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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.
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.
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!
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.
It's only called that because we haven't found a way to make it not true, yet. So no, it's not obvious, it's illuminating.
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.
That would violate the superposition principle. The superposition of two arbitrary one-photon states is again a one-photon state. But the superposition of two sinusoids under a gaussian envelope is almost never a sinusoid under a gaussian envelope.
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
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.
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
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.
> It's quite possible that time is in fact variable outside the Universe...
I know what you mean, if our universe were the product of a simulation computed by a 1mhz machine (with a damn lot of ram), from inside the simulation nobody could notice if the machine were upgraded to a 1ghz.
But, to reason like this we are assuming that the concept of time is definable outside the universe, and that is an insanely big assumption.
---- MISSING MISCELLANEOUS DATA SEGMENT --- [sigdash] trolololol
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.
By measuring the relative velocities of all of the galaxies, it can be extrapolated that everything (at least, everything within our visible radius) expanded from a single point. Which is not to say a central point -- for all observational purposes, the earth is the center of the universe thanks to the fixed speed of light. We can see back 13ish billion years in every direction -- there's no directional bias that would suggest we're not at the center.
However, the same thing happens in alpha centauri as well -- they'll be able to see a few lightyears further in one direction than we can, and a few lightyears less in the opposite direction because they're also at the "center" of the universe from their perspective. Of course, we can't make use of this feature by say, having an observatory in Alpha Centauri because the time it would take for AC to send their data to us would be no less than amount of time it would take the light from that piece of the universe to reach us directly. But in as much as we can imagine a universal "now", AC will have a slightly different view of the universe than we do -- yet we're both still justified in claiming we're at the center, thus eliminating any fundamental concept of "center" beyond just calling it an observational bias.
So back on topic, there's another couple of things that we can figure out:
- There is stuff we cannot see. Anything beyond our past lightcone is forever lost to us unless the universe turns around and starts collapsing again. It may well be that the universe is larger, perhaps many many orders of magnitude larger than what we can observe. Its possible that our specific singularity is a minor fluctuation in some phenominally larger structure (Ekpyrotic). But short of inventing FTL travel, we'll never be able to confirm that experimentally.
- The universe will eventually disappear. This is a combination of the finite speed of light with eternal expansion. Eventually the expansion even between the nearest pair of "objects" will exceed the speed of light (Observable Universe). At that point, there will simply be no universe left. I say "objects" as I'm not entirely sure at what scale gravity is able to overwhelm expansion and keep things held together. Definitely within a single galaxy, but to the scale of clusters and superclusters is something I'm less certain of. So the whole universe wouldn't disappear beyond the light cone horizon, just most of it. We're not at this point yet however -- we can see back to a time when the universe was completely opaque to light (Surface of Last Scattering) and we'll need to develop instruments that measure gravitational waves in order to see back any further.
- If expansion is speeding up as the big rip theory proposes, then it will eventually get to the point where the "force" of expansion exceeds gravity, then EM and finally the strong force, ripping everything apart even down to the subatomic level and there will literally be nothing left until the next big bang.
To sum up, we can make a pretty good estimate of how far back our observable universe goes, but whatever might be outside of our observable universe is entirely up for grabs, and the only way we can ever investigate it is to discover FTL travel, which has a very good chance of being fundamentally impossible (basically, we'd need not only new physics, but new physics that can be applied to macroscopic objects such as probes or people.)