Physicist suggests speed of light might be slower than thought

Physicist James Franson of the University of Maryland, Baltimore County has captured the attention of the physics community by posting an article to the peer-reviewed New Journal of Physics in which he claims to have found evidence that suggests the speed of light as described by the theory of general relativity, is actually slower than has been thought.

The theory of general relativity suggests that light travels at a constant speed of 299,792,458 meters per second in a vacuum. It's the c in Einstein's famous equation after all, and virtually everything measured in the cosmos is based on it—in short, it's pretty important. But, what if it's wrong?

Franson's arguments are based on observations made of the supernova SN 1987A–it exploded in February 1987. Measurements here on Earth picked up the arrival of both photons and neutrinos from the blast but there was a problem—the arrival of the photons was later than expected, by 4.7 hours. Scientists at the time attributed it to a likelihood that the photons were actually from another source. But what if that wasn't what it was, Franson wonders, what if light slows down as it travels due to a property of photons known as vacuum polarization—where a photon splits into a positron and an electron, for a very short time before recombining back into a photon. That should create a gravitational differential, he notes, between the pair of particles, which, he theorizes, would have a tiny energy impact when they recombine—enough to cause a slight bit of a slowdown during travel. If such splitting and rejoining occurred many times with many photons on a journey of 168,000 light years, the distance between us and SN 1987A, it could easily add up to the 4.7 hour delay, he suggests.

If Franson's ideas turn out to be correct, virtually every measurement taken and used as a basis for cosmological theory, will be wrong. Light from the sun for example, would take longer to reach us than thought, and light coming from much more distant objects, such as from the Messier 81 galaxy, a distance of 12 million light years, would arrive noticeably later than has been calculated—about two weeks later. The implications are staggering—distances for celestial bodies would have to be recalculated and theories that were created to describe what has been observed would be thrown out. In some cases, astrophysicists would have to start all over from scratch.

More information: Apparent correction to the speed of light in a gravitational potential, J D Franson 2014 New J. Phys. 16 065008 DOI: 10.1088/1367-2630/16/6/065008 . http://iopscience.iop.org/1367-2630/16/6/065008/

The effects of physical interactions are usually incorporated into the quantum theory by including the corresponding terms in the Hamiltonian. Here we consider the effects of including the gravitational potential energy of massive particles in the Hamiltonian of quantum electrodynamics. This results in a predicted correction to the speed of light that is proportional to the fine structure constant. The correction to the speed of light obtained in this way depends on the gravitational potential and not the gravitational field, which is not gauge invariant and presumably nonphysical. Nevertheless, the predicted results are in reasonable agreement with experimental observations from Supernova 1987a.

In any reaction, there is an "error factor" in that some of the mass goes missing. No one can find that missing mass.

Nobody could find it because until we had nuclear weapons nobody could measure it. Einstein predicted it, but in a typical chemical reaction it's so small it would never have been missed. For example, the binding energy of a water molecule is about 6 eV, whereas every nucleon has a mass of around 1 GeV [I'm going to do some rounding to make the math very simple.] For H₂O, the molecule has a mass of (2 x 1 protons Hydrogen + 8 x 1 protons, Oxygen + 8 x 1 neutrons Oxygen) GeV + 10 * 0.5 MeV [for all the electrons]. Throwing away the electron mass of 5 MeV as insignificant to the calculation, the "missing" mass of bound hydrogen and oxygen in water is 6 / (18 * 10 ⁹) or about 3 * 10^-10 less than hydrogen and oxygen constituents. We can't really measure three parts in ten billion of mass, especially when the isotope balance accounts for a lot more mass than that all by itself.

You can re-do the calculation for something like high explosive, or gasoline, which have much higher energy densities and therefore much more binding energy, but it doesn't change things by more than a factor of about 10.

So 5%? No. More like 300 millionths of a percent.

And incidentally, even for nuclear reactions, you don't get 5%. For a typical nuclear fission, say ²³⁵U, you have [to the same order of rounding] about 235 GeV [the actual number is around 218 GeV] in the bound nucleus. When it fissions, you get around 200 MeV. So that represents a binding energy of about 200/235000, or 0.085% of the mass would go "missing" as energy.

This entirely a function of how much stronger the strong nuclear force [which binds nucleons] is than the electromagnetic force, which binds molecules.

I recall reading somewhere the the hardest vacuum in interstellar space has something like one atom per cubic meter,

It's about 0.1 atom per cc, which is about 100,000 atoms per cubic meter.

the article refers to the fact that space is not a “perfect” vacuum, but it's pretty darn close to one.

Ultra High Vacuum is considered to be 10^-12 to 10^-15 atmospheres. [One trillionth, to one quadrillionth of an atmosphere.] There are some labs doing Extreme Ultra High Vacuum, which is anything less than 10^-15.

In contrast, interstellar space, over which this measurement is taken is on the order of 10^-20 atmosphere, which is around 100,000 times better [weaker] than what is considered an extreme terrestrial vacuum.

So, yeah. Pretty darn good.

the old analogy being an atomic nucleus being the size of a baseball, the electron (back in the days they were considered physical objects instead of probability clouds) would be the size of a grain of sand several hundred feet away from the nucleus and the next closest atom would be found a few miles away.

The modern picture is that a proton has a spatial extent of about 1.5 x 10^-15 meters, and the Bohr radius of hydrogen, where you expect to find an electron "most of the time" is about 5.3 x 10^-11 m. So, if the proton were a baseball, [regulation: 37 +/- 2 mm radius (though it looks bigger when it's a curve ball)] the probability cloud of the electron extends around the proton-baseball in a sphere with a radius of about 1.3 kilometers.

The "next closest atom" depends on a lot of things, but in condensed matter it would actually be "pretty close." For example, molecular hydrogen has an average size about twice the Bohr radius. So, the electron clouds actually get very close, and in favorable bonding orbitals the intra-molecular distances can actually be "smaller" than the constituent atoms. [But, order of magnitude, the overlap is usually about like what you see in atomic hydrogen vs. molecular hydrogen. Some, but "not much" overlap of the electron clouds of the respective single atoms. This is not to say that the molecular orbitals are qualitatively the same as atomic orbitals. They aren't, or there would be no such thing as chemistry.]

This being the case, the space inside a glass lens at the scale of a photon would be as empty as the interstellar medium, to the photon it would be a vacuum, but it travels much slower through the glass than through “space”. Why do you suppose this is?

The correct answer is that condensed matter [and even gases at atmospheric pressure] are not really very much like empty space, because the old, oversimplified picture is wrong. Even if you just consider the nucleons, it doesn't hold up all that well, because proton and electron are not inert baseballs or grains of sand. The electromagnetic force is tremendously powerful, so the likelihood that an electron or an atom or molecule will interact with a photon is quite high.

This is also the reason why condensed matter is largely impenetrable: the electromagnetic forces holding the structure together, and binding the electrons to the lattice, repel [at the atomic level] other condensed matter that tries to "push them too hard".

On the other hand, it also explains friction, because, as long as you don't "push too hard" the protons in the surface are trying to bond to electrons in the materials pushed up against them.

These are, of course huge oversimplifications. The real bottom line is that the electromagnetic interaction is so powerful, that photons, which are excitations of the electromagnetic field, couple so strongly to charge carriers that you don't have to get atoms too close together before it's impossible for photons to avoid them. In "Space" the distance between atoms isn't close enough, but in the atmosphere it is. Go back to the vacuum comparison: Vacuum is a direct function of how many particles there are per unit volume. At atmospheric pressure, there are 10¹⁹ molecules per cc, ~20 orders of magnitude more electromagnetic charge carriers than in empty space. That's a lot more chances for the electromagnetic interaction -- which is 10³⁶ times stronger than gravity -- to grab a photon flying by...

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