# Stillness of Systems

1. Mar 2, 2012

### StevieTNZ

Hi there,

I'm not sure if I read somewhere - but if we see a system (macroscopic usually) and it looks still, is it actually not still? Is there a universal equation that governs how 'still' an object is?

In the double-slit experiment, if we have the slits not being still, because we're calculating the paths possible for the system to reach point B as if the slits are in a certain position - they're 'moving' - can we safely ignore any stillness issues in any Physics equation without affecting the accuracy of the Physics equations?

2. Oct 17, 2013

### StevieTNZ

This question dates back quite a while, but am interested in knowing the answer.

3. Oct 18, 2013

### CWatters

Perhaps have a think about the implications of the Uncertainty principle. Prevents you knowing both the position and momentum at the same time. If the momentum is known (for example velocity = 0) then the position is unknown.

http://en.wikipedia.org/wiki/Uncertainty_principle

I'm not sure what the OP means by "safely ignore". Clearly this is a small scale effect that can be ignored in many situations.

4. Oct 18, 2013

### meBigGuy

The math for the double slit assumes infinitely thin perfect absorbers with perfect boundaries and interference free paths. To the extent that is not true, the results are distorted. The inherent vibration or uncertainty of the molecules at the boundaries and the resulting "non-ideal" interaction with the particle could be modeled. (but not by me)

So, stillness is but one of many factors that can effect experiments. I don't know of any universal treatment of stillness since it could be caused by uncertainty, vibration due to air currents, gravitational effects due to anything, or whatever.

5. Oct 18, 2013

### StevieTNZ

What do you mean by thin perfect absorbers with perfect boundaries?

6. Oct 19, 2013

### meBigGuy

What do you mean by what do you mean by? (lol)

I think I know what you are asking, but I'm not sure how far I can take it. I'm just saying that the simplistic math assumes the slits and barriers are ideal. There is no reflection from internal surfaces of a thick barrier, that the boundaries are not jagged, and when the barrier is contacted it results in perfect absorbtion (and none if not contacted). In determining the area of the slit there are no angle of approach issues caused by barrier thickness. I think I should have added that the particle is dimensionless. After all, the path integral assumes the particle takes all possible paths and a thick barrier has effects on the possible paths.