Wednesday, 25 January 2012


Let's talk about collisions. Pretty much all physics experiments can be broken down into two categories: scattering (for example, particle accelerators) and interference (for example, Young's double slit experiment).

Example of a scattering experiment. I have been trying to convince the guys in the lab next door (who are actually working on an interference experiment) that they need to buff up and dress like this in the lab. Incidentally, the laser safety glasses that he is wearing are correct for the wavelength of light being emitted in the tube, if I compare them to a similar model from Thorlabs.
In certain regimes, the line between the two kinds of experiments gets kind of hazy. You see, there is this thing called wave-particle duality. Sometimes, matter acts like a particle (like little balls smashing into each other), and other times the same matter can act like a wave (like ripples in water overlapping with other ripples, making an interference pattern). This is true even for some large molecules. It is most definitely true for cold atoms, like the ones I am working on.

In our lab, we are doing scattering experiments. We take two clouds of very cold atoms (less than a microkelvin!) and smash them against each other. However, they are so cold that they no longer behave like particles, as your everyday gas would. They instead act like waves and interfere with each other! The way they behave when they collide is nothing like what would happen to a particle. It looks like this:

These are two clouds of atoms trapped in some laser beams. They are about 400 nK and there are half a million atoms in each cloud. We accelerate one cloud towards the other and then release them from their trap so they can expand after the collision. The expanding halo is made up of scattered atoms.

These atoms are moving at pedestrian speeds. If we increase the collision energy a little bit more, the pattern of scattered atoms will look a bit different and develops lobes in funny places. It's one of the things next on the agenda, so watch this space!

I will add a final note about particle accelerator experiments. To do interesting physics and make a name for yourself, it's all about going for extremes. For example, the LHC was made to do higher energy collisions than any other existing accelerator. The other, often forgotten, end of the spectrum is the super slow collision regime. That's where we're at! We're doing the slowest, lowest possible energy collisions! That's pretty extreme.

1 comment:

  1. Massive biceps? Check.
    Laser safety glasses? Check.
    Cold atoms? Check.
    Infinite energy source in embedded in chest? Hmmm... still working on that one