Planck mass; time; length derived out of air?

1. Feb 18, 2006

Chaos' lil bro Order

Greetings,

Anyone know how the Planck mass, Planck time, and Planck length were derived in the first place? Please cite experiments or mathematics to support your comment as I'd like to intimately understand this, not just from a metaphorical standpoint.

Thanks a bunch,

Chaos' lil bro.

2. Feb 18, 2006

Staff Emeritus
The http://en.wikipedia.org/wiki/Planck_length" [Broken] are algebraic combinations of three measured constants of nature, the speed of light, c, Planck's constant, h, and Newton's constant of gravity, G. Those constants have been measured many many times.

Last edited by a moderator: May 2, 2017
3. Feb 20, 2006

dextercioby

They are the only possible algebraic combinations of those 3 constants which obey dimensional analysis.

Daniel.

4. Feb 27, 2006

Chaos' lil bro Order

Ok, then how was 'h' derived?

Ok, C, G, h.

Then how was Planck's constant 'h' derived? If possible please site an experiment, observation, or derivation since to help me understand the meaning beyond a google cut and paste.

Thanks.

5. Feb 28, 2006

AlphaNumeric

Do you mean the concept of h or the value of h?

The concept of h came from problems in thermdynamics/electromagnetism around 1900 which predicted that the black body radiation spectrum for a cavity (like a heated oven) should have infinite energy. In order to solve this Max Planck realised you could get the right spectrum if you assumed light was made up a particles which had energy hf, where f was their frequency and h a constant. He didn't initially agree with his own work, but the results were undeniable.

To get the value of h, experiments are done. I seem to remember reading a few years ago that Josephson junctions are excellent for determining h to a very high accuracy, but solid state physics isn't my thing and I can't be sure I'm remembering that correctly.

6. Mar 2, 2006

Chaos' lil bro Order

Alpha, great post!!

One little question:
Are you saying that the oven contains infinite energy because there can be infinite modes within it? As in a curve that extends towards but never reaches the x line?