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Single Photons Do Not Exceed the Speed of Light
GhigoRenzulli writes "A group of physicists at the Hong Kong University of Science and Technology (HKUST) led by Prof Shengwang Du reported the direct observation of optical precursor of a single photon and proved that single photons cannot travel faster than the speed of light in vacuum. HKUST's study reaffirms Einstein's theory that nothing travels faster than light and closes a decade-long debate about the speed of a single photon. ... Discovery of superluminal propagation of optical pulses in some specific medium 10 years ago has evoked the world's dream of time travel, but later scientists realized that it is only a visual effect where the superluminal 'group' velocity of many photons could not be used for transmitting any real information. Then people set their hope on single photons because in the strange quantum world nothing seems impossible — a single photon may be possible to travel faster than the speed limit in the classical world. Because of lack of experimental evidence of single photon velocity, this is also an open debate among physicists. To tackle the problem, Prof Du's team measured the ultimate speed of a single photon with controllable waveforms. The study, which showed that single photons also obey the speed limit c, confirms Einstein's causality; that is, an effect cannot occur before its cause."
QED says that the path light travels is a path of least action, one where the phasors of all the contributing paths consistently reinforce each other. Nothing in QED states that light must travel at the speed of light, it just does so because the paths where it travels at some other speed interfere with each other destructively. Over very short distance scales, light may propagate superluminally, at least, QED makes no statement that it is impossible. So this is a useful result.
Conclusion seems to be "light cannot exceed the speed of light"...
"Light doesn't travel faster than the speed of light" means nothing with regard to causality. Quantum entanglement still occurs and results in faster-than-light data transmission. This doesn't disprove causality, but it sure as hell proves the speed of light has nothing to do with causality.
pay higher insurance premiums because theyre single, regardless of their diligent adherence to light-speed.
that new convertible probably didnt help things either.
Good people go to bed earlier.
The universe hiding its secrets again...
Journal Article: http://prl.aps.org/abstract/PRL/v106/i24/e243602 fascinating, and it is important that information not travel faster than c, which entanglement hasn't been shown to violate (yet) see: http://en.wikipedia.org/wiki/Bell's_theorem
RTFA, Mr. Brainless. I suppose you've never heard (or at least not understood) the phrase "knowledge for the sake of knowledge".
Since light moves as fast as "C", the call "C", "Speed of Light".
Anyway - its not really news. If they found it could move faster, that would be news!
Every single SciFi hyperlight drive gets the mass of the ship out of normal space where, in theory, C is the same as C is in normal space.
Is buying a Harley Davidson as your first motorcycle since you were 16 at age 49 a midlife crisis issue?
I see the effect of them taxing my pay cheques and never see the cause...
Over-unity machines made from Lego and magnets: can they work?
The experiment proves that under some set of conditions covered by the experiment a photon does not move faster than c. You can't automatically generalize that and claim that under no conditions does a photon exceed c.
Read "A Brief History of Time". Or wikipedia.
Those who fail to understand communication protocols, are doomed to repeat them over port 80.
Really? I see nothing in this article that precludes stasis pods and wormholes!
Science is like that. It takes things thate seem to be given, and checks to be sure that taking them as given is a good idea.
This is newsworthy because, ever since the earlier experiments described in the summary, there's been a suggestion that maybe it wasn't true, and that makes it a big deal to prove it either way.
What does "single photon with controllable waveforms" mean? I thought photons were all sinusoids under a gaussian envelope.
Photons don't vote, so it doesn't matter what their perspective is.
Keep up with the program.
No. While experimental data can of course always only give an upper limit to the mass of a photon (therefore we cannot exclude a photon mass), all our observations are compatible with massless photons, and photons are massless in all our theories.
I'm a scientist, and I'm not aware of this "fact".
The Tao of math: The numbers you can count are not the real numbers.
Huh, I had no idea there was a debate about whether light travels at the speed of light.
sic transit gloria mundi
Yes, I know the speed of light is defined as a specific number, but the wording of the headline made me laugh.
I think you may have this backwards. The speed of light is measured*, while things like the second, the metre and the mole are defined (sometimes in terms of c).
*Meaning its value is independant of the system of units used.
If God forks the Universe every time you roll a die, he'd better have a damned good memory.
Einstein's causality; that is, an effect cannot occur before its cause.
/.
Except here on
I've got your sig, right here.
See, this is what pop culture teaches us: these photons were thought to be fast just because they're single.
No. It's a parameter called "c" in Relativity that describes how the universe works, at least as we understand it. Relativity, which is in accordance with numerous experiments, predicts that particles having non-zero rest mass (the mass when they're stationary) get heavier and heavier as their speed increases, becoming infinite at c. That's why matter can't go faster than light. Massless particles on the other hand, like photons, can't be at rest ('cause if their mass was zero they couldn't exist) or at any other speed below c where their mass would also be 0, so they travel at c where their mass is E/c^2. Light, having no rest mass, thus travels at c, which we can conveniently call the speed of light. As far as I know there are no other massless particles (except maybe gravitons which are hypothetical?), so the name is really appropriate. Relativity was developed in a framework where fields are continuous, whereas we now understand them to be 'made up' of discrete particles. The experiment verifies the hypothesis that relativity applies to individual particles that make up a field, not just to the field as a whole, which apparently hasn't been experimentally tested before.
This
strikes me as rather wrong-headed. It's just that formulations of quantum mechanics have been more rigorous and less "intuitive" than formulations of classical physics, thus nothing seems obviously or "intuitively" impossible. Or, for that matter, possible. The point is simply that quantum physics is neither obvious nor intuitive.
The previous posts noting that "photons do not exceed the speed of light" is actually a good example of this. What is a "photon"? How do you define "speed"? Given that the formalisms of relativistic quantum theory are still very much an active research area, this appears to be a result that requires a bit more than a Slashdot summary to grasp.
To give a rough idea of the problems involved in "intuitive explanations," individual photons are indistinguishable, thus we might say "we create a photon at point A at time t0" and "we detect a photon at point B at time t1," but how can we be sure it's the "same photon"? Does the question even make sense? Probably not, since photons are not "practically indistinguishable," they are indistinguishable even in theory — the underlying mathematical models do not admit the concept.
As an analogy, given particular conditions, sound travels at a certain finite speed, significantly slower than light. Now encode the sound and send the encoded representation as an optical signal through a vacuum to a decoder that reproduces the original sound from a speaker. Have we demonstrated that, under certain conditions, sound travels at the speed of light?
If not, why not?
Light has momentum, not mass.
Beside that, special relativity has been corroborated again and again, tachyons are shown to be unstable due to their imaginary mass component, and physics as we know it simply doesn't hold up well in the presence of closed spacelike paths.
This isn't to say FTL travel is impossible. *Maybe* some way exists that gets around these huge obstacles, but when they say there's no known way it could work, it's not for lack of imagination.
labeling C as the "speed of light" makes the article seem like a tautology -- but C is a constant in certain theories, not a proven wall. Light often fails to travel at the speed of light, like when light is passing through air or water -- or lead (albeit not so much slower as "halted"). Take from that the following postulation: Light can vary in speed, sometimes much slower than C. Then ask this question: Does that mean that light can exceed theoretical C under the right conditions (i.e. Vacume outside the influence of gravity)? If so, what does that mean?
I think its everything after the 'if' in that last line that explains the muddy second half of the article. (the time travel nonsense). The article does overstate the findings though -- what they did sounds pretty neat, isolating one part of the wave element of light for observation and measuring its speed in a vacum. However, observation never tells you what's impossible, only what's been observed. They have shown that the set of conditions they created support Einstein's theory. They haven't "demonstrated that light can't" do anything. They have made observations which suggests that light does not travel faster than C.
-GiH
Because of the relativity of simultaneity. Whenever something goes faster than light in one frame of reference, you can find another frame of reference where the temporal order of events is reversed, i.e. the object travels backwards in time. Moreover, the principle or relativity means that if you can achieve superluminal speeds in one frame of reference, you can reach the same superluminal speed in any frame of reference. Which is enough to construct closed loops of causality.
The relativity of simultaneity is a direct consequence of the principle of relativity and the invariance of the speed of light.
That's an entirely unrelated, purely optical effect.
The Tao of math: The numbers you can count are not the real numbers.
There are so many things wrong with your comment I don't event know where to start. Everything has mass, but light has no REST mass, meaning if it were to stop then it would have no mass, which would be impossible. Electrons and protons for example, and airplanes, do have rest mass so they can stand still. If you take electrons and pump energy into them they start moving faster and faster. If you pump more energy their speed increases, but the closer they get to the speed of light the smaller this increase becomes. There is no limit to the energy they can have, the more you pump the faster they go. If you want to push them from 0.999999c to 0.99999999c, then fine. Also, the mass of any particle is its energy divided by the speed of light squared. That's mass, not rest mass. It increases with speed. For photons which always travel at the speed of light, if you give them more energy they stay at the same speed, but they get heavier. You can also give them as much energy as you want. Finally, if photons had rest mass their speed would vary with their energy just as it happens with electrons. Experiments confirmed with great accuracy that this doesn't happen, i.e. red light from distant stars arrives at the same time as blue light. Please read the Wikipedia article on special relativity, and study the friendly equations, they're not *that* complicated and everything I said is actually very clearly explained by said equations. There's nothing that's unexplained, except maybe why are the equations like that. Answer: because all experiments to date, including this one, fit them. We don't know fundamentally why, that's just how we see the world work when we look closely enough. We keep looking to see if the current equations are possibly slightly wrong and enhance them to fit what we see.
What is defined is the value we assign to the speed of light. The speed of light would be exactly the same if we defined it as exactly 300,000,000 meters per second; however the meter would be a bit shorter.
The Tao of math: The numbers you can count are not the real numbers.
The words you use sound ominously potent and are difficult for me to understand. Is there a book or source you could refer me to that would help me better understand simultaneity, invariance, closed loops etc.. Something closer to layman speak perhaps? I tried Wikipedia, but more than clearing things up, it's tossing my brain into a spaghetti wok. :(
Geekism is your _only_ God!
Your confusing me, I'm just content with knowing that speed of light is always 299 792 458 m/s no matter how fast your going in relation to someone else. Thus, if you launch from earth at 149 896 229 m/s, and you measure light in a vacuum guess what, light is still going 299 792 458 m/s when you measure it. Knowing this, how could you possibly start even thinking about going the speed of light when in reality your no better than a horse trying to catch up to a carrot hanging from a stick on its head?
The problem is.. They say if you travel faster than light, time stops, and then starts rolling back on itself.. You'll be a time traveler and you'd be able to go into the past. Apparently, if you could catch that carrot, you could also go back in time, kill yourself as a kid and find yourself laughing paradoxically at the humor of it all.
My confusion is about how time could possibly roll back. It has no properties, except those we give it. Time does not travel... We just take time to travel.
Geekism is your _only_ God!
Why the only way to do time travel have to be going FTL? Couldnt be shortcuts or side approachs? Proving that one possible path won't work don't rules out any other unknown yet way to do it.
Of course, still there is that little trouble with causality, paradoxes, and blue butterflies. But being ruled out just because that speed limit maybe isnt necessary
First off, why does anything travelling faster _have_ to go back in time?
An effect cannot happen before its cause. That's understandable. However, just exceeding a certain velocity will not cause time to roll back. Or for that matter, to slow down. Ones _perception_ of time might be altered, but time itself does not change. It's all relative.
Time is relative, yes. Not just in how our minds perceive it, but measurably different for different reference frames in accordance with Relativity. This means different observers can see events occur in a different order, and there is no universal ordering because not everyone can agree on it.
They can however agree that Cause A occurs before Effect B. This is causality. And because in Relativity there is no preferred reference frame, all observers must agree that Cause A occurred before Effect B in order for causality to be maintained.
Now suppose you can send information faster than c, and alert the Nations of Earth that the Vogons are coming to destroy earth and make way for an interstellar expressway, or worse, to read poetry at us. From some observer's point of view, the recipients of the message on earth will have shat their pants before the message was sent. That observer will see causality violated, and remember it has to hold everywhere.
Also, given this ability to send superluminal information, it is pretty trivial to construct situations in which every observer would agree that causality was violated. Given two space ships with the ability to communicate faster than c, you can set up a relay where you send a message to them, and they send it back, and you receive the response before you even sent it!
This is the "time travel" case. It's limited by how fast the ships can go and how long they've been traveling, and just how super-luminal your message-sending is... You can't send the message back to the year 3000 BC for example So "time travel" might give the wrong idea; let's just stick with "causality violation".
The enemies of Democracy are
That's what is confusing to me, who are these people who even bother to think or make up theories about "What if you could catch the carrot" once we have proven time and time again that its impossible within the scope of the laws of this universe. That is far less productive than trying to figure out if catching up with the carrot could somehow be forced into happening in the first place.
Did you also read http://en.wikipedia.org/wiki/Relativity_of_simultaneity ? The first paragraph of "The train-and-platform thought experiment" actually contains the key part. I don't think there's an easier way to explain it. And then imagine a superluminal signal sent from the front end of the train, sent very shortly before the light arrives there, to the back end of the train, arriving very shortly after the light arrives there, and think about what it would look like from the platform.
The Tao of math: The numbers you can count are not the real numbers.
That's what is confusing to me, who are these people who even bother to think or make up theories about "What if you could catch the carrot" once we have proven time and time again that its impossible within the scope of the laws of this universe.
Imagining what would happen if you could catch the carrot is how we concluded that it is impossible in the first place. The only reason FTL is considered impossible is because according to Special Relativity, FTL implies causality violation.
Sure the relativistic kinetic energy equation says you could never accelerate a conventional space craft to c, much less beyond. But what about something more clever, like a Warp Drive? That and every other method of going faster than light runs into the causality problem.
Trying to figure out if every method of attaining FTL is impossible is much less productive than determining that FTL as a concept is impossible if our other assumptions (constancy of c for all observers, and causality) are true.
The enemies of Democracy are
Well technically your mind can be at an imaginary sun created within your mind, which while wonderful, is cheating for purposes of this experiment. :)
The enemies of Democracy are
They are too bright to.
-- I have a private email server in my basement.
Forgive me if I am wrong (I dont mean to sound like a total fuckwad or anything...).. But isn't it impossible for photon to be totally massless, given that it can decay into an electron/positron pair while traversing vaccuum?
A truly massless vector particle would have a local time of 0, since it would be traveling at the maximum allowed velocity...(and thus it would take an infinite amount of time in any other reference frame for it to even initiate decay...) This was part of why Neutrinos were re-evaluated, when it was discovered that they could change flavors en-route from the sun to the earth. (It means they cannot be massless, because they change over time/can decay.)
While the mass of a photon would be so tiny as to be unmeasurable, it MUST have a mass, because it is ABLE to decay.
No?
It is impossible for a photon to decay into an electron-positron pair while traversing vacuum. Only when the photon is scattered by a massive, charged particle, an electron-positron pair can be produced (provided it has enough energy).
But you are right, if that decay were possible, then it would prove that the photon were massive. Indeed, it would have to have more than twice the electron mass. Which would result in a very different behaviour of electromagnetic fields.
Wrong. The neutrino oscillations mean that the flavour eigenstates don't agree with the mass eigenstates. Which is only possible if there are neutrinos with different mass (because if all masses are equal, then every state is a mass eigenstate). Now if two neutrinos have different mass, at least one of them cannot be massless.
No. Your premise is wrong, therefore the conclusion doesn't follow.
The Tao of math: The numbers you can count are not the real numbers.
Isn't this tautological, since photons *are* light? Is this an example of yet another poor summary?
mass of photon = zero
although they do have a momentum.
Wealth is the gift that keeps on giving.
Some mod points for AC please? This is a brilliant post.
It is impossible for a photon to decay into an electron-positron pair while traversing vacuum. Only when the photon is scattered by a massive, charged particle, an electron-positron pair can be produced (provided it has enough energy).
Can't two photons colliding in a vacuum "decay" like that? It should be possible to time-reverse what happens when an electron and positron collide in a vacuum.
I am trolling
If two photons collide, it's not a decay. Most importantly, the photons don't need mass for that, since energy and momentum conservation can be fulfilled even with massless particles (since the photons have different momentum -- otherwise they wouldn't collide --, there's a frame of reference where the total momentum is zero; if in that frame of reference the energy is at least 2mc^2 (with m the electron mass), an electron-positron pair can be generated (however the probability of that happening is extremely low; I don't think you could actually observe it in experiment). A single particle needs rest mass to decay because otherwise it would be impossible to fulfil both energy and momentum conservation. For two colliding photons producing an electron-positron pair this is no problem as long as the energy is high enough.
The Tao of math: The numbers you can count are not the real numbers.
In quantum mechanics, lots of things that seem to be obviously impossible turn out to be possible after all. Particles can tunnel through energy barriers, etc. In some theories of quantum mechanics, there was a possibility for individual photons to travel faster than light. A bit like tunneling, they should not be able to do it but they do it anyway. They would only be able to do it over very short distances, and in an unpredictable way, so this could not be used to transfer information. Any observable collection of photons in an actual experiment would still obey the law.
So anyway, it seems that that theory has been rendered less likely now.
That's just because the Sun hacked your cell phone.
What I would like to know is how photons are accelerated to the speed of light. Let's say I energize a laser tube just enough and somewhere in the gas I create a single photon, it instantly shoots off at light-speed. What exactly is propelling that photon to light-speed? Why does it travel at all? Is there any period of time where it exists at a speed less than c as it accelerates to c?
Things will get more interesting when we increase the speed of light 197 years from now.
- Light does have a mass, even if it is near 0. What speed could a particle with no mass attain then?
This is utterly silly. Photons have no mass at rest because they only exist in motion. E=MC^2; the mass (intertia) of a photon like any other particle, or body, is its energy. In fact, there is no difference between energy and inertia; they are the same thing. A photon might have "low mass" because it has low energy. A particle with lower energy would have lower inertia. Zero inertia only occurs at zero energy, which is empty space. I.e., what you get when you remove everything. If C could vary for photons, then there would be a hidden variable (state) x, such that E=MC(x)^2. Or energy and intertia are two different things. Good luck constructing a working theory of physics that 1) fits observations, and 2) predicts something E=MC^2 doesn't. (I.e. finding a case where the x is actually necessary.) But never has a particle been observed where E=MC^2 is violated, so you'd very likely be wasting your time...
But didn't you move out of time t0 originally?
They don't need to be created at rest and accelerated, they're already propagating when created. Photons of light are not like tennis balls that you have to hit to get moving, they're always propagating. In fact they can't have any other speed than c because they have no rest mass. If you're not satisfied with this and must have a 'mechanical' mental picture of the process, imagine an antenna transmitting radio waves. There's a so-called near-field zone around the antenna where the field is not yet freely propagating as waves, but is interacting with the antenna that generates it. As you move away from the antenna this field "detaches" and becomes propagating waves. Quantum mechanics models this near-field as virtual photons, which have rest mass because of the interaction and don't propagate at c. These virtual photons are not light yet, just electromagnetic fields.
No. You wouldn't *be* at both places at the same time.
The only thing that will happen is that for an outside observer, you would seem to be in both places at the same time, or even be at a closer position before being in the original position.
Just if a supersonic airplane was flying your way, you'd first hear the noise from its later position and only after that you'll here the noise from the previous location.
This *isn't* time travel, but rather just an illusion.
^_^