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new school year

So, yesterday was the first day of school this year. Went to my very first Quantum Field Theory class yesterday, and Intro to Particle Physics today. Both were just review so far, but it looks like we'll be getting into the new stuff real soon. This quarter I'm TAing one astro class ("Overview of the Universe") and one physics class ("Mathematical Methods in Physics II"). Both of them should have a bit of material here and there which I may not have seen in full detail as an undergrad (I've never taken an astro class or a math methods class, just sort of picked them up on my own), so it should keep me on my toes and fill in a few gaps.

Today we made liquid nitrogen icecream again, our best batch ever! yummm... we put eggs in it this time, which made it much creamier and taste delicious. I think that was the secret ingredient we were missing before.

I officially got the word back that I passed all my quals, but I feel an odd combination of outrage, disappointment, and relief because my statmech score was far below what I projected, and I barely passed E&M at all. Apparently, I'm not nearly as good as I thought at self-assessment. (That's the last time I put errorbars on my projections!) I have yet to figure out what the hell the extra points they took off are for, since my answer agrees with the book and as far as I can see I didn't skip any steps or make any leaps in reasoning. But then again, it's said and done with. I narrowly escaped certain death by a closer margin than I thought possible, but I suppose it matters little now. :) I'm on the other side of the gateway, awaiting the next challenge.

What else? Oh, I never knew we had this here, but I was invited to join "journal club" today. I signed up for the email list and plan to attend (and hopefully) present at some point. Looks interesting:

Dear physics grads,
To promote a broader knowledge of physics, we have started a journal
club which was very successful last spring.  We wish to continue this
and invite (encourage) all interested grads to attend (and maybe
present).  Basically, we just get together on fridays at 11:30am and
have lunch and talk about a specific article that the presenter has
read and found worth sharing.  Unlike fight club, you can talk about
it.  Our first meeting is friday, Oct 1.  (old JCers, note new time).
we meet at the picnic tables outside ISB.
Prime Directives:
1) Go where no physicist has gone before by communicating complex
subject matter in a comprehensible way to a reasonably intelligent
sentient beings.
2) No extra work; present articles that you've already read and feel is


( 5 comments — Leave a comment )
Sep. 25th, 2004 11:29 am (UTC)
congrats, my good man!
Sep. 26th, 2004 11:59 pm (UTC)
Hi there. Long time reader first time commenter. Got a question if you don't mind. Re: general relativity, how can we say that space is warped unless there's something else beneath it to compare it to? For example, it's impossible to determine if time is running slow or fast for us if we're contained and can only view things whose time goes at the same rate. A CPU has no way of knowing how fast it's going unless it also has access to a clock and even then it can only make a comparison. Has no idea if it's fast or the clock's slow or vice versa. I think we can agree on that. Then how can these illustrations of warped space show a straight line being curved and an imaginary straight line of where it appears to be? There is no straight if space is all that exists and it is warped. Straight is whatever direction the warping occurs. These illustrations imply there's another "space" or whatever beneath that's perfectly flat and our space is curved relative to it, just like the clock and CPU example. Thanks for your time.
Sep. 27th, 2004 10:23 am (UTC)
Hey there,

excellent question!

There are some properties of spaces which work like your time example... you can't measure them unless the space is embedded in a higher dimensional space. For instance, a piece of paper is a two-dimensional space. From inside that space, there's no way to tell if it is folded up or crumpled or what direction it's turned in. These are meaningless questions to ask unless it's sitting in some larger-dimensional space such as the 3D world we're used to moving around in. But there are other properties, such as curvature, which are intrinsic to a space. You can measure the curvature at any point in space from inside, without reference to any larger space.

Notice that even though you can fold a piece of paper without changing its internal geometry, you cannot wrap a piece of paper smoothly around a baseball without warping it. In order to deform a flat piece of paper into a curved (warped) piece of paper you'd need it to be made out of rubber or something which stretches. As long as there is no stretching allowed, the paper has an intrinsically "flat geometry" to it.

One of the easiest ways to tell if a space is locally curved is to measure the sum of the angles in a small triangle. If it adds up to 180, then it is flat. If it adds up to more than 180 then it has positive curvature. If it adds up to less than 180, it has negative curvature. The surface of the earth is an example of a 2D surface which has positive curvature. Another consequence of positive curvature is that parallel lines will always eventually intersect. For instance, take two meridians on the earth which are parallel at the equator (both form 90-degree angles with the equator). Follow them up and they will intersect at the north pole.

A 3D (or 4D, in the case of spacetime) curved space works the same way; you can tell from inside that triangles sum up to more than 180 degrees, and parallel lines can intersect if you follow them far enough. Even if you can't see the whole space or if it isn't embedded in any higher dimensional space. Such is the case for spacetime. The mathematical study of these kinds of spaces is called "Non-Euclidean Geometry" because it violates many of Euclid's original postulates about geometry (such as parallel lines never intersecting). When you do calculus on such spaces it's called "differential geometry", one of the coolest classes I got to take as an undergrad.

Hope that helps!
Sep. 27th, 2004 02:50 pm (UTC)
Thanks, I understand completely now. That had to have been one of the best, clearest explanations I've ever gotten to a question like that. You up for more as I come across them? Feel free to set a rate limit at one a week or so.
Sep. 27th, 2004 06:43 pm (UTC)
It all depends on my workload (which should be going up pretty soon as the school year is kicking off.) Also depends on how in-depth the questions are. Some would take only a few minutes, others I would have to think about longer. But there's no harm in asking!

You could also ask on physics; I usually try to answer any questions on there if I have a good answer and nobody else has said it yet. Although be aware that some people there will give partially correct (or even incorrect) answers, and others might give correct answers that aren't very illuminating. You have to learn "who you can trust" to some extent. But if anyone is too far off base, someone will usually point it out.
( 5 comments — Leave a comment )


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