About
I’m a undergrad at UC Davis, I have been studying physics for two or three years. I started studying from the library at CalTech with a retired rocket scientist and have been fascinated by theoretical physics since. I have a few more years to go until they kick me out with a degree or two; next year I am going to be taking 13 to 15 math courses, so I don’t know how much I can keep up with this blog then. But for now, I read everything I get my hands on and try to read up on every technical paper I can (truth be told: Dr. Carlip is very intimidating with his demigod-like powers to mystically cite sources like one would recite one’s phone number! So that is something to aspire to!) and as a bad habit from my earlier years I tend to think critically (I’m a baaaad man!).
Consequently I reject String theory as it stands now (yes, I am a heathen heretic whatever). Hmmm…I think that if we had strings that are one planck length long, all we could really know about these strings are their relative position perhaps desribable by a graph? The various graph-states would be superpositioned and we easily recover quantum mechanics via a sum over histories method, etc. But that’s not what bothers me about string theory the most. What bothers me the most is that they propose the existence of the graviton, which I see as cartoonish. I’m certain that String theorists will have a host of insults to hurl at me about this, but it doesn’t change the fact that it makes no sense to have a graviton. Saying “Well quantum field theory demands the existence of a mediating boson” is no better than saying “Well, the bible demands that evolution be false”; it’s a simple appeal to authority.
Anyways, I thought about being an economist a while back (since fifth grade!). I must confess that I have a soft spot in my heart for economics…well, criticisms of Neoclassical and marginalist economics now. I’m an “old school” Neo-Ricardian type of fellow. I might actually post some stuff criticizing Neoclassical economics later on, keep your eyes peeled for it!
Back to my history, uh well, that’s it I guess for now.
12 May 2007 at 10:59 am
you sound interesting
I want to read more
I hang out some of the time at physicsforums.com
at the “beyond” and “cosmology” subforums, e.g.
http://physicsforums.com/forumdisplay.php?f=66
I will post your blog address there
12 May 2007 at 11:48 am
Dear angry physicist,
The presence of a graviton is no big mystery. When you take quantum field theory you will see a host of various lagrangians written down corresponding to particles of different spin, and you will study them one by one. Eventually you will write down the equation for a spin 2 particle and run the machinery, and something miraculous happens.
Out pops Einstein’s field equations for general relativity. Indeed, its quite intellectually satisfying that a classical field equation pops out of something distinctly quantum mechanical using distinctly quantum mechanical machinery. We thus take the not so subtle hint, and identify this boson with gravity, not unlike how the other bosons corresponded to other force carriers.
Now the controversy about the existence of the graviton is a little more subtle. Secretely we are using something called a weak field approximation, where perturbation theory makes good sense to use calculationally. It is in this regime where the graviton ‘lives’ and corresponds to general relativity, and I don’t think you will find many physcists who doubt this.
Of course what about non weak field approximations you might ask? Well, we run into technical problems, and everything is much less clear. But when people talk about gravitons, they are really talking about the not so controversial ‘regime’ where it makes sense to think of them as such.
As far as string theory goes. Its really not accurate to think of them like lego’s of 1 planck length and so forth. For instance you can have strings less than a planck length, and ultimately they really are quite ‘fuzzy’ just like all other quantum entities, only this time they are additionally making the very geometry that describes their own ‘length’ fuzzy as well.
12 May 2007 at 12:26 pm
The weak field approximation is not general relativity.
12 May 2007 at 12:33 pm
Just to tell a little bit more about myself, I have read Misner, Thorne, and Wheeler’s “Gravitation” (the phone book of a text!), Carrol’s “An Introduction to General Relativity: Spacetime and Geometry”, Hartle’s “Gravity”, and Anthony Zee’s “Quantum Field Theory in a nutshell” amongst a large number of other of texts; I’m worming my way through Wald’s “General Relativity” and - at a significantly slower pace due to time constraints - Peskin and Schroeder’s “An Introduction to Quantum Field Theory” (actually, I have too many textbooks; so many that there isn’t enough book case space for them!). I’m not just any old piker here, I’m a book-worming piker
The problem is that the weak field approximation tells us that general relativity is the “deformation” of some pre-existing spacetime…most unsatisfactory if you are a general relativist! Einstein remarked to a group of journalists about GR “I don’t know if you’ll understand this or not, but most people think if you remove all the matter and energy from the universe what is left is space and time. What General Relativity says is that if you remove all the matter and energy from the universe, you also remove space and time.” It would seem that full blown GR is telling us that energy and matter “generates” spacetime. I have just spent the last 48 hours on a caffeine induced marathon of calculations, but it seems to me in this sleep deprived state that if the metric were 0 you’d get the same result as the vacuum field equations.
Quantization of the weak field approximation may very well be an approximation to quantum gravity *at some appropriate scale*…I don’t know, and can’t say since we don’t have a final quantum theory of gravity. Perhaps parts of it are a proper approximation, perhaps not. One thing that is certain however is that the philosophy of the weak field approximation is nothing like the philosophy of full blown GR.
One problem that I have, and perhaps this is just me, String theory tries to unify all forces and that would result in a quantum theory of gravity. Well, we didn’t need to do that for quantizing electromagnetism. Nor was that needed for unifying two physical theories like Electricity and Magnetism. So why is it necessary for gravity? And if the answer is that gravity is a special force, then why use regular quantum field theory on it?
I am loosely aware that, using a spin 2 particle you can recover Einstein’s field equations; however (there is always a “however”!) is this really a physical phenomena, a sign from the quantum field theory gods, or is this just a mathematical coincidence? I tend to think it’s the latter, otherwise why would it be the weak field approximation and not the full blown field equations that is quantized?
As for strings not being of uniform length, that doesn’t really bother me too much either. What bothers me is that it’s not a background independent formulation of the approach. I have studied General Relativity before studying quantum theory, so I am incredibly biased towards background independence and so forth. A graph formulation of string theory, it seems, would be able to make string theory background independent. Frankly I think that there is too much dogma with string theory based on my interactions with the grad students who are string theorists (perhaps it’s coincidence, but they all seem like used-car salesmen), what has been laid down is holy writ and any challenge to it is blasphemy. Perhaps I am wrong, but I usually am a blasphemer.
12 May 2007 at 2:41 pm
Hi Angry Physicist,
Re: your string theory comments, I expect that Lubos is far too busy to respond, which is probably why you have not been called an aggressive crackpot moron f***wit idiot with a negative IQ … yet. Stick around, though.
I agree with you about the graviton. How many experimental tests of quantum gravity are there? None. Could it be that there is no such thing as QG at all? Possibly. Why, then, do the most vitriolic debates in fundamental physics seem about QG? Because, scientifically speaking, the stakes are so low, maybe?
12 May 2007 at 3:54 pm
The Electromagnetic force and the weak force are naturally united in field theory into the electroweak force . 2 of the 4 forces in nature combined, and coupled with the strong force we call this the standard model.
Beyond the standard model, most particle physicists strongly believe in a GUT that unifies the strong force with the other two (and theres very good indirect evidence for this), so that means 3 forces naturally combined into 1.
So it would be peculiar for this situation to not repeat itself with gravity at the last possible meaningful energy scale (the Planck scale)
12 May 2007 at 4:37 pm
Yes, I could see the intuitive appeal for a unified field theory, I do not contest that it is indeed something very desirable.
However, I should think that all the components should be quantized first, since reality is supposedly quantized. (Perhaps quantum theory is only an approximation to an underlying theory; perhaps both general relativity and quantum theory are both approximations to the same thing, who knows.)
The quantum theory of gravity that string theory delivers (which is little more than the graviton with some new packaging when you boil it all down to it) is unsatisfying. Worse, the quantization is not even based on what I would consider a legitimate version of general relativity, only an approximation to it.
So rather than try to eat a meal in one bite, and yield a quantum theory of gravity while at the same time yielding a UFT, perhaps we should do what we always do when we eat a meal: take it slow and do one thing at a time. Quantize gravity first, and then create a UFT.
12 May 2007 at 9:42 pm
Quantizing gravity by itself is nasty business, and incidentally why im glad I am not in the field. Already in the tamest possible regime of GR (where say the metric and curvature aren’t varying wildly and where we are focusing on some very local patch where microphysics should naively live) and where we have some trustworthy calculational techniques, the problem is almost (but not quite) intractable. So already pure gravity is a big technicle problem (so far only accessible in lower dimensions)
The far greater problem is the second you introduce matter into this ‘tame’ regime, its more or less guarenteed to absolutely destroy any solution you might find. And when I say destroy, I mean huge completely uncontrollable contributions over many orders of magnitude that will absolutely swamp anything you might write down.
Recall that gravity isn’t like other forces, the stress energy tensor recieves contributions from everything it can see, including itself. Even the lightest most inconsequential uncharged particles adds to its content.
There are various no go theorems that highly constrain how you can even proceed in this most tame of settings, it would be a big leap of faith to somehow conspire it so that the whole shebang doesn’t make matters *worse* rather than better.
13 May 2007 at 12:38 pm
Right, classical general relativity is a beast of a mathematical calculation. I completely agree (and the various “simplifications” like the ADM Hamiltonian formulation, although simpler, is still ridiculously hard!). From looking at a few books on numerical relativity, it appears that one would require a *supercomputer* to actually calculate out the solutions (of course, if one were such a genius as to resort to numerical calculations, one should be able to do all the iterations with pen and paper alone
).
Einstein himself rued how complicated General Relativity was.
The various approximations (weak field approximation, linearized gravity, etc.) make life far simpler, I agree. However, they do not contain the full meaning of general relativity.
The only legitimate simplification, I think, is the Cartan geometry form of General Relativity…and even then it isn’t a silver bullet that will end all complications! Especially in quantization!
When quantizing gravity, one has to suck it up and be a scientist about it! That’s my bias at least, that one should not be quantizing *approximations* to GR but actual GR itself. Easier said than done.
Perhaps it would actually be easier to come up with a model that works at the quantum scale that merely returns at the appropriate scale the classical solutions. I haven’t thought too hard about this, but it seems equally (if not more) difficult as quantizing the classical system. Maybe it will be more fruitful? Who knows.
It does seem, though, (just a priori) that there is some advantage for using the sum over histories method (since such a method uses action, and from the action you can get the stress energy tensor); perhaps one could somehow tie in general relativity to that (if I recall correctly, I think Hawking was working on something like this).
But overall, yeah, classical general relativity is a nightmarish calculation, and quantum general relativity would be doubly so. That’s life.
15 May 2007 at 4:27 am
Very nice blog. Keep up the good work. I think religion analogies, being a heretic, and being excommunicated and/or attacked by religious dogmatic doctors from other sects only shows that Doctors of Philosophy and Doctors of Theology are the sides of the same coin. Therefore, it will be more and more difficult for you to think independently as you move up in the hierarchy.
15 May 2007 at 10:18 pm
Anger with Lubos is a GOOD thing !
However, even if you are a prodigal grad student, with a voracious appetite for th.physics, you are in danger of burning out. This is why Oppenheimer made very few seminal contributions: He was consumed with EVERYTHING `physicsy’, and was quoted as saying “I need physics, more than I need friends”.
Judging from the length of this voluminous tirade about QG, you may be lost in the quagmire of your BLOG, thinking yourself tb possessed of superhuman intellectual abilities, but be forewarned: You are pissing away time & energy better spent focussing IN DEPTH on 1 or 2 problems, rather than emulating Lubos, & other blog-obsessed scientists who never accomplish much professionally, but spend hours prosletizing to the world their sagely opinions.
16 May 2007 at 4:02 pm
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16 May 2007 at 4:07 pm
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16 May 2007 at 11:49 pm
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