Why Do They Always Ask Tough Questions?

Posted on by Chad Orzel

I was away at Readercon this weekend, which meant a fair amount of hanging out in a hotel bar socializing with writer types. One of whom was working on a novel that will have some hard-science elements to it, and had been looking for a physicist to ask questions of. Having just sat down, and being well supplied with beer, I offered to give it a shot.

Unfortunately, the first question she asked was (paraphrasing slightly, as I was well supplied with beer):

Is the speed of light a property of space-time, or a property of light?

I still don’t quite know what to do with that one. Further elaboration suggested that the reason for the question had something to do with wanting to know why gravitational forces travel at the speed of light, which led me off into a whole bunch of stuff about the particle exchange picture of forces, and quantum field theories, and so on. It probably went downhill from there, as I don’t think my explanation of neutrino oscillations was very helpful…

If you’ve got any Deep Thoughts on these topics, this would be a good comment thread in which to post them. And, incidentally, help me test the recently tweaked spam countermeasures on the blog…

6 thoughts on “Why Do They Always Ask Tough Questions?”

  1. I would call it a property of spacetime, as we can think of the metric for our system as (1, -1, -1, -1), meaning light moves at the speed of light in all directions. However, light is so fundamental that it reveals the metric, so there is room for argument. Since gravity must travel at the same speed as light, it seems reasonable for it to be spacetime rather than light that gets the responsibility.

  2. Ah, that is actually a point I make when teaching modern physics: SR tells us there is a limiting speed, the fact that this speed limit is attained by light is inessential, though of course historically important. Ultimately SR has nothing to do with properties of light specifically.

  3. I agree — it’s definitely a property of spacetime. It’s the speed at which a massless particle will be observed to be moving relative to a local massive observer.

    Photons are massless. So are gravitons (insofar as we think that there are probably gravitons, even though we haven’t directly observed them as individual particles yet). Once upon a time we thought neutrinos were massless.

    The original motivation for relativity was the fact that Maxwell’s equations gave a universal speed for light…. But SR transcends electromagnetism.

    The name “speed of light” is perhaps thus not the most appropriate name, but it’s what we’ve got. It’s not really a light thing any more than “tidal forces” is a beach thing.

    -Rob

  4. Time is what an honest clock reads. If you eschew oscillators use radioactive decay. If you compare honest clocks and your physics was not accurately predictive, you change the physics. Spacetime is what a ray of light maps. If your vacuum triangles’ internal angles do not add to 180 degrees, you change your geometry.

    Vacuum is the interface. Unlike classical vacuum, the real world (lack of) stuff is filled with footnotes. Unshieldable gravitation negotiated as free fall; Casimir effect, Lamb shift, Rabi vacuum oscillations, electron anomalous g-factor… Heisenberg Uncertainty.

    The vacuum is measured with light. It is not cause and effect, it simply is – and must be internally consistent with the rest of observation plus its mathematical modeling as physics.

    Is the vacuum homogeneous and isotropic? Homogeneity looks good. Isotropy is untested in the mass sector. Does the vacuum have a chiral pseudoscalar background (teleparallel gravitation rather than metric) coupled to mass distributions?

    1) Do local left and right shoes vacuum free fall identically? Somebody should look.

    http://www.mazepath.com/uncleal/lajos.htm
    2 days, 2 calorimeters, 3×10^(-18) sensitivity
    http://www.mazepath.com/uncleal/qz3.pdf
    90 days, Eotvos balance, 10^(-13) sensitivity

    2) If gravitation has an EP parity violation then inertial and gravitational mass can be decoupled. General Relativity works as claimed, but it is still wrong for a falsified founding postulate. An EP parity violation empirically falsifies half of string theory, falsifies the isotropy of space, falsifies conservation of angular momentum for opposite parity mass distributions via Noether’s theorem… and does NOT contradict any existing observation. How is that for slippery?

    3) If the vacuum is chiral toward mass distributions then parity violations are intrinsic to theory, biological homochirality is a natural consequence, and physics is not point-like but instead has an emergent scale. It could be a whole lot of fun!

    That is a lot of stuff to come forth from averaged nothing.

  5. C is defined in terms of the permeability and permittivity of space(-time), right? Is there any definition for those that is not electromagnetism specific? If there is any way to assess those quantities in a non-EM way, then it’s uniquely the property of space, but if not, it would appear to be the interaction of the two, no?

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