7 ms·

It's like saying picking up a ball from point A and putting in point B, then taking it from point B and putting back in point A is "reversing time".

• adamc 8y ago
  I don't think that is true. When you pick up the ball and move it, you are part of the system... and it takes work (and produces heat). No second law violation, right?
  • krastanov 8y ago
    Indeed, no 2nd law violation. And this work does not have anything to do with the 2nd law. OP summary is great: they are just doing an operation and then doing it in reverse (albeit in a technically interesting way).
  • _Schizotypy 8y ago
    There is also no violation in the proposed experiment because they put energy into the system to do their "reversal"
• phkahler 8y ago
  Yeah, this is a poor use of words. I was actually hoping they had read some quantum information "from the future" but in a way that precludes transmitting useful information to the past - similar to how entangled pairs seem to transmit information faster than light, but in a way that doesn't enable FLT communication. But no, nothing about this suggests anything to me about reversing time. Reversing decoherence (if that's a better description) is nice but it's not about time reversal.
  • zwkrt 8y ago
    > similar to how entangled pairs seem to transmit information faster than light If your lover sends you a left-half heart locket through the postal service, when you get it you can assume he has the right-half of the locket "faster than the speed of light". However this is cheating because to know that he has the other half requires prior knowledge of the exchange--either he sent a previous letter where he stated his intent, or you just made an assumption about the state of the environment (namely, that your partner is the only one who would send you presents in the mail). In either case, no laws of physics need be broken.
    • ahelwer 8y ago
      This is absolutely not how entanglement works. This analogy requires the locket half you receive to be defined at time of "entanglement" (when the locket halves are separated & sent). Half a century of Bell experiments have told us that the result of measuring an entangled system is defined at time of measurement, not at time of entanglement. If it were defined at time of entanglement, the particles would have to carry information with them to "remember" how they decided to collapse (a local hidden variable). Bell experiments disprove the existence of local hidden variables as a means to explain entanglement. I wish people would stop posting these third-hand metaphors about quantum entanglement being like finding a single right-handed glove in your pocket or whatever. They are irredeemably flawed and completely misrepresent how entanglement works.
      • zwkrt 8y ago
        Yes my analogy had a hidden variable, but it is "less wrong" than saying that entanglement allows FTL communication, for reasons analogous to those that I provided at the end; entanglement can only convey information as fast as light because the two parties detecting the entangled particles aren't communicating they are just on the receiving end of a shared experience. I don't think I can explain entanglement to a lay person more succinctly than the locket/glove analogy, even though it does hand wave the most quantum-ey part of the experiment.
        • ahelwer 8y ago
          But I feel this harms more than it helps, like saying "quantum computers compute with every possible value simultaneously in superposition" which gives the audience a very flawed understanding of how QCs work (that they'll be able to solve NP-complete problems in polynomial time for example). By phrasing quantum phenomena in terms familiar to the classical audience, we do them a disservice; we are lying to them. In this case, the question "why doesn't quantum entanglement enable FTL communication?" is properly that the reduced density operator of an EPR half is the maximally mixed state. All we can do to simplify that is to tell them that quantum concepts exist outside of classical language, and in order to understand them they'll have to learn a new language - mathematics. The most you can say is quantum entanglement enables FTL "correlations" which are stronger than classical correlations but not strong enough to enable information transfer.
  • jeffwass 8y ago
    FYI, the term “Time Reversal” has long been used by physicists to describe a symmetry of quantum mechanical processes with regard to flipping the sign of t (ie, time). Just like Charge and Parity symmetries which flip the sign of electric charge and position. See https://en.m.wikipedia.org/wiki/CPT_symmetry https://en.m.wikipedia.org/wiki/CPT_symmetry
• ZeroFries 8y ago
  If all properties of the ball (and not just position) were "put back", sure.
  • throwawaymath 8y ago
    What other properties does your hypothetical ball have which aren't preserved?
    • neals 8y ago
      age?
      • throwawaymath 8y ago
        Okay, that's fair. That property can't really be linearly reversed. I was speaking to an idealized system consisting of a ball in a vacuum moved from one point to another. That's closer to a macro representation of what's happened here.
    • empath75 8y ago
      All the atoms making it up would have changed in various ways, which is to say that even if it’s been moved and moved back, one could tell that time has passed with an appropriate observation. For a single electron, this would not be the case.
      • throwawaymath 8y ago
        Yeah the analogy isn't perfect, but despite that I think it basically works as a macro representation. Obviously the ball isn't a fundamental particle and we wouldn't have all information about how it ages. But tracking just the ball's position is meant to analogize the fact that we have all information about e.g. a fundamental particle's position.
• have_faith 8y ago
  Reminds me of a thing people do when playing chess. Sometimes the best move is to move a piece "backwards". In reality the game doesn't care if a move is considered backwards, forwards or sideways, it's just conceptually a new position on the board. But most people see it as going backwards or retreating and struggle to get over this psychologically. Similar to seeing the ball in the previous position as reversing time.
  • matthewtoast 8y ago
    > most people see it as going backwards or retreating and struggle to get over this psychologically If you return a chess piece to a previous position, it's possible that doing so loses you a tempo. Even if, strictly speaking, the game/board doesn't care about the direction you move, saying it's "just" another position isn't quite accurate, since turns matter, and using up a turn to walk back a piece may give your opponent a +1 turn advantage?
• mises 8y ago
  It's more the idea that things can be reversed. How would they have such a "memory" of previous state? It presents such interesting questions. Imagine if we could coerce a human into the exact shape assumed 30 years earlier: that would effectively him backwards 30 years.
  • krastanov 8y ago
    In general, yes, but I think you will be very disappointed with this particular work: quantum gates are "linear", so they are readily inversible. All of the "memory" is explicitly baked in in the method proposed here.
• zerr 8y ago
  Well, if you put every atom back, like where they were e.g. in 1960, then it makes sense I think.
  • throwawaymath 8y ago
    That's not comparable unless we have all information about every position the balls have been in from 1960 to the present time. Theoretically plausible, but probably not feasible. The simple model of a ball shifting from one point to another is a fair macro analogy because, like the quantum system described here, we realistically have all information about the ball as it moved from point A to point B.