eh

What is quantum gravity? Well, if you find out, you'll become a very famous person. Just be sure to let me know, because I'm dying to know.

Seriously though, nothing beats finding some good books on the subject. Though I have yet to read three roads to quantum gravity (my local books store don't seem to carry it anymore) I would recommend starting out with some less advanced books first. The subject is hard to grasp, but interesting enough to make it worth while. Jumping into such a book without any previous reading would probably be confusing, and turn you off the subject.

So before reading about quantum gravity, try reading a little bit about quantum mechanics. The Elegant Universe, though dealing with string theory, does have a few chapters that explains the evolution of our understanding of the quantum world. But even before reading up on that, I would suggest reading a good book on General Relativity. To understand why a quantum theory of gravity has been such a difficult to come by, you should have an understanding of the classic understanding of gravity, as discovered by Einstein. Plus, GR is needed to explain the issue of the big bang, and cosmology in general.

Last on the list, I'll eventually have to recommend reading some work by Hawking. Since his books come with nice pictures along with the text, it makes understanding some of the concepts easier.

Duck!

I'm a bit confused by the whole concept of a massless particle.

Surely because of the mass-energy equivalance equation (E=mc^2), anything with mass should have energy and vice versa?

How can a massless particle affect anything if it contains no energy? Indeed, how can a massless and therefore energyless particle even exist?

Or am I taking a much too simplistic look at quantum mechanics....?


Duck!

Jesse

Duck of Death:
I'm a bit confused by the whole concept of a massless particle.

Surely because of the mass-energy equivalance equation (E=mc^2), anything with mass should have energy and vice versa?

How can a massless particle affect anything if it contains no energy? Indeed, how can a massless and therefore energyless particle even exist?

Good question. Actually E=mc^2 refers only to the energy contained in a mass at rest--the total energy of a particle is the rest energy plus the kinetic energy. The equation for a particle with rest mass m and momentum p is E^2 = m^2c^4 + p^2c^2, which as you can see would reduce to E = mc^2 if p=0. For a massless particle like a photon, on the other hand, m=0 and E = pc. Since p = mv in classical mechanics you might think that a massless particle would also have 0 momentum, but in relativity the equation for momentum is a little different, because mass appears to increase the higher the velocity relative to the observer. A particle with nonzero rest mass would have infinite mass if was traveling at light speed, which in a way explains why you can't accelerate a particle to the speed of light, and also why all particles which do move at light speed (like photons) must have zero rest mass. For a particle with 0 rest mass moving at light speed, the momentum becomes undefined--the equation is p=gamma*mv, with gamma = (1-(v^2/c^2))^-1/2, so for a photon m=0 but gamma is 1/0, so you get p=0/0 which doesn't tell you anything. This just means that the equation p=gamma*mv can only tell us the momentum of sublight particles, and does not tell us anything one way or another about the momentum of particles moving at light speed (as long as their rest mass is 0). Empirically we can see that particles moving at c do have finite well-defined momenta.

More details can be found here:

<a href="http://www2.slac.stanford.edu/vvc/theory/relativity.html" target="_blank">http://www2.slac.stanford.edu/vvc/theory/relativity.html</a>

[ September 06, 2002: Message edited by: Jesse ]</p>

Answerer

Oh sorry, I often thought that gravitational waves are more or less to be the same as gravity waves.

Answerer

Anyway if I'm not wrong, recently , there is a plan for an experiment that focused on uncovering the 'speed' of gravitional force. If gravitons are really massless, the obtained speed will be c, if not, most likely gravitons are not massless after all.
Well, I can't wait to know the results.

Duck!

Thanks Jesse.


Duck!

luvluv

Philosoft and eh:

Sure wish I had listened to you guys. I got through A Brief History of Time this weekend and I'm about 100 pages into Three Roads to Quantum Gravity. I have close to zero math background (The last real math course I took was in high school) so needless to say... my head hurts.

So, if I'm understanding this right, the theory of relativity works good for big stuff, but not so good for little stuff, and the quantum theories work good for little stuff, but not so good for big stuff. So they're looking for a way to unify those two things into a theory that works for both big things and little things?

The 3 roads to quantum gravity book kind of confused me. How can there be a quantum cosmology, I thought that quantum mechanics only referred to the very, very small? Some of the conclusions that are mentioned in passing in 3 roads regarding quantum cosmology are, frankly, absurd. Is this a real field? It seems to me that applying superpositions to stuff bigger than atoms will lead you to some strange conclusions.(Smolin quotes some scientist as giving a lecture stating that everything, even the past, is in a superposition, and that superpositions in different universes could overlap. She said that one could not therefore infer from dinasaur bones that there had ever been dinasaurs on the planet. It might be the overlapped superposition from another universe!)

I'm really interested in exploring Hawkings concept of infinite time. What is the opinion of cosmologists on this? Is it a genrally accepted theory?

I'm probably missing something, but it seems to me that he invented something which does not actually exist by virtue of the concessions he had to make in order to make the math in his equations work. It seems to me (and again, I am a mathematical dolt) that time only becomes a dimension like space (which can be traversed in any direction) in his equation, not in the real world. The spherical shape he gives to time is a function of his use of imaginary numbers to invent something called imaginary time (which he admits there is no evidence for). I can't understand how this concept has any validty, it seems to be a misleading by product of a mathematical process. Also, isn't it true that for his infinite time to work, the universe would have to eventually recollapse? I thought that we were pretty much assured that this was not going to happen?

Also, one big question I had on the uncertainty principle. On the one hand, it's stated that there must be quantum fluctuations because no field can have a state of absolute zero. But then on the other hand we're told that the entire universe, when all the positive and negative forces are added up, comes to absolute zero. So then does the entire universe violate the uncertainty principle?

Sorry, I know these questions are kind of scattershot, but I've got no chance of getting a good grasp on this stuff without some help. I'll check out elegant universe after I finish up 3 ways to quantum gravity (then I will reward myself with some really trashy fiction).

Answerer

Hi, I think I could only answer some of the question that you asked. Anyway, quantum cosmology is a very, very real research field in modern physics, it focus mainly upon the description of universe during the first few seconds of its creation(Big Bang) which is so small that it requires both unification of relativity and quantum mechanics to describe its state.

As far as I know, this is not a generally accepted theory, some agreed, some disagreed and others wanted no part in it.

Imaginary time is merely a conjecture which suggested that time has no beginning or end. For now, no one can really tell whether this conjecture is right or wrong?

The state of absolute zero refers to an isolated field, in which, in classical physics, was believed to have a state of absolute zero when a particle reaches the 'absolute zero' temperature. However, in quantum mechanics, the particle still emitts photons even if it is in the state of absolute zero degrees, due to quantum fluctations. Therefore, if the resultant field is zero, it doesn't mean that quantum fluctation isn't there or photons isn't emitted. Rather, the fields cancel out each other which appears on the surface that they violate the quantum physics.
Okay, for example, you are in a lift moves up at an acceleration of 9.8 , causing you to feel weightless, but this doesn't mean that Earth's gravity field is zero, got it? The same thing applies to the question you are asking.
Well, I'm not so sure about the imaginary time, perhaps others could tell you more.

[ September 10, 2002: Message edited by: Answerer ]</p>

Mark_Chid

Surely they can't measure that the speed is EXACTLY c, since perfect experiments don't exist, and therefore they can't be sure that the particles are massless - it strikes me as a pointless experiment. A result of 'c' with an accuracy of 0.00000000000000000000001 metres per second still allows for a particle with very very very small mass!

Indeed we don't even know what 'c' IS to that kind of accuracy, so how can this experiment be meaningful?

wade-w

<a href="http://www.newscientist.com/news/news.jsp?id=ns99992763" target="_blank">Here is a description of the experiment.</a> on the speed of gravity.