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Exactly what the question says;

If all the protons and electrons in every single atom in the universe were swapped for their anti-particles, what would essentially change?

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    $\begingroup$ relevant nature.com/articles/d41586-020-00384-y $\endgroup$
    – anna v
    Jan 19, 2023 at 7:14
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    $\begingroup$ Do you think something ought to change? $\endgroup$
    – Ghoster
    Jan 19, 2023 at 7:44
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    $\begingroup$ Did you also want antineutrons? If so, it would make either very little difference or very very very little difference. Research hasn't determined exactly how different antiatoms are. $\endgroup$
    – J.G.
    Jan 19, 2023 at 13:21
  • $\begingroup$ Where does that come from? Is it purely speculative or have you done at least an Einsteinian thought experiment, if not some research? $\endgroup$ Jan 19, 2023 at 21:57
  • $\begingroup$ Assuing you also include anti-neutrons, then if you do not also do the opposite swap for anti-matter then you have no 'opposite' particles. Which MAY be significant. $\endgroup$ Jan 21, 2023 at 9:22

3 Answers 3

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Changing all particles into antiparticles and vice versa is known as a charge symmetry operation (C) and for a long time it was believed this would leave everything totally unchanged. Similarly mirroring the universe (a parity symmetry operation, P) would seem to leave everything unchanged. However, in 1956 it was found that there are weak interactions that do not obey parity symmetry - the left and right handed forms have different probability. It was believed that the combined CP symmetry was still true, but in 1964 that was also found to fail: CP violation.

So the antimatter universe would be very slightly different from ours. The effect is small, and only occurs in weak interactions involving nonzero strangeness number. That means that the effects in everyday physics will be very small: while normal protons and neutrons have a pinch of strange quark presence it is tiny. It would be felt most strongly for weak interactions - that are also just a small part of what is going on. Presumably it would change energy levels in nuclear transitions to a tiny degree, which may change a few fusion pathways in stars.

So there would be a difference, but it would likely be almost imperceptible, in the form of different elemental abundances of heavier elements.

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    $\begingroup$ This analysis assumes a rather static universe. Taking into account chaos theory, changes could compound over time and result in very different structures across the universe. $\endgroup$
    – Joooeey
    Jan 19, 2023 at 18:31
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    $\begingroup$ @Dave Regarding beta decay, ¹³N → ¹³C is the key process regulating the CNO cycle. Even small differences could influence the stellar evolution! I wonder if this has been studied? $\endgroup$ Jan 19, 2023 at 18:36
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    $\begingroup$ @user1079505: The current hypothesis is one of the things that shows up in CP violation is the explanation for matter's dominance over antimatter. $\endgroup$
    – Joshua
    Jan 19, 2023 at 22:15
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    $\begingroup$ @Joooeey - I think the answer doesn't mean it would literally be exactly the same, I think they mean the laws of physics would appear almost exactly the same and our understanding of how the universe works would be almost exactly the same, not that every object would be in exactly the same state kind of deal. Arguably, if you take chaos theory and quantum randomness to their logical extreme, two universes with identical laws of physics could never evolve in the same way and would end up totally different. $\endgroup$ Jan 20, 2023 at 14:38
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    $\begingroup$ I wonder, though, can it somehow be quantified whether those "small effects" are "relevant" – i.e. their effects increase and will result in measurable differences over time, or whether they are "irrelevant", i.e. the small differences they introduce tend to decrease and disappear with time? $\endgroup$
    – printf
    Jan 20, 2023 at 23:44
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If protons become antiprotons, but neutrons don't become antineutrons, then atomic nuclei will blow up, since antiprotons are made of antiquarks but neutrons are made of quarks.

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    $\begingroup$ Of course most of the atomic nuclei in the universe are just single protons, so it’s not quite as bad. $\endgroup$ Jan 19, 2023 at 21:02
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    $\begingroup$ @RBarryYoung Which begs the question of how you could even get regular neutrons in such a universe. $\endgroup$
    – Spencer
    Jan 20, 2023 at 23:25
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    $\begingroup$ @RBarryYoung True, but I think that just having all of the atoms in the universe except for protium atoms (and $^2$He, which would vanish within a nanosecond anyway) would still be pretty bad. $\endgroup$
    – tparker
    Jan 21, 2023 at 15:00
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If the universe were made of antiprotons and positrons, Lorentz force and the Hall effects will work in the opposite direction. Instead of the right-hand rules, you have to take the left hand and vice versa.

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    $\begingroup$ Isn’t the direction of cross product vectors unphysical anyway? $\endgroup$ Jan 21, 2023 at 13:28
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    $\begingroup$ This is incorrect; see physics.stackexchange.com/a/733050/92058. $\endgroup$
    – tparker
    Jan 21, 2023 at 15:02
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    $\begingroup$ So we'd drive on the left in the anti-US and on the right in the anti-UK? $\endgroup$
    – Barmar
    Jan 21, 2023 at 16:23
  • $\begingroup$ Magnetic fields don't just exist on their own; they must be made from charges. In an alternate universe where every particle is instead its antiparticle, all the EM fields are also backwards. So all the forces, being products of charge with field, are in the same direction as in this universe. For EM, the left hand rule and the right hand rule are equivalent, and the choice is an arbitrary human convention. As long as you use same rule at all times, you get the right predictions. You can only "objectively" tell matter from antimatter or left from right by subtle weak force measurements. $\endgroup$
    – HTNW
    Jan 22, 2023 at 1:47

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