MadSci Network: Physics

Re: does an atom violate the laws of physics?

Date: Wed May 23 22:36:55 2001
Posted By: Vladimir Escalante-Ramírez, Faculty, Institute of Astronomy, National University of Mexico
Area of science: Physics
ID: 989961565.Ph

This is a rather philosophical question. Galileo and 
later Newton proposed as a physical law that a body 
will continue in motion, or will remain still, until 
a force changes its state. This is the way Nature 
works. Asking why is a very tricky question and can 
be answered in many ways. For example, suppose that observer "1"  
sees that an object moves in a vacuum, so that it never bumps into 
something. That would be perpetual motion, isn't it? 
Now suppose that observer "2" moves at exactly the same 
speed as the object and moves along it. This observer 
would say: "This object is not moving. It is permanently 
at rest. You are the ones who are moving in the opposite 
direction, and that's why you think the object is moving 
perpetually." Who is right? Physics says that both observers 
are equivalent. Nevertheless the laws of physics that 
observers "1" and "2" can determine about the object must 
be the same. Thus perpetual motion is compatible 
with physical laws, and even necessary if they are to make sense. 
I recommend that you read "The character of physical law" by 
Richard Feynman for further insight into how physical laws are made. 

Of course in real life, we never have a perfect vacuum. If 
I throw something, it will stop at some point because the 
force of friction with the air or floor will act against its 
movement. The explanation is that its energy is being transformed 
into heat due to that friction, and hence it is lost. 
Of course it can bounce off a wall and keep moving a little 
longer if it is made of some elastic material that looses 
little energy when it is deformed. 

What then keeps atoms moving? Atoms can be very elastic so to speak, 
or they can absorb energy very efficiently depending on the kind 
of collision they suffer. An atom that moves in a perfect vacuum 
won't stop moving. If, on the other hand, the atom moves within a 
container with lots of atoms, when it collides with an atom 
of the wall of the container or with a free atom within the container, 
it can recoil with the same energy, or it can pass its energy to 
the other atom and slow down, or it can receive more energy from 
the other atom and speed up. If there are enough atoms around, 
some atoms will speed up and some will slow down in every collision, 
so that the average energy, and the total energy of 
the container stay the same. We call this motion "heat", and 
as you know, a well insulated container will not cool down or 
heat up easily, like a thermos bottle, which means that atoms 
are exchanging energy all the time, but the energy of 
the whole stays about the same. 

It is possible to reduce the motion of atoms by cooling the container 
and it is possible to speed them up by reheating the container. If we 
could prevent the container from cooling down, the atoms in it would 
be loosing or gaining energy in every collision, but on average 
their total energy content would be the same. Their perpetual motion 
would be due to the fact that the energy in the container would 
never be lost, but then we would have the perfect thermos bottle, 
and this is impossible. We always have some kind of heat leak. 
I hope this is a satisfactory answer.

Vladimir Escalante Ramirez

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