Question

How did Max Planck determine the planck length?

Asked by: peter baghdasaria

Answer

What Planck did to get the Planck length is a clever bit of calculation. Firstly he considered some fundamental constants of nature.

General relativity is concerned with two constants of nature: G (the gravitational constant, which determines the strength of gravity) and c (the speed of light).

Quantum field theory, on the other hand, is concerned with c (the speed of light) and h (Planck's constant, which determines the amount of uncertainty in our knowledge).

Physicists are attempting to unite these theories into one theory of quantum gravity. If this happens, we can expect all three of these constants (G, c and h) to play a role in the new theory.

Now note that:

The units of G are (distance * distance * distance) / (mass * time * time) The units of c are distance / time

The units of h are distance * distance * mass / time

Planck observed that there is only one way to combine these constants to obtain a distance:

Distance = square root (h*G/c*c*c)

The resulting distance is called the Planck length.

It is a curious observation that Planck made: there is no other way these constants can be put together to get distance. This means that any theory using only these three fundamental constants to predict distance (as a theory of quantum gravity would have to) will give a distance that is this length multiplied by a numerical constant. This gives the Planck length its meaning, because physicists suspect that it is the distance at which the effects of quantum gravity will become apparent and may even be the smallest meaningful length that exists.

Answered by: Sally Riordan, M.A., Management Consultant, London

Search

Loading



Click here to get
a FREE ride with Uber!


Click here to
sign up for Birchbox






Science Quote

'Every creative act involves ... a new innocence of perception, liberated from the cataract of accepted belief.'

Arthur Koestler
(1905-1983)
Science Sidebar | Science Education Articles
10 Ways to Keep Your Kids Interested In Science

Young children are natural scientists: they ask questions, pick up sticks and bugs outside, and are curious about the world around them. But as they get a bit older, many kids gradually lose their interest in science. They might see it as just another task at school, something that doesn't apply to their lives. Of course nothing could be further from the truth, so here are ten ways you can remind your kids that science is everywhere. Most of these are fun for adults, too! Continue reading ...

Top Selling

Here are our physics & astronomy bestsellers:
Magnetic Levitator - Classic
Deluxe Water Rocket Set
Scorpion, Ant, Wasp and Flower Bug
Solar Radiometer
Enviro Battery 4M Kit
Tin Can Robot 4M Kit
Brush Robot 4M Kit
Revolving Multi-Color Fiberoptic Light
Wood Grain Newtons Cradle
Weather Station 4M Kit

Sponsors

USC University of Southern California Dornsife College Physics and Astronomy Department McMaster University Physics and Astronomy Department