Saturday, April 23, 2005

Beginning String Theory

Okay, I’ve been posting enough about my day job.

By night I’m a physics graduate student. Well, my work lets me go to class during the day, but (when I’m productive) night and weekend minutes are for physics (sounds like a mobile phone commercial).

I’m currently tackling String Theory — we finally have a graduate course at UCD after my petition to start one last year. Great stuff! The whole reason I wanted to study physics to begin with. It’s being taught by Professor John March-Russell, visiting us from Oxford.

Some great material on Philosophy of Physics by Oliver Pooley: I found it while reviewing general covariance. Of particular relevance is his lecture on General Covariance and gauge theories, which helped to clarify the muddled thoughts I had running in my head.

Paul Ginsparg’s review of Applied Conformal Field Theory posted on his brain-child, Necessary material since I learned the deep-connections between Conformal Field Theory in 2d and string theory are what tell us so much about 2d objects like strings propagating along a world-sheet.

Belavin, Polyakov, and Zamolodchikov’s classic article “INFINITE CONFORMAL SYMMETRY IN TWO-DIMENSIONAL QUANTUM FIELD THEORY” unfortunately isn’t on arxiv, and right now our electronic journals aren’t responding, so I may have to track down Nucl.Phys.B241:333–380,1984 on actual paper.

Well, this was fun but I should go back to reviewing my notes.


Anonymous said...

Hi -- I'm an Orion's Arm devotee, and much appreciated your explanation of wormholes. I had a few questions regarding wormholes I was wondering if you could answer.

1) So traversable wormholes have event horizons (that exotic matter can theoretically "banish"). Do they have singularities as well? Does exotic matter banish these as well?

2) Can a wormhole mouth open onto an event horizon or a singularity (e.g. of a black hole)?

3) re: aspheric wormholes (wormholes whose mouths are polyhedral and open onto multiple nonadjacent spaces) -- what might happen if instead of having a few facets, you had an infinite number of facets, and thus having no edges and corners (or, well, an infinite number of edges and corners)? Would such an infinite-faceted mouth be a singularity?

Sorry for posting these questions -- you seem like the most likely person to answer in a easily understood lay audience manner!

Best --
Charles Choi

Adam Getchell said...

1) Traversable wormholes don't have event horizons. The absence of the event horizon is the difference between a wormhole and a black hole. Since they don't have event horizons, they don't have singularities either (although there is unusual spacetime structure in the center not reachable by the traveler, for the modified Morris-Thorne-Kuhfittig solution).

2) Technically yes, those are Einstein-Rosen bridges. But they're not traversable in practice, because they pinch off faster than even a photon could manage (ie, they close before even a beam of light could get through).

3) That would be a spherical wormhole. Since its a wormhole, there's no singularities (since there are no event horizons).

Hope that helps!

Anonymous said...

Thanks much!

btw, Is it possible for a structure to 'thread' a wormhole, i.e. to constantly be stuck in one mouth and out the other? Could make for an interesting space-elevator-like structure...

Charles Choi

Adam Getchell said...

Yes, a structure can thread a wormhole. In fact, that's how Carl Sagan did it in Contact, after Kip Thorne's advice.

However, the structure will experience tidal stresses if it moves or rotates with respect to the wormhole mouth, so care must be taken with the structure. There will also be radial tides from the wormhole, so it must be of a certain minimal strength to stay together.