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Can anyone explain this passage?

dylancatlow
Posts: 12,242
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4/13/2014 5:50:29 PM
Posted: 2 years ago
From Hawkins' book 'A Very Brief History of Time' : There is a mathematical theorem that says that any theory that obeys quantum mechanics and relativity must always obey the combined symmetry of CBT. In other words, the universe would have to behave the same if one replaced particles by antiparticles, took the mirror image, and also reversed the direction of time. But Cronin and Fitch showed that if one replaces particles by antiparticles and takes the mirror image, but does not reverse the direction of time, then the universe does not behave the same. The laws of physics, therefore, must change if one reverses time -- they do not obey the symmetry T.

This appears to be saying something that doesn't follow at all. What am I not
getting?
tkubok
Posts: 5,044
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4/13/2014 6:24:31 PM
Posted: 2 years ago
At 4/13/2014 5:50:29 PM, dylancatlow wrote:
From Hawkins' book 'A Very Brief History of Time' : There is a mathematical theorem that says that any theory that obeys quantum mechanics and relativity must always obey the combined symmetry of CBT. In other words, the universe would have to behave the same if one replaced particles by antiparticles, took the mirror image, and also reversed the direction of time. But Cronin and Fitch showed that if one replaces particles by antiparticles and takes the mirror image, but does not reverse the direction of time, then the universe does not behave the same. The laws of physics, therefore, must change if one reverses time -- they do not obey the symmetry T.

This appears to be saying something that doesn't follow at all. What am I not
getting?

Although I am not a physicist, let alone a theoretical physicist, wouldnt the universe behave differently if we swapped out all the particles with anti-particles? Or if the universe was a mirror image of this one? So why would you be surprised if reversing time changes this universe into a different one?
Enji
Posts: 1,022
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4/13/2014 6:37:43 PM
Posted: 2 years ago
Presumably you mean CPT symmetry?

According to CPT invariance, the univese would behave the same way if charge, parity, and time were reversed, which means that you could use the same physical laws to describe the universe. CP invariance has been violated experimentally, so the same doesn't hold if you reversed charge and parity; the universe wouldn't behave the same way so you'd have to use different laws to describe the observations. Similarly, time symmetry violations have been observed; the universe wouldn't behave the same way if time were reversed.
dylancatlow
Posts: 12,242
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4/14/2014 10:21:44 AM
Posted: 2 years ago
Ok, I think I get what he's saying. If time were symmetric under reversal, then not reversing it shouldn't change anything (however, I was under the impression that all of them had to be done together). But I'm not sure why he rules out replacing particles by antiparticles and taking the mirror image as possible causes of the difference.
Enji
Posts: 1,022
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4/14/2014 11:55:21 AM
Posted: 2 years ago
At 4/14/2014 10:21:44 AM, dylancatlow wrote:
Ok, I think I get what he's saying. If time were symmetric under reversal, then not reversing it shouldn't change anything (however, I was under the impression that all of them had to be done together). But I'm not sure why he rules out replacing particles by antiparticles and taking the mirror image as possible causes of the difference.

The universe behaves the same way only if you reverse all three according to CPT invariance. Previously, it was believed that the universe would also behave the same way if only charge and polarity were reversed, but James Cronin and Val Fitch found that CP symmetry could be violated experimentally. I'm not exactly sure why Hawkins mentioned reversing time, but CP violation is relevant in explaining why there is more matter than anti-matter in the universe since if CP symmetry was preserved then both matter and anti-matter should have cancelled, leaving neither.
wheresthebeef
Posts: 1
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4/17/2014 12:12:09 PM
Posted: 2 years ago
Assuming all of the known particles in the universe were substituted for their physical opposites, "physics," as we see it, would not follow the same laws. This would be such that, if Dark Matter were the reigning force of the universe, everything would be reversed on a polar scale. This would mean that the particles of mass and energy would react directly in an opposing charge (as if protons became negative) and this would cause IMFs to change. Gravity would become a repelling force, instead of an attracting, and the universe would continually shrink back to pre-Big Bang. This being said, reversing time would cross over the negative equilibrium again. It would be simple to say that it would set "physics" in reverse. This is untrue, for time itself can be considered a standard of physics. The "reversed-time reversed-polarity" theory would be true, but there is no way to scientifically test the forces of time in reverse. The anti-universe would go back to standard IF time is seen as a single four-dimensional graph. you could also look at it as a three-dimensional graph, with particles being various points. If set backwards, the 3D graph would not follow through on the data, because the graph's domain constantly shifts as a result of ensuing time, so any point in the past would not be in the domain, therefore any reversed time would be UNDEFINED. Ergo, no, the universe may not be the same if it was inversed, and then reversed.