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March 26, 2007



Charlie says: First you have to define what you mean by "negative time." Do you mean it runs backwards? Otherwise, it's not a well-defined problem.

I say: Only in stories where you sometimes see the phrase "in less than no time," which is (now that I think about it) an interesting example of hyperbole (or one of those other fancy English-major words that I no longer remember). But there's a limerick (no, a clean one, really!):

There was a young lady named Bright
Whose speed was much faster than light;
She set out one day,
In a relative way
And returned on the previous night.

attributed to Arthur Henry Reginald Buller, in the December 19, 1923, issue of Punch. On the other hand (are we up to three hands yet?), the physicist Paul Dirac predicted an electron traveling backward in time, which would be observed in positive time as a positive charge traveling forward in time -- that is, a positron. And those do exist. But I'm in over my head now (in the Dirac sea?).

Katie Nestor

Yeah, I meant time running backwards. I will tell my class about the other stuff.


Okay, there's an interesting article at

It sounds as though generally speaking, the idea of time running backward is something that theoretical physicists play with for fun, but not something that people (or objects) can do, at least in four-dimensional space-time as we experience it.

Now if you're talking about MEASURING time, then there's certainly negative time (as in countdown for liftoff), where you're talking about time remaining before a (predicted) event, rather than time elapsed after an (observed) event. So in that sense, sure, you can have something like "at time T = -30 seconds." But it would be hard to spend half a class period on that one. (Ask people when the millennium ended, though -- if they say on December 31, 1999, you can tell them they're wrong, and I'll bet you can start something...)


Like she said, "42?"


I think the idea of a positron as an electron running backward in time was due to Feynman and Wheeler. Dirac's original idea was that all states of negative energy are full, except when enough energy is absorbed to pull an electron all the way into the positive energy continuum, so making a free electron. The positron was then the hole in the negative energy sea.

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