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The Planck length has no physical meaning. It's just a combination of physical constants that gives a length. It's considered a scale at which quantum gravitati

by gaze 9y ago

The Planck length has no physical meaning. It's just a combination of physical constants that gives a length. It's considered a scale at which quantum gravitational effects become significant, but it's just that, a scale. It might be a factor of 100 off from something significant, but what that thing is we have no idea.

iaw 9y ago
I have read alternate interpretations of the Planck length/time/volume/etc. One says that they are a mathematical construct with no meaning, the other says that they provide a bound for the limits of the measurement of reality as we understand it (which many people take to mean the smallest unit). Do you think these definitions disagree? Does one definitely supersede the other? If so, could you point me at some literature?
- gizmo686 9y ago
  Not a physisist, but I found this link [0] to be understandable. The Heisenberg uncertainty principle says that the more accurately we know position, the less accurately we know momentum. When you formalize this, the result is that as the size size of a particle you are trying to measure decreases, the energy necessary to measure it increases. Since energy causes gravity, there is a length, l, where measuring a particle would require so much energy that the resulting gravity creates a black hole. I think it is fair to say that l is just a mathematical construct, since nothing really happens at this point. But it is a useful mathematical construct that is, theoretically, relevant to our ability to observe the universe. Having said all of that, all of this is at a much smaller scale than we have ever been able to observe. Seeing as we needed to invent new physics (relativity) to explain scales as small Mercury's orbit [1], and as large as atoms, and that these two theories are still incompatible, it is highly likely that describing physics near the plank scale would require another reinvention of physics. [0] http://backreaction.blogspot.com/2012/01/planck-length-as-minimal-length.html http://backreaction.blogspot.com/2012/01/planck-length-as-mi... [1] Arguably smaller, since we have confirmed relativity without leaving Earth orbit.
  - gaze 9y ago
    Yes I too can contrive a thought experiment that combines a bunch of physical constants. It's hardly a construct. It's just a length. It might mean something. It might mean something when multiplied by pi or a tenth or whatever. It's the scale at which quantum gravity might maybe possibly could be important. Nothing more nothing less.
    - gizmo686 9y ago
      I would not consider the explanation I describe to be a thought experiment, but rather a mathematical derivation; although I might showing my roots as a mathematician instead of a physicist there. Additionally, all the argument, as I presented it, shows is that there is some length beyond which we cannot observe. It turns out that we can calculate this length, and the result happens to be precisely the plank length. As a mathematician, this seems highly unlikely to be a coincidence; however I am not familiar enough with physics to know if this is a deep result, or a trivial consequence of it's definition. To clarify the disagreement: do you agree that our current theory predicts that there is some length beyond which we cannot measure?
      - gaze 9y ago
        Yeah, that would make sense, but I don't imagine it's a hard cutoff. It may be that you get diminishing information per unit energy at a length scale related to the Planck length or something.
- gaze 9y ago
  The first one is true, the second is total speculation.
trhway 9y ago
>The Planck length has no physical meaning. It's just a combination of physical constants that gives a length. remember a combination of physical constants that gave a speed 100+ years ago? Happened to be pretty meaningful later :)
- gaze 9y ago
  But it's meaningless today.
Florin_Andrei 9y ago
> The Planck length has no physical meaning. Well, if current physics is worth anything, a photon with a wavelength around that scale is probably going to spontaneously collapse into a mini-black hole. So that probably does count as a threshold of some sort.

by gaze 9y ago

iaw 9y ago

I have read alternate interpretations of the Planck length/time/volume/etc. One says that they are a mathematical construct with no meaning, the other says that they provide a bound for the limits of the measurement of reality as we understand it (which many people take to mean the smallest unit). Do you think these definitions disagree? Does one definitely supersede the other? If so, could you point me at some literature?

gizmo686 9y ago
Not a physisist, but I found this link [0] to be understandable. The Heisenberg uncertainty principle says that the more accurately we know position, the less accurately we know momentum. When you formalize this, the result is that as the size size of a particle you are trying to measure decreases, the energy necessary to measure it increases. Since energy causes gravity, there is a length, l, where measuring a particle would require so much energy that the resulting gravity creates a black hole. I think it is fair to say that l is just a mathematical construct, since nothing really happens at this point. But it is a useful mathematical construct that is, theoretically, relevant to our ability to observe the universe. Having said all of that, all of this is at a much smaller scale than we have ever been able to observe. Seeing as we needed to invent new physics (relativity) to explain scales as small Mercury's orbit [1], and as large as atoms, and that these two theories are still incompatible, it is highly likely that describing physics near the plank scale would require another reinvention of physics. [0] http://backreaction.blogspot.com/2012/01/planck-length-as-minimal-length.html http://backreaction.blogspot.com/2012/01/planck-length-as-mi... [1] Arguably smaller, since we have confirmed relativity without leaving Earth orbit.
- gaze 9y ago
  Yes I too can contrive a thought experiment that combines a bunch of physical constants. It's hardly a construct. It's just a length. It might mean something. It might mean something when multiplied by pi or a tenth or whatever. It's the scale at which quantum gravity might maybe possibly could be important. Nothing more nothing less.
  - gizmo686 9y ago
    I would not consider the explanation I describe to be a thought experiment, but rather a mathematical derivation; although I might showing my roots as a mathematician instead of a physicist there. Additionally, all the argument, as I presented it, shows is that there is some length beyond which we cannot observe. It turns out that we can calculate this length, and the result happens to be precisely the plank length. As a mathematician, this seems highly unlikely to be a coincidence; however I am not familiar enough with physics to know if this is a deep result, or a trivial consequence of it's definition. To clarify the disagreement: do you agree that our current theory predicts that there is some length beyond which we cannot measure?
    - gaze 9y ago
      Yeah, that would make sense, but I don't imagine it's a hard cutoff. It may be that you get diminishing information per unit energy at a length scale related to the Planck length or something.
gaze 9y ago
The first one is true, the second is total speculation.

trhway 9y ago

>The Planck length has no physical meaning. It's just a combination of physical constants that gives a length. remember a combination of physical constants that gave a speed 100+ years ago? Happened to be pretty meaningful later :)

gaze 9y ago
But it's meaningless today.

Florin_Andrei 9y ago

> The Planck length has no physical meaning. Well, if current physics is worth anything, a photon with a wavelength around that scale is probably going to spontaneously collapse into a mini-black hole. So that probably does count as a threshold of some sort.