Speed of light slower than we thought? Probably not With the simple headline “Einstein was wrong”, yet another piece of questionable physics has garnered world attention. It starts with this kind of canned statement, which originated at Phys.org and apparently arose from this uncritical write-up at the Medium-hosted Physics at Arxiv blog. To quote Physics at Arxiv, “James …
Einstein recognised the significance of what Maxwell had stumbled upon (and proved) - that the speed of light is a constant - no matter what, not relative to anything (people kept asking the question 'constant relative to what what?'. Einstein through his theories of relativity explained why it isn't relative to anything.
Sorry - exactly wrong. SOL is always constant relative to an observer. So you get the same value regardless of whether you're moving towards or away from it. Unlike tennis balls.
Best way to get a reputation in science would be to disprove Einstein, so it's no wonder people keep trying. Which shouldn't be taken as me saying don't try, just be pretty damn sure before you publish.
The travel time of a beam of light/photons and the speed of light in a vacuum can be different. Because you can "stop" the beam/photon and re-emit it. A neutrino though is less likely to be interfered with as it is so small and weakly interacts.
Basically, it's the difference in the theoretical limit "my car can do 150mph" and the "but I'm stuck in traffic doing 1mph" limit. :P
Umm. IIRC light can be refracted by gravitational masses, but neutrinos...what path do they take?
Well they have mass so should also be bent by strong gravitational fields en passant, but to the same extent?
My A-level physics gives up at this point.
Also light slows down when passing through matter, but neutrinos do not I believe?
The thought occurred to me that we are observing a small fraction of a milliradian(?) of the light coming from the SN. Suppose it took 7.7 hours for the light to find a clear path through the debris from the SN, but the neutrinos weren't hampered at all by the debris. Simple explanation, no?
That is the actual theory so far. That the neutrinos escape before the light as the light gets caught up in debris, clouds etc.
No idea why I got downvotes. If I misspell something, or get some facts and details wrong, please correct me, I'm no genius that is for sure and not afraid to be told so.
Ok, so they're fine-tuning the speed of light, that means that the measuring equipment and techniques in early-ish 1900's wasn't as good as 2014. Yes, that's shocking, but true - 100 years DOES make a difference to accuracy.
That doesn't make Einstein "wrong", he just didn't factor in some guy farting in Thailand, to affect the weather in Canada.
Not really, the speed of light is very well established. Actually the speed of light is so well established, that it is defined as a constant and will never ever change from 299,792.458km/s. Any differences in measurement will not change c, instead it would change the definition of a metre.
That's an interesting point: while I agree that a metre is defined as the distance light travels in a vacuum in 1/299,792,458th of a second, arguably a metre as it exists at this point in time is more important than the accuracy of that calculation, in terms of the impact to society as a whole. i.e., if we (hypothetically) did discover that the speed of light is slightly different (as John suggested), wouldn't it make more sense to change that formula to keep the metre the same, rather than having to adjust all of our measurements?
Too early?
I think the conjecture is that the photons were emitted at the same time as the neutrinos but were slower to reach us.
If so, that's not saying so much that the 'speed of light' - as a constant - is not quite as big a number as was thought but saying that the speed of light (in a vacuum) is no longer the ultimate speed limit. I.e. - neutrinos move faster than the 'speed of light'.
Revising a measurement is one thing, but saying that that measurement no longer represents what you thought it did is quite another.
Is that the gist?
@frank ly
Within General Relativity, gravity is not really a force in the same way as, say, the electro-magnetic force - it is more a result of the curvature of spacetime by massive bodies.
Essentially, the Newtonian idea of the 'force' of gravity is (under Einstein) simply a manifestation of the curvature of spacetime, as influenced by mass or, more precisely, the 'stress-energy tensor', which is effectively a combination of mass, energy and momentum.
In other words, the curvature of spacetime that results in the apparent 'bending' of light is gravity.
Of course, Neutrinos are affected by gravity too! (And would be even if they were massless.)
At low relative speeds, Einstein's equations effectively reduce down to the Newtonian formulation of mass acting on mass. This works fine for celestial bodies moving at tiny fractions of c - the Earth, for example orbits the sun at ~1/1000 c. Of course, Newton's model can't account for light because light moves at 100% of c!
Thus, light is not affected by gravity as modelled by Newton but that model was explained by Einstein as essentially a special case of the larger picture, which is General Relativity.
Both are affected by gravity but
- neutrinos hardly interact and go 'straight out'
-light will interact with any matter in and around the supernova and will take some time to get out. How long depends on a lot of detailed understanding that we might not have got right.
for example photon diffusion time (getting absorbed, re-emitted in random walk style ) from the sun's core to its surface is estimated to be about 170,000 years, despite its radius being about 2.3 light seconds.
Franson has based his work on a conundrum raised by an old supernova explosion: that when supernova SN 1987A was observed in 1987, neutrinos were spotted 7.7 hours before the event became visible when photons arrived.
While we do not completely understand the physics underlying a supernova (the simulations show the shock wave stalling a few milliseconds after it rebounds from the collapsed core), scientists do pretty much agree that:
1. The neutrinos are generated during the initial core collapse;
2. The photons are generated when the shock wave reaches the surface of the star; and
3. The two preceding events are separated by several hours.
I guess Franson must have assumed that everything happens instantly. Just shows what an idiot he is.
There's certainly an idiot here, but it's not Franson.
If you'd read anything beyond the El Reg write up -- you don't even need to read the original paper, even some of the "Einstein wrong!!!" press reports got this part right -- you would know that this El Reg's cockup, not Franson's.
Yes, it's well known that there should be some hours between the arrival of the neutrinos and the photons. The problem with SN1987A is that the difference is several hours longer than our best theories of supernovae say it should be. Franson's proposal deals with that discrepancy quite neatly.
Whether he's correct remains to be seen, but if we're going to call somebody an idiot, I nominate the person who used the words "guess" and "assume" about a well-established physicist rather than read one link deeper.
Franson isn't questioning the speed of light. What he is actually saying, is that over those sort of distances, you can't consider space to be a true vaccum - you have to take into account quantum mechanical effects. Only an idiot would accuse him of being an idiot without first bothering to RTFM.
Ahhh... so he is "just" (though said in context, a lot of hard work goes into improving data and results) improving the theory and info about why the light takes so long to reach us. Not replacing it entirely (as that would be going against established observations and theories).
I do wonder, what other effects can the quantum mechanics, this "no true vacuum" cause?
Of course Einstein was wrong - this whole "photon" malarkey will soon go the way of the Aether, once people finally realize the whole process is about dark being sucked instead of light being emitted...
The speed of dark is obviously faster than the speed of light as you can demonstrate with a simple experiment:
Open your closet door. You can see light enter the closet, but dark moves so fast that you can't see it leave.
As for the heat question, Radiant heat is just another form of light, but shiftier, as it doesn't let us see it. Who knows what it's up to.
Conductive heat is just agitated matter. Some photons, usually from the shifty Radiant heat mentioned above, seek to keep matter agitated. Most respectable photons just move a little faster when they see agitated matter, they don't want to get involved.
Convective heat is just conductive heat trying to escape the gravity of the situation, therefore the same rules apply to it.
Sexual heat is the odd situation where intellect has shifted from between one set of limbs to between the other pair. This may cause passing photons to stop and stare, usually yelling "Get a Room!"
So, as you can see there is no easy answer to that question... Er, what was it again?
"And, is the speed of darkness just slightly faster than the speed of light?"
Cosmic distances are fascinating. The light from our sun takes a finite time to reach us. If it disappeared suddenly - we wouldn't know until about 8 minutes later. What really knots my mind is that apparently the effects of the sun's gravity would also only disappear after that time.
"His conclusion, in this paper at IOP, is that both Einstein's prediction of the speed of light – 299,792,458 metres per second in a vacuum – is wrong"
1) Einstein didn't "predict" the speed of light. That speed has to be measured. It is always 1.
2) He correctly posited that the most elegant way out of the pretzel-shaped contortions that physicists were making to adapt their theories to the logical implications of electromagnetism and the absence of any experimental sign of an absolute rest system in which light was moving was to go whole hog and posit that the speed of light must be the same in all reference systems. Which means that durations and lengths differ between reference systems
3) Quantum mechanics comes in later, whereby you can have "faster than light" events in the small (resolved by changing to antiparticles in other reference frames) and the world-famous "entanglements" (which have nothing to do with any speed at all, except the speed of thought).
Didn't someone discover a few years ago that neutrinos actually have a very small but non-zero mass?
It was found that neutrinos can change 'flavour' on their journey. And if they can change, they must therefore be travelling at less than the speed of light. Because at the speed of light, null time passes, and no change would be possible. If a neutrino had zero mass, it would have no option to travel at anything other than the cosmic speed limit, speed of light.
If you flick a bowling ball with your finger, it's not going to move very quickly. If you flick a marble, it's going to belt off a lot faster because it's less massive. If you flick something with no mass whatsoever, e.g. a photon, it's going to move as fast as is cosmically possible (subject to any interfering medium in which it is travelling). You might ask yourself the question why doesn't it move off at an infinite speed? Then things get quite complicated, and intuition lets you down. You will see the photon move off at the cosmic speed limit, but the photon itself will experience no time on its journey.
There's quite a good article about the constancy of the speed of light here.
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There is no contradiction, just a surprise if this pans out.
Einstein said that there was a fundamental speed - c -. As it turns out, a massless particle is constrained to travel at c. So photons should travel at c. However if there are interesting second order effects that make the effective path travelled a little longer, well the measured speed over a large distance (one that can't see these effects) will see a slightly slower speed.
The big problem for casual reading of all this is that for most people, c and the speed of light are thought of as synonymous. They are not. c is the fundamental. It may be that c is indeed a very very tiny fraction faster than we see photons travel, at those scales we are able measure over. No big deal here at all. Sort of a neat result, nothing more. Indeed, the current understanding of why the measured speed at macro scale might be different requires that Einstein still be perfectly correct. What we gain is understanding of just how weird the vacuum is.
Back when Einstein worked out the physics, only light was known to travel at c, so there was no reason to differentiate the fundamental from the speed of light as an exemplar. Neutrinos have been assumed to be another massless particle, and so should also travel at c. Although their mass is currently an open question. We don't talk about the speed of neutrinos as a metric. Yet why we should talk about the speed of photons as a metric is no less specific. It is simply an accident of history that we use light as a surrogate for c. That there might turn out to be a curious feature of the QED and the nature of the vacuum that makes the measure of a photon's speed on a macro scale slightly slower, is just a nice result.
My understanding is that the speed of light is the maximum, because that is the mechanism for the propagation of fields. The concept of vacuum was invented to allow this definition to be precise. Hence, speed of light in vacuum is c. Speed of light in jam <c. Tachyons(?)>c.
Only, since then (19th century), we now know the vacuum is not empty, but a "seething quantum foam", so it appears that things can get slower, just not faster - and the fastest thing we know is a photon.
Although it has not been measured (to my very amateur knowledge), I believe that if the sun was to disappear the loss of gravity and light would be simultaneous. If the sun exploded, the shockwave would probably take hours/days...any astrophysicists want to chip in with real numbers?
My understanding of this article was the SN model was not correct...
P.
"because that is the mechanism for the propagation of fields."
Ugh. Light isn't the mechanism for propagation of fields. Photons are the manifestation of the propagation of the EM field. The other force fields propagate by other force carriers. Not all of them travel at c - only the massless carriers. A field is nothing special - simply something that can be measured throughout 3 dimensional space. The fields that defined the fundamental forces are a bit special, but again, only the EM field involves photons.
One suspects you mean classical force fields.
For a classical field you only have light and gravity, and these propagate at c. But if you include quantum theory fields it all gets vastly messier.
The weak force is mediated by the W and Z boson. These have mass. The strrong force is mediated by mesons, and they have mass, although the residual strong force is arguably more to the point and mediated by the massless gluon.
A field is just some property you measure in 3D space. Pressure is a perfectly good field. Albeit not one that measures one of the fundamental forces. It propagates at the speed of sound. You can have a perfectly good quantum field theory for sound. The sound particles are phonons, and they mediate the pressure field, and travel at the speed of sound in the material - which need not be isotropic.
I once asked an academic astronomer about the sun disappearing. He said it would be about 8 minutes before we saw the light go out - and the gravitational pull would then simultaneously disappear.
Presumably at that point the Earth is on a new slingshot trajectory rather than an orbit - so any shockwave would be chasing us.
However if the sun merely exploded then some/all of the mass is now an expanding cloud. So - would that cloud have the same gravitational pull on the Earth as the same mass formerly coalesced into a ball?
James Clerk Maxwell developed four equations that derived the speed of light.
He had to find two constants (constants in a vacuum) to do this.
Good to resolve some arguments (speed of gravity being a compression wave does not care -pathetic fallacy be damned- about units of electric charge and permeability) and no other mediating particles travel at C so there.
Crap is all over empty space, random ions, solar winds, dead stars and wandering planets: the poor little photon never stands a chance.
As an aside the wiki page has a major (and very irritating) error on it
The same error that Microsoft's Encarta had
Any one else see it?
If it turned out that gamma rays travel through space somewhat more slowly than the speed of light because they sometimes turn into electron-positron pairs, then the speed they had when they weren't in this state, but instead going their fastest... would be the speed of electromagnetic radiation in a vacuum, the ultimate speed limit of the universe.
So Einstein wouldn't be wrong at all if it were discovered that real-world light happens to travel slightly slower than the "speed of light" - especially if this effect were stronger for high-energy gamma rays, and so the limit for really low-energy long radio waves as they approach having no energy at all could still be measured.
After all, when Einstein came along, it wasn't as if Newton was wrong.
So even if this is a valid discovery, it's no threat to the foundations of relativity.
The paper seems to rely on the idea that it took the light from the explosion 3 hours to rise to the surface of the star - it kinda bounces around in the star until it can get a free ride in space to us, the neutrinos just head on out.
The neutrinos were only detected in one detector and so there is no evidence they came from said star.
Even if they did come from said star the accuracy we can give to how long after the detonation the light would be able to head on its way to us is pretty low - there's still a lot of star to get through and there's no practical evidence for what goes on in these things so we're still making guesses: If its a spinning star are we looking at it through the pole or the equator?
What frequency were the photons?
It's possible photons of different frequencies have very very small differences in the speed of light -- which, given the huge distance to the supernova, equates to 7 hours difference travel time to Earth vs. the neutrinos.
This tiny difference would not be measurable in an experiment involving measuring the speed of light across distances many magnitudes smaller.
eg. If the largest distance we could measure the speed of light over was from here to Pluto (around 0.0007 light years) we'd need to measure the distance (assuming our measure of time is exact) to within 4x10^(-9) metres to be sure the speed of light we find isn't sufficiently different from neutrino speed to lead to a 7 hour delay from the supernova 1987a (168,000 light years away).
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