# Lower limit for speed

1. Apr 21, 2008

### Physonic

we all know that there is upper limit for speed that is speed of light, but is there a lower limit, I mean is there quanta for speed.

2. Apr 21, 2008

### lzkelley

There is no quanta for speed.
The effective "lower limit" for speed will be the uncertainty principle, because of the wave-particle duality. The speed of a particle can be arbitrarily close to zero as long as the uncertainty in the position is sufficiently large (i.e. the particle is delocalized).

If a particle is confined to any discrete region of space, there will be a lower limit on its velocity. Hope this helps.

3. Apr 21, 2008

### Physonic

Thank you that helped.

4. Apr 21, 2008

### dst

No, not at all.

The uncertainty principle applies for dXdP, not dXd(mv) which is a slight difference; in the case of light the uncertainty applies to its wavelength, frequency and energy via the relation P = hf/c where f is the frequency or alternatively, P = h/w where w is the wavelength.

Correct me if I'm wrong but that's how it seems.

Last edited: Apr 21, 2008
5. Apr 21, 2008

### lzkelley

In the case of light, the lower limit is quite well defined.... its still just c. but even in the not photon case, by the deBroglie equation the same relation applies (p=h/L).
dst : your point, thought true, is trivial and semantical. In quantum mechanics there is no way to define an instantaneous (accurate) velocity; however, the concept of spacial motion does apply very closely to momentum --> hence it is the term that we should be looking at, in the "lower limit of velocity."