I have started a little discussion on my about page and figured it be best to address the points made in a post dedicated to the heart of the matter: gravitons.
They are hypothetical massless particles that are the mediating boson of gravity…just like how photons are the mediating boson for electromagnetism. Intuitively, gravitons must be massless…otherwise they would generate graviton, since gravity is generated by mass (well, energy).
But, photons have energy (recall from special relativity: E=pc for a photon). Logically, gravitons would have energy. There is an infinite recursion here: energy creates gravitons, gravitons have energy, go back to step 1. This violates the first law of thermodynamics (energy conservation).
Perhaps we could abandon this notion of the graviton and *gasp* move forward? Although this is a string rant, it applies equally to loop quantum gravity due to its “graviton propagator” calculation by Rovelli a while back.
Perhaps one is approaching this the wrong way. Look at spacetime like a manifold. Each point (or rather quantized region) of spacetime has a certain curvature 
But, to sum up this rant, the notion of the graviton is an antiquity of the current paradigm of quantum field theory and either we need to apply this paradigm to geometry (instead of treating gravity as a field, go back to it being geometry) or we need a new paradigm. The latter seems preferable since the current paradigm has its problems (e.g. the renormalization problems - ignoring infinities is like ignoring depression, it’s not good for you!). The graviton it seems has problems with the conservation of energy, in addition to the problem that it has never been empirically observed.
14 May 2007 at 1:35 pm
The reason the graviton is generally thought to be massless is that the force appears to be infinite in range, like electromagnetism. The W and Z particles are massive, and so the weak force has finite range.
QCD also has finite range, but that is a product of color confinement which causes propagating states to become color-neutral.
14 May 2007 at 3:28 pm
Yes, that is true according to the current QFT paradigm…I am not contesting that one bit.
All I am saying is that if you think about this logically, there would be an infinite recursion: energy generates gravitons, gravitons have energy and thus generate more gravitons, and so on and so on. This violates energy conservation. True this is a mere intuitive argument that it appears that every graviton approach is non-renormalizable with a handwavy “proof” (if that), but it appears to be right if we think of this as a computer program recursion.
It seems that there is ground for a paradigm shift…or some important change (either in QFT or General Relativity)!
If there were actual empirical evidence for the graviton’s existence, I would change my tune. However, seeing as there is no empirical support for the existence of the particle, it seems like we should be thinking about something new.
Perhaps General Relativity isn’t exactly a field in the conventional sense? Perhaps we should then try a different way to quantize it rather than pretend it’s a regular old field?
15 May 2007 at 7:11 am
Angry Physicist,
You are demonstrating a worrying tendency to think for yourself.
It may well be that you are young enough and naive enough to think that this is a good thing; well, maybe it is but it will not help your career. Remember that the average age of physics faculty in the U.S. is about sixty (see Peter Woit’s post on “Physics Demographics” about 18 months ago for more details). What would you be thinking about at that age? How best to encourage lively-minded undergraduates, helping them develop their new ideas? Nope, you would be thinking about your retirement. You would be wanting to take it easy, and the only major project you would be contemplating would be the purchasing of a dwelling in some gated, geriatric community in Florida. If you were going to make recommendations about which of your students should be sponsored to do research, people like yourself would not even make the short list. I mean, what would happen if your “interesting new ideas” turned out to be controversial? How would that reflect on the sponsor? As I see it, you have two options:
1. Get a full frontal lobotomy. This should deal with your aggressive, angry, questioning tendencies. Once you have done this, you could, possibly, get a research post doing String Theory - but not before. Admittedly, there have been cases where the lobotomy has gone horribly wrong, but this is a risk you would have to take.
2. Get out after your Ph.D. and do something useful with your life. In some walks of life, the ability to think for yourself is actually an asset, and in some cases they actually pay you a salary you can live on as well.
15 May 2007 at 1:03 pm
“You are demonstrating a worrying tendency to think for yourself.”
I told you I was a bad man
But it’s not as though this “worrying tendency” is generating nonsensical ideas, like the existence of 27 dimensions or anything of the sort.
Indeed, this “tendency” is just my being conservative and cautious in the quantization of gravity. Personally I don’t think of this as a “bad” thing…it’d be a bad thing if I started thinking wacky ideas like “God is a string!” or some crackpot philistinism like that.
“Remember that the average age of physics faculty in the U.S. is about sixty (see Peter Woit’s post on “Physics Demographics” about 18 months ago for more details). What would you be thinking about at that age?”
This doesn’t tell me much statistically without the standard deviation, and it appears that Dr. Woit’s post didn’t have that information (the page I looked at might not have been it, here’s the url: http://www.math.columbia.edu/~woit/wordpress/?p=280 ).
I honestly don’t know what I’ll be thinking at that age, and I don’t honestly know what I’ll be thinking when I’m in grad school…the time will come soon enough. Who knows with my luck I’ll probably get hit by a car tomorrow!
I think the Ulful’s sums it up best:
“I’m young, and I have dreams
Surely some day, surely someday
You will understand.
There’s always tomorrow…”
“I mean, what would happen if your “interesting new ideas” turned out to be controversial? How would that reflect on the sponsor?”
This I think is a fundamental problem with the structure of science (theoretical physics specifically)…it’s a “high stakes” game involving “sponsors” and so forth. It reminds me of the Soviet Union’s “inner party” structure…and we all know how wonderful this structure worked for them.
What about that whole empiricism thing? Scientific materialism and so forth? (Well, no one pays attention to scientific materialism anymore, otherwise the multiverse idea would be shot down as Platonic.) It shouldn’t matter how “contraversial” my conservative approach would be, or the ideas generated by it, if the ideas are supported (or at least, not falsified) by empirical findings, it’s a good method.
“As I see it, you have two options:…”
And there actually is a third option, though I don’t think you’d like it: I could continue going into physics. True, I would become the “crazy conservative guy” in the physics community, and I might not even be employed as a professor…but it seems like that’s not too big of a deal. Not being a physics professor isn’t the end of the world, you know.
Sure one might say: “But you’ll live in poverty!” As if that’d be new to me, I’m a college student!
Just out of curiousity, not to be rude or anything, but what’s with the scare tactics? Perhaps you are trying to warn me how difficult it would be, and that shows a kind spirit, but in the words of my lab instructor Nelson Page: “You only get one chance to really change the world and do what you believe in, one chance. Sure the odds are nearly zero that you’ll actually change the world, but why waste it? If you fail, at least you gave your best shot, which is more than you could say about someone who walked away.” And I don’t intend on doing any walking away anytime before I die.
15 May 2007 at 3:11 pm
Yes, indeed, Angry One, I am trying to help you. If you take the attitude, “While I am working in fundamental physics, I am going to do what I think is best, and I don’t care whether it helps my career or not” is all very well, but you may well find, as I did, that you are no less devastated than the the next person when all your post-doctoral job applications lead nowhere.
15 May 2007 at 6:18 pm
It would be great to have a new paradigm, and it’s far from clear that there should be such a thing as a fundamental graviton. Nonetheless, we believe in the principle of effective field theory. In other words, there should exist a regime in which quantum gravity behaves like an ordinary quantum field theory (with an explicit cutoff). The idea of effective field theory is that whatever the theory of quantum gravity may be, it can be approximated by an effective field theory. This quantum field theory will exhibit a graviton.
Put another way, if you look at the equations of gravity in perturbation theory, they look a lot like electromagnetism, and solutions that correspond to gravitational waves. Thus, in this regime of perturbative gravity, the quantum theory should have something a lot like the photon. This is the graviton.
The fundamental theory of quantum gravity, whatever it may be, will surely involve some sort of paradigm shift, probably involving some radical restructuring of our nature of spacetime (most people who work on string theory, for example, believe this). But, that theory has to reduce to stuff we already know. Gravitational waves exist. If gravity is quantized, then those better be quantized, too.
And to Chris, (and at the risk of sounding condescending), the idea that there is no graviton is hardly unheard of or will get you kicked out of the field. I would think it occurs to most people that naively quantizing the metric may well not be the right idea. After all, in gauge theories it is the connection that gets quantized. Such things have long been considered, but as seems to be depressingly often the case, none of it has worked out.
16 May 2007 at 1:51 am
You will not get kicked out because you will not be allowed to enter it in the first place.
I would like to see an investigation of a much wider class of quantum theories whose classical limit agrees with what is known experimentally about gravity. As far as I know, what is available in the literature is that obtained by using the formal quantization procedure on G.R. or a string, stuff that has not even been demonstrated to have the right classical limit at all, or both (e.g. loop quantum gravity). I am only saying that more pragmatic approaches ought to be attempted as well. Given that quantum->classical limit is a many-to-one mapping, there ought to be a wide choice.
16 May 2007 at 5:49 am
You don’t think pragmatic attempts have been made? String theory and LQG are relatively recent developments in the history of quantum gravity. What do you think people were doing before that?
You say there ought to be a wide choice? People can’t even find one.
16 May 2007 at 6:43 am
Aaron,
Do you know of any useful books or review articles on QG prior to strings or LQG?
One problem, of course, is that failed approaches tend to get buried.
16 May 2007 at 8:11 am
Not really. The best thing to do is to ask people who were around then. I know DeWitt thought about it a bit, and he has a 1967 book on the subject, but I’ve never read it. I did run across this book in googleing
Quantum Theory of Gravity: Essays in honor of the 60th birthday of Bryce S. DeWitt.
But again I’ve never seen it myself.
16 May 2007 at 8:24 am
The idea that gravity is best quantized when regarded as a gauge theory is an old one. And I think this idea has only very partially be actually carried out.
It is of course precisely the premise of loop quantum gravity. However, it is technically demaning to quantize the space of all well behaved connections on some space. For some reason, people working in LQG have very early on decided that it is a good idea to sidestep this problem simply by passing to the much larger space of “generalized connections”.
This enlargement of configuration space is the reason behind much of the “discretized” flavor of work in LQG. It is not clear that this step is physically justified. As John Baez says here
So, I’d think that one good idea would be to go back to the very roots of gauge theory based quantum gravity and start the whole program over again — but now using the right configuration space of connections.
This is technically much more demanding. But the results by Freed, Moore and Segal show that it can be done explicitly at least in simple cases, and that it yields very interesting and suble quantum effects.
And there is every indication that 11-dimensional supergravity, hence M-theory, is itself best thought of as a gauge theory. A higher gauge theory in fact.
16 May 2007 at 8:46 am
Aaron,
Thanks. I will have a look.