Freethought & Rationalism Archive

spacer1 · 08-17-2003, 10:27 PM

the_cave,

Quote:

I don't want to sidetrack the topic, but I just wanted to add that for me, the biggest mystery was how there could be bottom units of things like time. I mean, if a unit of Planck time is the smallest that time gets, how do you know?

I found this explanation online:

Quote:

It is true and significant that the Planck length arises naturally when one considers the ultimate limits to measurement. Crudely, it happens because refined length measurement requires large momentum, according to Heisenberg's uncertainty principle, but when the momentum becomes too large, its gravitational effect becomes strong, curving space-time and distorting the interval one seeks to measure. Thus a fundamental difficulty arises in resolving lengths below the Planck scale.

http://www.physicstoday.org/pt/vol-54/iss-11/p15.html

Friar Bellows · 08-18-2003, 02:03 AM

Quote:

Originally posted by the_cave
I don't want to sidetrack the topic, but I just wanted to add that for me, the biggest mystery was how there could be bottom units of things like time. I mean, if a unit of Planck time is the smallest that time gets, how do you know?

As far as I know, the Planck time is the time it would take for light to travel the distance equal to the Planck length. One can do a hand-waving derivation of the Planck length on the back of an envelope. You might like to read this web page (esp. section 4) for more info:

http://math.ucr.edu/home/baez/lengths.html

To be clear, though: we don't know if the Planck length is the smallest length possible. It's just that we have no theory that accurately describes the world at such a length scale (or smaller). For that we would need a theory of quantum gravity, which we don't have yet.

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