Monday, April 5, 2010

Changing the Rules

Is the Large Hadron Collider a time machine?

Although I usually like Dennis Overbye's physics writing for the New York Times, I thought he misfired in answering this question yesterday, in the general-audience "Week in Review" section.

In a Q&A entitled A Primer on the Great Proton Smashup that discussed the scientific ideas that underlie research at the LHC, Overbye addressed the question:

"What does it mean to say that the collider will allow physicists to go back to the Big Bang? Is the collider a time machine?"

It may seem silly, but it's actually a good question, since I'd bet a lot of people get confused by the metaphors that writers use to motivate the research. These metaphors get repeated often enough that they are almost cliché, but, as with all metaphors, it's important to know which parts to take seriously and which parts are more poetic or even misleading. Not everybody will know which is which, and it's good to explain it every so often.

Here's Overbye's complete answer:

"Physicists suspect that the laws of physics evolved as the universe cooled from billions or trillions of degrees in the first moments of the Big Bang to superfrigid temperatures today (3 degrees Kelvin) — the way water changes from steam to liquid to ice as temperatures decline. As the universe cooled, physicists suspect, everything became more complicated. Particles and forces once indistinguishable developed their own identities, the way Spanish, French and Italian diverged from the original Latin.

By crashing together subatomic particles — protons — physicists create little fireballs that revisit the conditions of these earlier times and see what might have gone on back then, sort of like the scientists in Jurassic Park reincarnating dinosaurs."

I'll discuss in a moment what I think Overbye means by "the laws of physics evolved," but this notion is awfully subtle for a general reader. More importantly, it completely undercuts the whole thrust of the question: physicists believe they are learning about the early universe in high-energy particle collisions precisely because the laws of physics are the same. If the laws are the same, we can create the same conditions (mostly temperature) to learn about what might have happened in the early universe. (He eventually does say that.)

The confusion comes because the phrase "the laws of physics" can be mean quite different things.

In the context of LHC, it seems clear to me that we refer to the behavior at the deepest levels of the universe. These rules don't get repealed overnight.

In fact, as I understand the phrase, it refers not to the current human description of events, which changes as we learn more, but to the "truth," which doesn't. Otherwise, it wouldn't make sense to say that we want to learn about the laws of physics from the collider (since we already know the laws, even if they're wrong).

Still, we often say the laws of physics say that something is impossible. In that context, the phrase can only refer to our current understanding of the laws, as best as we can discern them.

In fact, when we talk about the laws of physics we're frequently not talking about the deep levels probed by the LHC. Instead, we're referring to laws that describe the more mundane behavior of objects in our cold everyday reality.

In one sense, these "laws" are just a manifestation of the deeper laws. Describing the world in terms of protons, or nuclei, or atoms, or molecules, or cells, or organs, or organisms, or societies, is often vastly more useful than describing it with quarks or strings.

In some cases, the higher-level description can be mathematically related to the deeper description, for example by "coarse graining" the description to smooth out fine details.

This is the sense in which we can say that the "laws of physics" evolve: when the universe was very hot, the description had to include a lot of ingredients that are no longer important now that the universe is much cooler. We can now accurately describe things using a simplified description that doesn't have to include the messier details. The "laws" are different now.

This is Overbye's answer. But I think it will confuse people, since the goal of the LHC is to learn about the immutable laws, not the simpler descriptions or approximations.

One further, mind-blowing complication. Many cosmologists are exploring the possibility that our universe is just one of an infinite number of universes that formed, like bubbles, out of a larger multiverse. According to this view, the "laws of physics" --perhaps even the dimension of space--may be entirely different in each of these universes.

Even if we will always see the laws of physics as unchanging, they may be not be the same everywhere.