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Could someone please explain to me Feynman and Wheeler's theory of waves acting forward and backward in time?

I have read about their idea of an antiparticle behaving as a particle travelling backward through time, but cannot see how no paradoxes emerge.

Qmechanic
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Physics
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  • Can you give an example of the paradox? There are certainly issues involving entropy/information loss, but in a simple particle system they may be different than we see in thermodynamics. – Digiproc May 17 '19 at 11:28
  • @Digiproc Time travel always risks paradox because, for example, a time machine can go back and interfere with its own construction. But quantum mechanics avoids this because all the time loops and so forth occur "out of sight" between measurements. – Mitchell Porter May 17 '19 at 11:32
  • Possible duplicates: https://physics.stackexchange.com/q/391/2451 and links therein. – Qmechanic May 17 '19 at 11:36
  • @Digiproc For example, one could design a mechanism with a target that would shut a gate when struck by a pellet, such that the advanced field cause by the pellet closed the gate before the pellet arrived, in which case the pellet could not strike the target, in which case the advanced field would not close the gate after all...and so on. – Physics May 17 '19 at 12:19
  • @Physics This is not a paradox, just a confusion that unfortunately is too wide spread. There is no paradox in time travel (e.g. no silly "grandfather paradox"). The logical flaw in your description is in the "in which case" statement that wrongly assumes the events unfolding as a sequence along a non existent "time dimension" different from real time. The 4D spacetime is static. We move in it as a slice of 3D space along the real time dimension. In 4D, everything has already happened, including time travel, if any. No paradox. You can't kill your grandfather, because he already has lived. – safesphere May 18 '19 at 01:08
  • @safesphere But if time travel is possible, what makes killing the grandfather impossible? Do you simply a posit a conspiracy of small causes that prevent any attempt to change what happened? – Mitchell Porter May 19 '19 at 05:16
  • @MitchellPorter "What" is a separate question (with no answer), but the rest is plain simple. The 4D spacetime is static. If you are to travel to see your grandpa in the past, then you visit already have happened in the past and your grandpa already knows all about it. So when you do go travel, you will do exactly what your grandpa's already seen you doing. If he already knows you said "Hi", then you cannot say "Hello". You also cannot change your mind and decide not to travel. I am not saying that travel is possible or not, but IF it is, then it has already happened and cannot be changed. – safesphere May 19 '19 at 05:34

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