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FTL != backwards time travel.

According to relativity it is. If you can send a message faster than light, you can take advantage of time dilation effects when moving at a high relative speed.

(A little more about time-travel in general here)

FTL or causality. Pick one and only one.

FTL combined with time dilation, which we know empirically to be accurate, means that a round trip message can arrive back at it's start before the message is sent. And do note that this holds true even if your method of FTL involves 'shortcuts', as long as a message effectively travels faster than light, ships at relativistic velocities can be be used to send the message back before it left.

Why this is the case is a bit hard to explain in a slashdot comment, I'd recommend looking here or searching google for "why does FTL imply time travel" or something similar.

Here's the problem: you're fundamentally wrong about quantum entanglement. It doesn't allow for instantaneous communication, even in the laboratory (let alone in our brains, but I'll leave that be). Even if our brains are utilizing vast amounts of entanglement that we just haven't noticed yet, they cannot possibly use entanglement for instantaneous communication. If they could, causality would be violated. See: Tachyon Pistols Thought Experiment. Talk to a physicist who knows a bit of relativity (better yet, one who teaches Special Relativity in an introductory course) and I'm sure they'll be happy to explain it in a bit more detail.

How, out of curiosity, do you propose that the brains have become entangled?

And if you could, then you could violate causality without breaking a sweat:
http://sheol.org/throopw/tachyon-pistols.html

Not sure I want to live in a universe where we've invented FTL communication, it would get really, really confusing.

IANAPhysist either, but I am pretty good at math.

Yes, FTL communication leads to causality violation. The "tachyon pistols" is a thought experiment that explains it:
http://sheol.org/throopw/tachyon-pistols.html
You can argue this, I guess, but it falls out of special relativity. If these experiments already done actually do propagate a signal faster than light then engineering a paradox would not be that hard, and that would be huge news.

"By carefully adjusting the frequency of the voltage and the phase displacement the researchers say they can make the wave travel at greater than the speed of light. However no physical quantity of charge travels faster than light speed."
The experiment in the article is fundamentally the same as sweeping a laser across the moon. As I read it, they're basically shoving the EM field enough that one part wiggles, then another part wiggles, and if you calculate the "speed" as if the wiggles were a wave moving from one place to another then you get a number faster than light. However, the wiggles aren't actually causing one another and don't transmit information in the direction of propagation.

One of the funny things about special relativity is that subjective time slows down the faster something moves. An atomic clock in orbit ticks slower than one on the ground. When you hit the speed of light (you can't, if you've got mass, but say you're a photon) then time stops entirely. Photons do not experience time.

Actually, all photons move at the speed of light. The apparent speed of light can slow down, by putting a bunch of atoms in the photon's way. The photon is absorbed and another is emitted, and that takes time. It's possible to take that emission and slow it down almost arbitrarily, "freezing" light.

Read this. If the physics is right, and all the physics guys agree that it is, then FTL stuff can cause time travel paradoxes.

One thing could save FTL, though: if there were a "true" underlying frame of reference that FTL somehow invoked. The FTL paradoxes stuff rise up from the differing frames of reference. But so far the experiments don't show any universal frame of reference, they just show that SR is correct.

If I punch a hole through space via a wormhole and travel at 45mph on a moped through the wormhole to travel a normal space distance of 600 light-years... I just traveled FTL. and THAT does not violate causality.

Except that I have been told repeatedly that it does violate causality. Here are some references:

http://www.theculture.org/rich/sharpblue/archives/000089.html

http://stason.org/TULARC/education-books/startrek-relativity-FTL/8-2-How-FTL-Travel-Implies-Violation-of-Causality.html

http://sheol.org/throopw/tachyon-pistols.html

But I still don't grok it. Probably a good thing I'm not a physicist.

steveha

Here's a link to help you out with that:
http://sheol.org/throopw/tachyon-pistols.html

End result is that true instantaneous effects like you suggest would imply time travel.

I've only read pop versions of quantum mechanics but the basic idea requires information to be transmitted from the sending location to the destination classically, slower than the speed of light. This rather large caveat tends to be left out of articles like this. The point of teleportation is not that it is instantaneous, its that it allows you to transmit quantum states without physically transporting the particles in the special state from one place to another.

In a relativistic sense, FTL communication would actually end up sending information back in time rather than in "real time" and cause all hell to break loose with causality; see this "tachyon pistols" thought-experiment

That reminds me about tachyon duel stuff..

"Effect never precedes cause in special relativity. In any reference frame, effect always comes after cause."

Orly?

The only reason effect never precedes cause is because we've yet to observe (let alone interact with) tachyons.

"Since you put "effect" and "cause" in quotation marks, maybe you already knew this."

No, I put them into quotes because causality goes out the window when you introduce tachyons.

The hugeness of the universe is surpassed by only its age. Besides, if we confine ourselves to the galaxy, in which there is a still a huge probability of intelligent life. If we further confine ourselves to the regions of the galaxy life and colonization is possible, , i.e. not too close to the galactic center nor too far out on the outer arms, the space, while still huge, should have been colonized by now if it FTL were possible.

Maybe they wouldn't have found earth, but to take your flu season analogy a step further, while you might not catch the flu every year, you undoubtedly know someone who did. That is to say that if FTL were possible, we should be able to detect a massive galactic civilization. If a interstellar capable society were able to expand its territory by only 1 light year per year (trivial if you have FTL), it would take only 150,000 years to travel from one side of the galaxy to the other, even if the colonizing society kept to the outer rim, and didn't cut across the middle. In other words, a very short amount of time even in geologic time scales, and a veritable blink in galactic time scales.

As far as we can tell relativity indicates that any FTL, including bending space-time or using wormholes, implies time travel, which enables causality violations. If you can send matter or information from point A to point B faster than light could traverse the distance you can still time travel. (Here's a decent explanation of why this is so: http://www.sheol.org/throopw/tachyon-pistols.html)

Simple geometry shows that FTL travel or communication leads directly to time travel.

"Why does it matter if a signal can get someplace faster than it could have gotten there via photons?"

Special relativity.

"If I send a message from A to B and it gets there instantaneously, and B sends a message back to A a fixed time later, it will be received by A after A sent the first message. No time travel."

No, B's signal will get there before A sent the original instantaneous signal. It is not a matter of "B's clock looks like it's behind A's," it is that B's clock is behind A's, and that instanatneous signal will actually arrive in B's (and A's) past. What you're doing is assuming a preferred frame of reference, that one station's measure of time is more valid than the other's, and special relativity says that cannot be.

"The only causality broken would be that A could tell B "there's some light headed your way, it's going to show you our sun going nova, the light should get there in about 4 years,"

No. If A was 4 light-years away, B would get the signal 8 years before the nova got there, or 4 years in the past. A, looking at B, would see B's calendar and see that B is 4 years behind (and it's not that B "appears to be" 4 years behind, otherwise it would be possible for A and B to get different measurements for the speed of light), so that instantaneous signal would reach B 4 years ago.

You're assuming A's reading of A's clock is more valid than A's reading of B's clock, and that's not allowable in special relativity.

"The pulses are encoded to distinguish one from the other (e.g. a time stamp as in NTP). When the pulses come back to A, A knows precisely how far apart they are, and so can send out a pre-arranged signal, and then (at the appropriate time) do something "simultaneously" with the other station (of course, B can also calculate the time difference, so they both know what time it is "now" on the other station). If there's a third station C, also at exactly the same time rate as the other two, is there any way that C won't be able to be in synch with both A and B (i.e. getting a consistent time difference for the two)?"

Time and space are not constant, only the speed of light is constant. Time and space change in relation to each other to maintain that constant ratio. According to C, moving at relativistic speeds, the distance between A and B is different than what A and B measure. And while A and B may see C as being half-way between them, C, moving towards A, will see itself as being closer to A than to B. Similarly, while A and B may believe they are in synch with each other, C, moving towards A, will see A's clock ticking faster than C's, and B's clock ticking slower than C's.

As in the barn and the pole "paradox",, if C is going at relativistic velocities towards A and away from B, A's signal will reach C before B's does, so they will not be simultaneous.

A's and B's clock interpretations cannot be "more right" than C's interpretation, or otherwise A's and B's measurement of the speed of light would be "more right." However, even if C is moving at half the speed of light towards A (accordin to A), A's light-based signal will reach C (according to C) at speed c (not 1.5c), and B's light-based signal will reach C at speed c (not 0.5c).

"Why can't A, B and C all "go off" simultaneously?"

It can only happen if A, B and C are all at rest with respect to each other. Otherwise, time dillation throws the clocks out of psynch. Once C moves, C's clock will start to tick slow (according to A and B), and even after C comes to rest again, C's clock will still be behind.

1. Special Relativity is wrong to some extent (no SR)
2. Tachyons cannot exist (no FTL)
3. The shooters are not free to act in such a way (no free will)

You can actually almost rationalize this. If you can go faster than light, you can access timelike trajectories and go backward in time.

Light goes 186,000 miles per second, so light can go all the way around the Earth (at the surface) just over seven times in one second. In the scene, Supes is at least 500 miles up, and doing more than seven laps per second, so he must be exceeding the speed of light. The blue glow of Cerenkov radiation around him confirms this.

So, it's actually not to unreasonable to suppose that he's exceeding the speed of light; the Earth isn't *really* spinning backward, he's just going back in time.

Then, of course, they spoil it by having him need so "spin up" the Earth again...

"That's because we're always in motion,"

Or always at rest. Or anything in between. You can proove that we're always in motion. I can proove we're always at rest. We'd both be right.

"and don't have the equipment to measure an absolute (i.e., relative to the universe as a whole) reference."

It's not that "we don't have the technology," it's that special relativity says there is no absolute. You can't use the fabric of space-time as an absolute because measurement of space and time is relative to the observer.

If there were an absolute frame of reference, then not all observers would measure c to be ~3E8 m/s relative to themselves, and they could use that difference to extrapolate an absolute frame of reference. It doesn't matter if you're standing on the earth's surface or flying away from it at .99c, you always measure the speed of a beam of light coming from the earth's surface as ~3E8 m/s relative to you.

In terms of Newtonian mechanics, c + .99 c = c

"When clock A looks at Clock B and sees 8:58, and it knows that clock B is two light-minutes away, clock A knows that clock B _should_ say 9:00"

"Should" has nothing to do with it. All physical evidence demonstrates that the clock says 8:58.

"and if we were to instantly go from B to A that we would see 9:00."

First off, there is no such thing as "instantly." Two events occur simultaneously only to a particular frame of reference.

Secondly, according to special relativity, if you were able to travel faster than light, when you got to clock B it would still say 8:58. As far as you would be concerned, you just travelled backwards in time.

More information on this here.

"that is, one moment always follows the next"

That's not relativity, that's the concept of causality. Relativity states that the time-order of two events can be whatever you want them to be so long as you can achieve the proper velocity. For an observer moving faster than light, the two events will be reversed ("effect" precedes "cause"). Causality is just a hunch that has survived to this day because of our inability to find any tachyons.

Causality, relativity, FTL. Pick any two.

"If the sun vanishes now, it doesn't vanish eight minutes later "for us," we just only notice it eight minutes later."

Physics is all about our ability to measure and interact with the rest of the universe. Special relativity assures us that any and all scientific, physical evidence for the sun's existence is still there on the earth. Therefore, for literally all intents and purposes, the sun is still there.

Saying that the sun "really" disappeared eight minutes ago presupposes a preferred frame of reference, an absolute frame that is somehow "more right" than another. Special relativity states that there is no such frame.

(Oh, and for an observer travelling sufficiently faster than c, the earth will leave its orbit eight minutes before the sun vanishes.)

"There really really is an objective reality and an absolute time frame & spaital system"

Proove it. All physical evidence collected in the past century or so in the study of special relativity says that you're wrong, but I will welcome hard evidence to the contrary if you can produce it.

"but it's not labled, and we have no way to see it,"

If it can't be detected/measured/etc., then as far as physics is concerned it doesn't exist.

"so those that "get" astrology tend to ignore the fact that we're just dealing with what we can percieve,"

I won't harp on your use of the word "astrology." However, you're getting hung up on the term "perception." As far as physics and pretty much all science is concerned, perception is reality. Science is the study of the measurable, observable universe. If it cannot be touched, heard, seen, measured, clocked, or otherwise detected, if it doesn't interact with the rest of the universe in any conceivable way, shape or form, then it is outside the realm of science.

You can say "It's there but we just can't detect it" all you want. Until you are able to detect it, it is purely conjecture. And, in this case, it happens to be conjecture that flies in the face of a century of physical evidence pointing to the contrary.

(I'm not saying it's impossible, I'm saying I'll believe it when I see it.)

"Why?"

Because c isn't just the speed of light, it's the speed of change through the universe. Every change in the universe, every interaction between two objects can only move at the speed of light. And if that change in the universe "hasn't reached you" yet, then for every conceivable purpose it simply hasn't happened yet. Period.

"I look out the window, with my powerful telescope and see yesterdays newspaper back on earth."

Here's the tricky part: It's not just that you're seeing an image that left your home yesterday, it's that you are actually looking at yesterday. Yesterday's reality is washing over you like a wave at the beach.

If you see yourself in that telescope, that's not just an image of you, that is you. Changes you made in the universe, everything you did "yesterday" is happening now, as you watch it. As far as space, time, and the universe (from where you sit) are concerned, you are now in two places at the same time. If you were able to step back into your teleportation device, you would end up back on the earth "yesterday."

There's a more drawn-out explaination of the circumstances and the consequences using the classic example of a duel with tachyon pistols here

I think you're missing something. Nothing can be imposed on anything because that denotes cause ("Senses cause phenomenon"). You cannot believe in anything becuse a belief is based on ("caused by") life experiences and learned knowledge. You cannot learn anything because that pre-supposes that something can change ("cause a difference in") your way of thinking. Any verb that isn't "is/to be" has no meaning without cause/effect. Including "think."

"Actually, our mind will try to come with scores of different explanations for a seemingly paradoxical event stemming from the confounding of coincidence with simultaneity that we call causality."

First off, part of this passage essentially says "Strange events cause the mind to try to cope."

Secondly, as long as special relativity continues to stand, simultaneity is meaningless, with or without FTL. You show me two events that happen at exactly the same time, and I'll move you at a different velocity (with respect to the events) and show you they didn't. And both views (simultaneous, non-simultaneous) are equally valid and equally true. It's this nature of space-time that brings about FTL paradoxes (like this one) to begin with.