Preposterous Universe

Tuesday, April 12, 2005
On colliders and telescopes

Among other responses to the post about fundamental physics in the U.S., there was a position that one occasionally hears: "Who cares about particle physics, we can just do astrophysics instead, it's cheaper and more fun." I've heard this claim even (especially?) from people who have been experimental particle physicists themselves, and have decided to move into astrophysics. This is actually quite an established career path, although not always the easiest one.

The truth is: that's a fine philosophy if your concern is with the employment prospects of physicists, but not if your concern is understanding deep truths about nature. Both astronomy and accelerator-based experiments can teach us something about fundamental physics, but there is no sense in which one is a replacement for the other. That's the point of the surveillance vs. interrogation metaphor. Astrophysics is like eavesdropping: you can overhear things that you wouldn't learn under direct questioning, but you have to take what you can get. Astrophysicists take advantage of the fact that the universe provides higher energies and longer timescales than anything we can duplicate in the lab. But if there's something specific you'd like to know, but it isn't an important astrophysical process, you can't learn anything about it. Particle physics is like interrogation: there are some questions that Nature will clam up and refuse to answer, but at least you can ask very detailed queries under well-controlled conditions.

To be somewhat less allegorical: imagine that we are able to detect dark matter, either "directly" (when we detect the collision of a dark matter particle with material in an underground cryogenic detector) or "indirectly" (when we observe radiation from the annihilation of dark matter particles in the centers of galaxies). Either scenario is quite plausible, if the dark matter is a weakly-interacting massive particle. But then you might like to know, so what is that particle? Is it the lightest supersymmetric partner? Is it a Kaluza-Klein state in a theory with universal extra dimensions? Is it something exotic and different, that we haven't already theorized about? Astrophysical observations will never tell us the answer to these questions. You need to not only see the particle, but to measure its interactions with other particles, known and yet-to-be-discovered. The only way to do that is to push the energy frontier forward at particle accelerators. Similar stories can be told for questions about baryogenesis, extra dimensions, technicolor, and any other theory of physics beyond the Standard Model; Mark has posted about this, and I have a talk I gave a while back.

To be sure, particle physics has issues. Mostly, it's extremely expensive. After the LHC at CERN, we'll want to build the International Linear Collider, for which the numbers look like eight billion dollars or so. That's a lot of cash to devote to pure intellectual curiosity. I think it's well worth the cost, but others might not. That's okay; it's a debate worth having, and I'd be happy to defend the side of devoting some tiny fraction of our wealth to discovering the laws of nature. But it should be clear that this is the choice with which we are faced: spend the money, or don't make the discoveries. Looking through telescopes will always be a complement to colliding particles, never a replacement.

Ideas on culture, science, politics.
Sean Carroll

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