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Name: Mitra
Status: other
Grade: N/A
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Date: 10/31/2005

What is the smallest unit of time? I have read that the smallest unit of time is "the time taken for light to travel across the surface of a speck of dust (that is visible in a shaft of light coming through a crack in the door of a dark room) that floats in the air". How can this be a repeatable measure?

Dear Mitra,

You are very perceptive to note that the time taken for light "to cross a speck of dust" is not a repeatable measure.

In fact, as far as is known today, there is no smallest unit of time. A clear indication of this is that the universe expanded by a larger factor in the first 1E-35 seconds of its existence that it has in the billions of years since that incredible time. If this is true (and there is actually pretty good evidence that it is), the smallest unit of time must be MUCH shorter than 1E-35 seconds. By 1E-35 I, of course, mean a number which can be written as 0.000...0001 with 34 zeroes between the decimal point and the digit 1.

I hope these comments are of some use to you. You are asking about a very subtle part of our understanding (or lack of understanding). Perhaps someday we will discover that time is indeed quantized (with the smallest quantum of time much shorter than 1E-35 seconds) and our understanding of our universe will be much increased.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University

The smallest units of physical variables -- time, length, charge, etc. are based on the value of a few fundamental constants: the speed of light in a vacuum, Planck's constant, the universal gravitational constant, the Coulomb force constant and Boltzmann's constant. The web sites below explain the Planck time specifically and the origin of the units in general. Other web sites can be found using a "Google" search on the phrase "Planck units".

In "Planck units" all these constants equal unity. In SI units the Planck time is about 5.4x10^-44 sec, the Planck length is about 1.6x10-35 meters,

2.2x10^-8kg, the Planck charge is about 1.9x10^-18 coulombs, and the Planck temperature is about 1.4x1032 kelvins.

A detailed discussion of the subject can be found at:

The laws of physics, as we know them, do not apply to values of the constants smaller than the Planck values.

Vince Calder

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