The Naked Scientists

The Naked Scientists Forum

Author Topic: What is the significance of the Planck units in QM?  (Read 5174 times)

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
While I've seen some of the Planck units, such as the Planck Distance and Planck Time, being described as the smallest values that make any sense in QM, some of the other (derived) Planck units are quite large, and the Planck Temperature represents a maximum rather than a minimum value, so it must be possible to work with sub-Planck unit values.  This seems contradictory to me.

There also seems to be another practical problem, regarding the Planck Distance and Time units, in that working with sub-light speed values requires using values of < 1 Planck Distance/Time.  For a large object, composed of many particles, I could see how its average speed could be < c by statistical averaging of each of the individual particles velocities but when you consider the movement of a single particle you hit problems, especially if the particle is accelerating/decelerating.

If you consider a very slowly moving particle, it seems to me that it must either be constantly moving over either < 1 Planck Distance for each Planck unit of time, or it must take a varying number of Planck units of time to move one Planck Distance unit.  At high speeds though, even taking a varying number of Planck Time units to move a single Planck Distance unit is problematic; when traveling at 0.5 c you'd take two Planck Time units to travel one Planck Distance, which is fair enough, but then what happens when you travel at 0.75 c?  You then need to deal with fractions of Planck Time units.


 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
What is the significance of the Planck units in QM?
« Reply #1 on: 20/05/2009 18:31:39 »
No thoughts anyone?
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3817
  • Thanked: 19 times
    • View Profile
What is the significance of the Planck units in QM?
« Reply #2 on: 20/05/2009 19:16:06 »
I think it a matter of how long a time you take to measure the velocity of your particle if you took 1000 Planck time units you could determine whether it had moved 749 or 751 length units and you could get a result accurate to 0.1% but the shorter the observing time the more difficult it becomes to make any meaningful measurement.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
What is the significance of the Planck units in QM?
« Reply #3 on: 20/05/2009 19:35:02 »
I think to answer your question, you'd need to know the physical meaning of Planck length and 1 Planck time and why they're thought to impose limits on what we can measure.  A theory of quantum gravity might end up changing things, since the Planck length is the length scale where quantum gravity might become important.

This is just a speculation, but I know that one speculative interpretation of the Planck length/time is the scale at which space-time becomes "foamy" or "discrete" due to quantum fluctuations.  This is entirely a guess, but you could probably account for different velocities by having a particle with different probabilities of jumping integer values of Planck units in integer Planck times. 
 

Offline Vern

  • Neilep Level Member
  • ******
  • Posts: 2072
    • View Profile
    • Photonics
What is the significance of the Planck units in QM?
« Reply #4 on: 20/05/2009 21:05:57 »
Planck's constant is energy-time. I suspect it is the energy-time of one cycle of electromagnetic energy. If this is true, there must be a more basic constant from which Planck's constant derives. Notice that the equation e = hv does not contain a unit for the electric and magnetic amplitude of the frequency. To me this means that the electric and magnetic amplitude is a constant. Maybe that is the more basic constant from which Planck's constant derives.

 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
What is the significance of the Planck units in QM?
« Reply #5 on: 22/05/2009 16:48:48 »
Thanks fellas.  The idea of probabilistic changing of integer values had occurred to me as one possibility but it seems to me as though there must be another factor constraining the range of probabilities.  For example, in probabilistic terms, an acceleration would be a 'trend', but what ensures that the 'trend', or in other words, the acceleration, is consistent?  This is still not clear to me though, so I might be barking up the wrong tree entirely.

Interesting food for thought.
 

Offline Vern

  • Neilep Level Member
  • ******
  • Posts: 2072
    • View Profile
    • Photonics
What is the significance of the Planck units in QM?
« Reply #6 on: 23/05/2009 16:20:02 »
The point I was trying to make is that we may be placing too much value on Planck units. We seem to be in a Quantum Mechanics bubble sparked by the success of QM theory. We want to see everything in terms of quantum values. So we invented Planck units to quantize space and time. To me that is purely ad hoc with no experimental evidence in support. :)

I suspect that the constant amplitude of photons is the root of the quantum nature of electric and magnetic fields. I see no evidence that this should apply to anything else. The link is to Dr. Robert Kemp's proposal of quantization of electric and magnetic fields.
« Last Edit: 24/05/2009 14:27:38 by Vern »
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
What is the significance of the Planck units in QM?
« Reply #7 on: 26/05/2009 18:56:46 »
I wouldn't exactly describe myself as an advocate of QM but the bottom line is that it works across a wider area than any other model and as such seems more complete.

I don't think it's correct to say that we want to see everything in terms of quantum values; it just that this is how it (mostly) seems to work, so we accept them.  Similarly, the Planck units are not just there because they're desirable but because they're implicit.  I see them more as something discovered than invented.
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
What is the significance of the Planck units in QM?
« Reply #8 on: 04/06/2009 08:20:07 »
LeeE - Would time dilation effects come into it?
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
What is the significance of the Planck units in QM?
« Reply #9 on: 05/06/2009 16:46:18 »
LeeE - Would time dilation effects come into it?

Only, I think, from the PoV of the traveling object/particle, but not from the Pov of an observer.
 

Offline Vern

  • Neilep Level Member
  • ******
  • Posts: 2072
    • View Profile
    • Photonics
What is the significance of the Planck units in QM?
« Reply #10 on: 06/06/2009 14:41:09 »
Quote from: LeeE
I don't think it's correct to say that we want to see everything in terms of quantum values; it just that this is how it (mostly) seems to work, so we accept them.  Similarly, the Planck units are not just there because they're desirable but because they're implicit.  I see them more as something discovered than invented.
Yes; QM is a useful tool and things seem to follow its predictions. But, its difficult for me to see how the quantum nature of light suggests that there is also a quantum of time and a quantum of distance. A quantum of light is an electromagnetically saturated point in space; or maybe two points; one positive; one negative to complete the wave. We know the electric and magnetic amplitude is a constant because it is not needed in the equations for Planks constant. So I assume the value is the maximum possible for electric and magnetic fields in space.
 

The Naked Scientists Forum

What is the significance of the Planck units in QM?
« Reply #10 on: 06/06/2009 14:41:09 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums