Freethought & Rationalism Archive

luvluv · 09-25-2002, 12:57 PM

Hi smart people.

I have been reading some books on physics lately and I have come to a bit of an impasse.

How can gravity be caused both by the curving of space and by gravitons?

If gravitons are found, would that mean that space does not curve?

One book I read, 3 Roads to Quantum Gravity, suggested that space itself is quantized. Could that mean that gravitons and space/time are the same entity? That a graviton is simply the smallest possible particle of space (I think the author, Lee Smolin, calls his theory quantum loop gravity).

Also, how can a photon have no mass? This string theory book I'm reading says that elementary particles have mass because of the energy of the vibration of strings. Why wouldn't photons also have mass because of the energy of their movement? A photon does have energy, so why doesn't it have mass? Why can't we just plug in it's energy to the equation e=mc^2 and figure out it's mass?

Finally, why do people think that quantum gravity would be the final theory. Do people really believe that a quantum theory of gravity will be able to calculate why it was that I fell in love for the first time in the 8th grade? I don't understand why this is considered a theory of everything.

Postscript: Is it me or are all of Hawkings popular books exactly the same but with different covers?

elwoodblues · 09-25-2002, 01:06 PM

Quote:

Finally, why do people think that quantum gravity would be the final theory.

They often say this because they believe, in theory, that it would acount for all known observed phenomenae, with only a few kinks to work out.

IMO, they're full of shit, because this has happened several times in history with roughly the same 'proof'. We thought we had everything figured out before quantum physics; how wrong we were. I'm not saying that a theory of quantum gravity would necessarily be some sort of useless phlogiston chemistry. But I doubt it'll be the end-all be-all of physics that some are predicting.

Quote:

Do people really believe that a quantum theory of gravity will be able to calculate why it was that I fell in love for the first time in the 8th grade?

A lot of people would say 'yes', and that it's only a matter of complexity that shrouds it (in the same way that applying particle physics to economics is difficult).

Jesse · 09-25-2002, 01:24 PM

luvluv:
Also, how can a photon have no mass? This string theory book I'm reading says that elementary particles have mass because of the energy of the vibration of strings. Why wouldn't photons also have mass because of the energy of their movement? A photon does have energy, so why doesn't it have mass? Why can't we just plug in it's energy to the equation e=mc^2 and figure out it's mass?

I have no idea how it relates to the concept of mass as string vibrations, but I addressed a similar question about the mass of photons and E=mc^2 on <a href="http://iidb.org/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=57&t=000494" target="_blank">What is quantum gravity?</a>

Quote:

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>

eh · 09-29-2002, 06:43 AM

Forgot about this thread. Maybe some links will help:

<a href="http://www.abc.net.au/science/k2/moments/s275021.htm" target="_blank">http://www.abc.net.au/science/k2/moments/s275021.htm</a>

<a href="http://www.lns.cornell.edu/spr/1999-02/msg0014677.html" target="_blank">http://www.lns.cornell.edu/spr/1999-02/msg0014677.html</a>

<a href="http://itss.raytheon.com/cafe/cosm/planck.html" target="_blank">http://itss.raytheon.com/cafe/cosm/planck.html</a>

And of course, there is always the superstring theory forums, at <a href="http://superstringtheory.com/forum/forums_i.html" target="_blank">http://superstringtheory.com/forum/forums_i.html</a>

Jesse · 09-29-2002, 06:50 AM

Another useful link is this <a href="http://www.physics.gatech.edu/academics/tutorial/phys2123/Chapter%2045/virtual_.htm" target="_blank">virtual particles FAQ</a>. From the FAQ:

Quote:

I hear physicists saying that the "quantum of the gravitational force" is something called a graviton. Doesn't general relativity say that gravity isn't a force at all?

You don't have to accept that gravity is a "force" in order to believe that gravitons might exist. According to QM, anything that behaves like a harmonic oscillator has discrete energy levels, as I said in part 1. General relativity allows gravitational waves, ripples in the geometry of spacetime which travel at the speed of light. Under a certain definition of gravitational energy (a tricky subject), the wave can be said to carry energy. If QM is ever successfully applied to GR, it seems sensible to expect that these oscillations will also possess discrete "gravitational energies," corresponding to different numbers of gravitons.

Quantum gravity is not yet a complete, established theory, so gravitons are still speculative. It is also unlikely that individual gravitons will be detected any time in the near future.

Furthermore, it is not at all clear that it will be useful to think of gravitational "forces," such as the one that sticks you to the earth's surface, as mediated by virtual gravitons. The notion of virtual particles mediating static forces comes from perturbation theory, and if there is one thing we know about quantum gravity, it's that the usual way of doing perturbation theory doesn't work.

Quantum field theory is plagued with infinities, which show up in diagrams in which virtual particles go in closed loops. Normally these infinities can be gotten rid of by "renormalization," in which infinite "counterterms" cancel the infinite parts of the diagrams, leaving finite results for experimentally observable quantities. Renormalization works for QED and the other field theories used to describe particle interactions, but it fails when applied to gravity. Graviton loops generate an infinite family of counterterms. The theory ends up with an infinite number of free parameters, and it's no theory at all. Other approaches to quantum gravity are needed, and they might not describe static fields with virtual gravitons.

Steven S · 09-29-2002, 08:12 AM

Quote:

Originally posted by luvluv:

Finally, why do people think that quantum gravity would be the final theory. Do people really believe that a quantum theory of gravity will be able to calculate why it was that I fell in love for the first time in the 8th grade? I don't understand why this is considered a theory of everything.

People don't think that if you have a quantum theory of gravity (QG) you will have a final theory. People who work on loop quantum gravity, lattice methods, etc are just looking for a means to get a consistent theory of QG. The string theorists are the ambitious ones who are trying to wrap up the strong, electroweak force, and gravity into one uber theory that explains all the known fundamental forces.

Coragyps · 09-29-2002, 12:18 PM

Quote:

Do people really believe that a quantum theory of gravity will be able to calculate why it was that I fell in love for the first time in the 8th grade?

Naah. If it was like my eighth grade, that was just the testosterone.

Edited to add: woohoo! a thousand posts!

[ September 29, 2002: Message edited by: Coragyps ]</p>

Answerer · 09-30-2002, 04:31 AM

I always thought gravitons are small packets of quantized spacetime, so am I wrong or not? Does anyone have any ideas?

[ September 30, 2002: Message edited by: Answerer ]</p>

09-25-2002, 12:57 PM	#1
luvluv Veteran Member Join Date: Mar 2002 Location: Planet Lovetron Posts: 3,919	gravitons vs curved space and other physics questions Hi smart people. I have been reading some books on physics lately and I have come to a bit of an impasse. How can gravity be caused both by the curving of space and by gravitons? If gravitons are found, would that mean that space does not curve? One book I read, 3 Roads to Quantum Gravity, suggested that space itself is quantized. Could that mean that gravitons and space/time are the same entity? That a graviton is simply the smallest possible particle of space (I think the author, Lee Smolin, calls his theory quantum loop gravity). Also, how can a photon have no mass? This string theory book I'm reading says that elementary particles have mass because of the energy of the vibration of strings. Why wouldn't photons also have mass because of the energy of their movement? A photon does have energy, so why doesn't it have mass? Why can't we just plug in it's energy to the equation e=mc^2 and figure out it's mass? Finally, why do people think that quantum gravity would be the final theory. Do people really believe that a quantum theory of gravity will be able to calculate why it was that I fell in love for the first time in the 8th grade? I don't understand why this is considered a theory of everything. Postscript: Is it me or are all of Hawkings popular books exactly the same but with different covers?

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09-29-2002, 06:43 AM	#4
eh Senior Member Join Date: Sep 2000 Location: Canada Posts: 624	Forgot about this thread. Maybe some links will help: <a href="http://www.abc.net.au/science/k2/moments/s275021.htm" target="_blank">http://www.abc.net.au/science/k2/moments/s275021.htm</a> <a href="http://www.lns.cornell.edu/spr/1999-02/msg0014677.html" target="_blank">http://www.lns.cornell.edu/spr/1999-02/msg0014677.html</a> <a href="http://itss.raytheon.com/cafe/cosm/planck.html" target="_blank">http://itss.raytheon.com/cafe/cosm/planck.html</a> And of course, there is always the superstring theory forums, at <a href="http://superstringtheory.com/forum/forums_i.html" target="_blank">http://superstringtheory.com/forum/forums_i.html</a>

09-30-2002, 04:31 AM	#8
Answerer Veteran Member Join Date: Feb 2002 Location: Singapore Posts: 3,956	I always thought gravitons are small packets of quantized spacetime, so am I wrong or not? Does anyone have any ideas? [ September 30, 2002: Message edited by: Answerer ]</p>

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