**Ask a monkey a physics question thread**

[OngBonga]

I feel like we got quite a lot of mileage out of that question posed by a random internet user. Cheers Ash.


It is interesting though. If you have two objects of vastly different mass "bounce" off each other such that the mathematical motion of the larger body is less than the Planck length, has the larger body moved at all? How would it be possible for such a transfer of energy to happen? Maybe in this case the ground does behave like a perfectly rigid surface.

[MadMojoMonkey]

Maybe in this case the ground does behave like a perfectly rigid surface.

Perfect rigidity is problematic in the same ways, it just takes a bit of Einstein to see it.
You can google Born Rigidity (named after physicist Max Born) to see why it's problematic, but it's pretty heady to understand.
IDK, though... your knowledge of Einstein's Relativity is way above the average Joe, so maybe you can dig it.

If an object is perfectly rigid, then it cannot experience length contraction, but then it cannot accelerate at all.
This takes more than a passing glance to prove, and an old model is to imagine 2 identical rockets connected by a thread. The 2 rockets are exactly the same and accelerate in exactly the same direction such that the tension on the string is constant. For all intents and purposes, they are 1 rigid object, just a funny shape.

The thread is needlessly thin to show that it simply must change length or break... and if it breaks, then that's a problem... 'cause it means that a perfectly rigid body must change shape in order to not break... which is a contradiction with "perfectly rigid."



Ergo, it cannot be real. It would be the hypothetical immovable object.

[OngBonga]

c tells us the speed of causality... basically how fast we move through time.

g tells us how much a given mass will bend spacetime.

h-bar tells us something too, we just haven't really figured out what.

[MadMojoMonkey]

c tells us the speed of causality... basically how fast we move through time.

g tells us how much a given mass will bend spacetime.

h-bar tells us something too, we just haven't really figured out what.

Why use h-bar and not h?

The difference is only a factor of 2 pi, but which one you choose to use in the calculation of the Planck Length will obviously change the value you calculate.

So which one gives the "real" Planck Length? Why is that the one?