# What is Planck time, Planck length, Planck particle

1. Oct 24, 2005

### Caesar_Rahil

Can someone explain what are they:
For egs. What is Planck time, PLanck length, Planck particle etc.

2. Oct 24, 2005

### Mk

Erm... this is difficult to explain.

The Planck time is the time it would take a photon to move a Planck length.

The Planck mass is a mass whose Schwarzschild radius and whose Compton length are equal distances. A mass' Schwarzchild radius is the radius of its event horizon. Its event horizon is the imaginary spherical boundary which divides the region surrounding the black hole, where the escape velocity is ≥c, and the rest of space.

If you compressed the Sun to its Schwartzchild radius, it would form a black hole and would be three kilometers wide - about 4 millionths, of its present radius. For Earth to meet the same fate, it would have to be squished into a sphere 18 millimeters across - about a billionth of its present diameter.

Compton wavelength can be thought of as the rough intrinsic quantum mechanical size of a particle. The Compton wavelength λ of a particle X is given by λX = h / mXc, where h is the Planck constant, mX is the particle's mass and c is the speed of light.

I've never heard of a Planck particle. Perhaps a particle with a Planck mass?

Last edited: Oct 24, 2005
3. Oct 24, 2005

Exactly, defined as a singularity of one particle whose Compton wavelength is equal to its SC radius. Just a theoretical device mostly.

4. Oct 24, 2005

### Caesar_Rahil

5. Oct 24, 2005

6. Oct 24, 2005

### rbj

the words of Mk above are correct, but i believe there is a much more fundamental way to first introduce oneself to the Planck Units. essentially, the Planck Units are a definition of units so that, in terms of these Planck Units, the numerical value of these dimensionful universal constants: $G$, $c$, and $\hbar$, $k = 1/(4 \pi \epsilon_0)$, $k_B$ (Boltzmann) are all set to 1. so all of those constants disappear in the physical equations of action. that makes them Natural units (although, i am convinced that it is more natural to normalize $4 \pi G$ and $\epsilon_0$ rather than what are the traditional Planck units. as it is, the dimensionless Fine-structure constant $\alpha$ is equal to the square of the elementary charge $e$ when measured in terms of the Planck charge.

Planck Units (or something very close to them) define where the "tick marks" are on Nature's ruler or clock or weighing scale or electroscope. if you think about it, Planck units help you understand why it is literally meaningless to talk of an operational difference if any of the dimensionful constants (like above) were to change. in Planck Units, even if some god were to somehow change the speed of light, the speed of light is still 1. we would not know the difference.

the base units are a unit time, length, mass, charge, and temperature. see http://en.wikipedia.org/wiki/Planck_units to get the low-down on this aspect.

there are other sorta speculated physical consequences of particles taking on the values of some of these quantities (but not always, the Planck Mass is the mass of some specks of dust). but these consequences are the results of working problems in physics and getting results that depend on those universal constants above.