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- The Planck units [0] are generally associated with the attempt to quantize space and time. - Macroscopic phenomena are in turn governed by microscopic phenom

- The Planck units [0] are generally associated with the attempt to quantize space and time. - Macroscopic phenomena are in turn governed by microscopic phenomena, where the strong and weak forces dominate, so I would say our dimension is equally governed by those. At high energies we know the electromagnetic and weak forces combine into the "electroweak" force, and it is commonly hypothesized that at even higher energies (such as those seen only during the very beginning of the Big Bang), all of the forces are unified. If you already knew that or were talking about something else, my apologies. - There is the concept of the Big Crunch, where gravity would pull the universe back to a single point, perhaps in an infinite cycle. To the best of our current knowledge though, the expansion of the universe is actually accelerating, and gravity will not overtake it. [0] https://en.wikipedia.org/wiki/Planck_units https://en.wikipedia.org/wiki/Planck_units

chiefofgxbxl 9y ago
From my understanding (just a Quantum Physics I class, I could be wrong here) the Planck units quantize space or time to the extent that they define the smallest thing we can practically measure due to Heisenberg's Uncertainty Principle, but they do not preclude that things can actually be smaller than that. That is, something like length or time can be continuous, but our measurements can only ever be discrete multiples of the corresponding Planck constants. For example, a really small object can have a length smaller than the Planck length, but we wouldn't truly know how long it was because the Planck length acts as a fundamental limit on the precision/granularity of our measuring instruments.
- richrichardsson 9y ago
  > For example, a really small object can have a length smaller than the Planck length, but we wouldn't truly know how long it was because the Planck length acts as a fundamental limit on the precision/granularity of our measuring instruments. Would that be, to all intents and purposes, undetectable by us then? If so then that might be a possible explanation of "dark matter", assuming all these things were individually smaller than the Planck length.
  - spynxic 9y ago
    > Would that be, to all intents and purposes, undetectable by us then? In so far as our means for measuring particles doesn't change; namely, smashing things together at extremely high energies. If you suppose that there are methods for measuring sub-Planck length areas then one must assume that they involve either: energies lower than those believed to be needed (which doesn't necessarily come to a solution, but pushes the problem to even higher energies and shorter lengths) OR that there is a default force that keeps space-time stable unless high energies are approached. Note that both notions are mere speculation, the first on logic and the second on intuition about phase changes in states of matter.
- FreeFull 9y ago
  Also, it's possible that the actual quantum of space is larger than the corresponding Planck unit.
andyjohnson0 9y ago
> the Big Crunch, where gravity would pull the universe back to a single point Just as the big bang didn't happen in a single place, a Big Crunch wouldn't be a contraction back to a single point. Rather, the scale factor [1] that characterises cosmic expansion would simply(!) decline back to zero. [1] https://en.wikipedia.org/wiki/Scale_factor_(cosmology) https://en.wikipedia.org/wiki/Scale_factor_(cosmology)
- raattgift 9y ago
  Well, "we don't know" applies here. To a first order approximation (e.g. in the FLRW perfect fluid model), a Big Crunch would resemble a time-reversal of the standard big bang cosmology. In our universe, time reversal results in galaxies appearing at a comoving observer's horizon, and everything inside the horizon becoming denser and hotter. Ultimately the density and heat is expected to result in beyond-the-Standard Model physics in the matter sector, and (hopefully) new physics in the gravitational sector. We don't really know what those physics will be. Everyone hopes for something that prevents a gravitational singularity from forming, but so far what we have in terms of possible stabilizers are conjectural at best. The time-reversals of Big Crunch and the usual-forward-time big bang cosmology with structure formation relate to the BH information loss problem. For a non-eternal BH when we time-reverse from i+ we have a gas of Hawking radiation that collapses into a time-reversed BH which eventually emits matter with much less (Boltzmann) entropy -- ions, molecules, dust, stars and even the things orbiting them. (A time-reversed stellar BH likewise will eventually spit out a white dwarf or neutron star with their respective complex layerings). How does a time-reversed BH "know" how to spit out whole stellar-mass objects when only time-reversed Hawking radiation fell into it? Most quantum gravity programmes hope that strong gravity[1] produces an environment where this sort of (time-reversed) structure formation is likely. Flipping the arrow(s)-of-time is extremely useful for reasoning in a setting in which strong gravity is important, and extra-underlines the question of why we have the arrow(s)-of-time we do in the first place. "Boundary conditions did it" is pretty unsatisfying. - -- [1] i.e., where the radius of curvature is on the order of the Planck length; this mostly comes from the non-renormalizability of perturbatively quantized gravity (where we quantize perturbations of a background metric) and from work in finding the effective field theory limit of semiclassical gravity; generally it's very close to a gravitational singularity and -- depending on the censorship conjecture -- always invisible to outside observers.

chiefofgxbxl 9y ago

From my understanding (just a Quantum Physics I class, I could be wrong here) the Planck units quantize space or time to the extent that they define the smallest thing we can practically measure due to Heisenberg's Uncertainty Principle, but they do not preclude that things can actually be smaller than that. That is, something like length or time can be continuous, but our measurements can only ever be discrete multiples of the corresponding Planck constants. For example, a really small object can have a length smaller than the Planck length, but we wouldn't truly know how long it was because the Planck length acts as a fundamental limit on the precision/granularity of our measuring instruments.

richrichardsson 9y ago
> For example, a really small object can have a length smaller than the Planck length, but we wouldn't truly know how long it was because the Planck length acts as a fundamental limit on the precision/granularity of our measuring instruments. Would that be, to all intents and purposes, undetectable by us then? If so then that might be a possible explanation of "dark matter", assuming all these things were individually smaller than the Planck length.
- spynxic 9y ago
  > Would that be, to all intents and purposes, undetectable by us then? In so far as our means for measuring particles doesn't change; namely, smashing things together at extremely high energies. If you suppose that there are methods for measuring sub-Planck length areas then one must assume that they involve either: energies lower than those believed to be needed (which doesn't necessarily come to a solution, but pushes the problem to even higher energies and shorter lengths) OR that there is a default force that keeps space-time stable unless high energies are approached. Note that both notions are mere speculation, the first on logic and the second on intuition about phase changes in states of matter.
FreeFull 9y ago
Also, it's possible that the actual quantum of space is larger than the corresponding Planck unit.

andyjohnson0 9y ago

> the Big Crunch, where gravity would pull the universe back to a single point Just as the big bang didn't happen in a single place, a Big Crunch wouldn't be a contraction back to a single point. Rather, the scale factor [1] that characterises cosmic expansion would simply(!) decline back to zero. [1] https://en.wikipedia.org/wiki/Scale_factor_(cosmology) https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

raattgift 9y ago
Well, "we don't know" applies here. To a first order approximation (e.g. in the FLRW perfect fluid model), a Big Crunch would resemble a time-reversal of the standard big bang cosmology. In our universe, time reversal results in galaxies appearing at a comoving observer's horizon, and everything inside the horizon becoming denser and hotter. Ultimately the density and heat is expected to result in beyond-the-Standard Model physics in the matter sector, and (hopefully) new physics in the gravitational sector. We don't really know what those physics will be. Everyone hopes for something that prevents a gravitational singularity from forming, but so far what we have in terms of possible stabilizers are conjectural at best. The time-reversals of Big Crunch and the usual-forward-time big bang cosmology with structure formation relate to the BH information loss problem. For a non-eternal BH when we time-reverse from i+ we have a gas of Hawking radiation that collapses into a time-reversed BH which eventually emits matter with much less (Boltzmann) entropy -- ions, molecules, dust, stars and even the things orbiting them. (A time-reversed stellar BH likewise will eventually spit out a white dwarf or neutron star with their respective complex layerings). How does a time-reversed BH "know" how to spit out whole stellar-mass objects when only time-reversed Hawking radiation fell into it? Most quantum gravity programmes hope that strong gravity[1] produces an environment where this sort of (time-reversed) structure formation is likely. Flipping the arrow(s)-of-time is extremely useful for reasoning in a setting in which strong gravity is important, and extra-underlines the question of why we have the arrow(s)-of-time we do in the first place. "Boundary conditions did it" is pretty unsatisfying. - -- [1] i.e., where the radius of curvature is on the order of the Planck length; this mostly comes from the non-renormalizability of perturbatively quantized gravity (where we quantize perturbations of a background metric) and from work in finding the effective field theory limit of semiclassical gravity; generally it's very close to a gravitational singularity and -- depending on the censorship conjecture -- always invisible to outside observers.