Quantum Mechanics

http://physicsweb.org/articles/news/10/4/7/1

Dr. Weiss et al. have produced a ``quantum Newton's cradle" -- the atomic equivalent of the popular desk toy that has five steel balls suspended from strings in a straight line.

We like toys, but are they ``relevant science"?

First of all, understanding helps invention. It can be argued that understanding is necessary for invention once one passes a threshold of complexity. A construction worker can build a small room. To construct a skyscraper we need understanding.

There is an exact mathematical solution for this quantum system of atoms in one dimension. Let me conclude by explaining what I think a quantum mechanical description is.

Inventors out there, pay attention, it is good to understand Quantum Mechanics to make good use of this result from the scientists at Penn State.

Newton's description of his cradle, contains the essential ingredients. If you took physics in high school, you may remember what those ingredients are. Position, time, rate of change of position or velocity, and rate of change of velocity. Mass, force, action and reaction, and inertia.

One ball hits the next, and gets hit in return, half a cycle of the chain reaction ends when the last ball does not find a next ball to hit, then it comes back and hits the ball that hit it; and there they go again, over and over until the end of time.

Actually friction will stop them. That is the beauty of the quantum case, as far as the scientists could determine in this case, the atoms do not stop hitting each other.

Now that the classical and a little bit of the quantum descriptions are laid down, let us look more attentively at the quantum case.

Position and velocity cannot be measured at the same time with infinite precision for atoms. Steel balls only give us the impresion that can be measured exactly. Any practical scientist can tell you what the measurement errors are, and how to reduce them. What quantum mechanics adds to this description is that no matter what the person does, there is an irreducible uncertainty in determining these two variables.

 Do atoms have a precise position and velocity?

At some point it becomes a very interesting philosophical discussion. I do not think that the Penn State scientists are loosing too much sleep understanding the philosophical implications of what they accomplished. I can assure you though, that this result has philosophical and practical implications. Very good work, Dr. Weiss!