The Sciences

The Flow of Time

Cosmic VarianceBy Sean CarrollAug 19, 2011 4:27 AM


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I Tweeted the following inscrutable remark. Probably best left unexplained, but upon reflection I can't resist.

My consciousness freely travels up and down my world line, but sadly it only carries the memories appropriate to the moment it inhabits.

The point is that (some) people don't think about the flow of time in the right way, and this leads to a couple of unfortunate consequences: a difficulty in understanding the psychology of time, and a scattering of entertaining but illogical science-fiction scenarios. Modern physics suggests that we can look at the entire history of the universe as a single four-dimensional thing. That includes our own personal path through it, which defines our world line. This seemingly conflicts with our intuitive idea that we exist at a moment, and move through time. Of course there is no real conflict -- just two different ways of looking at the same thing. There is a four-dimensional universe that includes all of our world line, from birth to death, once and for all; and each moment along that world line defines an instantaneous person with the perception that they are growing older, advancing through time. But if you don't play too much attention to the way these two views fit together, you are tempted to imagine that "you" might actually, in some set of laws of physics if not actually in our own, go visit different moments in your own life, carrying along the consciousness of your "present" self. Something like that happens in SF stories from Slaughterhouse-Five to Back to the Future. But it's not consistent -- it requires the implicit introduction of a kind of "meta-time" that keeps track of when we visit the ordinary time with which we are familiar. That's not how nature works; my tweet was trying to point out the inconsistency of taking this idea seriously, subject to the strictures of 140 characters or less. (To be earnestly explicit: if you did manage to travel up and down your world line at will, you would indeed have whatever memories were appropriate to the moment you were inhabiting -- which means it would be exactly like not traveling at all.) Sometimes, unfortunately, people go further than science fiction. I've run into folks who believe that our conscious perception of time passing is actually evidence against modern physics -- arguing that we need to change the known laws of physics to account for the flow of time. It's always conceivable, in principle, that what we think we understand at a basic level is completely wrong. But the evidence had better be pretty overwhelming. The brain is a complicated thing, and I don't think that our present inability to provide a complete and comprehensive theory of conscious perceptions should be held as compelling evidence that the laws of physics are in need of overthrowing.

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