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Quatom mechanics doesn't really have a concept of space-time. The notion of space-time is entirely a construction of relativity; and the two theories have yet t

Quatom mechanics doesn't really have a concept of space-time. The notion of space-time is entirely a construction of relativity; and the two theories have yet to be unified. Collapsing the wave function doesn't bring a system into the domain of relativity. That happens at a fuzzy point where a system become massive enough for relativistic effects to become significant. In thoery, we would probably expect to see some relativistic effects in entangled systems; but predicting what those effects are requires a theory of quantum gravity. There is semiclassical gravity; which essentially says that the quantum fields described quantum mechanics occurs in the curved spacetime of relativity. This is (by design) just an approximation of the still unknown theory of quantum gravity; and we don't have good experimental evidence telling us how accurate of an approximation it is.

c1ccccc1 6y ago
Quantum mechanics doesn't work well with general relativity (gravity), but it works just fine with special relativity. Special relativity still has a concept of spacetime, it's just that the spacetime is flat, not curved like in general relativity. (In fact, you can even do quantum mechanics in curved spacetime, as long as the spacetime has a predefined fixed curvature. But in GR, the curvature depends on how the matter and energy inhabiting the spacetime are distributed. That is what gives quantum gravity theorists trouble.)
tsimionescu 6y ago
Quantum mechanics is fully unified with special relativity, and has a clear concept of spacetime. The Dirac equation was discovered in 1928 and was the key to solving this problem. This means that quantum mechanics is fully compatible with time dilation at large speeds, that it respects the limits of the speed of light (current article notwithstanding) etc. The only incompatibility is between quantum mechanics and general relativity - quantum mechanics can't work with the general relativityequations for a curved spacetime or for the gravitational effects of particles being in multiple places at once. These are the things that a theory of quantum gravity must solve.
raattgift 6y ago
> Quatom mechanics doesn't really have a concept of space-time The Standard Model of Particle Physics incorporates the Poincaré group, which is the isometry group of the flat spacetime of Special Relativity, so it has an exact concept of spacetime built right into the formulation. Quantum mechanics and Special Relativity were unified in the 1920s by Dirac. Solutions for some curved spacetimes came not much later, and are now taught in several textbooks. The problem is that we don't have a general solution for quantum mechanics on arbitrary curved spacetimes (and some curved spacetimes are known to make quantum field theories insoluble with current methods). I refer you to my sibling comment https://news.ycombinator.com/item?id=24877746 https://news.ycombinator.com/item?id=24877746 for more details. > (by design) just an approximation of the still unknown theory of quantum gravity Kind-of. It is manifestly an approximative approach because the equation includes "mean" angle-brackets around the stress-energy tensor, which, speaking roughly, indicate that the curvature is generated by the averaged value of the stress-energy generated by the quantum field(s) are used, rather than an exact value at each and every infinitesimal point in the solution. This "classicalizes" the source term of the classical curvature, erasing quantum effects. It is not really in the strictest sense an approximation to a theory of quantum gravity, however any candidate quantum gravity theory had better explain results from semiclassical gravity that find some support in astronomical observations. Contrasting approaches -- like canonical quantum gravity -- "quantumize" the metric, leaving the source term fully quantum mechanical and non-averaged, and results from such approaches also find some support in astronomical observations. These are arguably approximations to a full theory of quantum gravity since one can cast them as a series of terms similar to a Taylor series; the problem is that the set of such terms appears to grow without bounds as one takes energy-momentum into the "ultraviolet" regime (UV ~ extreeeeeemely high energy-momentum densities). Worse, the relative weight of the contribution of some of the far-from-leading-order terms grow stronger in the UV limit, so not only can't we just ignore them, we also can't apply perturbative renormalization. Fortunately this probably only happens inside black hole event horizons and near the big bang, one of which is probably not observable, and the other of which is at high enough individual energies that the Standard Model doesn't predict results even if their collective mutual gravitation (and the contribution of dark matter and other energy-densities) could be factored out. > we would probably expect to see some relativistic effects in entangled systems; but predicting what those effects are requires a theory of quantum gravity We can calculate the effects of a boost, even an ultraboost, on particles with mutually-entangled properties just fine thanks to the Poincaré invariance baked into post-Dirac relativistic quantum mechanics. If you were flying past from an entanglement experiment in deep space at ultrarelativistic speeds, one could predict your observed redshift (etc) of the entangled properties exactly with no worries. (If you were flying radially to or from an Earthbound experiment at ultrarelativistic speeds you might want to incorporate some corrections from the https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultraboost https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultrab... for greater accuracy. The post-Newtonian expansion leading-order corrections attributable to Earth's gravity will not be large; linearized gravity is enough. Errors from the next-to-leading-order corrections will not be measurable with current technology, and we can brute force calculate at least next-to-next-to-leading order corrections, which is good enough for accelerating down towards the surface of a neutron star, like the material that is blown onto the Hulse-Taylor binary members.

c1ccccc1 6y ago

Quantum mechanics doesn't work well with general relativity (gravity), but it works just fine with special relativity. Special relativity still has a concept of spacetime, it's just that the spacetime is flat, not curved like in general relativity. (In fact, you can even do quantum mechanics in curved spacetime, as long as the spacetime has a predefined fixed curvature. But in GR, the curvature depends on how the matter and energy inhabiting the spacetime are distributed. That is what gives quantum gravity theorists trouble.)

tsimionescu 6y ago

Quantum mechanics is fully unified with special relativity, and has a clear concept of spacetime. The Dirac equation was discovered in 1928 and was the key to solving this problem. This means that quantum mechanics is fully compatible with time dilation at large speeds, that it respects the limits of the speed of light (current article notwithstanding) etc. The only incompatibility is between quantum mechanics and general relativity - quantum mechanics can't work with the general relativityequations for a curved spacetime or for the gravitational effects of particles being in multiple places at once. These are the things that a theory of quantum gravity must solve.

raattgift 6y ago

> Quatom mechanics doesn't really have a concept of space-time The Standard Model of Particle Physics incorporates the Poincaré group, which is the isometry group of the flat spacetime of Special Relativity, so it has an exact concept of spacetime built right into the formulation. Quantum mechanics and Special Relativity were unified in the 1920s by Dirac. Solutions for some curved spacetimes came not much later, and are now taught in several textbooks. The problem is that we don't have a general solution for quantum mechanics on arbitrary curved spacetimes (and some curved spacetimes are known to make quantum field theories insoluble with current methods). I refer you to my sibling comment https://news.ycombinator.com/item?id=24877746 https://news.ycombinator.com/item?id=24877746 for more details. > (by design) just an approximation of the still unknown theory of quantum gravity Kind-of. It is manifestly an approximative approach because the equation includes "mean" angle-brackets around the stress-energy tensor, which, speaking roughly, indicate that the curvature is generated by the averaged value of the stress-energy generated by the quantum field(s) are used, rather than an exact value at each and every infinitesimal point in the solution. This "classicalizes" the source term of the classical curvature, erasing quantum effects. It is not really in the strictest sense an approximation to a theory of quantum gravity, however any candidate quantum gravity theory had better explain results from semiclassical gravity that find some support in astronomical observations. Contrasting approaches -- like canonical quantum gravity -- "quantumize" the metric, leaving the source term fully quantum mechanical and non-averaged, and results from such approaches also find some support in astronomical observations. These are arguably approximations to a full theory of quantum gravity since one can cast them as a series of terms similar to a Taylor series; the problem is that the set of such terms appears to grow without bounds as one takes energy-momentum into the "ultraviolet" regime (UV ~ extreeeeeemely high energy-momentum densities). Worse, the relative weight of the contribution of some of the far-from-leading-order terms grow stronger in the UV limit, so not only can't we just ignore them, we also can't apply perturbative renormalization. Fortunately this probably only happens inside black hole event horizons and near the big bang, one of which is probably not observable, and the other of which is at high enough individual energies that the Standard Model doesn't predict results even if their collective mutual gravitation (and the contribution of dark matter and other energy-densities) could be factored out. > we would probably expect to see some relativistic effects in entangled systems; but predicting what those effects are requires a theory of quantum gravity We can calculate the effects of a boost, even an ultraboost, on particles with mutually-entangled properties just fine thanks to the Poincaré invariance baked into post-Dirac relativistic quantum mechanics. If you were flying past from an entanglement experiment in deep space at ultrarelativistic speeds, one could predict your observed redshift (etc) of the entangled properties exactly with no worries. (If you were flying radially to or from an Earthbound experiment at ultrarelativistic speeds you might want to incorporate some corrections from the https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultraboost https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultrab... for greater accuracy. The post-Newtonian expansion leading-order corrections attributable to Earth's gravity will not be large; linearized gravity is enough. Errors from the next-to-leading-order corrections will not be measurable with current technology, and we can brute force calculate at least next-to-next-to-leading order corrections, which is good enough for accelerating down towards the surface of a neutron star, like the material that is blown onto the Hulse-Taylor binary members.