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The problem with all these quantum gravity theories is they cannot be proven, or falsified, by experiment. Not the theorists fault, we'd need an LHC the circum

The problem with all these quantum gravity theories is they cannot be proven, or falsified, by experiment. Not the theorists fault, we'd need an LHC the circumference of the galaxy, or build a black hole in a lab, to test quantum gravity. But it means we will never know if the theory in question is actually how nature works.

JumpCrisscross 10y ago
> we will never know We as in our lifetimes? Or we as in our species?
- danbruc 10y ago
  I am not aware of any mechanism that definitively places experiments testing theories of quantum gravity outside of our reach. We may not be able to probe Planck scale effects in the laboratory for a very long time or possibly even never, but it seems to depend a lot on the specific theory one wants to test where and at which scales deviations from current theories occur. I don't see any reasons why deviations could not occur only an order of magnitude beyond our current experimental limits and therefore be experimentally accessible in a lifetime. And even if experiments in the laboratory were currently impossible, there is always the possibility that we can gather indirect evidence by observing distant galaxies, black holes, the cosmic microwave background or something along that line.
  - walrus1066 10y ago
    The Planck energy is where we can be 100% certain to observe quantum gravity. There are 16 orders of magnitude between the LHC collision energies and the Planck scale. That's a humongous energy range in which to look for new physics. You could build a collider that's 10x more powerful than the LHC, but it'd be a stab in the dark, as the new physics may occur at 100x the LHC energy, or much much more. To date there is no indirect evidence (e.g. Particle decay rates that deviate from the standard model predictions) that points to quantum gravity effects occuring below the Planck scale. This indirect evidence is required to make a case for building a super expensive particle collider. For example, the LHC was built because we knew through indirect evidence, that it would either find the higgs boson, or new physics if the higgs wasn't found. Without indirect evidence your experiment is a stab in the dark, which is fine, but for particle physics an insanely expensive stab in the dark. Regarding astronomical evidence, it can give hints on quantum phenomena, but can't shed much light on their cause. For instance, we know dark matter exists via astronomy, but it can't tell us what the stuff actually is, we need laboratory evidence for that, I.e. Produce it in a collider, measure its mass, spin, other quantum properties, and how it interacts with other particles.
- walrus1066 10y ago
  Certainly not within our lifetimes, within our species lifetimes, who knows. We need to be able to probe the planck energy scale to observe quantum gravity effects. That is around 10^16 TeV, to put that into perspective, the LHC collision energy is 14 TeV. Below the planck scale, gravity is too weak to be measured at the quantum level.

JumpCrisscross 10y ago

> we will never know We as in our lifetimes? Or we as in our species?

danbruc 10y ago
I am not aware of any mechanism that definitively places experiments testing theories of quantum gravity outside of our reach. We may not be able to probe Planck scale effects in the laboratory for a very long time or possibly even never, but it seems to depend a lot on the specific theory one wants to test where and at which scales deviations from current theories occur. I don't see any reasons why deviations could not occur only an order of magnitude beyond our current experimental limits and therefore be experimentally accessible in a lifetime. And even if experiments in the laboratory were currently impossible, there is always the possibility that we can gather indirect evidence by observing distant galaxies, black holes, the cosmic microwave background or something along that line.
- walrus1066 10y ago
  The Planck energy is where we can be 100% certain to observe quantum gravity. There are 16 orders of magnitude between the LHC collision energies and the Planck scale. That's a humongous energy range in which to look for new physics. You could build a collider that's 10x more powerful than the LHC, but it'd be a stab in the dark, as the new physics may occur at 100x the LHC energy, or much much more. To date there is no indirect evidence (e.g. Particle decay rates that deviate from the standard model predictions) that points to quantum gravity effects occuring below the Planck scale. This indirect evidence is required to make a case for building a super expensive particle collider. For example, the LHC was built because we knew through indirect evidence, that it would either find the higgs boson, or new physics if the higgs wasn't found. Without indirect evidence your experiment is a stab in the dark, which is fine, but for particle physics an insanely expensive stab in the dark. Regarding astronomical evidence, it can give hints on quantum phenomena, but can't shed much light on their cause. For instance, we know dark matter exists via astronomy, but it can't tell us what the stuff actually is, we need laboratory evidence for that, I.e. Produce it in a collider, measure its mass, spin, other quantum properties, and how it interacts with other particles.
walrus1066 10y ago
Certainly not within our lifetimes, within our species lifetimes, who knows. We need to be able to probe the planck energy scale to observe quantum gravity effects. That is around 10^16 TeV, to put that into perspective, the LHC collision energy is 14 TeV. Below the planck scale, gravity is too weak to be measured at the quantum level.