BenTheMan

Ugh. Ok I give up... I'll stick to particle physics

Great--so many questions! Take a number, and get your nickle out. YT---the string is the fundamental object. There are not ``types'' of strings as there are ``types'' or particles. Depending on which string theory you're working in, you can have open string, closed strings, or objects called d-branes. One should remember that string theory incorporates quantum field theory naturally. At energy scales where photons become important, the physics is exactly the same. This is one of the reasons why it's so hard to test string theory. You can think of the photon as a string vibrating with a specific frequency. (This is why I said Farsight's ideas were similar to string theory in the other thread, except he's calling `photons' fundamental.) Well, if you could controll strings, you could controll space-time. One would have to understand the theory very well before that could happen. This is a trick question I think strings is correct because of the fact that so many things naturally fall out of it. In other approaches to quantum gravity, you have to tickle things to make them work correctly. I was in a bar once drinking with Joe Polchinski, and I got him to drink a toast to Loop Quantum Gravity. I was with a group of grad students, and we talked a bit about the LQG program. Joe said that he liked string theory because he felt like he was ``discovering'' things, not ``inventing'' them. This is why I like string theory---things like chiral fermions, number of families, gauge symmetries... From studying the low energy data, we have absolutely no reason for any of the phenomena that we observe to be as they are---the standard model just IS. But string theory offers a nice way to understand WHY we have things like three families, gauge symmetries, and chiral fermions. As far as I know, these things are all put into other gravity approaches by hand, along with the number of dimensions, a cosmological constant, the black hole entropy, ... NEXT One dimension just means that you can describe any point on the string with one number. I think so. Well, four of our dimensions are infinite, so it's hard to be larger than that But if you're asking if all of the dimensions have to be the same size, then no---this is one of the papers that I (and tons of other people) am currently working on. The size of the extra dimensions has to be stabilized---this is a huge problem, called moduli stabilization. NEXT Think of the rope example. One dimension is big (the length of the rope), and one dimension is small and twisted up (the distance AROUND the rope). You can think of all kinds of funny spaces that have a similar property. Imagine a run of the mill torus. (Your profile says you know math, so I'll be a bit technical here.) You can make a torus by identifying the opposite sides of a sheet of paper, so you end up with a compact manifold (i.e. not infinite), upon which you can go in two different directions. Now imagine taking one of those dimensions to be big. If you live on the torus, you know that in one direction you can walk and end up back where you were, but the other direction to you looks essentially flat. Well, practically it can't. If you built a particle accelerator the size of the solar system or something, you could probably see evidence of the extra dimensions. But, barring that, you can't really test this assertion. You have to look for other generic predictions of the theory, and try to test those. But string theory is tricky, and it seems like there aren't very many generic predictions.

I thought the context was clear. I wish to answer question about science, not about long term goals of the field. Either way, it probably isn't good for any field to be downsized. When money stops going in to a field, when jobs become harder to find, more smart people would rather get a real job than worry about having to find the next post doc. A good friend of mine left strings for this reason, and now pulls down about ten times my graduate student salary in Chicago (before bonuses). And getting a post doc for him (and probably even a faculty position) would have been cake. He has over 150 citations of his work (http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+FRIESS&FORMAT=wwwcitesummary&SEQUENCE=), which is HUGE for a grad student. When more people work on a problem, progress is faster. There will always be people like Witten and Polchinski in the field who set trends and solve hard problems, but there are tons of small problems that a graduate student of average intelligence (enter: me) can solve. And all of these little problems have to be solved to have any progress whatsoever. The major thrust in the next decade will be explaining LHC/Planck data (more the former than the latter), and trying to get international support for the ILC, unless the LHC sees split SUSY or extra dimensions. Barring these unlikely events, more students will go into phenomenology because there will be tons of jobs for them (at the grad student level). Outside of high energy, recent advances in biology tell me that biophysics is the next big thing. And of all the grad students admitted to OSU with me, only two others (of 25 or so) were even remotely interested in high energy. Mostly people were interested in biophysics and condensed matter (OSU has a great AMO program). So the field I think is stagnant, and if the US institutions are downsizing, then China, India and Europe are on an upswing---there will be jobs in String Theory in the next ten years, but probably not as many in the US. This, again, is not necessarily a bad thing. The focus in the US will probably be towards experiment in the next 20 years, especially if the ILC is built here. Even longer term, if the VLHC is delayed, string theory will probably take another upturn, as it did after the SSC was killed during the Clinton administration.

Fair enough. I stand corrected. The illusion was to a popular book that has graced the bookshelves recently.

The decrease was in faculty, but the panel reccomended an INCREASE for graduate students and post-docs, I believe. If I am wrong, then I am wrong, but I remember reading on several physics blogs things to this effect. I don't know if there is evidence for a downsizing trend. More graduate students are studying string theory now because Elegant Universe came out when they were all in high school. As far as being good for the theory, who's to say? Obviously if there are more people working on a problem, progress is more likely to be quick. Martin---I'd really rather stick to the science, as opposed to the sociology. YT--- I will respond to your questions in a bit. I must go in to work now:)

Please do. You can find much information by googling my name on the internet. (I don't hide behind online handles.) Things you'll find... -->I am a self admitted redneck, and can skin a deer while drunk, AND tell you why compactifying the heterotic string on a six-torus gives you N=4 supersymmetry. -->I have an undergraduate degree in Chemistry, not Physics. -->There are probably 5-7 papers with my name on them floating around. -->I always post in physics forums under the same name. -->Farsight is the 5th crackpot I debunked (total 12). -->My woman is in med school. -->I'm never scared to be wrong.

Maritn---I can't speak about the size of the community, specifically. It has always been tough to find faculty positions in the US in string theory, and you can check the Rumor Mill---the numbers of string faculty hired for the past several years has been more or less constant. The HEPAP report recently suggested expanding the amount of money available for string grad students and post docs, so if anything I think there will be a slight upturn. The flip-side is that big, multi-national collaborations like ILC, LHC, and Planck will draw many workers into the field of phenomenology. The graduate students that I know are mostly accepting of the fact that they'll never get post docs, much less faculty positions. Many of them are just fine with going into industry and starting with six figure salaries. There are lots of positions in China, India, and Europe, so the people who really want to do physics generally don't mind going to these places. One can certainly say more than ten words here. The biggest problem with string theory is that it's a hard problem. I think that moduli stabilization (i.e. controlling the size of the six extra circles) is probably pretty poorly understood. For example, the Type IIA people (like Cvetic, Blumenhagem, et al) who build d-brane models can stabilize their moduli using fluxes, but can't get realistic phenomenology. Worse, there is no unique way to turn the fluxes on, so the moduli problem is restated as a ``flux turning-on'' problem. This aside, the fact that the moduli CAN be stabilized is a pretty big success. But the type IIA models don't have very much to do with reality---they have poor low energy phenomenology, and are fine-tuned. Conversely, our models (which have excellent low energy phenomenology) have no way to stabilize the moduli.

Hmmm. Hopefully I won't make any mistakes, but if ajb is floating around here, maybe he can correct them for me Well, these questions are pretty good, and some of them don't have answers. 1.) As far as we know, strings are made of...strings. IF string theory describes our universe, then a string is truly fundamental. This is a bit like asking, ``What are electrons made of?'' Well...electrons. Now, you could try to get a bit fancy and claim that electrons are made of energy, in which case you would be happy to accept that strings are also made out of energy. 2.) You can break a string. This is how you preform scattering experiments. This is also one of the triumphs of string theory---it gives general relativity in the low energy limit. So, suppose you compute two closed strings going to two closed strings, as above. The one loop corrections look like: You'll notice there are close analogues with particle physics. Now, once you compute these scattering amplitude, you find exactly general relativity! This is a BIT unexpected, as the theory was constructed from general quantum field theory considerations. So we didn't ENGINEER GR, we got it for free---it fell in our laps, so to speak. I can't describe this in any detail because I've never actually done the calculation, but I have it on good authority that this is the case In other approaches to quantum gravity, they start with GR. But then they have to ENGINEER particle physics. In some sense, we started with particle physics (quantum field theory), and got GR for free. 3.) Where did strings come from? This is a bit more philosophical. I could say ``The Big Bang'' and be done with it. Again, this question is a bit like asking where electrons come from. And it may be that this question doesn't really have an answer. Sorry if this is less than what you expected! But there are many questions that may never be answered, and you've stumbled on a few of them.

How can you have a spherical electromagnetic field? These are quasi-particles, not fundamental particles. They are formed from systems of electrons in strong magnetic fields. You are claiming that fundamental particles are no point-like. This is a MAJOR departure from observation. If the electron was not point-like, we should have seen new physics at the compton wavelength of the electron, but we haven't. So now you're relying on someone else's authority? I can find a collection of quotes that say the electron is point-like.

If you'll notice, I said that the first line of paragraph was good insight. The rest is just plain wrong. Oh, wait...it's worse... it's NOT EVEN WRONG. By all means start one. I know that it will be short lived.

Hi Glider--- I did read the abstract very closely. Specifically, some anti-abortion activists claim that fetuses can feel pain at eight weeks, which this research clearly contradicts. However, by the third trimester (30ish weeks), the fetus has the facilities to feel pain---at least, there seems to be little doubt about this. Either way, this much cannot be garnered from the abstract. This news article: http://health.dailynewscentral.com/content/view/0001552/40/, however DOES summarize the research. The scientists conclude that fetuses CAN feel pain at 29-30 weeks. Of course, I should clarify---by ``can feel pain'' I mean that they have the architecture in place. Thank you for clearing this up, I had not understood the acronym. From an animal rights website, admittedly not PETA: If one wants to accord a moral status to animals, in order that a consistent standard be applied across the board, shouldn't one accord the same standards to fetuses at the point where they can be shown to feel pain?

I started this thread for general stringy questions, if anyone is interested. Discussion is carried over from http://www.scienceforums.net/forum/showthread.php?t=28281 Wormwood--- Good question. This is actually one of the great successes of string theory---the fact that it predicts a number of dimensions is a success that no other approach to quantum gravity can claim, yet. The dimensions are predicted from the theory, by the requirement that the theory be consistent with quantum mechanics. In order to quantize a theory consistently, we have to introduce an idea called gauge invariance. This basically says that physics cannot depend on the way that we describe nature. In string theory, a one dimensional string moves through space, sweeping out a two dimensional world-sheet. Gauge invariance for the world sheet means that we can assign coordinates in any manner that we wish. In order to actually do a calculation, though, you have to PICK a set of coordinates---this is called choosing a gauge. The idea to use gauge symmetries to describe nature is not a new one, and is something that you would learn about in the second semester of a good quantum field theory class This is something that Glashow, Weinberg and Salam received the nobel prize for in 1979. Anyway, when you choose a gauge, you find that the theory has a bunch of unphysical stuff---stuff that would NORMALLY wreck a theory. But this stuff depends on the gauge that you choose, so we know that the stuff is truly unphysical. Sorry if this is confusing. Think of it like ths... Maybe you remember solving projectile motion problems in phsyics class. These are problems like, ``you fire a cannon with such and such an initial velocity and such and such a trajectory. Calculate how long it takes for the cannon ball to hit the ground.'' Anyway, you would solve the quadratic equation and get answers like t = 0 and t = 10 sec. You KNOW the t = 0 answer isn't right, so you toss it out and keep the other one. In string theory, though, you find out that consistently tossing the unphysical stuff out REQUIRES that we live in ten dimensions. This is a definite (but useless!) prediction of string theory---we live in 10 dimensions. I stress, this is a prediction of the theory. One can do general relativity in an arbitrary number of dimensions, for example. This is one thing that (I think) bugged Einstein---he couldn't explain why we lived in 4 dimensions. And, as far as I know, no other approach to gravity derives the number of space-time dimensions.

I'll start a string thread:) Wormwood--- Look here: http://www.scienceforums.net/forum/showthread.php?t=28298.

so... quite a bit?

Well, the simple one that Kaluza and Klein were working on, anyway. I do what I can.

Norm--- I seem to recall the tension in the string going to infinity, or some such.

I have talked with animal rights activists in the past, and the reason I always get is that we should protect things that suffer. What I don't understand is that it is proven that fetuses feel pain in the third trimester---see the link below. It is a systematic study, published in the Journal of the American Medical Association, detailing the finding that fetuses feel pain at 20 weeks. Why is it, then, that people like PETA aren't also campaigning for late term abortion bans? http://jama.ama-assn.org/cgi/content/short/294/8/947

Dan---you don't know what you're talking about.

The six little circles are the extra dimensions of string theory... We can describe OUR position with 4 numbers, but if we are small enough (like the ant on the rope), we need ten numbers. No, this was the first attempt by Einstein to unify electromagnetism and gravity. If you have five dimensions, where one of them is a little circle, then you can naturally incorporate electromagnetism with gravity. BUT, the problem is that there is an extra particle predicted that isn't observed, and we now know electromagnesim isn't fundamental.

Well, quantum gravity is a general term for a quantum description of gravitational phenomena. For example, the singularity occurs in the classical theory when a star collapses---that is, there is nothing to prohibit the matter from collapsing all the way to zero size, which is how you get a singularity. But we expect that there some new physics which prevents the singularity from forming---this is the catch-all phrase ``quantum gravity''.

I don't think you can say this. I think that the singularity forms classically---whatever quantum theory of gravity you want should prevent the singularity from forming, but classically you are stuck with it.

Despite the engineering challenges of having four beams travelling at 0.9999... times the speed of light collide at the same point, you are neglecting the fact that to get particles going this fast, you have to first accelerate them. LHC is so large becuase it takes a long time to get protons moving that fast. So you'd get twice the luminosity (two times as many collisions) but only half the center of mass energy. Keep thinking!

No, a black hole certainly exists, because an observer can watch an horizon form.

I don't understand any of this. What I think you said is that my calculation isn't valid because I am using a reference point at the center of the sphere. This clearly can't be right, as the velocity is measerued at the endge of the sphere, at r_e. No, that's exactly what we observe. Pointlike electrons---we have never preformed an experiment to make us believe that the electron is composite. You're assuming this, not showing it. Plus there is absolutely no experimental evidence that this is corect. Wrong again. We know what the mass is, we can measure it. Therefore we know the energy (or, rest energy, as you've written the equation.)

Nonono---for a dustball, or a dying star or something, a black hole forms in finite time, no problem. now if that dust was massless...