By Sean Carroll | June 17, 2009 10:04 am

After the FQXi Essay Contest, I was asked to comment on some of the essays besides my own, but I never did. Mostly because I didn’t take the time to read them all (there were an awful lot), but also because I just don’t know what to say about many of them. In her essay (which I liked), Fotini Markopoulou divides the world in two:

There are two kinds of people in quantum gravity. Those who think that timelessness is the most beautiful and deepest insight in general relativity, if not modern science, and those who simply cannot comprehend what timelessness can mean and see evidence for time in everything in nature. What sets this split of opinions apart form any other disagreement in science is that almost no one ever changes their mind…

That’s just about right (although perhaps there are also other splits with the same quality). Julian Barbour, whose essay finished first in the judging, has famously championed the view that time does not exist, even writing quite a successful book about it. In a recent Bloggingheads discussion with Craig Callender, Barbour talks a bit more about his view.

To which all I can muster is: I don’t get it. There are a set of technical arguments, which for the most part I do get, that can be used to make it seem as if time does not exist. In ordinary classical mechanics, we can perform some formal tricks to remove the time variable from the conventional equations of physics. More dramatically, in general relativity or quantum gravity we can express Einstein’s equation (at least in certain circumstances) in a form where time does not appear. On the other hand, we can usually re-write any of these equations in a form where time does appear (at least, again, in certain circumstances).

But none of these technical arguments are really the point. What I don’t understand — and this is a sincere lack of understanding on my part, not an indirect claim that this perspective is wrong — is what’s supposed to be so great about timelessness. What are we supposed to gain from thinking in this way? What problems is it supposed to solve?

Put it this way: clearly time appears to exist, at first glance. Even the timelessness crowd somehow manages to submit their essay competition entries by the deadline, and finish their Bloggingheads dialogues within an hour. So the claim “time does not exist” certainly doesn’t mean the same kind of thing as “unicorns do not exist.” It must mean (I suppose) that, while we all find time very useful in our everyday lives, there is a deeper level of description in which time doesn’t appear at all; it only emerges in some sort of approximate description of reality. But that approximate description seems extremely valid and useful, including all of the phenomena in the observable universe. Surely it behooves us to take this purportedly-non-fundamental notion seriously, and attempt to understand some of its puzzling features? Moreover, even if “time” doesn’t turn out to be fundamental, why would that tempt you into saying that it doesn’t exist? Protons are made of quarks, but you don’t hear particle physicists going around claiming that protons don’t exist.

The problem is not that I disagree with the timelessness crowd, it’s that I don’t see the point. I am not motivated to make the effort to carefully read what they are writing, because I am very unclear about what is to be gained by doing so. If anyone could spell out straightforwardly what I might be able to understand by thinking of the world in the language of timelessness, I’d be very happy to re-orient my attitude and take these works seriously.


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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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