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If the speed of light was greater, how would it show up?

  1. Nov 22, 2014 #1
    I have been reading some sites that claim that the speed of light was greater in the past. They do not say whether it was locally different or over the entire universe.

    My view is that either way, we could detect it. Locally would be an anomalous region of the universe looking very different to the rest, and that is not seen. If it C were different, then that too would show up. For example, E=mC2 would mean stars are considerably hotter, larger and may not form. The fine structure constant would change, and magnetic fields would be weaker. As the stars and galaxies show an amazing degree of uniformity and homogeneity, that must put very tight limits of any variation of C throughout the universe.

    If C were say 100 times greater, then the energy released would be far,far greater, so the core temperature would be large, the internal pressure correspondingly greater, and so harder for the gravitational effects to overcome, and so the star must be larger. If it is too large, then it will simply merge with neighbours and not really be a star, more a glowing hot cloud over many light years.

    Am I wrong on my assessment? Is there are value of C such that stars would not form?

  2. jcsd
  3. Nov 22, 2014 #2


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    Variable speed of light is an old idea that's like the smell of fish in the kitchen - it just won't go away. Were nature to orchestrate a carefully crafted conspiracy amongst the constants of nature, a variable speed of light could easily hide in the chaos. I fail to see any advantage in adding a conspiracy theory to current mysteries. It's like the thermos bottle fairy, it keep hot things hot and keep cold things cold, but, how do it know? Besides, I think nature is too lazy to go to that much trouble.
  4. Nov 22, 2014 #3


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    I'm not sure that analysis is correct. It's too simplistic. To do the real calculation, you'd need to examine how the speed of light factors in to the various forces. The first thing you'd have to do is not change the speed of light: the speed of light, as a dimensionful quantity, has no intrinsic meaning. To change something that will actually change how the universe behaves, you'd have to change some dimensionless quantity that depends upon the speed of light.

    Next, you'd have to follow through with all of the implications of this. Changing this dimensionless quantity (whatever it is: there are multiple choices) would impact nuclear physics by modifying the energies of various nuclear states and the rates of various nuclear decays. It would modify the electron orbital structure of atoms and thus chemistry would be very different with a different speed of light, as would the spectra of various atoms and molecules.

    So changing the fundamental constants would have far-reaching impacts in a large variety of contexts. So far no such change has been detected. Steve Carlip did a rather interesting post on this over at talkorigins a while ago:
  5. Nov 22, 2014 #4


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    Is there any difference between "c" as the "speed of light" and "c" as a "physical constant"?
  6. Nov 23, 2014 #5
    Hi Calnoth. Thanks for your detailed reply. I agree that in order to make detailed calculations, it would be necessary to explore via the dimensionless constants. Clearly Alpha already depends upon C, and is observed to be a constant to better than 1 part in a million, then other constants must change. Given that scenario, then the discussion would dissolve and digress into details, with virtually a blank canvas where any constant is not really a constant.

    I did post a message onto this stale three, that explores the visible consequences of C being very much slower


    I am thinking macroscopically rather microscopically. Gases can be considered in bulk ( thermodynamics ), or statistically, or quantum mechanically. They will broadly give the same answer, although not it detail.

    I am really looking for order of magnitude effects. If C were 10 times greater, or a million times greater. So how could we tell what the speed of light was in the past? Take a specific case. The galaxy Andromeda. That is 2.5 million light years away, +/- 5%. If the speed of light really had changed we should be able to detect it in Andromeda, as the astronomy has progressed so that individual stars and binary stars can be studied.
  7. Nov 23, 2014 #6


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    The most likely method for detecting a different speed of light in the past is to look at atomic spectra. The spectra of atoms are shifted in rather chaotic-seeming ways by changes in the speed of light (well, really changes in [itex]\alpha[/itex]), meaning it is in principle rather easy to estimate changes in the speed of light in the distant past just by observing the spectra of elements heavier than hydrogen. People have tried this and shown that the speed of light has varied by no more than a couple percent.
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