3 ms·
No, it's actually just like cleaning your room. Imagine your room (system) has three objects (quantum state) whose respective positions are (a, b), (x, y) and
by throwawaymath 8y ago
No, it's actually just like cleaning your room.
Imagine your room (system) has three objects (quantum state) whose respective positions are (a, b), (x, y) and (p, q). You move the objects in your room to positions (a + k_{1}, b + k_{2}), (x + j_{1}, y + j_{2}) and (p + m_{1}, q + m_{2}), respectively. You know where the objects were before and you move them by adding or scaling their coordinates within the room. Thus you know how to move them back to precisely where they were before.
Likewise, this process is linear and preserves all information inherent to the system. Therefore it's precisely invertible, and voila.
It probably has some kind of value and it's a neat result, but this doesn't constitute time travel (in any meaningful sense) in the nonlinear world we reside in.
EDIT: Come to think of it, this probably has value for debugging and auditing the states, as a perfect rewind stepping function.
- empath75 8y agoCleaning your room increases entropy.
- throwawaymath 8y agoYeah, though I don't think you're supposed to litigate an analogy that far. The analogy fits because it grounds what's happening as something mundane rather than mysterious. This isn't really "time reversal" in the meaningful sense of the word, because a linear system can always be reversed if you have sufficient information.
- jakeinspace 8y agoIt's only roughly equivalent to cleaning your room, in a hand-wavey way. Is your cleaned room in an identical quantum state as it was a week ago? No? Well, then the situations aren't the same.
- throwawaymath 8y agoYes of course it's hand wavey, all analogies are hand wavey. That's why they're called analogies and not lectures. You don't devise an analogy to deliver all the academic rigor of a topic, you devise it to ground something back into the realm of the intuitive and familiar. Not all analogies are useful, but the reason this one is useful is because it captures the heart of why this isn't meaningfully "time reversal." If you have all information about a linear system, yes you can transform it back into its previous state. That's not at all mysterious or unintuitive even if it's a technical achievement. Obviously reality is nonlinear, which is precisely the point the analogy is trying to capture. We're not trying to teach quantum mechanics here, we're trying to make sure people don't come away from the article thinking the second law of thermodynamics is (non-locally) violated or that time travel at the macro level is plausible. The basic idea is that you did a reversible thing, then reversed it.