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darky999
2015-Oct-17, 02:46 PM
First post on this forum - go easy!

I do have a PhD in real life, but in Superconductors (and NOT the theoretical side, I was a practical scientist), not theoretical physics. So I ask the below with some science knowledge, but i'm certainly not an expert.

The question is does the finite nature of the speed of light have anything to do with the quantum vacuum, and the constant creation/annihilation of particle/anti-partical pairs? It struck me that there is firstly no way this was an original thought (and some digging got me this recent paper from 2013: http://epjplus.epj.org/images/stories/news/2013/epj_d_2a-03-13.pdf which also references a 1957 paper by Dicke - R.H. Dicke, Rev. Mod. Phys. 29, 363 (1957) ), however I thought it would be a fascinating discussion.

My initial thought was that c is finite due to these quantum particles giving a form of ‘drag’ on the photons, perhaps c is infinite in a real ‘total’ vacuum (which we think is not possible to have). The above paper is more nuanced of course and talks about ultra-short range, but much longer than planck length, fluctuations.

Yet it’s a fairly obvious viewpoint if one considers the ‘fact’ that the speed of light, plus the permeability & permittivity of free space are numbers that have no derivation, they are what they are. Which is really quite unsatisfying. I have no idea if this is all easily explained, however I suspect it is not if there is a 2013 paper from proper scientists that is seriously discussing having a variable c, and also proposes an experiment to measure this.

Any thoughts much appreciated!

Cougar
2015-Oct-18, 01:43 AM
The question is does the finite nature of the speed of light have anything to do with the quantum vacuum....

I'm surely no expert either, but the problem is we have such a poor handle on the quantum vacuum. Quantum field theory estimates the magnitude of the vacuum energy 120 orders of magnitude bigger than the observed value... so we're apparently not looking at this in the right way!

Another problem with your idea: testing it would require modifying the vacuum, which basically involves creating a vacuum metastibility event, i.e., a bubble of lower-energy vacuum expanding at nearly the speed of light that catalyzes and converts our universe to a lower vacuum energy state, destroying all that we know without forewarning. I think we might move that experiment to the bottom of the list. :)

ShinAce
2015-Oct-18, 02:50 AM
The semi-colon at the end of your link causes it to fail.

I skimmed the paper quickly. I like it, but I need to take the time to go through it properly. I'm not sure how useful I will be as I'm not that well versed on vacuum polarization. Plus I don't know how comfortable I am with the idea that electric and magnetic properties led us to finding a constant speed of light, which led us back to defining the 'electric' and 'magnetic' properties of the vacuum as constant, which now leads us back to why the speed of light is constant. I fear there may be some circular reasoning, not necessarily hidden, in there.

Or, put another way. I don't know how comfortable I am with the idea that the authors seem only concerned with pair production of fermions to explain the speed of massless bosons which couple to electric charges. Meanwhile, work on supersymmetry continues. I don't think both ideas can be right and complete.

edit: They do propose a way to test it. Essentially, a setup using femtosecond laser pulses in a LIGO type setup(laser cavity) to detect pulse broadening.

ShinAce
2015-Oct-29, 03:46 AM
I'm still not 100% sure, but here's what I think.

The permittivity is affected by an external magnetic field. So they're predicting an easy to computer deviation in the speed of light under a uniform electric field. They're also predicting a pulse width broadening on something the size of a LIGO setup.

The pulse broadening, as I understand it, is theoretically detectable but pretty close to the detection limit.

The variation in light speed from an applied electric field shouldn't be too bad. Basically build a parallel plate capacitor around the specially made resonator cavity placed in the interferometer arm. So you'd need a specially engineered resonator cavity that can handle vacuum.

I can't shake the feeling that there must be better ways to test it. Where are the experimentalists?

Ken G
2015-Oct-30, 09:02 AM
One problem I have with saying the speed of light is caused to be what it is by physical effects in the vacuum is that the speed of light can be changed by other physical effects that are not in the vacuum, like things that happen in a glass prism. But relativity maintains that the speed of light in a vacuum is a kind of medium of exchange between space and time, which shows up in things like time dilation. But no one thinks a light clock made of glass is going to give you the time dilation inside that glass, so the "c" parameter in relativity does not include physical affects that alter the speed at which light actually propagates.

So the perspective of relativity is not that whatever speed light is moving at determines the relativistic effects, it is the opposite-- relativistic effects tell light what speed to propagate at if no other physical effects enter to change that. But we'd still have the c parameter in relativity, showing up in time dilation and so on, even if we had never discovered light at all. So c cannot depend specifically on anything that involves how light couples to matter. Put differently, all massless particles must move at the same speed-- there can only be one c. So gluons must move at c, and gravitons (if they exist) must move at c, even though they won't couple to electron-positron pairs the way light does. Also, particles with very small mass, like neutrinos, must also move at close to c, even though they also don't couple to positron-electron pairs the way light does.

So what I'm saying is, if it turns out that the speed of light is affected by strong electric fields, and that alteration can be modeled by saying virtual positron-electron pairs change the speed that light propagates, it will only mean that they are acting like glass. It won't mean that is why c has the value it has, because it won't explain why neutrinos move at nearly c, and it won't explain the c parameter that appears in relativity. I'd say these authors are looking for reasons that light does not move at exactly c, not that they are looking for why c is what it is.

As a final note, I would point out that it is always the nature of science to try to dig deeper, so you are right that not knowing why c has the value it has gives us a question to dig into. But a theory that explains why c has the value it does will likely have some new parameter that we will have the same question about. We don't know why G has the value it does, or h, so some deeper theory might "explain" these values, but that deeper theory will likely either need its own parameters, or else it will be a kind of trivial explanation like the anthropic principle in the multiverse picture (that all values exist in other universes, and we are in a universe that has the value that we see because that's just where we live). So I think it is just the nature of the beast to have elements to our theories that we cannot explain, though it is certainly worth looking for deeper theories that include their own mysteries. The way I like to put that is, science is not about eliminating mystery, it is about replacing more superficial mysteries with more profound and better unified ones.

darky999
2015-Nov-13, 01:35 AM
One problem I have with saying the speed of light is caused to be what it is by physical effects in the vacuum is that the speed of light can be changed by other physical effects that are not in the vacuum, like things that happen in a glass prism. But relativity maintains that the speed of light in a vacuum is a kind of medium of exchange between space and time, which shows up in things like time dilation. But no one thinks a light clock made of glass is going to give you the time dilation inside that glass, so the "c" parameter in relativity does not include physical affects that alter the speed at which light actually propagates.

So the perspective of relativity is not that whatever speed light is moving at determines the relativistic effects, it is the opposite-- relativistic effects tell light what speed to propagate at if no other physical effects enter to change that. But we'd still have the c parameter in relativity, showing up in time dilation and so on, even if we had never discovered light at all. So c cannot depend specifically on anything that involves how light couples to matter. Put differently, all massless particles must move at the same speed-- there can only be one c. So gluons must move at c, and gravitons (if they exist) must move at c, even though they won't couple to electron-positron pairs the way light does. Also, particles with very small mass, like neutrinos, must also move at close to c, even though they also don't couple to positron-electron pairs the way light does.

So what I'm saying is, if it turns out that the speed of light is affected by strong electric fields, and that alteration can be modeled by saying virtual positron-electron pairs change the speed that light propagates, it will only mean that they are acting like glass. It won't mean that is why c has the value it has, because it won't explain why neutrinos move at nearly c, and it won't explain the c parameter that appears in relativity. I'd say these authors are looking for reasons that light does not move at exactly c, not that they are looking for why c is what it is.

As a final note, I would point out that it is always the nature of science to try to dig deeper, so you are right that not knowing why c has the value it has gives us a question to dig into. But a theory that explains why c has the value it does will likely have some new parameter that we will have the same question about. We don't know why G has the value it does, or h, so some deeper theory might "explain" these values, but that deeper theory will likely either need its own parameters, or else it will be a kind of trivial explanation like the anthropic principle in the multiverse picture (that all values exist in other universes, and we are in a universe that has the value that we see because that's just where we live). So I think it is just the nature of the beast to have elements to our theories that we cannot explain, though it is certainly worth looking for deeper theories that include their own mysteries. The way I like to put that is, science is not about eliminating mystery, it is about replacing more superficial mysteries with more profound and better unified ones.


Thank you for a very well written and well thought out reply.

I absolutely get your point on the analogy with light in glass (for example). One thought it that glass is real, the atoms exist. But virtual particles are not real, they can't violate Heisenberg. So perhaps, just perhaps (and this is a long shot!) this breaks the analogy to glass.

As we are comparing the interaction of light with something real (glass) and something virtual in nature.

I'm thinking off the cuff.

I do agree that the whole topic of why the fundamental constants have their values is interesting.

Ken G
2015-Nov-13, 01:58 PM
I agree there are no bad ideas-- let's get all the ideas on the table.

John Mendenhall
2015-Nov-17, 09:20 PM
Thank you KenG, ShinAce, Cougsr, and Darky999. Darky's link to the relevant paper is great. If these guys are right in the paper . . .

Regards, John M.

ShinAce
2015-Nov-17, 10:45 PM
In case anyone is wondering, these authors haven't offered a 'one off' proposal for the speed of light not being a universal constant.

There's also this paper:
http://arxiv.org/pdf/0906.3018v1.pdf (see section 5)