# Smallest unit of time

1. Jan 13, 2008

### Mephisto

i read somewhere that the smallest unit of time that makes sense is the Planck time, ~10^-43 seconds. I'd like to know how physicists arrived at this number. Also, why is it so impossible to think of a time step lower than that? What equation exactly is in conflict, and what kind of a conflict is it? For example, do you get a division by 0 if you try to use a lower time, somehow? or what happens?

thanks

2. Jan 13, 2008

### nicksauce

From wikipedia:
"The following thought experiment illuminates this fact. The task is to measure an object's position by bouncing electromagnetic radiation, namely photons, off it. The shorter the wavelength of the photons, and hence the higher their energy, the more accurate the measurement. If the photons are sufficiently energetic to make possible a measurement more precise than a Planck length, their collision with the object would, in principle, create a minuscule black hole. This black hole would "swallow" the photon and thereby make it impossible to obtain a measurement. A simple calculation using dimensional analysis suggests that this problem arises if we attempt to measure an object's position with a precision to within a Planck length."

The Planck time is the time it takes for a particle traveling at c to cross the Planck length.

Also, dimensional analysis shows that the Planck length is the only length that can be obtained from some mix of the 3 important physical constants G, h, c (up to a scale factor), so this shows that it might (must?) have some physical importance. If space and time are discrete it would seem that the Planck length and time would be the natural units for discreteness.

Last edited: Jan 13, 2008
3. Jan 14, 2008

### pam

The "Planck time" is just a convenient unit of time for some applications.
It is not a "smallest unit of time" and there is no reason that shorter time intervals
don't exist.

4. Jan 14, 2008

### lazypast

that text from wikipedia is so clear. i always thought plank time was the time at which after the big bang no one can describe.

like from t=0s to t=1^-43s physics breaksdown and doesnt describe this period

is this all wrong in which case ive plucked from absolute nothing??

5. Jan 14, 2008

### yogi3939

To my knowledge there is no "shortest unit of time". There is only an inability to measure time to determine if one actually exists. So far conventional thinking is divided between the uniform flow camp and the discrete unit camp and no one has devised a way to prove or disprove either one.