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Name: Al R.
Status: educator
Grade: N/A
Location: CO
Country: N/A
Date: 12/28/2004


Question:
Background:
I was watching television when some journalists were discussing advances in science. One of them mentioned expanding universe. The phrase that really got my attention was something to the effect that the expanding universe might someday accelerate beyond the speed of light. I started looking around for information regarding faster-than-light experiments. The best I could ascertain was that under certain conditions it was possible to have the tails of wave impulses pass their respective heads--thus only energy surpassed the speed of light. No particles and no meaningful information were known to exceed the speed of light in a vacuum.

Question:
Is there a viable theory of the expansion of the universe that proposes that it might expand faster than the speed of light?


Replies:
This is a bit of a technical question, but here goes to the best of my comprehension.

The speed you refer to when you say that the "tail" of a wave packet overtakes the "head" refers to the 'phase velocity'. This term appears in the wave function of the photon in the exponential term and can exceed 'c'. The 'phase' cannot carry any information, so the apparent paradox is not real. The velocity of the wave packet (photon) itself cannot exceed 'c'. It is that velocity that is usually referred to when talking about the speed of light. You can follow-up on this distinction in Richard Feynman' Lectures on Physics Vol. 1, 48-5 to 48-7.

Theoretical physicist Joao Magueijo wrote a speculative book "Faster than the Speed of Light" in which he discusses the "what if" consequences of somehow overcoming the restriction of light (or any other things) traveling at "supra-luminal" speeds. He is a "hard core" theoretical physicist, so he isn't just spinning a tale, but he admits up front that the book is speculation. There is nothing wrong with speculation as a scientific tool (although you will not find it in traditional listings of the "scientific method").

There are three conceivable conditions where the limit of the speed of light is not experimentally determined. The first is the very first fractions of a second after the Big Bang when the physical laws of the state of energy (there was no "matter" yet) are not known. That is, we do not know if our current laws applied to that fraction of a second. Nobody has any evidence to the contrary, but nonetheless it is an open question. The second is the newly discovered observation that the Universe is actually expanding at an increasing rate, rather than "slowing down" due to the attractive force of gravity. Related to this is so called "dark energy" and "dark matter". That is, all the observable matter in the Universe is not nearly large enough to account for its experimentally observed behavior. No one knows what this "dark energy" and "dark matter" is. It is not just cold inter-galactic "stuff", it must be obeying a different set of physical laws than regular matter. So until these physical laws are elucidated, the question of the speed of light remains an open issue. The third is extremely small dimensions, for example time intervals less than < ~10^-43 seconds. Here the so-called "Standard Model" breaks down.

For all dimensions and spaces in between, however, the speed of light has been constant for the entire age of the Universe. At the small scale particle accelerators have experimentally verified this and on the large scale, observations of distance stars and galaxies have experimentally verified this. It is interesting to point out that the constant speed of light, and the fact that nothing travels faster is equivalent to the statement that energy is conserved. Another "fact" that has been verified millions of times.

Vince Calder



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