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Old 13th January 2019, 10:54 PM   #401
kellyb
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Originally Posted by Ziggurat View Post
No, not except for that. Including that. That's the whole point.

An electron moving forward in time is distinguishable from a positron moving forward in time. And a positron moving forward in time is distinguishable from a positron moving backward in time. But an electron moving forward in time and a positron moving backward in time are indistinguishable, because swapping both together produces no observable difference.
I have absolutely no idea how that could possibly work, but it sounds really cool!
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Old 13th January 2019, 10:55 PM   #402
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Originally Posted by RecoveringYuppy View Post
You know I'm aware of that, right?
A lot of people probably aren't, though.
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Old 13th January 2019, 11:05 PM   #403
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Originally Posted by kellyb View Post
I have absolutely no idea how that could possibly work, but it sounds really cool!
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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Old 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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Old 14th January 2019, 12:27 AM   #405
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It is happening all the time.
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Old 14th January 2019, 01:03 AM   #406
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The nearest planet to earth is Mercury.

https://www.bbc.co.uk/programmes/p06xvkjc
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Old 14th January 2019, 01:22 AM   #407
Ziggurat
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Originally Posted by MikeG View Post
Is this theory only, or can such going-back-in-time actually happen?
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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Old 14th January 2019, 01:54 AM   #408
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Originally Posted by Ziggurat View Post
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.
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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Old 14th January 2019, 02:27 AM   #409
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Originally Posted by The Great Zaganza View Post
That's just how reincarnation works.
That's just how reincarnationpetunian works.
Reincarnation is a totally different flower.
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Old 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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Old 14th January 2019, 07:29 AM   #411
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Originally Posted by Dave Rogers View Post
Is anybody else thinking, "Do not panic. Think only of yourself"?

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Old 14th January 2019, 08:24 AM   #412
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Originally Posted by arthwollipot View Post
An electron moving forward in time is in every way identical to a positron moving backward in time.

No need to bring politics into this.
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Old 14th January 2019, 08:28 AM   #413
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Originally Posted by xterra View Post
Why petunias?


Why not geraniums? Or asafoetidas?

It is what is written in the Great Book. Who are we to question it?
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Old 14th January 2019, 08:34 AM   #414
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Originally Posted by Roboramma View Post
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.

Symmetric? As in mirror image?
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Old 14th January 2019, 09:46 AM   #415
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Originally Posted by quadraginta View Post
It is what is written in the Great Book. Who are we to question it?

Originally Posted by Red Baron Farms View Post
That's just how reincarnationpetunian works.
Reincarnation is a totally different flower.


RBF got it right, and is the Floral Messenger to whom we must listen.
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Old 14th January 2019, 10:46 AM   #416
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Originally Posted by JoeMorgue View Post
The highest point in the continental contiguous United States is the peak Mount Whitney in California at 14,505 feet.
FTFY. The highest point in the continental United States is Mt. McKinley (Alaska), at 20,237 ft.
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Old 14th January 2019, 12:29 PM   #417
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Originally Posted by Ziggurat View Post
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.
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.

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Old 14th January 2019, 01:52 PM   #418
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Originally Posted by kellyb View Post
I have absolutely no idea how that could possibly work, but it sounds really cool!
It helps to notice that one of the first two "forward"s in the quote was meant to be a "backward".
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Old 14th January 2019, 02:40 PM   #419
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Originally Posted by RecoveringYuppy View Post
1. There is an arrow of time that seems at odds with the symmetry in our theories.
There doesn't seem to be a deeply satisfying answer on this one.

Quote:
2. There are known symmetry violations.
I know about CP violations. I am unaware of any observed CPT symmetry violations, and that's what you need to break the idea of antiparticles as backward-traveling particles.

Quote:
3. There is imbalance of matter/anti-matter.
That requires CP symmetry violations, including more than what we have observed so far. But I don't think it requires any CPT symmetry violation.

Quote:
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.
If we ever do establish the answer in either direction, it's probably worth a Nobel prize.
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Old 14th January 2019, 02:44 PM   #420
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Originally Posted by Red Baron Farms View Post
That's just how reincarnationpetunian works.
Reincarnation is a totally different flower.
Nope, Agrajag was reincarnated into a bowl of petunias so that he could be killed by Arthur Dent yet again.
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Old 14th January 2019, 04:42 PM   #421
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Originally Posted by Ziggurat View Post
I know about CP violations. I am unaware of any observed CPT symmetry violations, and that's what you need to break the idea of antiparticles as backward-traveling particles.

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.
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Old 14th January 2019, 05:09 PM   #422
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Originally Posted by RecoveringYuppy View Post
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.
Itís definitely only an approximation if CPT symmetry is violated. But if CPT symmetry holds exactly, then itís not an approximation. I have no idea if CPT symmetry holds exactly, but thatís what it depends upon.
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Old 14th January 2019, 06:09 PM   #423
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Originally Posted by RecoveringYuppy View Post
Symmetric? As in mirror image?
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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Old 14th January 2019, 06:12 PM   #424
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Originally Posted by Ziggurat View Post
If we ever do establish the answer in either direction, it's probably worth a Nobel prize.
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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Old 14th January 2019, 06:51 PM   #425
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Originally Posted by Roboramma View Post
I don't see how you can get two interacting systems to have entropy evolving in different directions.
I fully expect if we ever do the experiments, we'll see entropy increasing for antimatter just like matter, but as of now it's still untested, and reality doesn't have to make sense to us.
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Old 14th January 2019, 10:44 PM   #426
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Originally Posted by Thor 2 View Post
So that's how they moved those big stones around. This will blow Von Dšniken out of the water! Hang on, I think he got blown out of the water a long time ago.
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Old 14th January 2019, 11:39 PM   #427
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Originally Posted by Ziggurat View Post
I fully expect if we ever do the experiments, we'll see entropy increasing for antimatter just like matter, but as of now it's still untested, and reality doesn't have to make sense to us.
That makes sense, I just don't see any theoretical reasons to expect it.
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Old 15th January 2019, 12:06 AM   #428
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Originally Posted by Planigale View Post
The nearest planet to earth is Mercury.

https://www.bbc.co.uk/programmes/p06xvkjc
I couldnít get your link to work, but I looked it up. Amazing. If someone had have told me that at the moment Mercury was closer to earth than Venus I would have called them an idiot.
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Old 15th January 2019, 02:36 AM   #429
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Originally Posted by lionking View Post
I couldnít get your link to work, but I looked it up. Amazing. If someone had have told me that at the moment Mercury was closer to earth than Venus I would have called them an idiot.
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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Old 15th January 2019, 02:49 AM   #430
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Originally Posted by Planigale View Post
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.
I get it now.
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Old 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.
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Old 15th January 2019, 04:29 AM   #432
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Originally Posted by cullennz View Post
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.
Surely only on a flat earth though?
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Old 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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Old 15th January 2019, 07:21 AM   #434
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Originally Posted by Cheetah View Post
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.
Did you mean that the other way around? The smallest animal has the largest genome?
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Old 15th January 2019, 07:36 AM   #435
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Originally Posted by attempt5001 View Post
Did you mean that the other way around? The smallest animal has the largest genome?

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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Old 15th January 2019, 07:53 AM   #436
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Originally Posted by cullennz View Post
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.
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.
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Old 15th January 2019, 07:59 AM   #437
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Originally Posted by WhatRoughBeast View Post
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.
I believe a spinning bullet gets a tiny bit of lift as well.
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Old 15th January 2019, 08:00 AM   #438
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Originally Posted by cullennz View Post
Lottery:

12345 is just as likely to be drawn as any other numbers.
If you flip a perfectly balanced coin 100 times and they all land as heads. The odds of it landing heads on the 101st time is still 50/50.
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Old 15th January 2019, 08:03 AM   #439
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Originally Posted by attempt5001 View Post
Did you mean that the other way around? The smallest animal has the largest genome?
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".
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Old 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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