Thursday, May 10, 2007

Shameless Plug

I am a member of the LIGO Scientific Collaboration, which is a group using kilometer scale interferometers to look for ripples in spacetime called gravitational waves. Please don't tell anyone who doesn't already know. Anyway, these waves, which were discussed in a paper by Einstein in 1918, have still not been observed. But we're getting close ... very close. If you'd like to help out, why don't you donate the spare CPU cycles of your computer. Check out and join the Einstein@Home project. Not only will you be helping launch the new field of gravitational wave astronomy, but the screensaver is pretty cool too.


Anonymous said...

Warren, do you have any insight why both photons and gravitons travel at the same speed through space?
If we substitute all instances of c in relativity with 'speed of gravity', will nothing in the theory change?

I just find it amazing that we have two types of radiations with utterly different properties yet they travels not only at the same speed, but cannot exceed that speed. What is it that controls their speed limit?

Warren said...

We call the mandatory speed limit in the universe the "speed of light" because Einstein was thinking about light when he first identified it. If he had been thinking about gravitational waves, we might call it the "speed of gravitational waves" now, which would be considerably more of a mouthful. It doesn't matter what it is called, in fact, for special relativity to hold, it doesn't even really matter that light or gravitational waves or anything else travels at that speed - what matters is that there is a speed limit and that everything with a mass travels slower thans it. If we find out tomorrow that photons and gravitons and all other massless particles really have a small mass (we won't, but suppose we did), as we found that neutrinos did, relativity will still be fine. At least, the special theory of relativity will be - Maxwell's electromagnetic theory and Einstein's general theory of relativity will no longer be quite as exact as we now believe, however.

So, the reason that both these things travel at the same speed and it is the speed limit is that neither carries a mass and in our universe, that's what massless things do. Which begs the questions of why they're massless and why special relativity, with it's speed limit, holds in our universe rather than some other description of motion - these are deep questions, and, I would contend, we don't really understand the answers at present.

Anonymous said...

Thanks for the reply Warren. Yes indeed the question of why there is a light speed limit is one of the deepest mysteries. It begs the question: is speed of light fundamental. I think not - something more fundamental is controlling it and relativity says nothing about it. All I know is Smolin of Perimeter proposed a mechanism that can change c ever so slightly the effect of which accumulate over billions of years of travel and become just big enough to be detected. But what's important is that he suggested a deeper reason responsible for c. Don't know seriously other physicists take this suggestion.

Warren said...

Indeed, the hope is that Smolin and others who work on trying to unify all of physics into a single framework will provide answers to some of these deep mysteries. As of yet, however, while there are some candidates for a "Theory of Everything", none of them has been able to pass the gold standard for acceptance of a new theory, predicting something unexpected which can then be confirmed experimentally. Conversely, it is possible that there is no Theory of everything, that the deeper you dig the better the approximation, but that the convergence to exactness is asymptotic - that is, it is possible that it will take an infinite number of iterations to "get it exactly right". I'm personally hoping that's not the case.

Buck said...

My take (for what's worth which is likely nil) is light/gravity propagation speed limit and dark energy are linked. Both must originate at time of universe birth, and likely from the the same mechanism.

And my relative ignorance gives me the high optimism that there is indeed a TOE. Because the universe has operated on a single consistent exact non-asymptotic physics that had remained stable for over 14.7 billions years. The limitations reside in the human minds.