For all you tumblr-ers out there (tumblr-ists? tumblr-dwellers?), 4 gravitons is now on tumblr. It’s mostly going to be links to my blog posts, with the occasional re-blog of someone else’s work if something catches my eye.

Nima Arkani-Hamed gave a public lecture at Perimeter yesterday, which I encourage you to watch if you have time, once it’s up on the Perimeter site. He also gave a technical talk earlier in the day, where he finished up by making the following (intentionally) provocative statement:

There is no direct evidence of what happened during the Big Bang that could have survived till today.

He clarified that he doesn’t just mean “evidence we can currently detect”. Rather, there’s a limit on what we can know, even with the most precise equipment possible. The details of what happened at the Big Bang (the sorts of precise details that would tell you, for example, whether it is best described by String Theory or some other picture) would get diluted as the universe expands, until today they would be so subtle and so rare that they fall below the level we could even in principle detect. We simply don’t have enough information available, no matter how good our technology gets, to detect them in a statistically significant way.

If this talk had happened last week, I could have used this in my spooky Halloween post. This is exactly the sort of thing that keeps physicists up at night: the idea that, fundamentally, there may be things we can never truly know about the universe, truths lost to the ages.

It’s not quite as dire as it sounds, though. To explain why, let me mention another great physics piece, Tom Stoppard’s Arcadia.

Despite appearances, this is in fact a great work of physics popularization.

Arcadia is a play about entropy. The play depicts two time periods, the early 19th century and the present day. In the present day a pair of scholars, Hannah and Bernard, argue about the events of the 19th century, when the house was occupied by a mathematically precocious girl named Thomasina and her tutor Septimus. Thomasina makes early discoveries about fractals and (to some extent) chaos theory, while Septimus gradually falls in love with her. In the present, the two scholars gradually get closer to the truth, going from a false theory that one of the guests at the house was killed by Lord Byron, to speculation that Septimus was the one to discover fractals, to finally getting a reasonably accurate idea of how the events of the story unfolded. Still, they never know everything, and the play emphasizes that certain details (documents burned in a fire, the true feelings of some of the people) will be forever lost to the ages.

The key point here is that, even with incomplete information, even without the ability to fully test their hypotheses and get all the details, the scholars can still make progress. They can propose accounts of what happened, accounts that have implications they can test, that might be proven wrong or right by future discoveries. Their accounts will also have implications they can’t test: lost letters, feelings never written down. But the better their account, the more it will explain, and the longer it will agree with anything new they manage to turn up.

That’s the way out of the problem Nima posed. We can’t know the truth of what happened at the Big Bang directly. But if we have a theory of physics that describes everything we can test, it’s likely to also make a prediction for what happened in the Big Bang. In science, most of the time you don’t have direct evidence. Rather, you have a successful theory, one that has succeeded under scrutiny many times in many contexts, enough that you trust it even when it goes out of the area you’re comfortable testing. That’s why physicists can make statements about what it’s like on the inside of a black hole, and it’s why it’s still good science to think about the Big Bang even if we can’t gather direct evidence about the details of how it took place.

All that said, Nima is well aware of this, and the problem still makes him uncomfortable. It makes me uncomfortable too. Saying that something is completely outside of our ability to measure, especially something as fundamental and important as the Big Bang, is not something we physicists can generally be content with. Time will tell whether there’s a way around the problem.