Discussion: approaches and thought experiment - consequences of being able to send information back in time?

Do physics allow to send information back in time?

  • Yes, but it is very difficult (e.g. wormholes)

    Votes: 1 100.0%
  • No, it is absolutely forbidden

    Votes: 0 0.0%

  • Total voters


Mar 9, 2019
CPT theorem is at heart of modern physics, suggesting causality working in both time directions – what are the arguments that it can only work past -> future? (it is different from statistical 2nd law of thermodynamics)
What approaches for reversed causality were considered (like wormholes, John Cramer’s, CPT analogue of laser)?
Anyway, I believe it is stimulating thought experiment: imagine scenario that such retrocausality device is invented – how would you imagine the consequences?
Related: Novikov self-consistency principle - Wikipedia

Here is my approach and brief answer for the last question (last paragraph of updated Section IV.A of https://arxiv.org/pdf/0910.2724 ):
Assuming CPT remains valid for macroscopic physics, still construction of such time-loop computer would be technically extremely challenging, but it might be reachable - should be at least taken into considerations. Especially that it might allow to break currently used cryptography (if reaching sufficient delay and strength): with verifier checking if a given key leads to decoded file not being just a noise. Protection against such attacks might be done by adding computationally costly initialization: necessary before application of a new cryptographic key - to require much longer delays and stronger channels. Further improvements of such channels could allow to use verifier e.g. testing molecules for desired properties for drug design, testing possible algorithms/methodologies for desired outcomes, etc. Finally, being able to send information back for macroscopic time differences, could allow to prevent currently unpredictable unwanted events: there would be attempt to send such missing information - creating inconstancy, hence action optimization should modify the weakest links of this reason-result chain (e.g. in quantum measurement level) to get self-consistent time loop, e.g with satisfying outcome: not requiring to use such channel. In other words, just having access to such channel, we would enforce physics to make its best to prevent our bad choices, optimize randomness for outcomes preferred by us. While there would be many dangers on the way, if well balancing strengths of such channels worldwide, it might lead to a much more harmonic world based on trust, without various types of gambling, with choices made optimizing their actual future consequences.
In the distant future it will be possible up to a point - that point being the invention of a working time machine. After that, information will indeed be able to be sent back in time as far as that date.
There are basically two possibilities: physics allows for manipulation of time e.g. through wormholes, or it doesn't.
If it does, sending information seems the simplest, and the required technology might turn out very close - like this CPT analogue of laser.

Cramer's approach was based on Delayed-choice quantum eraser - Wikipedia
- which I agree cannot send information back in time.

CPT analogue of laser is completely different approach: CPT theorem says that physics should be the same after transforming by CPT symmetry - if building CPT analogue of a device explicitly using past -> future causality, wouldn't it explicitly use future -> past causality?

So could we build such CPT analogue of a device? E.g. for free electron laser it seems doable - it is just electrons traveling in magnetic field causing later excitation of target ... wouldn't its CPT analogue be positrons traveling in the opposite direction, causing earlier deexcitation of target?

The basic experiment would be just placing gas discharge lamp e.g. behind segment of synchrotron (needed transparent window). Powering this lamp excites its atoms - usually deexciting with nearly symmetric angle distribution.

The hope is that stimulated absorption could increase deexcitation rate toward this segment (?) If so, it should be noticeable if monitoring energy balance between lamp power source and detectors around.

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