Monday, September 26, 2011

Raman Sundrum at UNC

Today for our colloquium we had Raman Sundrum come and speak. He is of course known for the Randall-Sundrum model of particle physics (which was mentioned briefly on this site about three years ago). The RS1 model was established in three different papers published back in 1998 and 1999, and their papers have been cited 1022, 4321 and 5043 times respectively.

His talk was not unfathomable, and was actually quite understandable. One thing that I found quite interesting was that every single one of his power point slides had been written and drawn by hand. Seriously, every graph, figure, title and word he had drawn by hand. I was quite impressed, especially since it was so ascetically pleasing to look at (and colorful too).

He did not go into depth about the his models, but rather gave a brief overview and then introduced some key ideas. The two that I will mention here involve particles and anti-particles, and how mass is involved in interactions (spoiler: it isn't).

In explaining particle interactions he mentioned the standard space-time diagrams with the world lines of the particles. Imagine a particle moving in space and then at some point in time it interacts with another particle, as shown below. The arrows on the lines show the "direction" of the particles through time, that is, the direction of the arrow of time for the individual particles. The two particles exchange a virtual particle and thus have an "interaction". The virtual particle carries the exchange of energy and momentum from one particle to the other. So at some instant in time the particles interact and exchange energy through a particle that is created for just that purpose.
Now lets shake things up a little. We take the above diagram and rotate the particle interaction by 90°. The arrows still show the perceived "direction" of the particle as it sees itself moving, not how we would perceive it to be moving. Our percetion is governed by the "lab frame" denoted by the axes.
In this case we observe two particles, a normal particle and an anti-particle, moving towards each other (think of a slice at t = 0, that steadily moves up the graph. That slice is what we would see.). At some point the two particles meet and annihilate in a flash of light. The photon resulting from the collision goes off somewhere else in the universe. But if we observed that photon long enough we would see it split into two particles which would head off in different directions. But as we see from the above graph, what is actually happening is that the is a collision, much like the collision we saw before, but instead of happening in an instant of time, this one happens in an instant of space. Thus the anti-particle that we saw was not really a separate particle from the normal particle but they were actually the same particle, it's just that we observed the "anti-"particle moving backwards in time at a different location. From the perspective of the particle it was moving along and then at a particular point in space it interacted with another particle at some point in the future, thereby causing it to reverse direction in time and, from our perspective it became an anti-particle.

If that doesn't make sense, don't worry you're not the only one to get befuddled by that thought. It is quite mind blowing when you get it though.

The other thing that Raman Sundrum mentioned, and I won't give any explanation, I'll just mention it, but he said that when two particles interact, in the actual moment of interaction, they may as well have no mass. That is, it does not matter what the mass of a particle is in determining an interaction. The collision is entirely mass independent, but how the particles travel before and after the collision is entirely dependent on their mass. The strength of the particles' interactions does not depend on their mass. Thus they travel as a massive particle, but interact as a massless particle. Kind of like particle-wave duality, where particles travel as a wave, but interact like a particle. Interesting thoughts.

It was a rather interesting talk and really cleared up a few things for me about particle physics and why we built CERN in the first place.


  1. "Thus they travel as a massive particle, but interact as a massless particle."  Wow that is a cool thought.  Amazing how you can stare at these equations and compute things and never think of stuff in these ways.  

  2. Very interesting.  I heard that Feynman had a thought along those lines,
    that maybe all the electrons and positrons we see are just the same
    particle moving backwards and forwards through time.  The trouble then,
    he realized, was why do we see more electrons than positrons. 

  3. I guess that would be a symmetry issue, but Raman Sundrum was not trying to imply that that is where all particles come from, just the anti-particles that we see. In other words, most matter is normal matter but only occasionally it interacts with a particle in the future and thus becomes an anti-particle.


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