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Is the Speed of Light truly constant?

January 23, 2009 on 11:53 am | In cosmology, relativity |

This is an interesting question, for a number of reasons. First off, many of the things we think of as fundamental constants may not be constant. First off, most macroscopic physical things change over time. Second off, the conditions in the Universe are not the same today as they were in the past; the Universe was hotter, denser, and more energetic in the past. And third, we already know that some physical constants, such as the fine structure constant, do change at extremely high energies. This is where the idea of the unification of forces comes from:

Well, the fine structure constant is made up of a combination of three other fundamental constants: the electron charge, Planck’s constant, and the speed of light. One of these must change, at least a little bit, as a function of either energy or time. Well, yesterday, a scientist named Lorenzo Iorio published a paper on the limits of changes in the speed of light. This is really interesting for a number of reasons. Ready?

If the speed of light was faster in the past, the Universe is much younger than we think it is now. It also means that it’s possible that there was no inflation. Since our understanding of the Universe hinges on the constancy of the speed of light, this is an important thing to measure.

It may be possible for other constants to change over time, too. I know of one person who things that Planck’s constant changes over time as well. Her theory is that the speed of light and Planck’s constant both change proportional to (the age of the Universe)^(1/3). Although I disagree with her conclusions, it is certainly a possibility.

Or, that is to say, it was until this recent paper came out. If you watch the planets move in orbits around the Sun, the point of closest approach, or their perihelia, changes over time in a very predictable way.

This change, observable in Mercury, Venus, Earth, and Mars over multiple centuries, is very sensitive to the speed of light! So you predict the motion of the planets based on the laws we know, and then you look for anomalies, which will give you evidence that the speed of light is changing.

These measurements are incredibly precise over the last century or so, and the conclusion is reached that over that time, with more than a 99.7% certainty, the speed of light has not changed at all.

Phew! So it’s still possible that the speed of light changed in the past at some point, but this piece of research indicates that, as best as we can tell, it isn’t changing now. So you can keep looking for evidence of changes, past and present, if you like, but I’m convinced that it’s 299,792,458 m/s now, that it always was in the past, and that it always will be in the future.

That’s why we do the experiments, though, because you never know what new evidence is going to come up! And speaking of what comes up, the new Carnival of Space is up, and the Martian Chronicles is hosting. How appropriate that I just wrote about terraforming Mars! Have a great weekend, folks!


8 Comments »

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  1. I don’t believe it for one second. The speed of light was fixed from the very beginning. Co-moving spaces of the metric was what occurred in the inflationary beginning not variance. Also, Planck’s constant changing, give me a break. In Quantum Field Theory (QFT) h will be constant at least up to unification energy so that it is good in the semi classical approximation too. Aka QFT in curved space-times are the approximate theories which give the excellent Bekenstein-Hawking entropies of black holes. Change the speed of light and or Planck’s constant at any point in the evaporative history of a black hole (which replicates the vacuum state history of the universe going back in time)and you destroy both the classical and quantum nature of it (especially thermodynamics).

    Comment by mark a. thomas — January 23, 2009 #

  2. It should be mentioned that when someone (a researcher for example)believes that they have discovered an anomalous effect or have a new point idea that would rock a theory that any such approach should be considered dubious or highly suspect. Instead of explaining more of what we know these approaches tend to tear down the existing science paradigm and do not really explain much of anything. The saying is ‘extraordinary claims require extraordinary evidence.’ or something like that. These type of approaches look for single ‘chinks in the armor’ or a single chip in the ‘foundation stone’ to overturn the science consensus. It is laying a heavy bet (with very bad odds) and it usually involves the person’s overriding ego(like in wow, I am going down in history as the person who brought down the paradigm of General Relativity or Quantum Mechanics). Unfortunately, sometimes these anomalous approaches get far too much attention in the press. Science is bigger than the individual and is self correcting. It takes more than taking an idea or an anomalous effect into a theoretical framework to change the paradigm. You would have to show how the new theory change is better and how it explains more, not that it tears down what exists. Another point is that paradigm change in Science is very slow. Even famous researchers when they publish ‘peer reviewed articles’ do not necessarily get their views accepted at all or if their view is accepted IT TAKES YEARS. I repeat IT TAKES YEARS. I followed a paper of Stephen Hawking and I wondered why it did not get rapid acceptance overnight. It was a very cool paper. I took over 4 years for the insights of this particular paper to get around and accepted for the most part. I am sure Einstein’s theories did not get accepted overnight as well.

    Comment by mark a. thomas — January 24, 2009 #

  3. No way that the speed of light is a constant. There have been multiple universities that have slowed light, sped it up (by 8000 times) and have taken it to a dead stop. I solemnly believe that the earth is extremely younger than we have predicted. Less than 10,000 years to be exact.

    Comment by Philos — May 3, 2009 #

  4. xrDeQN comment6 ,

    Comment by Mdufomaq — June 25, 2009 #

  5. Is not the speed of light dependant upon the size of the universe?

    Comment by JM — September 30, 2009 #

  6. Philos your absolutely right. How in the universe can light be a constant when it can be affected by gravity? We can blatently see that when we look at a black hole. The further away an object is the older it is? How can that be when when light can be slowed down by simple particles of any nature. The further away an object is the harder it is for light to get to your eyes because of the obstacles it has to get through. You don’t need the highest qualification in astonomy to figure that out. Man, what other lies must we believe just because they have a degree.

    Comment by Junior — January 17, 2010 #

  7. The link to where you disagree with L. Riofrio’s conclusions is of great interest to me, as her viewpoint has just come to my attention. Can you please repair the link and let me know when you do? You are the best critic of her work I have found with a fair amount of digging, but I feel that is more due to lack of mention than lack of disagreement.

    Comment by Josh Black — August 11, 2010 #

  8. Einstein says nothing can travel faster than light, but according to the inflation theory of the universe it inflated to the size of the solar system in a fraction of a second. So for this to happen, it should have expanded at a speed faster than the speed of light. How is it possible, can anyone explain. If no one is able to explain, I doubt that either the theory that speed of light is constant is wrong or the inflation theory is wrong…. Kindly send the answer to me on my email (ahmadmail@indiatimes.com)

    Comment by Ahmad — August 23, 2010 #

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