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13th January 2019, 10:54 PM | #401 |
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"We are enjoined, no matter how uncomfortable it might be, to consider ourselves and our cultural institutions scientifically — not to accept uncritically whatever we’re told; to surmount as best we can our hopes, conceits, and unexamined beliefs; to view ourselves as we really are." - Carl Sagan |
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13th January 2019, 10:55 PM | #402 |
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"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law |
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13th January 2019, 11:05 PM | #403 |
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Here's one way to play around with it conceptually:
Make a graph with space along x axis and time along y axis. An electron comes in from the left, a photon comes in from the right, and the bounce off each other, the electron leaving towards the left and the photon leaving towards the right, making a sort of X where they collide. Now turn the graph 90 degrees counter-clockwise. You've still got an X, but now you have a reverse-time electron (ie, a positron) coming in from the left, a forward-time electron from the right, and when they collide, they annihilate, sending out two photons. Why isn't one of the photons traveling backwards in time? Well, it kind of is, but photons are their own anti-particle, so it doesn't matter which way they're going. |
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"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law |
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14th January 2019, 12:15 AM | #404 |
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Is this theory only, or can such going-back-in-time actually happen?
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14th January 2019, 12:27 AM | #405 |
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It is happening all the time.
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"... when people thought the Earth was flat, they were wrong. When people thought the Earth was spherical they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together." Isaac Asimov |
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14th January 2019, 01:03 AM | #406 |
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14th January 2019, 01:22 AM | #407 |
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The theory says that there's no difference. Whether you want to consider a positron to be an electron going backwards in time or just mathematically equivalent to an electron going back in time is a bit of a philosophical irrelevancy.
Note, though, that even if you think a positron is literally an electron going back in time, that doesn't give you anything like useful time travel. After all, what did we need in order to "bounce" that electron backwards? We needed the positron that it would become, and we need to bring them together in an annihilation event. And that positron already did whatever it did, so you were already unable to use that electron to send any signal backwards in time. Could we make a person go backwards in time? Actually, yes. But there's a giant caveat which makes the whole thing rather unattractive. And that's the details of how you do it: get a lump of equal antimatter, and let it combine with the person. Bouncing an entire person backwards in time not only turns them into antimatter, it turns them into mush, because you're never going to find antimatter already assembled as a person. So we can send you back in time, but only by killing you. There's one additional fly in the soup which doesn't matter practically but may cause philosophical headaches: the whole entropy as an arrow of time thing. Antimatter should still increase its entropy when going forward in our positive time direction. That's a little hard to test directly, though. |
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"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law |
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14th January 2019, 01:54 AM | #408 |
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I'm not sure I see the issue here. Given that the laws of physics are symmetrical with respect to time, the same argument that tells us that entropy should increase toward the future actually also applies to the past. Take a system at time t, and let it evolve toward the future. There will be vastly more possible states (based on our imperfect information about the current system) of that future evolution with higher than lower entropy, so we see expect to see it evolve toward higher entropy.
But take the same system at time t and evolve it backward in time. The same argument applies. The issue here is simply that we already know that entropy was low in the past. Given that knowledge we expect entropy to give us an arrow of time. If you take a system in thermodynamic equilibrium and leave it until there is some fluctuation toward lower entropy, then look at that fluctuation in detail you'll see the entropy went down then up in a way that is symmetric with respect to time. My point is simply this: we already know that entropy was low in the past. That's enough to tell us both how we expect ordinary matter and anti-matter to behave. |
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"... when people thought the Earth was flat, they were wrong. When people thought the Earth was spherical they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together." Isaac Asimov |
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14th January 2019, 02:27 AM | #409 |
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14th January 2019, 07:19 AM | #410 |
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The lowest point in the continental United States is Badwater Basin in Death Valley, California at -279 feet.
The highest point in the continental United States is the peak Mount Whitney in California at 14,505 feet. These two spots are only about 85 miles apart. They are in the same county. You can drive from Badwater Basin to the Mt. Whitney Portal Picnic area (the closest road to the peak) in about 2 and a half hours. |
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14th January 2019, 07:29 AM | #411 |
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14th January 2019, 08:24 AM | #412 |
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14th January 2019, 08:28 AM | #413 |
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"A great deal of intelligence can be invested in ignorance when the need for illusion is deep." "Ninety percent of the politicians give the other ten percent a bad reputation." |
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14th January 2019, 08:34 AM | #414 |
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14th January 2019, 09:46 AM | #415 |
So far, so good...
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Over we go.... |
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14th January 2019, 10:46 AM | #416 |
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"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law |
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14th January 2019, 12:29 PM | #417 |
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Entropy was one of the things on my mind when I questioned the original assertion. I wouldn't have objected if the statement had been qualified along the lines of "isolated electrons/positrons" or "considered in isolation" or "as portrayed in space-time diagrams". But the statement as presented, which seems to me to be hand waving away (ETA: the poster is just repeating the hand waving, it's repeated lots of places) at least three concerns:
1. There is an arrow of time that seems at odds with the symmetry in our theories. 2. There are known symmetry violations. 3. There is imbalance of matter/anti-matter. I also hadn't considered that anti-matter moving forward in time might lose entropy. It hadn't occurred to me that that isn't well established, but it now seems obvious that it can't be well established yet. |
14th January 2019, 01:52 PM | #418 |
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14th January 2019, 02:40 PM | #419 |
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There doesn't seem to be a deeply satisfying answer on this one.
Quote:
Quote:
Quote:
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"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law |
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14th January 2019, 02:44 PM | #420 |
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So take that quantum equation and recalculate the wave by a factor of hoopty doo! The answer is not my problem, it's yours. Three Word Story Wisdom |
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14th January 2019, 04:42 PM | #421 |
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But I don't actually care about breaking that. You posed the question more rigorously when you stated that we can't distinguish between the electron/forward and positron/backward states (and vice versa). My contention is that we have reason to suspect that we will discover there is a distinction there. That is probably an approximation. |
14th January 2019, 05:09 PM | #422 |
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"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law |
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14th January 2019, 06:09 PM | #423 |
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If that's a reference to CP violations then I take your point that it's a little more complicated than I am making out.
If not, let me just clarify what I mean: Look at a system in thermodynamic equilibrium and wait for a fluctuation that brings it out of equilibrium. During that fluctuation the entropy goes down to some minimum value, then goes back up again. The graph of it's evolution toward that minimum and the graph of the evolution from that minimum to return to maximum entropy will be the time reverse of each other. So if you look at the whole of the fluctuation there's still no way to distinguish an arrow of time. It's only when you look only at the system starting from some low entropy state that it will evolve toward higher entropy. |
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"... when people thought the Earth was flat, they were wrong. When people thought the Earth was spherical they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together." Isaac Asimov |
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14th January 2019, 06:12 PM | #424 |
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I don't see how you can get two interacting systems to have entropy evolving in different directions. Matter and anti-matter interact, so how does the entropy of one increase while the other decreases? I really can't see that working statistically.
As I generally learn a lot from your posts on physics (and in general) I may simply be being dense here. |
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"... when people thought the Earth was flat, they were wrong. When people thought the Earth was spherical they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together." Isaac Asimov |
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14th January 2019, 06:51 PM | #425 |
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"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law |
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14th January 2019, 10:44 PM | #426 |
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14th January 2019, 11:39 PM | #427 |
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"... when people thought the Earth was flat, they were wrong. When people thought the Earth was spherical they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together." Isaac Asimov |
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15th January 2019, 12:06 AM | #428 |
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15th January 2019, 02:36 AM | #429 |
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One of those obvious things once it is pointed out. Because Mercury is close to the sun when it is on the far side of the sun from Earth it is much closer to the Earth than Venus or Mars are when they are on the far side of the sun. A 'friend of the BBC' wrote code to average the distance that Mercury, Venus and Mars are from the Earth over the last 50 years. On average over time Mercury is the closest planet to Earth.
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15th January 2019, 02:49 AM | #430 |
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A fanatic is one who can't change his mind and won't change the subject. Sir Winston Churchill |
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15th January 2019, 04:04 AM | #431 |
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Couple of fairly obvious ones which I always like, and a lot of people still don't believe
Gravity: If you simultaneously drop a bullet and fire from a gun a bullet from the same height. They land at the same time. Lottery: 12345 is just as likely to be drawn as any other numbers. |
15th January 2019, 04:29 AM | #432 |
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15th January 2019, 05:50 AM | #433 |
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The largest terrestrial animal native to Antarctica has the smallest genome.
It's a flightless midge, Belgica antarctica at 6mm and a genome of 99 megabases. Old Article on LiveScience. |
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15th January 2019, 07:21 AM | #434 |
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15th January 2019, 07:36 AM | #435 |
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NO! Edit: I probably should have said: The largest fully terrestrial animal native to Antarctica has the smallest genome of any terrestrial animal/insect/arthropod* so far sequenced. *I'm not quite sure, definitely insect, arthropod as well I think. Maybe there are some parasites with small genomes. |
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15th January 2019, 07:53 AM | #436 |
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Well, I don't believe it for a minute. And neither should you.
If I drop a bullet at the same time I fire my gun straight down, the bullet from my gun will hit first (unless I had a misfire). If I drop a bullet at the same time I fire my gun straight up, the dropped bullet will hit first. If I drop a bullet at the same time I fire my gun perfectly horizontally, the dropped bullet will hit first (by a very small amount for practical weapons). The effect here is that as soon as the bullet has travelled a very small distance horizontally, due to the radial nature of Earth's gravitational field it will effectively gain an upward component. See Newton's Cannon. |
15th January 2019, 07:59 AM | #437 |
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15th January 2019, 08:00 AM | #438 |
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15th January 2019, 08:03 AM | #439 |
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You should keep in mind that the animals we think of in Antarctica, (penguins, mostly, and seals) are aquatic, not terrestrial.
There are a number of bird species which are found in Antarctica, but except for penguins they all head north for the winter, so are arguably not "native". |
15th January 2019, 08:32 AM | #440 |
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Roughly all mammals take the same average amount of time to pee, about 21 seconds, regardless of body mass, metabolism, bladder size, or urethra size.
https://www.newscientist.com/article...nd-in-mammals/ |
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