4 ms·

Is there an experiment which can test whether the Planck length really is the shortest possible length?

by panic 10y ago

maggit 10y ago
As I understand it, this is indeed a fundamental feature of the Planck length; The universe might have structure smaller than this, but we wouldn't be able to tell. So the Planck length is the shortest meaningful length, in that we would never be able to discern structure smaller than this.
- criddell 10y ago
  So, doesn't that suggest there's a Planck length for time? So the distance between two points in space-time is sqrt(x^2 + y^2 + z^2 - ct). If you try to find the distance of a point in space at two different times, does the Plank length dictate some type of Plank interval?
  - heydenberk 10y ago
    Indeed. https://en.wikipedia.org/wiki/Planck_time https://en.wikipedia.org/wiki/Planck_time
    - criddell 10y ago
      That page says the Planck time is the time it takes for light in a vacuum to travel one Planck length. That doesn't imply it's the smallest interval, does it?
      - qubex 10y ago
        Insofar as you can only really measure time as the interval it takes for something to happen, and insofar as light's travel is the fastest process in the universe, and that the plank scale is the smallest distinguishable distance, it follows that the measurement of time across that smallest interval is the shortest interval that can be reliably measured.
        criddell 10y ago
        Doesn't that imply that every process in the universe proceeds in lock-step? If two virtual particles were to randomly pop into existence, does the time between the first and second particle appearing have to happen in some whole multiple of Planck time?
        notduncansmith 10y ago
        To my understanding, Planck time describes a limit on sample rate, not frequency. That is, we can't perceive time at a finer grain than whole multiples of Planck time. But even starting with only integers, we can theorize and simulate floats. The speed of light is as arbitrary a scale as anything else when you're talking about absolutes, hence "it's all relative".
        lgas 10y ago
        Virtual particles actually exist for less than a planck unit of time. I'm not a physicist and I'm having trouble finding a good clear reference, but this page mentions it (cmd-f 'less than') - http://abyss.uoregon.edu/~js/ast123/lectures/lec17.html http://abyss.uoregon.edu/~js/ast123/lectures/lec17.html
      - 725686 10y ago
        That would be the chronon: https://en.wikipedia.org/wiki/Chronon https://en.wikipedia.org/wiki/Chronon
      - taneq 10y ago
        Well, if light (the fastest possible thing) takes this amount of time to travel the Planck length (the shortest possible thing), then does that not imply that nothing could possibly happen faster?
nabla9 10y ago
There is no reason to expect that Planck length is the literally the shortest possible length. I'm not aware of any theory that suggests that. It's more accurate to talk about Planck scale and not think it as strict limit. Planck scale is where the structure of spacetime itself becomes dominated by quantum effects and the structure of spacetime may start looking really strange. Strange theories like loop quantum gravity, causal sets, causal dynamical triangulation, fractal cosmology, etc. work at Planck scale. According to fractal cosmology spacetime is 2-dimensional in Planck scale and gradually becomes 4-dimensional in larger scales. Loop quantum gravity sees it as "foamy". Planck scale is also the area where measuring distances (differentiating with different positions in space) becomes impossible.
- TheOtherHobbes 10y ago
  I love these ideas, but they all assume that spacetime quanta, or nodes, or whatever the hell they are, can still communicate and/or change state in some way. Which suggests internal structure of some kind. I'm wary of any explanation that says "Well, it's just random", because randomness turns out to be a complicated process. It's hard to imagine that some kind of prototypical base quantum would be inherently random just because. I suppose it's possible. But it would be unexpected.
xioxox 10y ago
The operating energy of the LHC is around 15 orders of magnitude less than the Planck energy (if I worked it out correctly). Therefore we cannot directly probe Planck length scales.
dogma1138 10y ago
Planck length, nor any other Planck unit isn't the "shortest/smallest" anything* at least it was never intended to be that. These units are very useful in QM and as "constants" (not the best choice of words since Planck units intentionally ignore constant values to some extent) that can be used to describe a relativistic universe. *You can use Planck Length/Time to put a lower limit on any possible wavelength that below it a wave cannot exist, other Planck units can also be in the ballpark of the smallest possible unit/quanta of various things in various theories.
gizmo686 10y ago
Not really. The uncertainty principle states that the product of the standard deviation of the momentum and posisition of a particle is bounded from below by h/4pi, where h is the plank constant. This means that, in order to probe small distances, we must have great uncertainty in the momentum invovled, which means that there must be a lot of kinetic energy in the system. As the distances we probe become smaller, we have an increasing amount of energy in a decreasing amount of space. E=mc^2 tells us that energy and mass are interchangeable. Specifically, energy, like mass, causes gravity. In order to probe below the plank length, we would need to put so much gravity in so small a space that we would form a black hole. However, because we cannot observe the inside of a blackhole, this prevents us from observing anything smaller than the plank length.

Is there an experiment which can test whether the Planck length really is the shortest possible length?

by panic 10y ago

Is there an experiment which can test whether the Planck length really is the shortest possible length?

As I understand it, this is indeed a fundamental feature of the Planck length; The universe might have structure smaller than this, but we wouldn't be able to tell. So the Planck length is the shortest meaningful length, in that we would never be able to discern structure smaller than this.

criddell 10y ago
So, doesn't that suggest there's a Planck length for time? So the distance between two points in space-time is sqrt(x^2 + y^2 + z^2 - ct). If you try to find the distance of a point in space at two different times, does the Plank length dictate some type of Plank interval?
- heydenberk 10y ago
  Indeed. https://en.wikipedia.org/wiki/Planck_time https://en.wikipedia.org/wiki/Planck_time
  - criddell 10y ago
    That page says the Planck time is the time it takes for light in a vacuum to travel one Planck length. That doesn't imply it's the smallest interval, does it?
    - qubex 10y ago
      Insofar as you can only really measure time as the interval it takes for something to happen, and insofar as light's travel is the fastest process in the universe, and that the plank scale is the smallest distinguishable distance, it follows that the measurement of time across that smallest interval is the shortest interval that can be reliably measured.
      - criddell 10y ago
        Doesn't that imply that every process in the universe proceeds in lock-step? If two virtual particles were to randomly pop into existence, does the time between the first and second particle appearing have to happen in some whole multiple of Planck time?
        notduncansmith 10y ago
        To my understanding, Planck time describes a limit on sample rate, not frequency. That is, we can't perceive time at a finer grain than whole multiples of Planck time. But even starting with only integers, we can theorize and simulate floats. The speed of light is as arbitrary a scale as anything else when you're talking about absolutes, hence "it's all relative".
        lgas 10y ago
        Virtual particles actually exist for less than a planck unit of time. I'm not a physicist and I'm having trouble finding a good clear reference, but this page mentions it (cmd-f 'less than') - http://abyss.uoregon.edu/~js/ast123/lectures/lec17.html http://abyss.uoregon.edu/~js/ast123/lectures/lec17.html
    - 725686 10y ago
      That would be the chronon: https://en.wikipedia.org/wiki/Chronon https://en.wikipedia.org/wiki/Chronon
    - taneq 10y ago
      Well, if light (the fastest possible thing) takes this amount of time to travel the Planck length (the shortest possible thing), then does that not imply that nothing could possibly happen faster?

nabla9 10y ago

There is no reason to expect that Planck length is the literally the shortest possible length. I'm not aware of any theory that suggests that. It's more accurate to talk about Planck scale and not think it as strict limit. Planck scale is where the structure of spacetime itself becomes dominated by quantum effects and the structure of spacetime may start looking really strange. Strange theories like loop quantum gravity, causal sets, causal dynamical triangulation, fractal cosmology, etc. work at Planck scale. According to fractal cosmology spacetime is 2-dimensional in Planck scale and gradually becomes 4-dimensional in larger scales. Loop quantum gravity sees it as "foamy". Planck scale is also the area where measuring distances (differentiating with different positions in space) becomes impossible.

TheOtherHobbes 10y ago
I love these ideas, but they all assume that spacetime quanta, or nodes, or whatever the hell they are, can still communicate and/or change state in some way. Which suggests internal structure of some kind. I'm wary of any explanation that says "Well, it's just random", because randomness turns out to be a complicated process. It's hard to imagine that some kind of prototypical base quantum would be inherently random just because. I suppose it's possible. But it would be unexpected.

xioxox 10y ago

The operating energy of the LHC is around 15 orders of magnitude less than the Planck energy (if I worked it out correctly). Therefore we cannot directly probe Planck length scales.

dogma1138 10y ago

Planck length, nor any other Planck unit isn't the "shortest/smallest" anything* at least it was never intended to be that. These units are very useful in QM and as "constants" (not the best choice of words since Planck units intentionally ignore constant values to some extent) that can be used to describe a relativistic universe. *You can use Planck Length/Time to put a lower limit on any possible wavelength that below it a wave cannot exist, other Planck units can also be in the ballpark of the smallest possible unit/quanta of various things in various theories.

gizmo686 10y ago

Not really. The uncertainty principle states that the product of the standard deviation of the momentum and posisition of a particle is bounded from below by h/4pi, where h is the plank constant. This means that, in order to probe small distances, we must have great uncertainty in the momentum invovled, which means that there must be a lot of kinetic energy in the system. As the distances we probe become smaller, we have an increasing amount of energy in a decreasing amount of space. E=mc^2 tells us that energy and mass are interchangeable. Specifically, energy, like mass, causes gravity. In order to probe below the plank length, we would need to put so much gravity in so small a space that we would form a black hole. However, because we cannot observe the inside of a blackhole, this prevents us from observing anything smaller than the plank length.