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 …
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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