Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yamo on 11/05/2011 08:36:43
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Nothing can be smaller than a planck length. Nothing can go faster than c. Nothing can be denser than...?
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Nothing can be smaller than a planck length.
The Planck length is the limit at which our current theories of quantum mechanics break down. Things can be going on below that length, but we just don't know how to model them yet (and we don't have powerful enough experiments to measure them.)
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My guess would be the Planck Density (about 5.1 x 1096 kg/m3).
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It is often quoted that the 'Bigbang' started from an object the size of a grapefruit, that presumably would be the size of the universe compressed to Planck density.
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Are you stating that we know the size of the universe, as we apparently then know its density? To be able to come to this grapefruit
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Ah wait, they are reasoning from a isotropic model, isn't they? Where you might assume that the density of the universes mass should be the same relative a distance anywhere. Okay, I see it.
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Ok, but why is the Planck density the densest anything can become?
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There is a difference between Planks scales and what is 'possible'. The thing with Planck scales seems to be that outside those we can't define it any longer. There is a breakdown of the physics we use. But the exact same can be said for the center of a black hole, and most of us do believe that Black holes exists, don't we :)
As JP already stated btw :)
But then we come to 'density'. Does a photon have a density? I don't think so, not any measurable by us anyway. But it do have a energy that can vary, and that is a 'density' of sorts I might assume. What we need to consider if how we define density here. If we do it only from using fermions then we need to stay at, at least quark size and probably bigger, Perhaps we need to consider atoms? To me their invariant mass only seems to become 'touchable' as those elementary particles combine?
So maybe we need to define what we mean by 'density' here?
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So maybe we need to define what we mean by 'density' here?
Very good point! "Density" actually has big problems when you try to define it in the realm of quantum mechanics, since particles are "smeared out" over their whole wave function. But according to theory, they are also point-particles, meaning that they can interact at a single point.
So how would you go about defining the density of an electron floating in space?
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Are we coming to the conclusion that density is, like temperature, only really applicable to an aggregate of particles and not to the single individual particle?
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What I'm pointing out is that the definition of mass density as mass/volume might have issues when you're dealing with quantum particles, since volume of a particle can mean different things. I'm not sure if this means it's not applicable or not. I imagine in some circumstances, dividing mass by the region of space occupied by the wave function might be useful...
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Density is a fascinating subject when we come to quantum fluctuations. I looked up if there was a common use of the word when it came to light and to my everlasting delight :) I found many use the expression. In some ways words are so much more flexible, allowing for so many interpretations.
"aggregate of particles" is a very nice expression imatfaal and seems to catch what I think seems applicable practically. After all what would the density be of a electron able to be found in two orbitals, simultaneously. Is it even possible to assign it a 'density' other than in the way JP suggests?
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Agree with both Yoron and JP to an extent - it could be a useful measure to calculate the region of space that is the wave function divided by the mass, but I am not sure it is commensurate or cognate to the density per se.