eh

luvluv,

Quantum gravity is a hard place to jump right into, and even Hawking can be difficult to read without some previous reading on relativity on quantum mechanics. Did you get the illustrated version of a brief history of time? That might help, and I know I certainly wouldn't buy his non illustrated books. The Universe in a nutshell is full of pictures to help visualize the concepts, and might also be of help.

But if you're seriously thinking about reading a lot of the subject of cosmology, you'll probably find Physics Forums very useful. The forums are filled with friendly people with a lot more knowledge in the field of cosmology than I can ever hope to reach, and will take the time to explain things. The URL for the forum is at <a href="http://physicsforums.com" target="_blank">http://physicsforums.com</a>

For what it's worth, quantum cosmology is merely taking the wave function of particles, and applying it to the entire universe. That is, there are an infinite amount of possible states the universe can reach, with the improbable ones canceling out. This is much like the many worlds interpretation of quantum mechanics.

As for the net energy of zero state of the universe, quantum fluctuations would mean the total charge would be a little over zero. But the universe under classic physics has a net charge of zero, and the creation of something with zero energy would not violate the law of energy conservation, no matter how big it gets. Though the uncertainty principle makes the charge a little higher, the basic principle remains.

Coragyps

Sort of down this line of thinking, and one of you physicists correct me if I'm wrong: Neutrinos must have mass to be able to change 'flavors' as they have been found to do, correct? And the neutrino burst from Supernova 1987A arrived within hours, at worst, of the photons from the same event (though the neutrinos did make it out of the explosion first.) So has this result been used to constrain the maximum mass a neutrino can have?

And Mark - I may well be wrong, but a truly massless particle may be required to travel exactly at c.

Mark_Chid

Yes, but the point is that they are trying to prove that a) the particles travel at excatly c and are therefore b) massless - but they can't really do a) so b) becomes moot!

Answerer

Well, I'm not sure about the details of the experiment, anyway, physicists do enjoy making approximate calculations and deductions.

wade-w

From the above description of the experiment, it appears that what is proposed will compare the speed of gravity to the speed of light. Thus your argument about inaccuracies in measuremnets is moot.

Jesse

Mark_chid:
Yes, but the point is that they are trying to prove that a) the particles travel at excatly c and are therefore b) massless - but they can't really do a) so b) becomes moot!

Where did you get the idea that they were trying to "prove" that gravitational influences (not gravitons, necessarily, since we have no direct evidence of those) move at "exactly" c? I'm sure this experiment is just trying to get a more accurate measurement of the speed of gravitational influences, which will either show that it is smaller than c or that it is as close to c as the resolution of the experiment can tell us. This would either falsify or lend support to theoretical models which require gravitational influences to move at exactly c.

wade-w

From the url above which describes the experiment:

The experiment is not an attempt to find a value for the speed of gravity, so much as tp try and determine if gravity has a finite speed, as predicted by special relativity, or if it is instantaneous (or nearly so.)