It's not so clear that physics forbids backwards time travel (forwards time travel is trivial thanks to time dilation), since Einstein's theory of general relativity does have solutions involving backwards time travel. GR is thought to be an approximation to a future theory of quantum gravity and there are some arguments that such a theory would rule out backwards time travel, but this is speculation since we don't have a complete theory of quantum gravity yet.

@Hypnosifl: You're making the mistake that simply because GR has certain 'mathematical' solutions then they must be physical. No, in physics, its physics that comes first. Can you show me a real, live timelike loop? No, of course not. No-one has found one, not that many physicists have bothered to look for one.

@Hypnosifil: And as for Hawkings 'chronology protection conjecture', well that's not an argument of quantum gravity as you put it but exactly the same reasoning I said that physics disbarred time travel. I just haven't bothered to give it some formal name because no physicist would have thought otherwise.

There's no physical proof of time travel, but there is no physical proof of all sorts of things that are predicted by our best theoretical models of physics, like Hawking radiation or quark stars. Certainly you can say we should not confidently claim either is real, but you have presented no argument for being certain they are unreal, or thinking time travel is less likely to be real than other theoretical predictions like the ones I mentioned. I don't know what you mean by "same reasoning", as I said you didn't make any argument for your assertion, and Hawking had a detailed physics argument.

@Hypnosifil: Whilst there's no ready to hand experiment that can show Hawking radiation, the principle of causality is encoded into language, and physical practise since time immemorial. If you want to dispute it then you are going to have to provide some very strong proof, like a replicable experiment. It won't do to simply say because QM is bizarre we can freely speculate any old idea. Schrodinger, Planck, Einstein, Lorentz were forced to their conclusions because of experiment.

There is no experimental evidence for CTCs, but also no experimental evidence to rule out GR's prediction that they are physically possible in certain situations, like ones involving wormholes. Physicists do not make confident claims about what is and isn't physically possible when there is no experimental evidence either way, the idea that you can confidently rule out CTCs based on the way causality is "encoded into language" or based on armchair considerations of "basic ontological categories of physical nature" is not one that would be taken seriously by the physics community.

R. Feyman: "The next question is whether, when trying to guess a new law, we should use seat-of-the-pants feeling and philosophical principles — ‘I don’t like the minimum principle’, or ‘I do like the minimum principle’, ‘I don’t like action at a distance’, or ‘I do like action at a distance’. ... The method of guessing the equation seems to be a pretty effective way of guessing new laws. This shows again that mathematics is a deep way of expressing nature, and any attempt to express nature in philosophical principles, or in seat-of-the-pants mechanical feelings, is not an efficient way."

That quote is from ch. 2 of Richard Feynman's The Character of Physical Law, he argues that based on experience, attempts to ground physics theories in conceptual pictures or philosophical principles are generally useless, and it's better to just think in terms of physics theories as more or less arbitrary mathematical equations that can accurately predict the results of physical experiments (though he also notes that intuitions can play a useful role in finding new theories, or in helping physicists find the simplest approach to deriving a theory's predictions abt a particular problem).

@Hypnosifil: Ever since physics stole a march on philosophy in the modern era, physicists have been knocking philosophy. Never mind the subject arose from philisophy. Physicists are well known not to know the history of their own subject. In other places Feynman makes fun of mathematicians for making silly physical mistakes because they're relying on purely mathematical results without thinking about the physical principles involved. See his story about the Banach-Tarski paradox.

Philosophy (like physics) is about the rational search for truth, unlike with religious texts neither philosophers nor physicists are bound to respect the ideas of a classic thinker simply because they are old. Aristotle never really provided much in the way of argument for his particular ideas about causality, nor did the medieval scholastics who treated his ideas as canonical, and both physics and modern analytic philosophy owe more to the early modern mechanical philosophy which was reacting against Aristotle's ideas.

@Hypnosifl: No, Aristotle provided lots of argument. Newton respected Plato and Aristotle enough that he called them "his friends". He was referring to Aristotle when he admitted that his notion of gravity, being action-at-a-distance, was philosophically untenable. This argument comes from Aristotle.

@Hynosifl: Moreover, early modern physics wasn't "reacting" to Aristotle, this is a story we've been telling ourselves or rather we've been told it. It doesn't stand up to any serious reflection. It merely reflects a modern European arrogance that they are something new in the world. Its the same parochial arrogance that Plato accused his compatriots of for thinking the Greeks were so superior to everything that came before. It only goes to show how when so much has changed, nothing has changed at all.

@Hypnosifl: All this modern European arrogance is what lead to the Europe comitting suicide in the first and second world wars. Just when they thought they were the best, they were the worst.

Newton respected Plato and Aristotle enough that he called them "his friends". Again, neither science nor philosophy are based on religious style-authority, we don't have to lend any special weight to Newton's personal beliefs if they weren't supported by either evidence or convincing argument. No, Aristotle provided lots of argument. Not really, for ex. he couldn't imagine that there could be an atomist-style mechanical explanation for organic growth, so he inferred a separate category of causality, "final causes". Early modern thinkers def. reacted against that, so does modern science.

Similarly, neither Aristotle or any other ancient philosopher provides any real argument for the supposed asymmetry of cause and effect, that earlier ones cause later events but not vice versa--but we now know that there is no reason to think there is any such asymmetry in fundamental physics, it's more a statistical matter owing to the low-entropy conditions of the early universe.

It merely reflects a modern European arrogance that they are something new in the world. Clearly science has discovered a lot of new specifics about the world. But in general terms, the broad picture of modern science is fairly similar to the atomism of Democritus, including the idea that the basic constituents of reality are defined solely by mathematical properties rather than qualitative features like colors (this also has some resemblance to the Mahayana Buddhist idea that every part of reality exists only in relation to everything else, with no intrinsic properties).

So it's not that science proves all ancient philosophies are nonsense, it's more that there were a lot of basic metaphysical rivalries in ancient thought, and modern science aligns much better with anti-Aristotelian thinkers like Democritus, and the more abstract or structuralist ancient systems as opposed to the essentialist ones.

@Hypnosifil: No, you're wrong. Aristotle could imagine atomism but he argued against it. The algebraist Rene Thom said that Aristotle was the only significant philosopher who argued for the continuum for a millenia. Given that modern physics do not have particles as a fundamental category, its not clear that he was wrong there. Science is built upon authority as much as argument. Why were you quoting Feynman at me if you didn't think his authority as a physicist wasn't important? Its quite clear to me that science and philosophy relies as much on authority as religion - that some people ...

@Hynosifil: ... due to their philosophical prejudices. You're also wrong about Democritus. I don't recall he suggested that mathematics was important in the study of nature - have you read De Rerum Natura? - that was Plato, and he got that from the Pythagoreans. More, as I've already pointed out in modern physics the notion of a particle is incoherent. QFT on flat space thinks of particles as irreducible representations of the Poincare group. But spacetime is not flat - far from - as Einstein taught us - and when taking account of general covariance the notion of a distinguished ...

@Hypnosifil: ... spacetime symmetry group disappears and with it too the notion of a particle. The fundamental concept is that of a quantised field and this does not mean particle. That is atom.

Aristotle could imagine atomism but he argued against it Obviously he could imagine it since he knew of Democritus, but he never really gave a good argument for why it was wrong. If you think he did, can you give a specific reference to one of his texts? Why were you quoting Feynman at me if you didn't think his authority as a physicist wasn't important? Not his authority per se, just his knowledge and arguments about what strategies had proved successful in developing new physics theories (there is more argument in that chapter).

...also to dispute your claim that causality has been part of "physical practise since time immemorial"--in fact I think almost no 20th century physics pay any heed to a "principle of causality" that goes beyond the basic idea of mathematical laws of nature that allow predictions of states at one time based on states at a different time. And note the comments I posted in the chat showing Hawking/Einstein held no notion of "causality" that would rule out CTCs a priori.

"You're also wrong about Democritus. I don't recall he suggested that mathematics was important in the study of nature" He did say that things like color and taste were mere human "conventions", and that "in truth there are only atoms and the void". Few of Democritus' original writings survive, but other contemporaries describe his view that atoms had only quantitative geometric properties, for example Plutarch said that the atomists believed the only properties of atoms were "size and shape", (p. 88 of Taylor's "The Atomists"),

and Stobaeus said that "Democritus says that by nature there is no such thing as colour" and that atoms by themselves "are propertyless", that the appearance of color is created by "arrangement, shape, and position; for the appearances arise from these." (p. 120-121) Aristotle says that the atomists see everything as generated by "combination and interlocking" of bodies which "differ in shape" (geometric properties again), and that in that way "they too make everything into numbers or composed of numbers; even if they do not say so clearly, all the same that is what they mean." (p. 82)

It's impossible to understand people and their doings without thinking about purposes and intentionality Sure, but in light of the assumption that all behavior is in principle derivable from physics, this can be viewed as a useful approximate model for systems when it's not practically possible (or desirable) to predict their behavior just from physical laws. See Hawking's comments here, and Sean Carroll's here

'when taking account of general covariance the notion of a distinguished spacetime symmetry group disappears and with it too the notion of a particle' GR still has a distinguished local spacetime symmetry group, that's built into it with the equivalence principle. In any case, I'm not saying Democritus' view of atoms exactly matches the notion of excitations of quantum fields, just that the basic idea above of all objects built out of simple units defined by mathematical properties alone, with all behavior understandable in terms of something like "efficient cause", has been vindicated.

@Hypnosifl: I've already given you one good argument as to why atomism is fundamentally not correct. There is no notion of particle that makes sense in terms of general covariance. Aristotles argument was against the existence of the void which ancient atomism presupposes. It's the same argument, as I have already pointed out, that Newton used to admit action-at-a-distance made no sense. You are relying on Feynman as an authority in tge same way you are relying on the general disparagement of Aristotle by physicists for your own argument. And you're cherry-picking your arguments. Feynman ...

@Hypnosifl: ... wasn't a systematic philosopher. Its easy enough to find other arguments of his which diametrically oppose your mathematically inspired phikosophy. After all, he made fun of mathematicians who thought using the Banach-Tarski paradox to describe matter as a good idea. There's no need to wuote Democritus or Aristotle at me. I've reaf them both. Still, I note that nothing in your quotes bu Democritus mentions mathematics as the only way to understand the universe. This should be obvious. I have zero apples in my hands. And then look, I have minus one apple in my left hand and ...

@Hypnosifl: ... plus one apple in my right hand. And thats true because 1 + (-1) = 0. Of course that just nonsense. Because we already know that on the level of apples that kimd of arithmetic doesn't obtain. We still need physical intuition amd we get this through experience, experimentation and theory not blindly chasing mathematical chimeras. No, its not possible to derive peoples behaviour and that of life from physics. This is another myth from the physicalists stall. Even then, its not correct to say that when parts make a thing, then the parts are more fundamental. Try for example ...

@Hypnosifl: ... attempting to understand a book by analysing its atomic composition. Most people would just laugh and say you're on a fools errand. I've already pointed out that Hawkings Chronology Conjecture is based upon a long-standing implicit physical principal of causality. The ADM formulation of Hamiltonian GR relies on time evolution. Yvonne Choquet-Bruhats proof of the stability and existence of Einstein's field equations relies on the ADM framework and so on temporal evolution. The notion of generally hyperbolic manifolds requires a time orientation. Your idea that ...

@Hypnosifl: ... real physicists take any serious notice of timelike loops and acausality is frankly not true. That some do so is merely an exploration of the mathematical consequences of a theory so as to understand the limits of the theory. Its not because anyone - well apart from you - believe it to be true.

@Hypnosifl: Oh, and one last thing. Democritus explained the mental world through what he called "soul atoms". It's strange how his fans never seem to mention this.

@MoziburUllah "There is no notion of particle that makes sense in terms of general covariance" Is this an argument you've seen made by physicists, or is it your own original argument? If it's your own argument, I've already given a possible counterargument, than general relativity still has local Lorentz-symmetry built into it. Also, physicists have extended quantum field theory to curved spacetime, used to derive Hawking radiation (which is usually spoken of as involving particles).

@MoziburUllah "You are relying on Feynman as an authority" I already explained that I was relying on him for his experience of what approaches have been more or less successful in developing new theories in the 20th century, as well as to counter your specific claim that physicists rely on some non-mathematical notion of "causality" in their thinking.

@MoziburUllah "After all, he made fun of mathematicians who thought using the Banach-Tarski paradox to describe matter as a good idea" He was making the point that the construction has no physical meaning in current theories, if someone proposed a new mathematical theory of physics in which it did have physical consequences, his attitude expressed in "The Character of Physical Law" makes clear he wouldn't dismiss it based on "physical intuitions" prior to experimental testing.

@MoziburUllah "Still, I note that nothing in your quotes bu Democritus mentions mathematics as the only way to understand the universe" I only provided one quote by Democritus, and others by contemporary philosophers who would have had access to writings by him that are now lost, like Aristotle. Aristotle literally said that the atomists "make everything into numbers or composed of numbers", do you dismiss that?

@MoziburUllah "No, its not possible to derive peoples behaviour and that of life from physics. This is another myth from the physicalists stall." That's just an assertion, not an argument. Note that I didn't assert it was definitely possible, I just said "in light of the assumption that all behavior is in principle derivable from physics, this can be viewed as a useful approximate model".

@MoziburUllah In other words, if we use the hypothetico-deductive method, both a hypothesis that says reductionist explanations of behavior are not possible and a hypothesis that says they are possible in principle (but not in practice with current limited computing power) would lead to the same consequence that we need distinct ways of understanding human behavior in everyday life, so that fact can't be taken to prove one hypothesis over the other.

@MoziburUllah "I've already pointed out that Hawkings Chronology Conjecture is based upon a long-standing implicit physical principal of causality" I've already pointed out that Hawking's paper describing the conjecture makes no reference to a "principle of causality", only to specific results in quantum field theory in curved spacetime, and that he himself treats CTCs as a viable possibility, and you presented no counterargument, so this claim is clearly completely unfounded.

@MoziburUllah "The ADM formulation of Hamiltonian GR relies on time evolution" The ADM formulation is a way of reformulating GR only in the case of "globally hyperbolic" spacetimes, see here for example. But there are valid solutions the field equations of GR that are not globally hyperbolic so the ADM method can't be used, and spacetimes with CTCs are examples.

@MoziburUllah "Your idea that real physicists take any serious notice of timelike loops and acausality is frankly not true" It depends what you mean by "serious notice". Obviously many physicists might have strong intuitions that such things will not turn out to be possible in a final theory of quantum gravity (I share such intuitions myself), but that's not the same thing as taking a non-mathematical "principle of causality" as some sort of a priori truth that rules them out absolutely.

@MoziburUllah According to contemporaries Democritus did not say each of us has an individual "soul atom", rather that the soul was composed of atoms. For example Aristotle said "some say that the soul is chiefly and primarily the cause of motion, and as they believed that what was not itself in motion could not move anything else, they conceived of the soul as something in motion. Which is why Democritus says that it is hot, a sort of fire; for while there are infinitely many shapes,

he says that that the spherical ones compose fire and the soul (like the so-called motes in the air, which are seen in sunbeams coming through windows)".

@MoziburUllah Presumably "soul" would just refer to the animating element of living organisms and wouldn't involve any dualistic Cartesian notions of mind as metaphysically distinct from matter--if Democritus learned of later results of electrochemical impulses in nervous tissue leading to bodily motion, he might well say that this just pins down the "soul atoms" as being the ions and neurotransmitters whose motions make up these impulses.

@MoziburUllah The earlier atomists like Leucippus and Democritus also seemed to have believed everything happened by "necessity", ruling out "chance" as well as later concepts of "free will" (though Lucretius introduced the notion of an unpredictable 'swerve' in atomic motion, in part to allow for free will, an idea adopted by Epicurus). The evidence for this doctrine is discussed at length in a section starting on p. 188 of Taylor's book "The Atomists: Leucippus and Democritus".

@MoziburUllah Some of the support for this given in the book is Aristotle's comment (p. 93) that "Democritus neglects the final cause, reducing all the operations of nature to necessity." That page also quotes Cicero saying Democritus "preferred to accept that everything happens of necessity, rather than to deprive the indivisible corpuscles of their natural movements, as Epicurus did."

@MoziburUllah And it also quotes Plutarch clarifying what Democritus meant by "necessity": "Democritus says that it is impact and motion and a blow of matter." And Eusebius derides Democritus for his rejection of free will, saying that Democritus "has it in mind to show that the noblest of human things [i.e., free will] is a slave."