6 ms·
The past state of the universe is encoded in the present state only in the same way, and to the same extent, that the future is.
by rst 9y ago
The past state of the universe is encoded in the present state only in the same way, and to the same extent, that the future is.
- lisper 9y agoOnly in a classical universe, not a quantum one. The past is encoded in a way that the future is not because our classical universe emerges from a quantum universe through the creation of entanglements, which is what defines the arrow of time.
- eru 9y agoWhy? The wave function evolves symmetrically and deterministically, doesn't it?
- lisper 9y agoYes, it does. But the classical universe emerges because sub-systems of the wave function become classically correlated as a result of entanglements [1]. These entanglements are what define the arrow of time in the (or perhaps it would be more accurate to say "a" instead of "the" here) classical universe [2]. Information about the past is the classical correlations that arise from entanglements. Entanglements can only arise in (sub-)systems that were initially co-located, so relativity insures that these propagate only from the past into the future. [1] http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf [2] http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
- eru 9y agoDo we really understand quantum mechanics well enough to say how our classic approximation arises from it? Edit: interesting paper on quantum information theory and measurement vs entanglement!
- lisper 9y agoRead reference 1 (or watch the video: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc) and decide for yourself.
- Osmium 9y agoAny idea what the scientific background of the author of [2] is?
- lisper 9y agoYes :-) The author is me. I have a Ph.D. in computer science. But I worked for 15 years at JPL so I learned my quantum physics from Caltech professors.
- Osmium 9y agoThank you kindly for your reply! I was impressed by the article, and it made a lot of sense to me -- but I've also found that, given sufficiently persuasive writing, I'm easily persuaded that something incorrect is correct in QM, and the only way to get the 'right' answer is to do actually the math. This is why I was curious about your background: if I could reasonably surmise that you had 'done the math' so I didn't have to! (: (Note that I realise this is not how science is meant to work, but is a quite useful filter in practice)
- lisper 9y agoThe math is in reference [1], and also here: https://arxiv.org/abs/quant-ph/9605002 https://arxiv.org/abs/quant-ph/9605002
- xvedejas 9y agoExcept classical physics doesn't really tackle causation, and you'll find that Newtonian mechanics is not deterministic: http://www.pitt.edu/~jdnorton/Goodies/Dome/ http://www.pitt.edu/~jdnorton/Goodies/Dome/
- lisper 9y agoThat's a very interesting example, but it doesn't contradict what I said. Let's recall how we got here: > > The past state of the universe is encoded in the present state only in the same way, and to the same extent, that the future is. > Only in a classical universe, not a quantum one. In in Norton's dome, the indeterminacy extends equally into the past and the future: you can't tell when the ball will leave the dome, and you can't tell when it arrived.
- kobeya 9y agoThat was bad physics. Even assuming poetical cow physics with perfect spheres, domes, no friction, no air, and no external forces, the sphere will simply never move. There is no underlying instability to trigger its motion.
- xvedejas 9y agoExcept Newtonian mechanics doesn't say anything about causation, it simply says which equations are valid solutions. Most people find it counter-intuitive, but it's totally possible for multiple solutions to exist in the Newtonian framework for some circumstances.
- kobeya 9y agoNo what I'm saying is that the single Newtonian solution in this problem is no motion. The ball stays at the top of the dome. If there is a problem with multiple Newtonian solutions, this isn't it.
- xvedejas 9y agoHow did you determine that? There are infinitely many solutions that follow all laws of Newtonian mechanics. The burden of proof is yours to show uniqueness of a solution here, which I'm certain you cannot do with the canonical Newtonian laws alone.
- szemet 9y agoonly in the same way, and to the same extent Yes, but entropy can make it asymetrical - in practice past usually have less legal states (a lower entropy point) than the future. Past and future can be completely encoded by the direction of entropy change, if we start from, or end in one legal state. (You can exactly tell the past of the Logo turtle (or something similar physical thing) by following its trajectory line. You can have an inverse turtle that follows and erase an existing line, then you can tell its future, but not its past by looking only its present state) This kind of entropy increase is very common in human life - like forming memories, filling empty screens and white papers with signs, leaving footsteps in the snow etc... So the subjective feeling of asymetry is not completely ungrounded.