Particle Prediction

[Riot]

I recently was told that given the known velocity of a particle and the position of every particle at a given time, it could be predicted.

 

However, my father disagrees and says that you cannot find the location and the elocity at once and therefore a particle cannot be predicted.

 

Unfortunately, I don't have any evidence or theories to back myself up. Any help?

Edited June 3, 2011 by Riot

[DrRocket]

I recently was told that given the known velocity of a particle and the position of every particle at a given time, it could be predicted.

 

However, my father disagrees and says that you cannot find the location and the elocity at once and therefore a particle cannot be predicted.

 

Unfortunately, I don't have any evidence or theories to back myself up. Any help?

 

Classically one can determine the position and velocity of every particle in a closed system at some point in time and predict the behavior of the system at all subsequent times, although the equations are difficult for more than two bodies and must be solved numerically. . This is a reasonably accurate model for macroscopic bodies. It is how the positons of the planets are calculated.

 

However, this model is not completely accurate and fails badly at the atomic level. Quantum mechanics tells us that exact determination of the simultaneous position and velocity of a particle is not possible. This is known as the Heisenberg uncertainty principle. At the atomic and sub-atomic level this effects is very important. It applies to macroscopic bodies, but in that case the uncertainty is very swmall and the classical model is a good approximation.

[IM Egdall]

I like to think of the Uncertainty Principle in quantum mechanics as a kind of see-saw. One side goes up when the other goes down. For example, the more accurately I know the location of a particle, the less accurately I can know its momentum (mass times velocity). And vice-versa.

 

Say I know to great accuracy where a particle is. This means I can only know its speed and direction (velocity) to very poor accuracy. So since I do not know exactly where the particle is going or how fast, I cannot predict where it will be in the future (or where it was in the past).

[Riot]

Thanks!