Evidence of a Correction To the Speed of Light

KentuckyFC writes: In the early hours of the morning on 24 February 1987, a neutrino detector deep beneath Mont Blanc in northern Italy picked up a sudden burst of neutrinos. Three hours later, neutrino detectors at two other locations picked up a second burst. These turned out to have been produced by the collapse of the core of a star in the Large Magellanic Cloud that orbits our galaxy. And sure enough, some 4.7 hours after this, astronomers noticed the tell-tale brightening of a blue supergiant in that region, as it became a supernova, now known as SN1987a. But why the delay of 7.7 hours from the first burst of neutrinos to the arrival of the photons? Astrophysicists soon realized that since neutrinos rarely interact with ordinary matter, they can escape from the star's core immediately. By contrast, photons have to diffuse through the star, a process that would have delayed them by about 3 hours. That accounts for some of the delay but what of the rest? Now one physicist has the answer: the speed of light through space requires a correction.

As a photon travels through space, there is a finite chance that it will form an electron-positron pair. This pair exists for only a brief period of time and then goes on to recombine creating another photon which continues along the same path. This is a well-known process called vacuum polarization. The new idea is that the gravitational potential of the Milky Way must influence the electron-positron pair because they have mass. This changes the energy of the virtual electron-positron pair, which in turn produces a small change in the energy and speed of the photon. And since the analogous effect on neutrinos is negligible, light will travel more slowly than them through a gravitational potential. According to the new calculations which combine quantum electrodynamics with general relativity, the change in speed accounts more or less exactly for the mysterious time difference.

Don't mess with "c"

[NReitzel]

There's an alternative explanation. Space-Time could have non-zero viscosity, and slow down photons.

There are a lot of reasons to consider that space might have a viscosity. For one thing, it would neatly explain the expansion of the universe, without the necessity of invoking dark matter and dark energy.

We live in interesting times!

-- Norm Reitzel

Re:Which means

[VernonNemitz]

There seems to me to be a slight error in the original article. Neutrinos have been determined to possess mass. It is only a slight amount of mass, but it precludes them from being able to travel at exactly the speed of light. How close to light-speed do they normally travel? I can't say. But it is reasonable to think that the distance from Supernova 1987A to Earth should have led to a slightly later arrival time, for neutrinos, than if they had actually traveled at light-speed.

The preceding relates to another thing, the quantum-mechanical mechanism for interfering with the actual speed of light. Those pairs of virtual particles that form also have mass. That means, while they temporarily exist, they also cannot be traveling at exactly light-speed; they have to be traveling slightly slower.

Re:So, what's the correction?

[Anonymous Coward]

"c" is defined as 299,792,458 meters per second, which is the idealized speed of light. The practical speed of light can be slower, like when propagating through glass. If this new data is correct, then neutrinos move faster than photons through a gravitational potential.

That's not what I took away from this...

[SXO148]

From what I gathered, the basis of Franson's hypothesis is the idea that a photon can spontaneously split into a matter-antimatter pair (this is also the idea behind Hawking radiation). Unless something crazy happens (again, see: Hawking radiation), that pair will almost instantly recombine, creating a photon with the same size and energy as the original photon.

Franson's idea, as I understand it, is that during the small window between creation and annihilation, the massive particles are under the influence of gravity, which bleeds off energy. When the pair recombines, it results in a reduced velocity of the photon.

Now, as I understand it, reducing the energy of a photon would merely reduce its frequency (red-shifting), not affect its actual velocity.

However, over long distances, the total time required for a photon to travel distance X would thus be slightly more than X/c, based on the proportion of time spent as a pair of massive particles, rather than as a massless photon. From a statistical perspective, this yields an average velocity of slightly less than /c/ (the speed of light in a vaccuum).

This seems reasonable to me, at least at first.

mrsquid0 raises an issue, though: Photons in the visible light range are not sufficiently energetic to create an electron-positron pair. I do not know if the photons in question were in the visible light range or not.

NoNonAlphaCharsHere also raises an important point: the electron-positron pair *cannot* travel at the speed of light. In fact, he/she raises an even better idea than Franson; my reading of Franson's explanation is that gravity is slowing down the particles (gravity field behind the photon), but there's just as much opportunity for gravity to *speed up* the particles (gravity field in front of the photon).

Now, I don't feel like doing all the math for this one little message, so here are the things I would consider before taking this article (and the original paper) at face value:

• This is predicated upon the idea that gravitational fields affect neutrinos less than they do photons and ordinary matter. Do we know this to be true?
• For the slowing down of the e-p pairs bit, there are two opposing forces:
  • I expect that higher-energy (higher-frequency) photons are more likely to generate an e-p pair than lower-energy photons. This means they are more likely to be slowed down
  • When a photon of energy /E/ forms an e-p pair with combined mass /m/, there is E - m*c*c energy unaccounted for. I expect that that energy ends up as kinetic energy, resulting in a velocity v = sqrt(2 (E - m*c*c) / m). Therefore, higher energy photons will have more energy left over and the e-p pair will thus being going faster.
• According to the Wikipedia article on Pair production, the spontaneous formation of a matter-antimatter pair can only occur inside a nucleus (or momentum could not be conserved). However, this necessarily involves the photon traveling through a non-vacuum, which will necessarily slow it down.

Re:So, what's the correction?

[lgw]

An easier example of this: light moves much slower than c in glass, or in water. The open question is: does light move non-trivially slower than c in the vacuum of space (which is not an idealized vacuum).

I am dubious

[mbone]

When 1987A happened, it is fair to say that an enormous amount of attention was placed on those neutrinos - >> 1 paper per neutrino. The report of an earlier neutrino burst from the Mt Blanc LSD was discussed at length - see Arnett 1987 [harvard.edu] Table 1 for the time line.

The facts are these - the optical supernova could not be accurately timed, it wasn't bright at Feb 23.10 and it was at 2 / 23.443. The Mt Blanc LSD burst was at 2 / 23.12, while the other two detectors had a mutual burst at 2 / 23.316. Note that both neutrino bursts occurred before the optical SN was detected, and also that none of the other detected picked up the Mt Blanc LSD burst.

All of this has been known a long time, and numerous theories have been introduced to explain it.

- formation of a nlack hole [harvard.edu] (from the neutron star)
- formation of a quark star [harvard.edu] (from the neutron star)
- the Mt Blanc data were unrelated to the SN (that appears to be Arnett's viewpoint).

So, this is another explanation, and not a super compelling one to me. It will clearly never be proven from the SN 1987A data - the next such close supernova should have a lot of neutrino data, and maybe will resolve the issue.

Re:So, what's the correction?

[Bryan Ischo]

In my conception (which may be flawed; I came to this conclusion after university physics classes that I didn't always understand as well as I should have, and these were 20+ years ago), the speed of light is governed by "the rate at which things can happen".

Electromagnetic waves propogate because a changing electric field produces a changing magnetic field which produces a changing electric field, etc. For reasons that I can't remember these changing fields occur in a slightly offset position each time, so that the fields move through space as they create each other.

If causes and effects could occur at an infinite rate, the waves would move infinitely fast; but since there always has to be a time gap between a cause and an effect, there is a fixed upper bounds for the rate at which these fields can produce each other.

There is also a fixed lower bounds on the minimum offset that can occur between the electric and magnetic fields.

So what you have is essentially effects occurring as quickly as possible over distances as small as possible. The ratio of the smallest possible time between a cause and an effect, and the smallest possible distance between an electric field and the magnetic field it produces and vice versa is ... the speed of light.

So why can't light go faster than c? Two reasons really: a) things "can't happen" faster than the cause-effect relationship of a magnetic field producing an electric field, and vice-versa; and b) distances between an electric field and the magnetic field it produces, and vice-versa, can't be smaller.

I vaguely remember that this is related to one of the cool aspects of Calculus - the ability to take the ratio of an infinitesimally small number to another infinitesimally small number, each expressed as a limit approaching zero, and get a calculatable, real number result.

In this case, if you take the limit as distance approaches zero, divided by time as it approaches zero, you get the speed of light - the ratio of two infinitesimally small numbers (the smallest unit of distance over the smallest unit of time).

Anyway that's how I explain it to myself.