# Does particle at rest behave as wave?

1. Aug 26, 2011

### cltaylor23

Wouldn't you be able to determine both the momentum and position of a particle at rest? Wouldn't a particle at rest behave only as a particle and not as a wave?

Never mind I get it now you would never be able to observe a particle at rest.

Last edited: Aug 26, 2011
2. Aug 26, 2011

### Drakkith

Staff Emeritus
Why wouldn't you be able to observe it at rest? I can easily bounce an X-Ray photon off an atom. The problem is that it wouldn't be at reast anymore after that! The issue is determing the position or momentum of a particle at some point in the future after you observe it. IE you want to determine where it is now AND where it will be in the future.

3. Aug 26, 2011

### cltaylor23

Gotcha.

Is this a valid way of thinking about it? A picture gives you 0 information about what's happened before or after, while a video doesn't allow you to observe an instant.

4. Aug 26, 2011

### Drakkith

Staff Emeritus
I think so, but I could be wrong. In truth, most people interpret the uncertainty principle to mean that the particle NEVER has a set position or momentum until you measure one or the other. However I don't know enough about QM to really say for sure what is true and proven and what is merely interpretation. I keep getting mixed opinions between the forums here, books I read, and articles online. It is kind of confusing...

5. Aug 26, 2011

### cltaylor23

I understand and can accept the wave particle duality and uncertainty principles but the thing that bothers me so much is apparently particles with momentum take on wave like properties for no reason. I'm going to either figure this out or go crazy. Why physics!?! Whyyyyyyy!?!!?!!

Thanks for helping though.

6. Aug 26, 2011

### Drakkith

Staff Emeritus
What do you mean? Everything has both particle and wavelike properties at ALL times. (As far as I know anyways)

7. Aug 26, 2011

### cltaylor23

Alright so even though a particle at rest has a wavelength of infinity it's still a wave?

8. Aug 26, 2011

### Drakkith

Staff Emeritus
It does not have a wavelength of infinity. A particle at rest can be described using a wavefunction, which is composed of many different waves of different frequencies. This describes the probability of finding the particle at any specific point and what the momentum might be. Note that this only describes how to observe the particle and predict what it might do, not what it is. We KNOW it obeys wavelike and particlelike rules, but trying to say that it IS a wave or a particle is irrelevent. To describe anything one only needs to explain how it interacts and what it's properties are. Whether or not an electron is a particle, wave, both, or none has no real meaning. It obeys certain rules that classic waves obey, acts like a single object, and has set properties such as rest mass, spin, etc.

9. Aug 26, 2011

### cltaylor23

Fair enough. Correct me if I'm wrong so we're basically at the point where we know what a particle might do but have no idea why it might do it.

10. Aug 26, 2011

### Drakkith

Staff Emeritus
More like we know HOW things work and behave within limits. I don't like to use the word "why" because of how inaccurate it is. Remember the little kid who keeps asking why...

11. Aug 26, 2011

### cltaylor23

I appreciate this haha. You're the man

12. Aug 26, 2011

### soothsayer

So you know the equation, $\lambda$= h/p ?

p is momentum, theroretically, if a particle had zero momentum, p=0, wavelength ($\lambda$) would be infinite

But this is an impossibility, even theoretically, due to the uncertainty principle and also the laws of thermodynamics which state that you can never reach absolute zero temperature (but you can get really close)

Actually, I think the fact that you can reach absolute zero might be explained via the Uncertainty Principle.

Even a particle "at rest" has some finite wavelength, thus, it can be modeled as a wave.

13. Aug 26, 2011

### Drakkith

Staff Emeritus
It has to do with what a wavefunction is. Normally the uncertainty of the position and momentum causes the wavefunction to interfere with itself after a certain distance, meaning that the particle cannot be found outside that point. If you calculate the momentum of a particle to the maximum precision you can get, the wavefunction turns into a single wave which never interferes with itself and as such has an infinte length. Conversly, if you calculate or measure the position as accurate as possible, the wavefunction becomes so small that you have no idea what the frequency of the waves are and the frequency determines the momentum, so now that is uncertain.

See here for a good visual aid: http://en.wikipedia.org/wiki/Wavefunction#Spatial_interpretation

14. Aug 26, 2011

### soothsayer

cltaylor23, you must have deleted your post before I got a chance to respond, what I was saying is that even though wavelength WOULD be infinite if its momentum is zero, it's impossible to do because momentum CAN'T be zero. Even a particle at rest has some momentum, which dictates a finite wavelength.