable of disproving that assertion - and it's hard to think of anything capable of supporting it.

60's TV Sci Fi plot outline - just to show that other things are, at least in principle, worth thinking about:

Planet Halcyon 5 is critically short of the energy it needs to prevent its core imploding. To resolve the crisis, Spock proposes the use of "entangled" Sigmatau particles, Alpha and Beta, each of which is in a superposition of state X and state Y. He creates the pair, and beams particle Alpha to the surface of Halcyon 5, where Kirk is waiting. Particle Beta he beams to a space beacon in orbit around the Halcyon star. Using his tricorder, Kirk examines particle Alpha to trigger the state collapse. Instead, what happens is that Kirk himself becomes entangled - there's a Kirk X who sees particle Alpha in state X (call that Alpha X), and a Kirk Y who sees it in state Y (call that Alpha Y). Independently, back on the Enterprise, Spock examines particle B - and becomes similarly entangled, as Spock X and Spock Y. But now Kirk X is entangled, via Alpha X and Beta X, with Spock X - so when they compare results, bingo, they match. Kirk X flips a switch down on the planet, and limitless power flows from the star to the planet for no readily-identifiable reason other than plot necessity (a security officer in a red uniform is too slow in avoiding the incoming energy surge and dies as a brief, glowing outline, but otherwise everything goes smoothly). Meanwhile, Kirk Y is similarly entangled with Spock Y, with the same result; Spock Y draws a similar conclusion, the same energy flows, but when he flips the switch things go horribly wrong, and the Enterprise, planet and whole solar system are destroyed in an explosion of enormous proportions. But that's OK, because Kirk Y and Spock Y both had pointed beards. Cut to Spock X speculating about what *might* have happened; roll credits.

Re:Oooh gravity experiment

[slashmydots]

Everything was sci fi before it was science. But before sci fi, it's theory. So, it is a valid theory. It makes perfect sense too. 1 particle, 2 locations in space. You affect one, the other reacts without a particle or wave traveling between them. It just "happens" because it "is." And causality violations are just made-up logical bullshit with no basis in physics. Black holes violate physics logic too and yet, there they are. Reading the data off just one without affecting it is the hard part but they already solved that over a year ago....and it was on Slashdot *facepalm*

Re:Oooh gravity experiment

[bill_mcgonigle]

"It's a quantum thing! You wouldn't understand!"

I think it's more like, "It's a quantum thing! Nobody understands!". Yes, we have equations that balance, but to my knowledge there's no mechanism explaining simultaneous action separated by light years.

I have my money on that separation being merely illusory - that the two particles aren't actually two, but just how we perceive the mapping of reality onto (or into) 4D spacetime, but that's just my speculation.

Re:Oooh gravity experiment

[MyLongNickName]

You affect one, the other reacts without a particle or wave traveling between them,

And there is the absolute proof that you don't understand anything about quantum entanglement. You do not affect one in the slightest. You are only measuring the state of one the member pairs. You are not setting any kind of state, and attempting to do so would break the entanglement.

You have thrown out a lot of insults, and then when asked to provide links to your purported NASA information refuse to do so. Fact is, you haven't the faintest idea of what you are talking about.

Re:Oooh gravity experiment

[einyen]

Let's hope the Universe doesn't give us a BSoD one day if we poke too much in the "code" :-)

Re:Oooh gravity experiment

[delt0r]

It is in fact not that complicated and is generally obfuscated with well wishing "science journalism" or other sci fi fan desires. Even perhaps some of the debates Einstein had about it has made it more magical that in is.

Entanglement though not mathematically trivial but is not too hard to explain with an analogy. Consider that i entangle two die (aka photons) in such a way that the sum will always be 8. I cannot, due to the way physics work control what value each die has, only that the sum will always be 8. So i do this and each dice is in a box (ie entangled and unobserved). I keep one here and i take one to the space station. Now if i look at the dice i have, and its a 5 I instantly know that the die on the space station is 3! Note that the space station people don't know this without also looking at their box. Clearly i did not communicate this value 3 in anyway. Some people are happier with the analogy when the die are constantly being rolled till one box is opened. However this changes nothing.

In quantum physics the box where we don't see things is important part of the physics. So looking at my box changes the other box from my perspective, 100 or even thousands of miles way to also be open. Of course unless the person at the other box also takes a measurement (aka opens the box), they can't know its an already opened box for the observer on earth. By confusing these things, one can get the impression that there is an effect on the other particle (dice) despite that this cannot be observed/measured in any way.

We know that quantum physics and entanglement does not permit FLT messages.

We also know that if you can have FLT messages in 2 or more reference frames, you can communicate with your past.

Re:Oooh gravity experiment

[slashmydots]

Here you go, you stupid, ignorant troll hiding behind anonymity: science.slashdot.org/story/12/04/24/2031211/quantum-experiment-shows-effect-before-cause
science.slashdot.org/story/12/10/04/010223/quantum-measurements-leave-schrdingers-cat-alive "Physicists have managed to probe a delicate quantum state without destroying it – the equivalent of taking a peek at Schrodinger's metaphorical cat without killing it. The result should make it easier to handle systems such as quantum computers that exploit the exotic properties of the quantum world."

So don't forget to shut the fuck up.

Re:Oooh gravity experiment

[MyLongNickName]

Actually, your link just shows you still don't understand what quantum entanglement is. In another post, you indicated that it was manipulating one entity in a pair and measuring it on the other pair. This is blatantly false.

Again, your article does nothing to indicate that data is transmitted. If it were, it would be an absolute stunner for the physics community, and you would see thousands of stories about science scrapping the theories that underlay everything we know about relativity.

No, I am not a physicist, but there was another physicist who called you out, and apparently you have at least one other person who thinks you are pretty offensive.

Thanks for providing the link. It proved that you are insulting without having a clue about what you are talking about.

Re:Oooh gravity experiment

[MyLongNickName]

Oh, and in case you still think your article proves you right, you might want to read ALL of the article. The last paragraph states

"As always with entanglement, it's important to note that no information is passing between Alice, Bob, and Victor: the settings on the detectors and the BiSA are set independently, and there's no way to communicate faster than the speed of light. Nevertheless, this experiment provides a realization of one of the fundamental paradoxes of quantum mechanics: that measurements taken at different points in space and time appear to affect each other, even though there is no mechanism that allows information to travel between them."

So. while you may thing we are all "stupid" "ignorant" and "idiots", you cannot be arsed to read the entire article before forming an opinion based on nothing.

Not Much Advantage Gained

[biohazardpb4]

With the ISS being in LEO, not much distance is truly gained from a far simpler terrestrial experiment at opposite equatorial points, it would seem.

Re:Not Much Advantage Gained

[femtobyte]

Presumably, the ability to shoot a beam of light >250 miles, without needing to build a 250-mile-long evacuated beamline, is a major advantage gained. The Earth is surrounded by this annoying thing called "the atmosphere," which wreaks havoc with light traveling only a few miles; the faster you can get out of the atmosphere (by, e.g., shooting straight up), the easier it'll be to get any useful amount of light to the other end.

Re:Not Much Advantage Gained

[camperdave]

The far simpler explanation is that if you are testing the effect of gravity, it's essential to have two different points in the gravity well.

Re:Not Much Advantage Gained

[lgw]

Perhaps, but the ISS is only ~5% farther from the center of the Earth than sea level, so that's only a ~10% difference. Seems like gravity would have to have a huge effect on quantum entanglement for that to matter.

Re:Not Much Advantage Gained

[femtobyte]

Depending on the experiment, 10% differences can be pretty obvious to measure. With the best atomic clocks, we can now see relativistic effects due to gravitational potential differences corresponding to 1m height change in the lab. Without understanding the experiment, you have no way to judge whether 10% differences are negligible or whoppingly huge compared to experimental sensitivity.

Always a letdown.

[mikeulus]

Can someone please explain to me why this can't be used for instantaneous communication purposes? From everything I've understood so far, the answer is still no, it can't be used to transmit information, just measure state of the particle at a particular point in time.

Re:Always a letdown.

[Shimbo]

Can someone please explain to me why this can't be used for instantaneous communication purposes?

QE is rather like being married. You know that whatever you decide, your partner will want to do the opposite. However, no actual communication is involved.

Re:Always a letdown.

[jabuzz]

Basic explanation. So I have two entangled particles, and we move them apart so you have one and I have one. At this point we have no idea what the spin on either of them is, in fact it is not determined till we try and measure it, but they must be different. I now measure the spin on mine and find it is +1, meaning yours is -1 "instantly". You can now measure the spin on yours to confirm that.

The problem is because the spin of the particle is undetermined until I read it and when I do read it the result will be random, there is no way to transmit any useful information.

Re:Always a letdown.

[spyke252]

Generally, a communicant needs to borrow a classical communications channel to express which state the pair collapsed into- Wikipedia has a good article on this. http://en.wikipedia.org/wiki/Quantum_teleportation#Protocol

Re:Always a letdown.

[mikeulus]

I like you.

Re:Always a letdown.

[nsaspook]

Can someone please explain to me why this can't be used for instantaneous communication purposes?

Because that would require FTL transfer of energy/information.

It's like if three people were in a room and #3 put a nickle in #1's pocket and a dime in #2's pocket completely randomly. They all know there is only the possibility of a nickle or a dime but 1&2 won't know what coin until they actually look in the pocket.

#1 flies to Mars on a rocket.
#2 stays on earth and looks in his pocket. He now knows instantaneously the value of the coin in #1's pocket on Mars.

Re:Always a letdown.

[femtobyte]

Because the universe doesn't seem to like causation violation, so all its operating principles preclude faster-than-light (which, in Einsteinian relativity, is equivalent to "faster-than-causality") information transmission.

A rough "classical" analogy for quantum entanglement is: seal two cards, one white and one black, in a pair of envelopes. Shuffle the envelopes, and give one to a person who travels to the Moon. Whenever they open their envelope, they'll instantaneously know what the other envelope contains. However, this doesn't instantaneously "transmit" any information: all the information was "transmitted" when the person carried their envelope to the moon, at under the speed of light.

The "quantum" part of Quantum Entanglement adds some fun not-in-classical-physics features to this analogy. For example, you can make a machine that will flip a black card to white and white to black (without telling you which); when the person on the moon puts their envelope through such a device, it can still stay "in sync" with the other envelope (when they are both opened afterwards, they'll still have opposite-colored cards). However, no information is transmitted: the Earth person has no way of knowing (unless you tell them through speed-of-light-or-slower channels) whether or not the Moon person has used the card-flipping machine; once they've checked their own envelope, the entanglement is broken and changing the Moon envelope's contents no longer changes the one on Earth.

Re:Always a letdown.

[mikeulus]

This is the first explanation that has actually clicked with me.

Re:Always a letdown.

[MyLongNickName]

Not yet. anyway. Someone will borrow another sci fi concept and magically make it work.

No they won't. There is a certain cult that treats science as a religion and refuses to understand that there are basic laws of physics that constrain us. FTL communication causes a litany of paradoxes and trying to turn quantum entanglement into a FTL communication device shows a fundamental lack of understanding of what is going on.

Re:Always a letdown.

[iggymanz]

you can transmit your finding to the other person, at equal or less than lightspeed, so they don't have to measure theirs. BUT they won't get that information instantly, so the whole thing is useless for FTL comm. What is can be useful for is secure communications as "reading" one of two or more entangled particles affects the state of them all.

Re:Always a letdown.

[Anonymous Coward]

So, you can't control the spin on either particle?

Re:Always a letdown.

[jellomizer]

So they are two particles that are are in sync with each other. So the information doesn't travel faster then light, it is just implied.

So if we try to change the spin in a meaningful way we would break the sync thus they will no longer be entangled?

Re:Always a letdown.

[DahGhostfacedFiddlah]

This still bothers me. Just because a spin is unobserved doesn't mean it's not there.

I still don't understand how this is different from sending a red card and a blue card to two friends. When one opens theirs, they "instantaneously" know the contents of the other. But that doesn't mean the contents weren't predetermined the moment I sent the cards.

Re:Always a letdown.

[iris-n]

Jackpot! This is the whole issue. Just observing these correlations means nothing, as you said, the same data comes out of our friends with the cards.

But the reason we are fascinated by entanglement is that there is more to it. We can actually prove that no theory in which the contents of the letters were predetermined (and FTL communication does not exist) can reproduce the correlations that can be produced via entangled states. This is the famous Bell's theorem.

Re:Always a letdown.

[femtobyte]

Up to the point you describe, the classical (a pair of cards with different colors) and quantum (a pair of photons with different polarizations) systems behave the same. However, there are some nifty features of the quantum system that can't happen classically. For example, you can rotate the polarization of one entangled photon (without knowing what it is) --- then, measure both photons, and they'll still be in opposite polarizations, even if you carried out the polarization rotation and measurement so far apart that no "signal" had time to travel, even at the speed of light, from one to the other (what Einstein would call "spooky action at a distance").

Re:Always a letdown.

[MouseTheLuckyDog]

Hmm. Take ten entangled pairs. Sequence them 1-10. ( It doesn't have to be ten. )
Split the pairs up.
Use an encoding invariant under bitwise not set one set of pairs to some value.
Read the other set of pairs value.
Seems to me information was transmitted.

Of course QE is something that most people didn't talk about till after I finished grad school.

Re:Always a letdown.

[lgw]

So they are two particles that are are in sync with each other. So the information doesn't travel faster then light, it is just implied.

Not quite, but the reasoning is subtle. Say you have 2 spin-entangled particles, such that the spins must be opposites. You have to pick an angle to measure spin (up or down). If the same angle is used for measuring both particles, the results will always be 100% correlated (one up, one down).

However, if the measurements are taken at different angles, the results will be somewhat random. If the measurements are at 90 degrees to one another, the results will be 0% correlated. If the measurements are at some angle not a multiple of 90 degrees, something surprising happens. If the information were in a "hidden variable", predetermined, then the correlation would be linear with the angle between the detectors. It's not. It varies with cosine^2 (IIRC - Wikipedia says cosine).

Re:Always a letdown.

[wonkey_monkey]

Try Bell's theorem. I once read a very good explanation involving boxes with three hidden lights, but I can't for the life of me remember which book it was in. Sorry!

Re:Always a letdown.

[lgw]

See here for amore detailed answer. The difference is subtle.

Re:Always a letdown.

[femtobyte]

The problem in your scheme is "set one set of pairs to some value": you can't do that. Quantum mechanics forbids you from setting one side of the pair to values you want (without first breaking the entanglement, so the other side won't see the results).

Re:Always a letdown.

[Lemmeoutada Collecti]

If we can influence them to rotate, can we influence them to not rotate as well? Because if that were the case, we would have the basic for a 2 bit communication system.

For example: measure the receiving end at 1 second intervals. Three rotations = start of message. From there, rotate = 1 and no rotate = 0. Rinse, repeat.

I would imagine that the reason we don't have this is because we can't influence them not to rotate.

Re:Always a letdown.

[lgw]

Because the universe doesn't seem to like causation violation, so all its operating principles preclude faster-than-light (which, in Einsteinian relativity, is equivalent to "faster-than-causality") information transmission.

I take issue with that somewhat: as I understand it, FTL communication will only interfere with causality in highly contrived circumstances, where the endpoints of the communication are moving at relativistic speed relative to one another. An ansible that would break if the distance between the endpoints changed rapidly would protect causality.

Re:Always a letdown.

[deadweight]

+1 Every other explanation I have read was lame - this is such a simple answer too.

Re:Always a letdown.

[femtobyte]

Doesn't work --- on the receiving end, you have no way to tell whether or not the bit was rotated on the opposite side. You see a random bit; suppose it's a 1. There's no way to tell whether this was a 0 "to start with" then rotated, or a 1 "unrotated." The original states of the bits are random: you can't produce entangled bits knowing that one side starts with all 1's, and the other starts with all 0's.

Re:Always a letdown.

[femtobyte]

Nope. You can't tell on the receiving end whether or not the bit you're measuring was or was not rotated. A measured '1' could be a 'rotated 0' or an 'unrotated 1', so you know absolutely nothing about what the message sender is doing (only that, if she measured her bits right after you measured yours, she'd see the opposite values).

Re:Always a letdown.

[nsaspook]

Even in the classic universe groups of 'things' can have properties that can change at FTL speeds but they like "spooky action at a distance" effects with entangled photons can't transmit information FTL.

For example in Maxwell equations
Induced polarization current P has no constraints about speed.

http://arxiv.org/pdf/physics/0405062v1.pdf
http://www.iasf-roma.inaf.it/congress/public_html/Congress/Ardavan%20Rome.pdf

Re:Always a letdown.

[Anonymous Coward]

No you don't.

Re:Always a letdown.

[femtobyte]

FTL communication interferes with causality (within Einsteinian relativity, which so far seems to be a pretty solid theory) because if you have something that looks like FTL communication in one reference frame (a signal is sent from spacetime point A which arrives 1 year later at spacetime point B, which is 2 light years away from A), then there exists some other frame in which causality is violated: B receives the signal *before* A sends it (in this case, there's another frame where A and B are at the *same location,* except B is the time *a year before* A sends the signal; this would allow all sorts of causality-violating paradoxes, like B leaving a message for A saying "don't send the message we just received from you!"). You may consider the circumstances "contrived" in the sense that you need huge velocity differences between the frame in which A precedes B, and that in which B precedes A --- however, "contrived" or not, the theory breaks down. A theory describing how the universe works shouldn't depend on how good your ansible structural engineers are; and, we actually can create particles traveling much faster than the required frame boost for this example, and observe that they behave according to relativistic theory.

Re:Always a letdown.

[femtobyte]

Another example of "classical superluminal" properties is sweeping a laser pointer across a distant wall: there's no classical physical law which prevents you from rotating a laser pointer so the point of light on a distant wall moves from point A to B "faster than the speed of light." In this case also, there's no way to transmit information from A to B faster than light via this method. Your linked paper is a clever fancier version of this --- setting up "extended systems," so it appears that a polarization ripple is moving across a system FTL (like a bright point moving FTL across a wall), due to slower-than-light "preparation" of the system (like sending initial photons from the laser in the right directions to produce the "FTL" spot). Quantum "FTL" effects may indeed fall into the same "philosophical" category of "apparently-FTL due to clever preparation of the system" (initial production and non-FTL transport of the entangled particles).

Re:Always a letdown.

[zAPPzAPP]

I know the analogy, but I always wondered how you can tell your switching-machine works, if you are not allowed to check.

As far as I can tell, the result of checking is random and it is still random after the machine 'switched' something. So nothing really happened.

Re:Always a letdown.

[femtobyte]

In a sense, you can't "tell" the switching machine works on particles you aren't allowed to measure on both sides --- for the same reason that you can't "tell" there isn't a giraffe that appears in your closet whenever the door is closed and you can't look. However, the switching machine can be one that's proven to work whenever you pass a known-state particle through (just like you can prove there's no giraffe in every instance that you open the closet door); you can make a polarization rotator that works every single time you check a particle's state before and after. Furthermore, the observable data that you measure is never inconsistent with the polarization also working on the unobserved particles. Thus, it's sensible to think about the universe in terms of the polarization-rotator always working (instead of magically "breaking" just and only for the cases you don't check), just like it's sensible to operate with a no-giraffes-in-closet theory (even though an unobservable-giraffe-in-closet-only-when-you-don't-check theory is just as good a description of known data).

Re:Always a letdown.

[DahGhostfacedFiddlah]

But you should know statistically. If you suddenly stop receiving 0's for thousands of samples, wouldn't it be a good guess that your partner up on the ISS has turned on the '1' filter?

Re:Always a letdown.

[femtobyte]

Sorry, stupid slip-up (need more coffee!) --- there's no reference frame where A and B are at the same location and reverse time order. However, you can pass through a frame where A and B are closer together and at the same time, to where A and B are further apart but in reverse time order (which you can't do for points that are closer in space than in time in any frame): FTL communications forwards-in-time would also allow communications backwards-in-time if the sending/receiving devices where thrown at the right velocity across the lab, and from there you can build all sorts of paradoxes using causality violation.

Re:Always a letdown.

[femtobyte]

Your ISS partner doesn't have a '1' filter. They have an "all 1's become zeros, all 0's become 1's" filter. In both cases, you just see a random stream of zeros and ones. As soon as your ISS partner "looks" at a bit to tell whether its a 0 or 1 (e.g., if they want to apply a filter only to the 1's), they break the entanglement on that bit --- after that point, they can flip the bit all they want and it does nothing on your end (you just see the random bit that's opposite of whatever they measured at the point of breaking disentanglement).

Re:Always a letdown.

[MyLongNickName]

Do you thoroughly investigate every claim about perpetual motion machines and hope that some day, someone will figure out how to circumvent the laws of thermodynamics?

Re:Always a letdown.

[femtobyte]

Information is basically the "minimum" stuff that needs to be sent to have "causality". For example, I can cause you to be or not be punched in the face by either walking over and punching you in the face or not; but that requires a lot of work on my side. What's the "minimum" I have to do? Well, perhaps you're already sitting in front of a face-punching machine. All I need to be able to do to have "causal" impact on your face is send some "information" to the face-puncher machine; a tiny electrical pulse to the "start punch!" circuit. If I can't even send "information," then I can't do anything at all; I'm no longer causally connected to your face getting punched.

Thus, the ability to send information (hence meet the minimum requirements for causality) is very important. If not even information can be sent between two regions, then they are not in causal contact. Some things can "move faster than c," so long as they carry no causality-relevant content (i.e. no information). Transmitting information, however, creates causal contact --- and any causal contact faster than c permits causal paradoxes, where the effect of a cause can precede the cause (!), which, so far as we know, can't happen (and doesn't make "sense" if it could, since "sense" is based on non-paradoxical causality).

Re:Always a letdown.

[lgw]

You may consider the circumstances "contrived" in the sense that you need huge velocity differences between the frame in which A precedes B, and that in which B precedes A

More than that: as I understand it (and it's been a while), it's not enough for some third reference frame (neither A nor B, but some observer) to be moving fast relative to A/B to get a paradox. You actually need A moving fast relative to B to get a paradox. Therefore, a mechanism for FTL communication which cannot work (not a matter of how good your engineers are) if A is moving fast relative to B protects causality.

For example, if keeping a wormhole open required power dependent on the relative speed of the endpoints, but beyond a certain level of power input the energy density itself must cause the wormhole to collapse.

Re:Always a letdown.

[femtobyte]

In my example, "A" and "B" refer to points in space time. They have no velocity of their own; there's no meaning to "A moving fast relative to B", or "moving fast relative to A" ("5pm at the corner of Central and Main" doesn't have a velocity, even though "the corner of Central and Main" does). Using somewhat loose language, I also used them to refer to an observer whose world-line passes through point A or B --- someone who is at a particular place at a particular time ("A" or "B"), but could be moving at any velocity passing through said point. Depending on your reference frame, A and B may be separated differently in time and space (and do not themselves depend on choice of frame). My argument shows causality violation if you have FTL transmission in any reference frame, since there is some other reference frame where the message is received before being sent.

Re:Always a letdown.

[avandesande]

I still don't understand the novelty of this. Maybe when you entangle photons it sets their spins (to opposite). Its like sending a black and white ball down two tunnels and because the one that comes down your tunnel is black you know the other one is white... instantaneously!

Re:Always a letdown.

[lgw]

My argument shows causality violation if you have FTL transmission in any reference frame, since there is some other reference frame where the message is received before being sent.

That is necessary but not sufficient for paradox. By itself "there is some other reference frame where the message is received before being sent" isn't enough to create any sort of worrisome contradiction. Further, AFAIK any scenario in which "data sent FTL from A to B creates a causal paradox" requires A and B to be rushing together at relativistic speed.

For example, for co-moving A and B, if A sent B a FTL message, and B immediately replied FTL, A still gets the reply after the message was sent. From some fast-moving frame C, the communication would appear to go back in time in one direction, but would appear equally delayed in the other direction.

Re:Always a letdown.

[holmstar]

You're describing a local hidden variable theory. Local HVTs have been dis-proven in multiple experiments.

Re:Always a letdown.

[femtobyte]

Again, you don't understand the statements I'm making. There is no such thing as "co-moving A and B": observers can be co-moving; it makes no sense for space time *points* (a location through which an observer can move). Suppose you have a pair of black boxes that, when sitting still in your reference frame, can send FTL messages in *your* reference frame (one box sends a message at space,time point A, and the other receives it at space,time point B, which don't have a velocity of their own, but in your frame A precedes B). Then you can use these exact same pair of boxes to send a message *backwards* in time in *your* rest frame. In order to do that, you'll need to throw the boxes so they're moving fast relative to you --- in their own reference frame, they're still sending a forward-in-time message. However, in your reference frame, the "receiver" box whizzing through one lab will be able to get a message from the "sender" box whizzing through a different lab *LATER*. By chaining a couple of these together, you can relay a message back to your past self to tell your future self not to send the message.

Re:Always a letdown.

[binary01]

First point - If we as a species perceived everything by sound I'm quite certain the same statements would be made by us regarding breaking the sound barrier. And yet, we have

Regardless of whether it is possible to do anything faster than light, this statement demonstrates a profound ignorance of modern physics.

Hint: it has nothing to do with light itself. The c constant is a fundamental property of space-time in the universe we live in. Light happens to travel at that speed in vacuum because photons have no mass, but the real problems with communicating at higher speeds has nothing whatsoever to do with light or with anything else we perceive. Also, physics does not actually say that you cannot communicate faster than light. It "only" says that if you manage to do that you can violate causality and create paradoxes.

Re:Always a letdown.

[binary01]

No, it will cause problems if *any* two objects communicate FTL, not just the endpoints. So, even if your ansible only works if the two endpoints are at rest relative to each other, you can still easily cause problems by having the endpoints communicate (using standard EM waves) with other objects which are free to move at any (slower than light) speed. We know very well that there is no problem with two objects in different frames communicating with each other using EM waves, even in cases where the time dilation is significant (e.g. satellites communicating with Earth). So, your scheme would not only have to require keeping the two endpoints at rest but it would have to prevent any standard communication between the endpoints and other objects that are not at rest relative to the endpoints. Now, *that* is contrived and would also make the whole system not very useful.

Re:Always a letdown.

[Xest]

No I don't think that's what he's saying. Though I didn't quite grasp whether there was a purpose to him selecting 10 pairs I think I understand what he's getting at, so I'll simplify what I think he's getting at, or at least explain how I foresee information being transmitted.

You don't necessarily need to know what the values are, the simple act of change is enough for data transfer, for example, say you instead have 2 pairs that have been separated, let's call the first pair 1_1 and 1_2, and the second pair that are a hundred miles away or whatever 2_1 and 2_2.

You could surely just set up a scheme where by if you cause a state change in 1_1 and 1_2 simultaneously, you can also detect that 2_1 and 2_2 changed simultaneously - forget what they changed to for now, that doesn't matter, just recognise that they changed. Then if of course 1_1 changes without 1_2 changing the person at the other end sees that only 2_1 changes and not 2_2.

So what if you say well, if 2_1 and 2_2 both change you treat that as binary 0, but if 2_1 changes without 2_2 changing, then you treat that as binary 1. This means that you could encode the simple act of what particles change into a form of data transfer. If you expand this out to 8 pairs of entangled pairs, you could surely transmit a byte using this mechanism.

I assume there is some difficulty in causing state change simultaneously so it may be that your measuring equipment would define simultaneous change between two pairs as something a bit loser than absolutely instantaneously such as both changing with 10^-100 seconds (I made up that number) or whatever. I assume also that things could still go wrong, what if they just happen to "change" to the same state they were already in giving the appearance of no change if that's a think that can happen, but at that point it's no big deal, at that point you're simply at the point where you can apply the well established field of coding theory to create meaningful data transfer.

Does that makes sense? As I say I don't know the first thing about quantum mechanics, but I do get data encoding and coding theory, and it seems to me that if you can simply measure the fact something has changed, even if not necessarily what it's changed to, then that is already enough to create a meaningful data transfer scheme.

I'm sure that the act of encoding into binary means we lose some of the potential benefits of quantum physics given that we can already punt binary round the globe at the speed of light through fibre cable anyway and from what little I do know the perceived benefit of quantum computing would be in being able to measure spin because you're then not limited to the mere two states of binary, but this would still at least allow for instantaneous uninterceptable wireless data transfer anywhere in the world (universe?) wouldn't it?

Re:Always a letdown.

[dmgxmichael]

It "only" says that if you manage to do that you can violate causality and create paradoxes.

How? I've read the descriptions - they all hinge on the principle that for something to exist it has to be observed. To me that's as stupid as a man claiming the sun doesn't exist when he can't see it in the sky.

Re:Always a letdown.

[binary01]

they all hinge on the principle that for something to exist it has to be observed. To me that's as stupid as a man claiming the sun doesn't exist when he can't see it in the sky.

No, it has nothing to do with that (which is indeed garbage). You actually do influence your past. For example you can send your FTL message to a satellite and have it send it back to you and depending on the speeds of the message and the satellite you can receive the message before you sent it. This can create a paradox because you can setup your experiment so that you only send the message if you haven't received it at time t. But after you bounce it off the satellite it can arrive at time t-1, which would cause you not to send it, but if you didn't send it, then how did you get it in the first place?

FTL communication resulting in being able to send a message to your past follows from special relativity. Here are a few resources that show the details of how it works:
http://www.theculture.org/rich/sharpblue/archives/000089.html
https://en.wikipedia.org/wiki/Tachyonic_antitelephone#Two-way_example
http://www.physicsguy.com/ftl/html/FTL_part4.html (unlike the others this also covers the case of non-flat spacetime, such as warp drives or wormholes)

Re:Always a letdown.

[femtobyte]

You could surely just set up a scheme where by if you cause a state change in 1_1 and 1_2 simultaneously, you can also detect that 2_1 and 2_2 changed simultaneously

No, you surely could not (well, unless you have some radical new theory to replace quantum mechanics, and said theory is correct in its relevant predictions). There is no way to tell when a state is changing, whether that's a single-particle state or any number of particle states. No fancier encoding scheme gets you around this.

Re:Always a letdown.

[lgw]

observers can be co-moving; it makes no sense for space time *points*

Ahh, suddenly this looks like the best troll of 2012 on the XKCD forums. Points in space indeed.

Re:Always a letdown.

[lgw]

No, it will cause problems if *any* two objects communicate FTL, not just the endpoints.

Can you give me an example? Or is that just you something you heard once? Every example I've seen of how FTL communication can create a paradox (e.g., you receive a message before you send it) requires ansibles moving near the speed of light relative to one another, not merely moving fast relative to an ansible.

Re:Always a letdown.

[femtobyte]

??? Do you have problems with the idea of a "point" in Cartesian coordinates, either? A problem with the concept of "5pm at the corner of Main and Central" defining a spacetime point independent of the motion of Main and Central and what happens at any time other than 5pm? Do you ask questions like "where is the point (x,y)=(1,3) when y=1.8"? An observer is "someone" who follows a path through space-time (their "world line"); along this path, you can measure things like velocity, and whether said observer is "co-moving" or not. Like any other curve, the path is an (infinite) set of points in space-time, just like a points along a line in a Cartesian space (which don't have a "slope" even through the line does). Take the most introductory course on special relativity and you'll be talking about spacetime points right at the beginning --- they are quite well defined, and critical to basic concepts like the "invariant interval" between two points ds^2 = (space distance)^2-c^2*(time difference)^2, which remains the same no matter what frame you observe from (though the space and time separations between two points vary depending on your reference frame, as do coordinates that you might use to label said points).

Re:Always a letdown.

[femtobyte]

P.S.: maybe it will help clarify things to you to read the Wikipedia article on Spacetime, and to note that another common term (when discussing spacetime) for what I'm calling 'points' is 'events'. From the Wikipedia article:

an event is a point in spacetime specified by its time and place.

Because events are spacetime points, an example of an event in classical relativistic physics is (x,y,z,t), the location of an elementary (point-like) particle at a particular time. A spacetime itself can be viewed as the union of all events in the same way that a line is the union of all of its points, formally organized into a manifold, a space which can be described at small scales using coordinates systems.

In a Euclidean space, the separation between two points is measured by the distance between the two points. A distance is purely spatial, and is always positive. In spacetime, the separation between two events is measured by the invariant interval between the two events, which takes into account not only the spatial separation between the events, but also their temporal separation.

The causal structure of a spacetime describes causal relationships between pairs of points in the spacetime based on the existence of certain types of curves joining the points.

Re:Always a letdown.

[lgw]

There was an exceedingly long and effective troll on the XKCD forums last year that rabbited on endlessly about this sort of thing, exposing some crackpot theory about why relativity was wrong, successfully drawing out dozens of pages of argument about the finer points (ahem) of "points in spacetime", the difference between a space and a coordinate system embedded in that space, etc, ad nauseum. The thread was glorious, but "no, I'm talking about points in space, not observers" joins "airplane on a treadmill" and "does .999 repeating = 1" as pure trollbait.

Can you explain your example in terms of participants, ansible endpoints, and the relative movement of each?

Re:Always a letdown.

[femtobyte]

This is a fundamental problem that I can't solve: to an uneducated reader, a sufficiently slick troll might be indistinguishable from a genuine expert. As I said, I can't solve this --- but you can on your end, by working to not be an uneducated reader. At this point, I'd recommend that you pick up a basic undergraduate introductory text on special relativity, and get a firm grip on the most elementary principles (which will likely start with events/points and invariant intervals). Your problem is that you've confused how science works with how science fiction works: you're trying to "explain" basic concepts by piling on layers of ever-more complex terminology, which is just gobbledygook that you don't fundamentally understand at all. Having taken graduate-level courses in relativistic physics from top researchers in the field, I can assure you that the word "ansible" does not appear anywhere in the concepts needed to answer your questions --- it's a Sci-Fi plot device, not a component of Einstein's theory of relativity. So no, I'm not going to attempt further explanation (since it won't help any if you're already assuming I'm a troll and too uneducated to tell otherwise for yourself), and hell no I won't use the word "ansible" to explain physics, because tossing around Sci-Fi jargon is the opposite of helpful in explaining real basic science concepts (no matter how fun it may be as a plot device).

Re:Always a letdown.

[Xest]

Fair enough then. The posts above seemed to suggest you could detect change, just not to what state.

How does quantum computing and communication work then exactly?

Re:Always a letdown.

[femtobyte]

If I could tell you how quantum computing worked exactly, I'd be too busy preparing my Nobel Prize acceptance speech to comment on Slashdot. But roughly, just because there are some things you can't do according to the laws Quantum Mechanics (like precisely measure both the position and momentum of a particle at the same time --- the Heisenberg Uncertainty Principle puts certain hard limits on exactly how much / what kind of information you can extract from a quantum system), doesn't mean that there aren't also interesting things that you can do. Quantum computing requires setting up systems where the final "answer" can be read out in a measured state, instead of being "hidden" in some fundamentally unobservable quantity. Likewise for "quantum communication" schemes where you encode transmitted information in quantum properties that you *can* directly control/measure (though you can devise clever schemes where, if the recipient doesn't know exactly which things to measure, they'll accidentally measure the wrong thing and irrecoverably destroy the real information, making it difficult for an impostor to eavesdrop).

Re:Always a letdown.

[lgw]

Ahh, the "I can't explain it because you're just a big stupid-head!" response. Always classy, that one.

Feel free to invent your own word for "hypothetical device capable of communicating faster than light" if you don't like the accepted one. There are still people who insist on saying "remote manipulator" instead of "waldo" too. But "hypothetical device capable of communicating faster than light" is a bit of a mouthful, so if you'd like to give an example of how a "hypothetical device capable of communicating faster than light" could create a causal paradox, you might choose some word for "hypothetical device capable of communicating faster than light", just for terseness.

Still waiting for that example, BTW. How would a hypothetical device capable of communicating faster than light be problematic, if the endpoints of that communication were co-moving? How could I arrange to receive a message before I sent it, were a hypothetical device capable of communicating faster than light to exit?

Re:Always a letdown.

[femtobyte]

Ahh, the "I can't explain it because you're just a big stupid-head!" response. Always classy, that one.

Always classy, sometimes true. You're acting like someone who asks a mathematician to explain Green's Theorem, but gets confused when concepts like "addition" and "fractions" appear in the explanation. Sigh; I'll give one more try to address some points using your choice of terms.

Your big sticking point seems to be that, as soon as you need something other than "co-moving endpoints" to demonstrate a paradox, you'll rip a new asshole in the fabric of spacetime and new-physics monkeys will fly out (allowing for exotic possibilities like FTL). Yes, the examples I was giving above on how to construct a paradox did rely on the possibility of boosting the endpoints so they "see" a different reference frame. However, this doesn't require coming anywhere close to the boundaries of "unknown physics," where strong gravitational effects may over-rule the laws of known physics. While accelerating an object to 0.999*c in the lab frame is an "engineering" challenge (note, we do a whole lot more all the time with atomic nuclei, so this would be no problem if your "device" was a quantum-entangled electron), you're nowhere near the point of worrying about general relativistic effects (much less the point of forming black holes where known physics details get iffy). And if your "FTL theory" produces paradoxes while operating well within the realm of known physics (regardless of what "engineering challenges" are required), then it's not compatible with or derivable from known physics (doesn't mean its wrong, but the burden is on you to demonstrate why your whole new hyper-relativistic grand unified superphysics isn't a load of crap).

If you have a pair of FTL-communicator endpoints which, when co-moving along with your own reference frame, can send a message 2 LY away in 1 year, then you can also boost the speed of the endpoint pair relative to your frame so that when a "2 LY in +1 year" message is sent in the endpoint pair's reference frame, in your reference frame the message was sent 2 LY in -1 years (shows up at the receiver a year before being sent). The precise Lorentz boost required to do this is left as an exercise to the reader (if you can't handle this, you're **way** too far out of your depth). Now, chain two of these systems together in a loop: your first message shows up at the remote lab a year ago, who relays a message back that arrives in the sending lab 2 years ago, saying "success! no need to run your experiment 2 years from now!".

Re:Always a letdown.

[binary01]

I may not have been clear but what I mean is that an ansible can allow any two other observers (one on each side) to communicate FTL in a flat space-time and these observers do not need to be at rest (even if the endpoints of your ansible must be at rest) and this is all that's needed to run into problems. Also, the speed of the observers does not necessarily need to be close to the speed of light and the farther away the two observers are the lower the minimum necessary speed for causing violations. A more concrete example:

Let's say observers A and B are at point (0,0,0) and C and D are at (1000ly,0,0). A and C are at rest relative to each other and have an ansible connection between them that will break if they are not at rest anymore. B and D are not in anyway restricted so they can travel and any slower-than-light speed that they want. So, B starts going at 0.5c relative to A and to the left of A and and D starts going at 0.5c relative to C and to the right of C. Thus B and D are going at a significant speed in each other's frame. A sends a radio message to B, which immediately relays it through the ansible to C, which sends it via radio message to D. Then it goes the other way from D to C by radio, through the ansible to B and by radio from B to A. So, what you have is A and D communicating much faster than light, while also moving thus easily allowing them to send messages into their past. In this case the fact that the ansible only works if the two endpoints are at rest relative to each other does not help you unless you also make up something that prevents A and C from communicating with nearby observers.

Another example with a somewhat different setup but still exploiting the same problem is shown here, with pretty space-time diagrams: http://www.theculture.org/rich/sharpblue/archives/000089.html. In this case you have A and B with an ansible and C and D with a separate ansible. A and B are at rest relative to each other. And so are C and D. However A and B are moving relative to C and D and are able to communicate by regular methods, which violates causality.

Re:Always a letdown.

[lgw]

That wasn't actually my sticking point, but lets leave that aside, as your example doesn't seem to violate the constraint I was placing anyhow.

I think I understand your example, but correct me if I'm wrong. We have:
* 2 labs (co-moving in frame 1),
* a FTL communication device with 2 endpoints (co-moving in frame 2, very fast compared to frame 1)
* a second FTL communication device with 2 endpoints (co-moving in frame 3, very fast compared to frame 1, in the opposite direction of frame 2)

So we tilt the coordinate system (so to speak) one way for the first message, and the other way for the second message, and thus paradox. I'll have to work through that when I have some time.

Re:Always a letdown.

[lgw]

Also, the speed of the observers does not necessarily need to be close to the speed of light and the farther away the two observers are the lower the minimum necessary speed for causing violations

You know, that's a very good point that I hadn't considered at all. I worked this out once: at the edge of the visible universe (in any given direction along the plane of Earth's orbit), the difference in speed over 6 months gives us something like 300 M years of "time parallax".

Thanks for the clear example! And the link: "almost every explanation Iâ(TM)ve seen of the effects of superluminal travel on causality has been badly garbled" indeed.

There is one (quite horrible) loophole, however: if the ansible is a wormhole, such that the distance between A and C is shortened and the communication is not FTL along the path through the wormhole (just FTL through "normal space") I believe you're fine, causality-wise. But I call that horrible because it has a worse problem: you could construct a triangle where the sum of 2 sides was shorter then the 3rd, if one side passes through the wormhole. That would mean we don't even live in a metric space, and have very few mathematical tools to work with. I somehow suspect that's possible, based only on how annoying that would be.

Re:Always a letdown.

[binary01]

If it is a wormhole (or by using a "warp" drive) then the scenario I described may not work for producing a paradox. However, if you have two separate wormholes, one connecting points A and B and another connecting points C and D and you position them such that they are far enough from each other, not to affect each other's regions of spacetime but close enough that you can travel between A and C and between B and D at slower-than-light speeds then you can still violate causality and create paradoxes. There is a short description of that here: http://www.physicsguy.com/ftl/html/FTL_part4.html#sec:stmanipulation and there is a more detailed discussion in Everett's book Time Travel and Warp Drives: A Scientific Guide to Shortcuts through Time and Space (this covers both warp drives and wormholes, as well as other topics related to FTL and causality).

Quantum-entanglement deniers?

[DavidHumus]

Wow - a dozen or so messages and not yet one from quantum-entanglement (QE) deniers.

Re:Quantum-entanglement deniers?

[i kan reed]

What is quantum entanglement denial? I mean, I understand people will believe lots science to be false for a ton of stupid reasons, but I haven't heard of this one. Could you explain their position?

Re:Quantum-entanglement deniers?

[femtobyte]

I don't know what particular form of QE denial DavidHumus has run across, but what I've seen usually (like most denialisms) starts with some critical misunderstanding of QE: e.g "QE lets you transmit information faster than the speed of light"; then concludes "you can't transmit information faster than c, thus QE is bunk!".

Re:Quantum-entanglement deniers?

[ArcadeMan]

Talk about a quantum mistake.

Re:Quantum-entanglement deniers?

[lgw]

Well, QE deniers still have a leg to stand on. The experiments so far aren't quite conclusive yet. But since all the evidence thus far points to QE and not intuition, it's likely only a matter of time and experiments like this one until that door is shut.

Re:Quantum-entanglement deniers?

[Digital Vomit]

Quantum particles do not become "entangled" so that one entangled particle's spin magically becomes the opposite of the measured entangled particle's spin. The US Government has a secret program in charge of flipping the spins of quantum particles in order to make them appear entangled in order to benefit the rich and powerful.

Wake up, sheeple!

Re:Entangled Photons?

[SJHillman]

Very few scientific advances have been made without a few Unforeseen Consequences, but that's no reason for Apprehension or Questionable Ethics so we may just need to Forget About Freeman.

Re:Ansible

[bsane]

Enders Game

Re:Ansible

[SteveFoerster]

Rocannon's World by Ursula K. Le Guin

Quantum Entanglement Does Not Transmit Info

[MyLongNickName]

No, the article isn't suggesting this, but every time quantum entanglement gets brought up on Slashdot, someone suggests how we can use it to communicate FTL. Quantum entanglement is the equivalent of instantaneously sending a random message (more complicate than that, really). No information is actually transmitted. The first time I tried to wrap my head around Quantum Entanglement, I thought it could be used to communicate to far-away places (even other planets) with no latency, but as I understood more, my hopes were dashed.

Re:Quantum Entanglement Does Not Transmit Info

[slashmydots]

Actually, you were wrong before and you're wrong now. Get over it. NASA knows what they're talking about. You still do not. There was a slashdot article years ago about a method discovered to transmit data between entangled particles.

Re:Quantum Entanglement Does Not Transmit Info

[MyLongNickName]

Care to post a link?

Re:Quantum Entanglement Does Not Transmit Info

[femtobyte]

Perhaps there was a Slashdot article with a lousy summary (shocking!) that mislead uninformed people into thinking NASA had a method for transmitting data between entangled particles, but NASA does not have a method for transmitting data between entangled particles.

Re:Quantum Entanglement Does Not Transmit Info

[Immerman]

Heh, many years ago I actually designed the theory for a FTL communication channel - all I needed was a way to use N remotely entangled particles to generate N+1 of them (or a serious boatload of particles to start with). The essential element being that, while random, the probability distribution can in fact be manipulated. It makes for a very "noisy" transmission medium, but with enough redundant bandwidth it should be possible to get a message through.

But then despite multiple attempts I've yet to understand how FTL communications, even instantaneous, would in any way break causailty. So maybe there's something in QM I've overlooked as well. If anyone can explain that one I'd love to hear it - I can see several ways in which it would result in *apparent* causality violations to a 3rd observer unaware of the FTL channel, but no way in which a signal could possibly be looped back to be received before it was sent, so no *actual* causality disruption could occur.

Re:Quantum Entanglement Does Not Transmit Info

[femtobyte]

Here's how FTL communication can break causality.
In your reference frame, spacetime points A and B are 2 light years apart, with B "happening" 1 year after A.
A sends an FTL message to B (arriving 2 light years away, but 1 year later).

Here's where relativity messes you up: by changing your reference frame, you can trade off between the "space" and "time" differences between A and B, so long as the "invariant interval" s^2 =(space difference)^2 - c^2*(time difference)^2 remains constant, and continuously connected on the same branch of the hyperbola s^2 = constant (in this case, s^2 = 2^2-1^2 = 3). In particular, you can boost through a reference frame where (space difference) = sqrt(3), (time difference) = 0, continuously to a point where (space difference) = 2, (time difference) = -1. In this frame, A and B are the same difference apart, but occur in the opposite time order (B a year before A!). In the case of space-difference s^2 = dl^2 > 0 (contradiction 0>0).

Thus, in the alternate frame, B receives a FTL message a year before A sends it! The same technology that allows A to send a FTL message "forward in time" in one frame allows messaging "backwards in time" in another. This allows construction of paradoxical triangles: B sends a message to A (2 LY away, a year earlier); A sends a reply to B' (in the same location but 2 years earlier than B) saying "don't send your message"; B' leaves a post-it on the lab wall saying "two years from now, don't send a message back to A".

Re:Quantum Entanglement Does Not Transmit Info

[MyLongNickName]

Heh, many years ago I actually designed the theory for a FTL communication channel - all I needed was a way to use N remotely entangled particles to generate N+1 of them (or a serious boatload of particles to start with). The essential element being that, while random, the probability distribution can in fact be manipulated. It makes for a very "noisy" transmission medium, but with enough redundant bandwidth it should be possible to get a message through.

Essentially your design boils down to "I have a process which transmits zero information. If I multiply the process x times I will transmit a positive amount of information".

The sender is doing ZERO information transmittal. None. And no amount of handwaving will change that. This is the equivalent of perpetual motion machine designers arguing that they added the right combination of cogs and parts to violate the laws of thermodynamics. If you understand the underlying principle, you no longer need to try to understand how the perpetual motion machine is design to know that it impossible.

Re:Quantum Entanglement Does Not Transmit Info

[Immerman]

Not true - it's possible to manipulate the probabilities of quantum states without collapsing them, in fact that's what much of quantum computing involves itself with. And if I can shift the probability of measuring a given spin from 50:50 to even 51:49 then, given enough data points, that shift can be detected in the entangled particles. In reality it's easy to shift the probabilities much farther than that, though most of the naive techniques involve repeated collapsing of the quantum state which would disrupt entanglement (the fatal flaw in my old design, which I've yet to be interested enough to revisit).

Simple non-entangled example: Normally if you measure the spin of a particle, and then make a second measurement at 90* there will be no correlation and the second measurement outcome will be 50:50. On the other hand if your second measurement is at 45* the probabilities will be asymmetric - enough so that a third measurement at an additional 45* (90* from the original) will still be correlated with the original rather than being 50:50 odds, in effect allowing you to "twist" the spin of the particle. This is essentially what the "three polarized sheets" trick is doing, where inserting a third sheet between two perpendicularly polarized sheets that block 100% of light suddenly allows much of the light to pass through.

Re:Quantum Entanglement Does Not Transmit Info

[nekad]

I underwent a similar struggle grasping quantum entanglement and came to the same conclusion (instantaneous communication is not possible via QE) years ago. However, recently I've been starting to explore this concept again and have found a new caveat. I can't seem to find a definitive answer that outright dismisses this caveat. Instead of simply measuring the entangled particles, does current theory forbid simply manipulating the spin, polarization, ETC? If two particles are entangled and I could simply change the spin on one, I've just sent information FTL. I surmise that "manipulation" is equivalent to measurement and will yield same consequence (collapsing the wave function/destroying the entangled particle), but I haven't found a decisive "yes" to this question yet. At any rate, QE has always fascinated me, regardless it's FTL communication shortcomings.

Re:Quantum Entanglement Does Not Transmit Info

[femtobyte]

The problem is your forms of manipulation on the "non-entangled" partner don't "transmit" to the entangled partner --- no matter what you do, it'll still be 50/50. To the extent that you "partly" measure the polarization and gently tweak the distribution of particles on your end, you also "partly" break the entanglement: there is no longer 100% correlation. The more you "coerce" your particles into a better-known state, the weaker the entanglement correlation becomes, in proportion such that you *never* get even 1% transmission of information. Like a lot of "perpetual motion machine" plans, you're trying to skim a small change in probability on one side (saying it's big enough to add up to something useful), while ignoring the corresponding small changes in correlation on the other (assuming they're negligible because they're so small).

Re:Quantum Entanglement Does Not Transmit Info

[MyLongNickName]

Quantum entanglement involves measuring current state not manipulating current state. If I manipulate either entity in the pair, I have broken entanglement. Yeah, I wish it weren't true either, but it is what it is.

Re:Quantum Entanglement Does Not Transmit Info

[MyLongNickName]

Wouldn't it be more accurate to say there is zero corresponding change not "a small change in probability"? As long as it is non-zero there is some information transmitted and this clearly isn't the case.

Re:Quantum Entanglement Does Not Transmit Info

[femtobyte]

The "small change in probability" I was referring to was the probability distribution for, e.g., polarizations on the "sender" side that he's manipulating. It's easy to make a small, or extremely large, change in probabilities on your side (you can re-polarize 100% of your electrons to be "spin up" if you want, but you'll 100% break entanglement in the process ; trying to slip past smaller changes doesn't help either). It is indeed accurate to say there is "zero corresponding change" on the other side, which was the point of my post.

Re:Quantum Entanglement Does Not Transmit Info

[MyLongNickName]

Okay, I am self-taught in this area, so wanted to make sure I understood what you were saying. I think the crux of the matter is so many people WANT this FTL communication to be possible (like me) that they refuse to accept the underlying principle.

Largest quantum?

[Chas]

Cognitive dissonance at its finest!

Re:Largest quantum?

[dlzbub]

Whatever you do, don't cross the streams!

Re:Ansible

[wonkey_monkey]

Sounds like, but wouldn't be anything like.

No

[iris-n]

This is bullshit. The scientific content behind this claim is that "nonlocal realistic models that reproduce the results of quantum mechanics must have speed of communication at least 10,000 faster than the speed of light in some arbitrary ference frame that we've chosen".

This means that this number is completely irrelevant, i.e., does not measure anyhting related to the real world.

What can be said, scientifically, about the speed of this channel is that it is the speed of light, because we can only actually measure the presence of the information on the other side after a light signal is sent from one party to the other.

The fact that it looks instantaneous is more of an artifact of our mathematical formalism, and a common philosophical misunderstanding about the nature of the quantum state (i.e., people regard it as objective rather than subjective).

Re:No

[lgw]

What can be said, scientifically, about the speed of this channel is that it is the speed of light, because we can only actually measure the presence of the information on the other side after a light signal is sent from one party to the other.

The fact that it looks instantaneous is more of an artifact of our mathematical formalism, and a common philosophical misunderstanding about the nature of the quantum state (i.e., people regard it as objective rather than subjective).

That doesn't follow. You can only confirm the measurements agree after a speed of light delay, but you can perform the measurements at very close to the same time (the 10000x speed of light number is just the resolution of the clocks that confirm when the measurements were taken).

And WTF is "subjective vs objective" about the spin of a particle? Sure, the word "spin" is arbitrary (likely nothing is actually spinning), but it's an objectively measurable property just like charge or mass.

Re:No

[iris-n]

I don't mean to insult you, but you seem to be talking without having read TFP, which I did. It is here. (Incidentally, this paper just repeats the arguments first published here, but with a better experimental setup.)

I'm saying this because, as the paper says explicitly, the number 10,000 c has nothing to do with resolution of the clocks, but with their choice of preferred reference frame.

And the experiment I had in mind was quantum teleportation, where it is a little bit less insane talking about the "speed of quantum information".

So what you do is to send a quantum state from Alice to Bob. If you do the calculations, they show that quantum teleportation instantly transfers the state from Alice to Bob. But Bob is only capable of acessing the state after A also sends him some auxiliary classical information (two bits, to be precise). That's why I said that the only scientific thing that you can claim is that the information propagates at lightspeed, because before receiving also the classical information no measurement that Bob can do can say whether he received the state or not.

About the subjectivity, I know this is weird, but there dosn't seem to be any way around it. The state that the calculations show changing instantaneously is Alice's knowledge of the spin, which in fact does change as soon as she makes the measurement. But it makes no sense to say that this is also Bob's knowledge about the state of the particle, since he has no idea about what's going on.

QM is complete? Really?

[Immerman]

Not quite true. The equations appear to be complete - i.e. they appear accurate to within the limits of current experimental error; however, assuming we're not living in The Matrix, the equations are only a mathematical model of a physical reality that we still have very little understanding of, hence the various superstring, etc. theories that seek to explain the equations. Prediction is only half of what science strives for, and the understanding still eludes us.

Moreover, there are theoretical inconsistencies between QM and Relativity, which implies that one or both mathematical models are still incomplete. Glossing over that fact is likely to prove quite similar to the preeminent experts in the late 1800's who stated that everything in science was known except for a few minor unexplained phenomena such as black-body radiation, etc. Investigating those inconsistencies eventually exposed almost every established theory in physics as fundamentally incomplete, and in the absence of evidence to the contrary that seems to be the safe bet for the current situation as well. After all, rigorous science is only a few centuries old, and every major advance to date has revealed far more questions than answers.

at least the 5th story in a row

[slashmydots]

This has got to be the 5th story in a row where I will attempt to convince incredibly stupid people that quantum entangled particles have a practical use EXACTLY LIKE THIS. I believe we last left off at people screaming that entangled photons can't transmit data between "them" (it's really sort of not 2 particles). Also that data can't travel faster than the speed of light, which is doesn't and yet does. You're all 1.5 years worth of wrong so far.
Let's see, how many times do you think I have to copy and paste "entanglement really is instantaneous over long distances" because some slashdotters believe that professional physicists believe it's true. This is a real thing! You're stuck in the 1980's with your high school physics degree, modding me down because I'm saying it's possible. Well, here it fucking is, right in your damn face.

Re:at least the 5th story in a row

[MyLongNickName]

You still think you can transmit data faster than light. How cute.

Re:at least the 5th story in a row

[femtobyte]

I am a professional physicist. Entanglement does indeed appear to be "instantaneous over long distances". However, no data is transmitted through this process. You're half-right, half-wrong: right on the part most people don't argue against, and entirely wrong on the important question of data transmission. And you're all wrong on thinking you're in any way qualified to comment on this question, because you're obviously not.

Re:Ansible

[bsane]

Oh- right... I remember now that in the book its named after some other SciFi, but I never knew the source.

Re:Ansible

[SteveFoerster]

I am nerd. Hear me roar.

Simulation

[RoccamOccam]

Hasn't entanglement and "spooky action at a distance" been proposed as evidence of the universe as simulation? In other words, the simulation doesn't resolve an observation until it is needed?

Re:Simulation

[femtobyte]

Since "the universe as simulation" is an un-disprovable proposition that could be made regardless of what we observe (really, what would indicate a non-simulated universe? flaming letters in the sky saying "THIS IS NOT A TEST"?), folks use all sorts of "evidence" to support it (especially "wacky" quantum concepts poorly understood by the general public and some metaphysical philosophers). This doesn't prove the universe isn't a simulation, either; only that, so long as the proposition remains untestable (we can't "break through" to observe the "outer layer"), such metaphysical arguments about the universe are entirely useless within the realm of science.

Re:Simulation

[cwsumner]

Hasn't entanglement and "spooky action at a distance" been proposed as evidence of the universe as simulation? In other words, the simulation doesn't resolve an observation until it is needed?

Or, is it just "instument loading" ? Like reading a high impedance circuit with a cheap voltmeter. I suspect -all- of out instruments for quantum states are still pretty crude!

Or, is it that we don't find out which (infinitelly) parallel universe -we- are in, until we look to see...

This is great

[TheSkepticalOptimist]

But really, we can't find some better form of energy? We can soon instantly transmit data faster than light across hundreds of miles yet still need to explode dinosaur juice to make our mechanical wagons move?

Re:This is great

[geekoid]

" instantly transmit data "
No information will be transferred instantaneously.

And why are you trying to compare apples to the sqr root of -1?

Re:This is great

[White Flame]

We do have a better form of energy: nuclear. It's already in portable use, in satellites and naval craft.

Beyond that, "dinosaur juice" offers a far greater power density over most of the other "safe" energy storage devices.

Re:Entangled Photons?

[miknix]

When the hell are we going to get HL3?? I don't even ask for that much, a new HL2 episode would be nice!

(pardon moi a bit of internet rage just in case some valve guy is actually reading this)

Re:Entangled Photons?

[k3vlar]

What happens to the Anomalous Materials they need for their experiments? Do they have the means for Residue Processing, or do they just seal all the waste in a Blast Pit? As far as pollutants go, We've Got Hostiles leaking into our atmosphere, oceans and rivers. So before you get On A Rail, and Power Up your Lambda Core, you need to consider the environmental Interloper.

Re:Entangled Photons?

[rot26]

I would say that by the way the valve/steam developers are making weekly random and unnecessary changes to the code, apparently for the sole purpose of breaking servers, that they're trying to wind down the franchise. You can see on Steam that the number of players on the HL2 servers is dropping steadily.

Not faster than light

[Roger W Moore]

What the article fails to point out is that actually nothing is travelling faster than light. This is the fancy equivalent of shining a bright laser on the moon and moving it around so that it appears that the bright spot on the surface moves at a velocity in excess of c. There is no problem with this because no information is transmitted from one point on the moon to another point on the moon faster than c - the only information which is transmitted is from the person pointing the laser to the moon. In the same way no data is transmitted between the two people making the measurements because neither has any control over the outcome of their measurement.

FTL communcation with a stick

[locopuyo]

1. Put a really long stick so that the ends reach the two points you want to communicate.
2. Push the stick from one end and it moves at the same time on the other end for instantaneous FTL communication.
3. Sell sticks to day traders.
4. Profit.

Does the qe information have to be verified?

[zacherynuk]

They do not know it works until they have verified the information transmission is intact.
If they didn't verify the information integrity... would any information have been transmitted ?

Re:Does the qe information have to be verified?

[femtobyte]

First, for pedantic clarity, no "information" is transmitted through acts of fiddling with entangled bits. The "information" is transmitted as the entangled particles are moved to their respective endpoints (at no more than the speed of light), and though further actions may "change the outcome" (the exact meaning of which requires rather subtle interpretation, and can be framed within many not-incompatible understandings of QM), they cannot "transmit information".

Your question, then, is no different in the "quantum" case than for any classical information transfer. If I mail you a letter, has any information been transmitted if you don't verify with me the integrity of the information in the received letter? How do you verify the integrity of the information, except through a transfer of information that itself requires verification? If you make the basic assumption that science relies on, that the universe operates according to repeatable principles, then you can achieve at least a statistical certainty that information is properly transmitted without an endless chain of uncertainty ("this method worked the 99 times when we checked before, so I bet this 100th transmission also conveyed information").

Re:Does the qe information have to be verified?

[zacherynuk]

Great stuff, thanks for that
Do you know, I may have missed something of the point in all of this.
If we cannot communicate FTL by fiddling ones qubits AND we already have SSL v2 I honestly see no point. *flicks hair and wonders off*

Re:Does the qe information have to be verified?

[zacherynuk]

PS - This is a good read - but my FTL expectations down: http://www.physicsforums.com/showthread.php?t=231008

Re:Does the qe information have to be verified?

[femtobyte]

If you absolutely trust SSL v2 to be un-crackable, then there's no "point" to better communications. However, if there is the least bit of worry that classical cryptography can be broken (either due to flaws in the algorithm that a super-smart mathematician might discover, or "brute-force" solutions by quantum computers --- the jury is still out whether such is possible), then quantum transmission channels offer a new "layer" of security not vulnerable to clever mathematicians or super-powerful (quantum) computers.

From another perspective, even if quantum entanglement has no commercial application, it's still cool to learn how the universe works simply for the sake of curiosity.

Re:QM is complete? Really?

[geekoid]

So they appear complete by all known methods, but you insist that they aren't without any evidences or maths? how cute.

Re:Entangled Photons?

[geekoid]

Ou need to understand Valve counting.

1
2
new game.
Or
Excited about new game
perfecting the game in the sequal
3 - I'm bored with that, time for something new

If not having a 3 is what it take to ave a company still make great 1/2 then fine.
Would you want them to do a game they aren't excited about making?

Re:Entangled Photons?

[geekoid]

Less people playing? gosh, who would expect that in an ancient game.

Re:Ansible

[macson_g]

+3.5 internets and my personal kudos to you, my nerdy friend!

Re:QM is complete? Really?

[sjames]

No. We KNOW they are incomplete because they cannot be reconciled with relativity which we have also tested. That means that there is necessarily something missing.

Re:Entangled Photons?

[denmarkw00t]

Umm...Fall 2013. It's in Portal 2. Ta-da! Also, how are you modded 4 for that? 'Sounds like some real Gordon Freeman shit to me.' gets a 0 offtopic, "Where's my HL3 I'd take a new HL2 ep" gets 4? Neat. No offense, I'm just as eager as you, but I'm also confused.

Re:Entangled Photons?

[rot26]

Developers making [destructive|unnecessary|unexplained|frequent] changes to working code for no apparent purpose? Gosh, who would expect that in an ancient game?