Wikipedia:Reference desk/Archives/Science/2023 January 2

= January 2 =

Relativistic Quantum Mechanics exists, so why many say the reconciliation between QM and SR is impossible?
Many phylosophers of science write that it's not possible to reconcile QM and Special Relativity. A major problem is that the experimental violation of Bell's inequalities implies that any theory which makes the same prediction of classical QM has to be non-local, whereas in SR everything is local. So my question is: since one can modify the Schroedinger equation to make it Lorentz-invariant, why doesn't this solve all the issues? We have a more general theory, which is local and makes predicions at least as accurately as non-relativistic QM, don't we?

Thanks! 2.42.134.212 (talk) 15:51, 2 January 2023 (UTC)
 * Relativistic quantum mechanics is indeed well-studied. The major unsolved problem is reconciling quantum mechanics and general relativity, i.e., finding a consistent theory of quantum gravity. Could you perhaps provide a source of philosophers of science challenging the reconciliation of QM and SR? Complex / Rational  17:15, 2 January 2023 (UTC)

Tim Maudlin, a leading expert in the phylosophy of QM, says so in his 2-volume book on the phylosophy of physics, and also around min. 31:40 in this video interview: https://www.youtube.com/watch?v=OduDEz77h9U. Maudlin does not say explicitly "special relativity", but he argues that the main problem is the nonexistence of the concept of simultaneity in relativity, which applies to SR.

— Preceding unsigned comment added by 2.42.134.212 (talk) 19:43, 2 January 2023 (UTC)
 * You probably shouldn't be getting your information from someone claiming to be a "philosopher of quantum mechanics" or writing books on the "philosophy of physics." These aren't philosophical topics. You are better off using actual physicists or physical chemists to discuss this topic, rather than someone who "who studies the metaphysical foundations of physics and logic." I'm literally running a quantum mechanical calculation right now that uses a modified Hamiltonian explicitly account for relativistic effects in core electrons of large atoms while finding solutions to the Schrodinger equation. That Tim Maudlin doesn't think that is possible is, well, his problem and not a problem of the actual scientific fields of knowledge. Indeed, our understanding of quantum mechanics as a model to explain observable is dependent upon relativity. It explains why, for example, gold atoms have smaller atomic radii than we would expect from the amount of shielding from the large number of core electrons. Relativistic effects cause a contraction of the radii. That lead-acid batteries work better than tin-acid batteries is another example. --OuroborosCobra (talk) 22:04, 2 January 2023 (UTC)
 * Maudlin graduated in physics and Trevis Norsen is a physicist. However, is your answer simply that they are telling nonsense? 2.42.134.212 (talk) 22:10, 2 January 2023 (UTC)
 * Well my unerstanding is that GR is basically non-linear dealing with bent space-time whereas QM currently only deals nicely with a flat space-time, and that is is a major reason the Holographic principle is so exciting - it indicates there may be a way of making GR linear - but there's still lots of efforts to make QM deal with bent space-time. So nothing to do with what either of thos philosophers said which I find surprising - are you sure you followed what they were saying right? NadVolum (talk) 22:40, 2 January 2023 (UTC)
 * I had a look at the bit pointed out at 3:40 in the video with Tim Maudlin and I'm pretty certain he is just wrong. Entanglement is definitely weird but there's nothing there conflicting with relativity. I'm not sure where he gets the idea that simultaneity is required i QUantum Mechanics - it's not needed for anything to do with entanglement. NadVolum (talk) 23:00, 2 January 2023 (UTC)
 * Relativistic quantum mechanics does not "solve the issue", which is the spooky action at a distance. Bell's inequalities continue to hold under Lorentz invariance, so there is some coordination between events (observations of entangled particles) whose separation in spacetime is not a timelike interval. They are not causally related in any theory admitting only local reality. It is hard to assign a meaning to Maudlin's statement that relativity was "designed" [his words] for local theories. Relativity did away with the concept of absolute simultaneity of Newtonian physics. But although Newtonian universal time allows the concept of simultaneity, it offered no role for nonlocal instantaneous coordination. So it seems more reasonable to say that relativity was not conceived to deal with nonlocal spacelike coordination. Neither was QM; the spookiness was an unexpected consequence of the theory. So I am tempted to say that if someone says it is hard to reconcile special relativity with quantum theory, it is not because of any specific aspect of special relativity but because it is hard to reconcile macroscopic preconceptions of how the world ought to behave with quantum theory. If this an issue that needs to be solved while holding on to these preconceptions, special relativity is not the solution. Neither does it stand in the way. --Lambiam 01:50, 3 January 2023 (UTC)
 * Thanks, Lambian, this seems a good answer to me. In my understanding, the main difference between QFT and NRQM is in the degrees of freedom that the wavefunction is a function of. For the rest, the mathematical structure is the same and so I cann see why the problems with the spooky action at a distance don't go away. My only doubt is in which sense there is a "relativistic" QM... So I formulate a more direct question: is relativistic QM able to predict the outcomes of the experiments made by Aspect? (My guess: yes). If so, the violation of Bell's inequalities shows that it's a nonlocal theory...but then how can it be "relativistic"? 2.42.134.212 (talk) 07:45, 3 January 2023 (UTC)
 * RQM is relativistic in the sense of invariance under the Lorentz transformations. Since NRQM is a a special case of RQM, the violation is equally predicted by RQM. Consider that Aspect's experimental set-up did not get anywhere close to lightspeed w.r.t. the observer. :) --Lambiam 11:04, 3 January 2023 (UTC)
 * Invariance under LT implies that the same two events can be simultaneous in a reference frame and non simultaneous in another. Now if two objects A,B are entangled, NRQM prescribes that the state of A can become determined when the state of B is measured, and here "when" means "in the same exact moment". How to make sense of this in relativistic QM? 2.42.134.212 (talk) 19:37, 3 January 2023 (UTC)
 * There is no requirement for anything to be simultaneous. If one person does a measurement on one particle then they know what the result would be if the same measurement is performed on a second particle entangled with it.It does not matter if one is causally before or after the other or somewhere there cannot be a causal link. There is no way of detecting an effect from the entanglement until one gets to a time and place where both experiments have been done. The same exact moment only refers to th person doing an experiment, it does not affect the person with the second particle. Our common experience way of dealing with it is to think one experiment does something to the other and therefore that one must be first but that is not so. NadVolum (talk) 23:31, 3 January 2023 (UTC)


 * Because Einstein objected to the "spooky action at a distance" of QM (or, more concisely in German, its requiring spukhafte Fernwirkung), and Einstein was the father of the theory of relativity, there is a misunderstanding that his objection was informed by an implied incompatibility between QM and his cherished theory. But, clearly, the instantaneous action at a distance in the classical Newtonian setting of absolute time is no less spooky. --Lambiam 09:04, 4 January 2023 (UTC)

Dead hand grip
It appears that Viktor Vasnetsov's painting After the Battle of Igor Svyatoslavich with the Cumans contains an odd part, where a dead warrior on the upper right still grasps a bow firmly with a raised hand (even if resting on something else). Can a dead body actually hold an object like this or is it more like an error on Vasnetsov's part? 212.180.235.46 (talk) 19:29, 2 January 2023 (UTC)
 * Perhaps the warrior is wounded rather than dead. Cullen328 (talk) 19:33, 2 January 2023 (UTC)
 * Rigor mortis can set in very quickly indeed if a person has been engaged in hard work - like fighting in a battle. I like the way he's omitted any blood! More likely his glove is such that he would have to deliberately open his hand to let go of the bow. Or it could just be artistic license like the lack of blood! NadVolum (talk) 23:14, 2 January 2023 (UTC)
 * Whether or not a dead body can do so in reality, the idea that it can is widespread, so it is perhaps allowable artistic licence in such a work. {The poster formerly known as 87.81.230.195} 51.194.245.235 (talk) 02:36, 3 January 2023 (UTC)
 * From my cold, dead hands... Alansplodge (talk) 09:58, 3 January 2023 (UTC)
 * It looks look like the hand is just resting on the bow as opposed to grasping it. The bow itself leans on a dead body and on the ground. Ruslik_ Zero 10:19, 4 January 2023 (UTC)
 * Or it might be a case of cadaveric spasm... —  Wasell ( T ) 🌻🇺🇦 16:55, 5 January 2023 (UTC)