Wikipedia:Reference desk/Archives/Science/2015 October 18

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October 18

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If you saw Interstellar movie, you'll understand what I am talking about clearly. I know we can't prove that something doesn't exist or won't happen. Future humans communicating with us? Come on. How can they possibly do that? Time travel to future is possible but past, I think, is not possible. I cannot prove it, but I can show you that it has very very low possibility. If time travel to the past is true, a person will come back from future and kill himself. is that logically possible? but this is not enough to throw away the time travel to the past idea. If our universe is a simulation, then things get different. If somebody is watching the universe but is not part of the universe and he knows what happened and what happens and but can't change the things inside the universe, once the simulation starts, but he can change the initial conditions of the universe. He can change the initial conditions of the universe in such a way that all the information about our universe is already present inside universe and any intelligent enough being can decipher it. It isn't same as communication but its like interstellar movie.

All the information about our universe, can be put on to something physical thing, which will never change, right? Let's think about that thing. I know something which will never change, initial settings. I mean the particles which were there 0.000001 secs after the big bang. If we know the exact position and speed of those particles, we can know our past, future, everything we could possibly think of. Is it really possible to know the exact position and speed and direction of those particles? Heisenberg, a homo sapien, said no. For us it is not possible but for another intelligence, it may be possible, maybe for future humans it's possible. Even after knowing the exact position and speed of the particles, is it really possible to know the past and future? <-----This is my question.

If we know all the information, is there any need to time travel? We can watch our solar system forming, earth turning blue and sun engulfing earth on our computer. We are virtually time traveling. Supdiop (T🔹C) 00:01, 18 October 2015 (UTC)

Wow, lots of topics there. You really need to narrow it down to a few specific Q's, preferably numbered.

Regarding time travel paradoxes, under the infinite worlds hypothesis, you can get around those problems by instead going into a universe precisely like ours, just at a different time. Thus anything you do there affects that world, but not our own. Of course, finding the right world and devising a way to jump into it are far from trivial problems.

Note that under quantum mechanics, there are truly random events, that can not be predicted by knowing the initial conditions.

As for the need to travel to other times, consider the case of traveling to other planets, as this topics has recently come up with respect to Mars. Yes, we can get all the info we need from robotic probes, but this just doesn't excite the people like going there in person does. And, for political reasons, if you don't excite people, you don't get the funding you need. I imagine the same would apply to time travel. StuRat (talk) 00:25, 18 October 2015 (UTC)

Incidentally, that assumes the Copenhagen interpretation is correct, which is by no means universally accepted. Tevildo (talk) 00:58, 18 October 2015 (UTC)

Perhaps not quite universally accepted, but, as our article says, it "remains one of the most commonly taught interpretations of quantum mechanics". StuRat (talk) 01:51, 18 October 2015 (UTC)

Laplace's demon and hard determinism might prove useful here. Tevildo (talk) 00:52, 18 October 2015 (UTC)

Thanks for the responses. I pointed out my question. Wow, many people thought about this idea before me, I thought, I am the first one, lol. Laplace's demon is exactly what I am talking about, thanks for linking it. Supdiop (T🔹C) 01:39, 18 October 2015 (UTC)

The idea of simulating the universe has some logical problems, but it's a plot device in Hitchhiker's Guide to the Galaxy. Many of the other paradoxes here can be recognized from precognition. Your assumption that you can decide to kill your grandfather may be defective. I think it is; more specifically, I think that knowledge of the immutable future is actually the basis of human free will; such causal loops allow phenomena that are neither random nor predetermined from past events. I would also suggest that precognition outside of that context is a pathological, hazardous phenomenon. Wnt (talk) 10:48, 18 October 2015 (UTC)

Those who cannot remember the past are condemned to repeat it. Bestfaith (talk) 13:05, 18 October 2015 (UTC)

I once heard someone quote that, and follow up with, "I'll keep that in mind the next time I'm thinking about invading Poland." ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:27, 18 October 2015 (UTC)

History repeats itself. Historians repeat each other.. DMacks (talk) 20:45, 18 October 2015 (UTC)

Hence the expression, "History is a set of lies agreed upon." ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:33, 19 October 2015 (UTC)

As I understand it, quantum mechanics is time-reversible. If you know the present, you can't perfectly predict the future or the past. In whatever sense the future is indeterminate, so is the past. That said, if multiple pasts correspond to our present, does that mean that information has been erased? And as per Maxwell's demon, does that create entropy? (The universe's entropy does increase, but does it increase like this?) Wnt (talk) 00:49, 19 October 2015 (UTC)

There is only one past. You don't "predict" the past, because it has already happened. Though you can predict what you might expect to find when exploring evidence of the past. However, memory and recording techniques are imperfect and are only a fraction of the "total" past, so your memories and mine will tend to be different. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:38, 19 October 2015 (UTC)

Schroedinger's Cat. The past didn't happen until you know it happened. And if you forget it happened, at the QM level, it didn't happen. (Despite these things, I am nonetheless tempted to believe in a "true" past, and a "true" future; but that isn't quantum mechanics - that's some kind of religious belief that God uses quantum mechanics to keep the 4D universe in approximate synch with previous versions, permitting incremental revisions to be made) Wnt (talk) 13:00, 19 October 2015 (UTC)

Not true. Whether you know an event happened or not, it still happened. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:09, 19 October 2015 (UTC)

Well, under the many-worlds interpretation described before, it happened in all possible ways in worlds that are, at present, indistinguishable. But for what you say, do you agree that whether you can calculate if a future event is going to happen or not (even if it appears random and unpredictable) it is still going to happen? Wnt (talk) 14:44, 19 October 2015 (UTC)

There is no evidence to support the sci-fi notion of multiple pasts. As to the future, do you mean like predicting eclipses? ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:06, 19 October 2015 (UTC)

Consider the particle horizon. It is continually expanding, admitting more and more of the universe to observability. There is a spacelike interval between us and anything outside of it. So we don't know what happened there, and in QM terms, it is completely unknown, like Schroedinger's cat. Now there's also a spacelike interval, same kind of interval, between us and "what will happen on Mars in five minutes". So if there is only one true past that describes what "went on" past the particle horizon that will later be revealed to us, there is also only one true future that describes what will happen on Mars five minutes from now. The difference between the direction of these spacelike intervals is strictly a matter of frame of reference. Wnt (talk) 19:38, 19 October 2015 (UTC)

Us not knowing the condition of the cat has nothing to do with the actual condition of the cat. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:28, 19 October 2015 (UTC)

The QM position seems to be that there are no hidden variables - the hypothetical cat really is both dead and alive. The trick though is that if you stand in a big box when you open the cat's box, now you are both seeing a dead cat and seeing a live cat. So the question is, do you "really" experience all the bazillion possibilities of life, or are all the others not actually feeling qualia while one real one is? Which is one reason why (as I said above) the true past/true future is a religious concept, not a scientific one, because science can't touch qualia. Wnt (talk) 22:40, 19 October 2015 (UTC)

The arrogance of the cat story is the notion that what we know somehow affects the reality of the situation. By that logic, dinosaurs didn't exist until we found their fossilized bones. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:44, 20 October 2015 (UTC)

Well, the idea of Schroedinger's Cat is really just an extension of Schroedinger's Photon (I mad that up), which is well established experimentally; you probably know the idea, but I'll run through it anyway, if you shoot photons through a slit they pile up behind the slit as you expect if they were particles, if you shoot them through another slit they pile up behind that one as you expect, if you shoot them through both slits at the same time, instead of the two piles behind, you get a diffraction pattern; i.e. now they act likes waves not particles. The philosophical interpretation comes from "no action at a distance" and/or "no Godlike POV"; you can't know anything about the photon or anything else unless/until you interact with it in some way; ask a question or do an experiment, if you will. And the result of the experiment depends on what experiment you do, and the answer you get depends on your interpretation of that result. Logically, then, without "observation", i.e. poking the thing to see what happens, you can't say that the photon is in one state or the other, because you could then do the opposite experiment and "prove" that it's in the other state. So, there is no "true" state of the photon in its solitude that you can discover; instead, if you do an experiment that looks for what we consider to be particle properties, you find them, if you look for wave properties you find them; thus, in the absence of the experiment, it must be in neither/both states.

And, since there's no apparent reason why this wouldn't scale up (or down, quarkwise), the same principle would presumably apply to everything in the universe; until you poke at it in some way to see, not only do you not know what state it's in, it has the potential to be in any state depending on your test. Certain amount of reductio ad absurdum here; if the cat is in the box, then it has been interacted with at some point, which has already collapsed its universe of possible states, so really it's not something that comes up in real life.

As you can tell, I see similar logic behind Heisenberg's uncertainty principle; if you do the experiment that looks for position of a particle, you've messed up its momentum, and vice versa; you can't do an experiment that measures both arbitrarily precisely, so thinking about both simultaneously and exactly is just discussing the results of an experiment that cannot be done, even in imagination. Note also that this doesn't involve an anthropomorphism-type thing here, as often interpreted; it's not that an intelligence, or consciousness, or human or some such has "observed" the thing that makes it gel into what we consider reality; it's almost mechanical, i.e. it's the fact that such observation involves bouncing a photon off whatever item to find the location, or adsorbing it onto a surface to find the momentum, etc. in some way, and that has a physical result that we think of as "finding out" whether the photon is alive or dead, when in fact we haven't really found out what the photon (or cat) "really" is in the absence of us tinkering with it. OK, quiz next week. Oh wait, never mind.Gzuckier (talk) 05:35, 20 October 2015 (UTC)

Of course, Bugs's understanding is the correct one here based on Schroedinger's own purpose behind his cat story. He was attempting to demonstrate exactly what Bugs notes above, the "The arrogance of...the notion that what we know somehow affects the reality of the situation". Schroedinger felt similarly about it. The Cat story is not meant to prove the Copenhagen Interpretation, it is meant to ridicule it. He's trolling his fellow scientists. Lest anyone doubt that, the Schroedinger's cat article states directly, with citation, " Schrödinger did not wish to promote the idea of dead-and-alive cats as a serious possibility; on the contrary, he intended the example to illustrate the absurdity of the existing view of quantum mechanics." --Jayron32 15:17, 20 October 2015 (UTC)

Funny. I just watched a rerun of "Through the Wormhole" on this very subject last night. Quantum entanglement theoretically allows a "message" (a photon state or something of that sort) to be sent anywhere in the universe, and, if I understood correctly, the "receiver" will actually receive the message before the "sender" has sent it (only nanoseconds, but hey, it works). I'm not a physicist so my understanding may be a bit off. Justin15w (talk) 18:24, 19 October 2015 (UTC)

Identity of the flower species

Unknown flower id

Can some one please identify the flower?? Thanks in advance. Nikhil (talk) 13:01, 18 October 2015 (UTC)

I'm no good with flowers, but I can tell you from experience that you're going to need to provide provenience for the photo - where in the world was the photo taken, what kind of area, that kind of thing. If you have additional photos of the leaves - or even just of the general plant from a distance - that can be a big help. For example, is this a solitary blossom growing straight up from the dirt or is it off a woody shrub? The more details you can provide, the better! 99.235.223.170 (talk) 13:39, 18 October 2015 (UTC)

Thanks for the reply, this was taken in Hyderabad, India. Unfortunately, I don't have many photos. I only have two photos, the other one not much different from this as it is also a close up shot. I think it is a vegetable plant, not off woody shrub. Nikhil (talk) 13:58, 18 October 2015 (UTC)

It could belong to Hibiscus genus. Supdiop (T🔹C) 14:30, 18 October 2015 (UTC)

Thank you for the hint! 😊