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Old 19th November 2022, 03:44 PM   #81
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Originally Posted by Thermal View Post
I'm not sure what this array of clocks would be synchronized to. Each other? Seems redundant. Adjust one to the other travelers time and keep one at your own? That's only two. I guess they would read the same if we correct the perceptual distortion that either is at rest as compared to the other. Each clock in the array would run accounting for the dilation that Thermal sees relative to earth's time frame. Different results then, yes?
All clocks synchronised to each other at the start.

You have an array of clocks because Bob and Alice can't instantaneously read each other's clocks.

But Bob can read a clock that is synchronised with Alice's clock and vice versa and this is just the same as if they had read each other's clocks instantaneously.

So this setup would demonstrate that there is a real difference in their time and that it is not just an illusion.

If it was just an illusion or a perceptual issue then all of the clocks would continue to read the same since everyone is reading a clock that is adjacent to them.

If two clocks that are right next to each other are showing different times then you could hardly call that difference a perceptual difference could you?

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Old 20th November 2022, 12:37 PM   #82
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@pzkpfw and Robin:

You both seem to take offense to my use of "perception", like it's a dirty word. I don't at all mean anything disparaging by it.

We all know the Glass Elevator thought experiments, right (and train cars and all)? You guys seem to conclude that both perceptions of the ball's movement are equally real. I agree, but saying "real" is baking a problem in, that pops up later. The ball's movements are equally real, in exactly the same sense that neither is "real". Each is only valid for its frame of reference, so I call it the perception, not the reality.

Do you guys disagree? I mean, the frame of reference is entirely subjective, and by definition, if the observer's actually interact with each other, one perception the another is blown. You guys (I think) stroke your metaphorical beards and say the realities are both valid, and "real". I scratch my crotch and say the perceptions are both perfectly valid, but neither is "real".

As soon as you say something has its own subjective experience of reality, it is the same as saying it is their perception of reality. Does that at all clear up where my head is at?
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Old 20th November 2022, 12:46 PM   #83
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Originally Posted by Robin View Post
All clocks synchronised to each other at the start.

You have an array of clocks because Bob and Alice can't instantaneously read each other's clocks.

But Bob can read a clock that is synchronised with Alice's clock and vice versa and this is just the same as if they had read each other's clocks instantaneously.

So this setup would demonstrate that there is a real difference in their time and that it is not just an illusion.

If it was just an illusion or a perceptual issue then all of the clocks would continue to read the same since everyone is reading a clock that is adjacent to them.

If two clocks that are right next to each other are showing different times then you could hardly call that difference a perceptual difference could you?
Ok. First off, what is up with this "array" terminology? An array is usually meant as a display or gridwork of something. What you and pzkpfw seem to be suggesting is a chain of brazillians of clocks connecting Alice and Bob's ships, synchronized to Bob's time. Do I have that right?

If so, none but the one on Bob's ship and the one on Alice's matter. Those two clocks need only to be connected by a magic wire that instantaneously keeps them synchronized at lunatic distances (and you'd need a lot of slack as the distance increased at near the speed of light). So I don't know what the other clocks are supposed to accomplish. Can you explain?

Also, the clock at Alice's end would be moving through Alice's time dilation, so wouldn't be reading the same as Bob's even if they were synchronized. It would have to me modified to account for both their relative velocities, making it basically a made-up time clock, which I also don't understand what that would be proving.
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Old 20th November 2022, 12:57 PM   #84
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And again, to both of you: I am not challenging Relativity. If I started studying physics right now and continued till I died, I would in no way be able to absorb enough to even pretend to that level of hubris.

I have two potential outcomes here: a poster here could get inside my head enough to create a model that would make the penny drop on my end. The other is that I put the subject in a cloud tagged with "Things Thermal Just Isn't Getting Yet".

A third possibility, which likely factors in to one degree or another, is that there is a dose of semantic distortion at play. When you say it's "real" for B or A, I would say it is merely their perception. If it became actually real, that would require the magic phones, and the causality impossibilities rear their ugly heads , which circles back to the OP problem.
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Old 20th November 2022, 01:05 PM   #85
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Originally Posted by Thermal View Post
@pzkpfw and Robin:

You both seem to take offense to my use of "perception", like it's a dirty word. I don't at all mean anything disparaging by it....
That's the trouble: the way you use "perception" to imply "not 'really real', just temporary illusion".

(I think that's what you were trying to show with your video - that while Alice and Bob are zooming about their clocks are weird due to their (personal, illusary) perception of reality, but once they all get back together and watch the (the? which? how?) video they'll have a good laugh and see it was all just a trick.)

But it's deeper than that. Sure, each observer has their own "perception" of reality. But that is reality for them. There is not some underlying single "real reality" that their perception is a distortion of.


Muons really do appear to last longer (to us) than they should. http://hyperphysics.phy-astr.gsu.edu...ativ/muon.html
The Hafele–Keating experiment showed actual differences in elapsed time. https://en.wikipedia.org/wiki/Hafele...ing_experiment
These are not illusions.


You know that speed relates distance and time. At a certain speed, you might cover a certain distance in a certain time. Drive faster, you'll cover more disatance in less time, or the same distance in less time.

Once it was shown that the speeed of light is the same for all observers, that has an implication: for observers in relative motion, for them all to agree on the speed of light, it must be their time/distance that are not the same. This is not an illusion, it is reality for those observers, and will have actual effects (like the one that started this thread).

Last edited by pzkpfw; 20th November 2022 at 01:27 PM. Reason: Add quote
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Old 20th November 2022, 01:17 PM   #86
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Originally Posted by Thermal View Post
Ok. First off, what is up with this "array" terminology? An array is usually meant as a display or gridwork of something.
Yes, though possibly just a line of clocks along the path of motion.

Imagine you are in a train running down some tracks. You have your own clock moving with you. There's a clock back at the station you want to compare your clock to. The trouble is, that station clock gets further and further away. It gets harder to know what it reads "now" (your "now") - this was an issue with your half-designed video experiment. Seeing that clock means you have to wait for light from the clock to reach your eyes, and the further away you get the longer that takes.

So what you do, is before your experiment you stick clocks all along the track, maybe every 100 cm. You synchronise these clocks to the station clock. Since they are at rest with regards to the station, they will stay synchronised.

Now you do your experiment. And when you want to know what time it is according to the station clock, you just have to look out your window at one of the nearby at-rest-with-the-station clocks.

Originally Posted by Thermal View Post
What you and pzkpfw seem to be suggesting is a chain of brazillians of clocks connecting Alice and Bob's ships, synchronized to Bob's time. Do I have that right?
Yes. (ETA: not "connected", just pre-synchronised.) (ETA2: you'd need (depending on exactly what you are trying to show) two separate sets of clocks. One set at rest with Alice, for Bob to look at, and one set at rest with Bob, for Alice to look at. The Bob and Alice clocks are not at rest with each other, because Alice and Bob are not.)

Originally Posted by Thermal View Post
If so, none but the one on Bob's ship and the one on Alice's matter. Those two clocks need only to be connected by a magic wire that instantaneously keeps them synchronized at lunatic distances (and you'd need a lot of slack as the distance increased at near the speed of light). So I don't know what the other clocks are supposed to accomplish. Can you explain?
You complained about magic telephones before, now you want magic wire?

The clocks make for an understandable with known physics way to see what the time is for another observer.

Originally Posted by Thermal View Post
Also, the clock at Alice's end would be moving through Alice's time dilation, ...
What does that mean? In the train-track example above, do you think that the train passing by clocks on the track, will affect those clocks?

No! The train is not carrying some kind of Star Trek Warp Bubble that makes time change for nearby stuff outside of it.

Originally Posted by Thermal View Post
...so wouldn't be reading the same as Bob's even if they were synchronized. It would have to me modified to account for both their relative velocities, making it basically a made-up time clock, which I also don't understand what that would be proving.
No. The clocks stationary with respect to an observer will record time the same. There's no reason why they'd be different.

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Old 20th November 2022, 01:20 PM   #87
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Right, and again, I get that. When you say that I was trying to "imply", no, I wasn't. I just think "perception" is the more accurate word, language-wise.

Like, we can all go all new-agey and say everyone has their own personal reality. We can extrapolate that to the current discussion, with lots of justification.

But remember how you have been lightly chastising me for trying to force an objective reality? That's the flip side of this coin. If there is no objective reality, it is all subjective perception, till a reference frame is agreed upon. B&A's perception of the other's reality is not real in any sense if they move to the same frame of reference. That's where Thermal and his horn rimmed glasses came in at MEBE control. If we don't declare something to be a neutral reference frame, it's all subjective perceptions with new "reality labeled" haircuts.
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Old 20th November 2022, 01:23 PM   #88
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Originally Posted by Thermal View Post
And again, to both of you: I am not challenging Relativity. If I started studying physics right now and continued till I died, I would in no way be able to absorb enough to even pretend to that level of hubris.

I have two potential outcomes here: a poster here could get inside my head enough to create a model that would make the penny drop on my end. The other is that I put the subject in a cloud tagged with "Things Thermal Just Isn't Getting Yet".

A third possibility, which likely factors in to one degree or another, is that there is a dose of semantic distortion at play. When you say it's "real" for B or A, I would say it is merely their perception. If it became actually real, that would require the magic phones, and the causality impossibilities rear their ugly heads , which circles back to the OP problem.
When I was about 15 my physics teacher told me: if a train is going at 20 kph and a Man runs along the top at 20 kph, his speed is actually 39.99999999 kph. (To be clear, that's not an infinite number of nines.)

I just refused to accept it. No way. 20 plus 20 is 40, why the heck would it be 39.99999999 ?????

The problem was, he presented it as a fact I just had to accept, there was no context. Now I am in no way an expert, but the "penny drop" for me was thing I mentioned before: the speed of light is the same for everyone. So it's time and distance that must be wibbly wobbly.

Last edited by pzkpfw; 20th November 2022 at 01:24 PM. Reason: I was 15, not 155
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Old 20th November 2022, 01:26 PM   #89
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Originally Posted by Thermal View Post
Right, and again, I get that. When you say that I was trying to "imply", no, I wasn't. I just think "perception" is the more accurate word, language-wise.

Like, we can all go all new-agey and say everyone has their own personal reality. We can extrapolate that to the current discussion, with lots of justification.

But remember how you have been lightly chastising me for trying to force an objective reality? That's the flip side of this coin. If there is no objective reality, it is all subjective perception, till a reference frame is agreed upon. B&A's perception of the other's reality is not real in any sense if they move to the same frame of reference. That's where Thermal and his horn rimmed glasses came in at MEBE control. If we don't declare something to be a neutral reference frame, it's all subjective perceptions with new "reality labeled" haircuts.
Sorry, but my reply to this really is "meh".

I had to let go of the idea that there is one true clock somewhere that's got the "real" time on it.

As shown, there are real measurable effects. You can't just blow them off and claim illusion or whatever. (Seriously, it's like you don't read replies and just jump to the end to just say the end is wrong.)

(ETA: the closest thing we have to a "neutral reference frame" - is the clock you hold in your own hands. It's at rest with you, and ticks your time. (And yes it's different to a clock held by someone else moving with respect to you.))

Last edited by pzkpfw; 20th November 2022 at 02:00 PM. Reason: typo
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Old 20th November 2022, 02:18 PM   #90
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Originally Posted by Thermal View Post
You both seem to take offense to my use of "perception", like it's a dirty word. I don't at all mean anything disparaging by it.
Not at all, just pointing out that it is incorrect.
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Old 20th November 2022, 02:21 PM   #91
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Originally Posted by Thermal View Post
And again, to both of you: I am not challenging Relativity.
That's right. You are just misunderstanding it by insisting time dilation is an illusion.
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Old 20th November 2022, 02:43 PM   #92
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Originally Posted by Thermal View Post
Ok. First off, what is up with this "array" terminology?
There are lots of clocks lined up at regular intervals in front of Alice, comoving with her so that Bob passes each as he moves away.

There are lots of clocks lined up behind Bob at regular intervals, comoving with him so that Alice passes each as she moves away.

Initially all clocks show the same time.
Quote:
If so, none but the one on Bob's ship and the one on Alice's matter. Those two clocks need only to be connected by a magic wire that instantaneously keeps them synchronized at lunatic distances.
There is no such thing as a magic wire.

Quote:
It would have to me modified to account for both their relative velocities, making it basically a made-up time clock.
They are just ordinary clocks. At the start they all show the same time. They are not modified to account for anything.
Quote:
Also, the clock at Alice's end would be moving through Alice's time dilation, so wouldn't be reading the same as Bob's even if they were synchronized.
Only if time dilation is real. If time dilation was just a perception then all the adjacent clocks would read the same.
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Old 20th November 2022, 04:19 PM   #93
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Here is the setup:



Then after a period of time:



Then after another period of time:



And so on for as long as you like. At the end we take all those photographs and assemble them in the same place.



So, say they are travelling at their respective directions at 70.71% of the speed of light and the time initially shows t=0 seconds and they pass by the clocks at a distance of, say, a couple of metres. We have to assume that the camera is fast enough to capture the photographs at that speed.

After a couple of seconds will the two clocks in the same photograph show the same time (adjusted for the time that light travels that couple of metres)? For example, will the two clocks in one of Alice's photographs show the same time? Will the two clocks in one of Bob's photographs show the same time?

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Old 20th November 2022, 05:22 PM   #94
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If they were moving at the same speed (opposite directions or not) wouldn’t they both experience the same amount of dialation and the array and ship clocks all stay in sync?

Earlier in the thread it’s mentioned that because the speed of light is the same for everyone, each traveller knows the other’s clock is slower compared to theirs, but I don’t understand that part. Is it the same reason that you can’t tell what someone else’s frame of reference is (ie who is moving and who is at rest?) or do I have that misremembered as well

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Old 20th November 2022, 06:19 PM   #95
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Originally Posted by Lithrael View Post
If they were moving at the same speed (opposite directions or not) wouldnít they both experience the same amount of dialation and the array and ship clocks all stay in sync?
Their clocks would dilate the same according to a third observer who stays at the middle of them (in this scenario).

But for each, it's the other who is moving, and the others clock that will dilate.

Watch out for terms like "moving at the same speed". Without realising it, you can be assuming there's some underlying single base against which "real speed" can be measured. Say a fourth observer is moving towards Alice in this example. To that fourth observer Alice and Bob are moving at different speeds compared to each other. Who is "right"? Alice? Bob? The third middle observer? This new fourth observer?

Originally Posted by Lithrael View Post
Earlier in the thread itís mentioned that because the speed of light is the same for everyone, each traveller knows the otherís clock is slower compared to theirs, but I donít understand that part. Is it the same reason that you canít tell what someone elseís frame of reference is (ie who is moving and who is at rest?) or do I have that misremembered as well
Read post #79, and compare the basic non-relativity example of the ball with the torch example.
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Old 20th November 2022, 06:37 PM   #96
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[quote=Lithrael;13949419]If they were moving at the same speed (opposite directions or not) wouldnít they both experience the same amount of dialation and the array and ship clocks all stay in sync?
/QUOTE]
The clocks going the same velocity as Alice will remain in synch with Alice's clock and the clocks going the same velocity as Bob remain in synch with Bob's clock.

But since Bob and Alice are travelling at different velocities wrt each other, then one array of clocks won't remain in synch with the other array.

So when Bob reads a clock that is adjacent to him he is reading the same time he would read if he could instantaneously read Alice's clock.

So, for example, with the velocities I quoted earlier, when Bob's clock reads 3 the adjacent clock will read 1.
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Old 20th November 2022, 08:41 PM   #97
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Ah, ok. So the counterintuitive part is that if you had two arrays of clocks moving past each other near c, a third observer would (ETA: I was thinking I should be more verbose, pzkpfw’s now done it for me, ty) see them as reading the same time, and yet each clock pilot would see the other set of clocks running slow. Is that more like it?

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Old 20th November 2022, 08:53 PM   #98
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Originally Posted by Lithrael View Post
Ah, ok. So the counterintuitive part is that if you had two arrays of clocks moving past each other near c, a third observer would see them as reading the same time, and yet each clock pilot would see the other set of clocks running slow. Is that more like it?
A third observer could, but might not. Depends on their movement relative to Alice and Bob. If they were in the middle, for example, with Alice and Bob moving away at what the third observer considered the same speed, then yes.
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Old 20th November 2022, 09:42 PM   #99
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Originally Posted by Robin View Post
There are lots of clocks lined up at regular intervals in front of Alice, comoving with her so that Bob passes each as he moves away.

There are lots of clocks lined up behind Bob at regular intervals, comoving with him so that Alice passes each as she moves away.

Initially all clocks show the same time.
Gotcha. Do you see how that description would not have sprung to mind by just using the word "array"? If we maybe toss in those nouns and verbs up front instead of speaking in riddles, we might get somewhere a bit faster.

Quote:
There is no such thing as a magic wire.
a.) I was being facetious. I was not actually suggesting we use ******* magic.
b.) There is no such thing as virtually anything we are talking about, starting with people making breakfast in space traveling at 3/4 the speed of light, followed by a chain of brazillions of clocks (the manufacturing alone of which would probably deplete the natural resources of the planet), Alice and Bob being able to read the clocks passing their windows at said 3/4 the speed of light, and any technology that could actually propel and carry humans at that speed and the inherent interstellar distances.

There is no such thing as any of this ****, ducky.
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Old 20th November 2022, 09:49 PM   #100
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Originally Posted by Robin View Post
Not at all, just pointing out that it is incorrect.
Perception means detecting with the senses. Observing. ******* seeing something. If you think calling it reality is a better word than observing, vaya con dios.

Originally Posted by Robin View Post
That's right. You are just misunderstanding it by insisting time dilation is an illusion.
OK, I've been repetitively clear on this point. Remember how I even explained that I put Thermal at mission control, to demonstrate that he clocked the mission at 10 minutes, but A&B clocked it at 8?

There is no fig leaf to hide behind. You know damn right well I am not denying dilation. On that note, this discussion seems to be about to tip over to the toxic, so I'm gonna shelf it for now, thanks. See ya on the next one.
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Old 20th November 2022, 09:53 PM   #101
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Originally Posted by Lithrael View Post
Ah, ok. So the counterintuitive part is that if you had two arrays of clocks moving past each other near c, a third observer would (ETA: I was thinking I should be more verbose, pzkpfwís now done it for me, ty) see them as reading the same time, and yet each clock pilot would see the other set of clocks running slow. Is that more like it?
Yes.
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Old 20th November 2022, 09:59 PM   #102
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Originally Posted by Thermal View Post
b.) There is no such thing as virtually anything we are talking about, starting with people making breakfast in space traveling at 3/4 the speed of light, followed by a chain of brazillions of clocks (the manufacturing alone of which would probably deplete the natural resources of the planet), Alice and Bob being able to read the clocks passing their windows at said 3/4 the speed of light, and any technology that could actually propel and carry humans at that speed and the inherent interstellar distances.

There is no such thing as any of this ****, ducky.
The experiment is doable just as long as we adjust it to clock signals encoded in binary.

It would require about ten clocks or so to reliably check the trend and it is all within the laws of physics.

Extremely expensive, certainly.
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Old 20th November 2022, 10:11 PM   #103
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Originally Posted by Thermal View Post
There is no fig leaf to hide behind. You know damn right well I am not denying dilation. On that note, this discussion seems to be about to tip over to the toxic, so I'm gonna shelf it for now, thanks. See ya on the next one.
You do sound as though you are denying dilation. For example:

Originally Posted by Thermal
Right, and I get that, but where I am floundering is where their respective 'nows' would be anything other than a warped relativistic perception.
That makes it sound as though you thought that the different "nows" were not real, but just a trick of the perception.

My thought experiment shows that it is not a trick of perception, but very real.

And it has been tested in reality that a clock accelerating away and then towards another clock will have ticked for less time.

I would help me understand your position if you would state whether or not you agree that when Bob reads a clock adjacent to him it shows a different time to his own.

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Old 20th November 2022, 10:30 PM   #104
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Quote:
Originally Posted by Lithrael View Post
Ah, ok. So the counterintuitive part is that if you had two arrays of clocks moving past each other near c, a third observer would (ETA: I was thinking I should be more verbose, pzkpfwís now done it for me, ty) see them as reading the same time, and yet each clock pilot would see the other set of clocks running slow. Is that more like it?
Yes.
Just to clarify, a third observer who's velocity was midway between Bob and Alice in the third observer's inertial frame.

Other third observers would see them as having different time.
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Old 21st November 2022, 03:36 AM   #105
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Originally Posted by pzkpfw View Post
Watch out for terms like "moving at the same speed". Without realising it, you can be assuming there's some underlying single base against which "real speed" can be measured. Say a fourth observer is moving towards Alice in this example. To that fourth observer Alice and Bob are moving at different speeds compared to each other. Who is "right"? Alice? Bob? The third middle observer? This new fourth observer?
I had to think about this for a while.

Actually Alice and Bob are travelling at different velocities for any observer. Things that move with the same velocity as each other will move with the same velocity as each other to all observers.

The key word is velocity (not speed).

If the clocks in front of Alice move with the same velocity as her then they are guaranteed to move with the same velocity as her to any observer.

No single base or "real" velocity is assumed because the velocity is relative.

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Old 21st November 2022, 07:36 AM   #106
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Originally Posted by Thermal View Post
Perception means detecting with the senses. Observing. ******* seeing something. If you think calling it reality is a better word than observing, vaya con dios.
I haven't been following the scenario being debated, but I want to step in here on a semantic issue, which might further cloudy the waters but might potentially help clear things up.

"Observe" has special meaning in relativity. What you observe is NOT what you see with your senses. Observations are idealized measurements, and can be quite different than what you see. I'll give an example.

Let's say you're standing in a field, and you hear a jet plane flying overhead. You look up, and you see the plane. But the sound is coming from a direction behind the plane, not where the plane is. You hear the sound coming from behind the plane.

But you also know that sound travels at a finite speed, and that it took time for the sound to reach you from where it originated. You also know that the plane is moving. Using what you know about the speed of sound, the speed of the jet, and how far away it is, you can figure out that the sound was emitted from the jet itself, not from a point behind the jet. You OBSERVE sound coming from the jet, even though that's not what you hear.

Observations of relativity are kind of like that, but even more idealized. They represent perfect knowledge, from your reference frame. They are reality, even if it's a reference frame dependent reality. Here's another example. Length contraction is an observation, it is not an optical illusion. But length contractions also often NOT what you see. The finite velocity of light, along with different flight times from different parts of an object, create optical illusions which can be much weirder than length contraction. For example, a spherical object moving at relativistic speeds will not look length contracted. Instead, it will look rotated. There's even a specific term for this, Terrell rotation. Two people watching a non-rotating relativistic ball pass between them will see the ball rotated (not dilated) in opposite directions. But they will both still observe that the ball dilated without rotating, and they will agree on how much.

One last example. The twin paradox is often resolved in terms of observations, but you can also solve it using what each twin sees. What they see if they look at the other twin's clock rate is a combination of time dilation (the same from the point of view of each twin) and red (outbound)/blue (inbound) shifts (same magnitudes for each twin). What differs is that for the traveling twin, the earthbound twin's clock switches from red shifted to blue shifted the moment he turns around (halfway through his trip), whereas for the earthbound twin, the traveling twin's clock isn't blue shifted until the signal from the turnaround point reaches him, which is much later than halfway through his stay. The final result is the same, but the process of getting there is a bit different.
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Old 21st November 2022, 10:07 AM   #107
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Originally Posted by Ziggurat View Post
I haven't been following the scenario being debated, but I want to step in here on a semantic issue, which might further cloudy the waters but might potentially help clear things up.

"Observe" has special meaning in relativity. What you observe is NOT what you see with your senses. Observations are idealized measurements, and can be quite different than what you see. I'll give an example.

Let's say you're standing in a field, and you hear a jet plane flying overhead. You look up, and you see the plane. But the sound is coming from a direction behind the plane, not where the plane is. You hear the sound coming from behind the plane.

But you also know that sound travels at a finite speed, and that it took time for the sound to reach you from where it originated. You also know that the plane is moving. Using what you know about the speed of sound, the speed of the jet, and how far away it is, you can figure out that the sound was emitted from the jet itself, not from a point behind the jet. You OBSERVE sound coming from the jet, even though that's not what you hear.
That is pretty much how I was using it, although I was using 'perception'.

The discussion was about two people, A & B, moving away from each other at 3/4 c (accurate math unimportant for the thought experiment). Each perceives themselves at rest, and the other's time as being dilated.

My use of 'perceives' in the above got two posters all upset, because they think it means 'illusion' or 'trick'. I mean it as observe, since they can't really interact with the other referential frame without becoming part of it, so their knowledge is based on distant observation/perception rather than experiencing the direct reality.

Which ties back to the OP question about why FTL would create causality problems. As I see it, the causality problem is that the relative frames can't interact simultaneously, and introducing FTL communication (actually near infinitely FTL information transfer), it forces a shared reference frame/reality that kind of can't exist, logistically.
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Old 21st November 2022, 11:15 AM   #108
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Originally Posted by pzkpfw View Post
Yes, though possibly just a line of clocks along the path of motion.

Imagine you are in a train running down some tracks. You have your own clock moving with you. There's a clock back at the station you want to compare your clock to. The trouble is, that station clock gets further and further away. It gets harder to know what it reads "now" (your "now") - this was an issue with your half-designed video experiment.
eta: just a quick aside: I am not clear on what your problem is with the video thing. It was pretty straightforward: play the videos side by side in the proverbial bar after the dilated flights to compare time passage. You keep asking me which video we are watching. What do you mean?

Quote:
Seeing that clock means you have to wait for light from the clock to reach your eyes, and the further away you get the longer that takes.

So what you do, is before your experiment you stick clocks all along the track, maybe every 100 cm. You synchronise these clocks to the station clock. Since they are at rest with regards to the station, they will stay synchronised.

Now you do your experiment. And when you want to know what time it is according to the station clock, you just have to look out your window at one of the nearby at-rest-with-the-station clocks.



Yes. (ETA: not "connected", just pre-synchronised.) (ETA2: you'd need (depending on exactly what you are trying to show) two separate sets of clocks. One set at rest with Alice, for Bob to look at, and one set at rest with Bob, for Alice to look at. The Bob and Alice clocks are not at rest with each other, because Alice and Bob are not.)
OK, I see at least part of the problem. You and Robin are responding to the same post, with completely different setup descriptions.

Robin has two lines of clocks presyncronized, but 'comoving' with their respective subjects. That would mean each set of clocks were spaced out on a chain brazillians of miles long, and reflecting the dilated time of the one who the chain is connected to, relative to their counterpart.

Yours has stationary clocks, which would reflect only the dilation that the observer experiences, but not their counterpart.

Do I have that right? Are you two using completely different setups, one stationary, and the other 'comoving'? Or do you mean that each could be considered comoving or stationary, if depending on the observer's perspective? Either way, it would be bonzer helpful if we could get on the same example, just for readability.

Quote:
You complained about magic telephones before, now you want magic wire?
No. By describing the clocks as an 'array', with no elaboration, I took a wild guess at WTF that was supposed to mean. My understanding of what you were (not) describing would have necessitated a 'magic wire', which of course I would have objected to, hence the facetious use of 'magic wire'.

I can't believe I have to say this, but: I was not suggesting that we use magic.
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Old 21st November 2022, 11:58 AM   #109
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First a note about reference frames: everything is in every reference frame. Alice and Bob are in the same Universe, they can see each other.
The real point is whether something is at rest with respect to a reference frame. When Alice and Bob are moving with respect to each other, they can't both be at rest with respect to the same reference frame.

Originally Posted by Thermal View Post
OK, I see at least part of the problem. You and Robin are responding to the same post, with completely different descriptions.
I'm trying to use simpler language. I'm very sure Robin knows more than me (I do disagree with some things he's written but don't want to add extra layers to this thread) and is being more formal.

Part of the problem is in trying to re-word things to say them again and again, it's difficult to maintain 100% consistency in the language used over many posts - even for one writer let alone two.

Originally Posted by Thermal View Post
Robin has two lines of clocks presyncronized, but 'comoving' with their respective subjects. That would mean each set of clocks were spaced out on a chain brazillians of miles long, and reflecting the dilated time of the observer relative to their counterpart.

Yours has stationary clocks, which would reflect only the dilation that the observer experiences, but not their counterpart.

Do I have that right? Are you two using completely different setups, one stationary, and the other 'comoving'?
We are actually using the same clocks!

Comoving means moving with something that's (possibly) moving. That means stationary with respect to. (Every time I say "stationary" I try to be very careful to specify who that's with respect to.)

Alice is stationary, she considers Bob to be moving.
Bob is stationary, he considers Alice to be moving.

According to Alice, clocks that are at rest with respect to her are stationary, and clocks that are at rest with respect to Bob are comoving with him.
and
According to Bob, clocks that are at rest with respect to him are stationary, and clocks that are at rest with respect to Alice are comoving with her.
Both sets of clocks can be called "stationary" or "comoving".

In shorthand, you can just say clocks comoving with Alice and clocks comoving with Bob - and leave whether either Alice or Bob is moving or not out of it. Still really the same thing; it just comes down to clocks that stay in sync with Alice and clocks that stay in sync with Bob, that Alice and Bob can both see.


In the post #86 example, where I was trying to lay out the very basics of what the array of clocks gave: the clocks along the tracks are stationary with respect to the tracks and the train station. But according to you, you are still and it's the station that's moving away. The track and the clocks are moving away too. So you could say: the clocks are comoving with the station, or the clocks are stationary with respect to the station.

I was just showing (in that part of post #86) how an array of clocks helps, so didn't show the symetrical scenario - how the station master could know the trains' clock. For that you could have a very long train - with a clock on each wagon. In that case, the clocks are stationary with respect to the train - and you; but the station master (anyone) could say those clocks are comoving with the train. And now, even when the train engine is very very far away the station master can know it's time by looking at the wagon just passing through the station.

Those two sets of clocks are now just like the clocks comoving with Alice and Bob.

Originally Posted by Thermal View Post
No. By describing the clocks as an 'array', with no elaboration, I took a wild guess at WTF that was supposed to mean. My understanding of what you were (not) describing would have necessitated a 'magic wire', which of course I would have objected to, hence the facetious use of 'magic wire'.

I can't believe I have to say this, but: I was not suggesting that we use magic.
It was you who first mentioned magic in this thread (post #30) and actually I don't mind at all the use of magic in a hypothetical. e.g. Olmstead in post #39 wrote "Physicists in relevant fields agree that FTL messages, especially magical instantenous phones, would lead to time paradoxes". All good.



ETA: I think post #79 explained the array of clocks well enough. Please stop and ask for clarification if things are not clear.

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Old 21st November 2022, 12:11 PM   #110
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Originally Posted by Thermal View Post
eta: just a quick aside: I am not clear on what your problem is with the video thing. It was pretty straightforward: play the videos side by side in the proverbial bar after the dilated flights to compare time passage. You keep asking me which video we are watching. What do you mean?
You just say "video" and "compare" but leave out too much detail in between for it to be much use. You are assuming it'll (they'll?) record what you expect.

How many videos are there?
How do you start and stop them?
How do you know the starts and stops are synchronised?
How do you manage the accelerations needed for Alice and Bob to leave and come back? (Are they part of the running of the experiment?)
How do you account for the acceleration effects?
If their time runs at different rates, how do you actually run the videos side by side?
Are they just recording their own clocks, or also the clocks of someone else?
How?
And if so: how do you account for the light travel delay in seeing someone else's clock?
...
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Old 21st November 2022, 12:23 PM   #111
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Originally Posted by pzkpfw View Post
First a note about reference frames: everything is in every reference frame. Alice and Bob are in the same Universe, they can see each other.
The real point is whether something is at rest with respect to a reference frame. When Alice and Bob are moving with respect to each other, they can't both be at rest with respect to the same reference frame.



I'm trying to use simpler language. I'm very sure Robin knows more than me (I do disagree with some things he's written but but don't want to add extra layers to this thread) and is being more formal.

Part of the problem is in trying to re-word things to say them again and again, it's difficult to maintain 100% consistency in the language used over many posts - even for one writer let alone two.



We are actually using the same clocks!

Comoving means moving with something that's moving. That means stationary with respect to.

Alice is stationary, she considers Bob to be moving.
Bob is stationary, he considers Alice to be moving.

According to Alice, clocks that are at rest with respect to her are stationary, and clocks that are at rest with respect to Bob are comoving with him.
and
According to Bob, clocks that are at rest with respect to him are stationary, and clocks that are at rest with respect to Alice are comoving with her.
Both sets of clocks can be called "stationary" or "comoving" - depending on which observers' view you are taking.

In shorthand, you can just say clocks comoving with Alice and clocks comoving with Bob - and leave whether eaither Alice or Bob is moving or not out of it. Still really the same thing; it just comes down to clocks that stay in sync with Alice and clocks that stay in sync with Bob, that Alice and Bob can both see.


In the post #86 example, where I was trying to lay out the very basics of what the array of clocks gave: the clocks along the tracks are stationary with respect to the tracks and the train station. But according to you, you are still and it's the station that's moving away. The track and the clocks are moving away too. So you would say the clocks are comoving with the station.

I was just showing (in that part of post #86) how an array of clocks helps, so didn't show the symetrical scenario - how the station master could know the trains' clock. For that you could have a very long train - with a clock on each wagon. In that case, the clocks are stationary with respect to the train - according to you on the train; but according to the station master those clocks are comoving with the train. And now, even when the train engine is very very far away the station master can know it's time by looking at the wagon just passing through the station.

Those two sets of clocks are now just like the clocks comoving with Alice and Bob.
Yeah, I had eta'ed my post to clarify the 'wouldn't matter' aspect.

I totally get the string of syncronized clocks. You want to know what the time is across a vast distance without being able to experience it directly, because it is too far away. Cool.

But when the description is of stationary clocks, and our subjects are moving away from each other, do you see how that would translate on my end as introducing an objective time frame? Again, when fleshed out, I get where you were going, and agree. Just gets confusing when two wildly different descriptions are put up front when we are trying to explain a setup, that might be interpreted differently than intended.

Quote:
It was you who first mentioned magic in this thread (post #30) and actually I don't mind at all the use of magic in a hypothetical. e.g. Olmstead in post #39 wrote "Physicists in relevant fields agree that FTL messages, especially magical instantenous phones, would lead to time paradoxes". All good.
And I am using 'magic' entirely facetiously, to remind us that 'this can't happen, so be careful with drawing any conclusions from its involvement'.

Quote:
ETA: I think post #79 explained the array of clocks well enough. Please stop and ask for clarification if things are not clear.
It's very clear now, thanks.

Ok: I acknowledge that I am having a lot of trouble spitting out my end. I'm at work, and not giving this the thought it needs. But here is my (I think) basic conundrum:

Alice and Bob would not experience any distortion if they were traveling together. Time would pass the same for them, although much slower relative to Thermal on Earth.

The problem comes when distance increases between them. To me, that implies that vast differences in space create literal separate realities, that because of limitations of the speed of light, cannot be experienced concurrently by different observers, but can only be observed at a distance. That drops a kind of stone wall with the magic IM/telephones, which is why the OP question drops out re causality. Do I have that +/- right?
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Old 21st November 2022, 12:38 PM   #112
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Originally Posted by pzkpfw View Post
You just say "video" and "compare" but leave out too much detail in between for it to be much use. You are assuming it'll (they'll?) record what you expect.

How many videos are there?
How do you start and stop them?
How do you know the starts and stops are synchronised?
How do you manage the accelerations needed for Alice and Bob to leave and come back? (Are they part of the running of the experiment?)
I literally covered every one of those explicitly in my proposal.

Operation MEBE starts on Earth, and has a planned duration of 10 minutes, Earth referential time. A&B take off on their nifty approaching lightspeed ships, with distance increasing between them. They land back on earth. Thermal had put a timestamped camera in each of their ships. We run the two videos side by side.

At this point, my question was 'what would we see?'

I think the first thing was that their timestamps would rea, say, 8 minutes, while Thermal's read 10. But A&B would have perceived time normally, but their duration would have seemed to have been cut short by two minutes. The videos would show each moving in slow motion in Earth's reference frame.

Quote:
How do you account for the acceleration effects?
If their time runs at different rates, how do you actually run the videos side by side?
That's one of the possible answers, not the question. The question is 'what would we expect to see when pushing both play buttons, side by side?

Quote:
Are they just recording their own clocks, or also the clocks of someone else?
How?
Again, answered in the setup description. Just a camera recording and timestamping what was happening and when inside the ship. One in each ship.

Quote:
And if so: how do you account for the light travel delay in seeing someone else's clock?
...
If we are going there, we have the same question applied to y'alls A&B experiment. A hell of a lot more of them, too.
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Old 21st November 2022, 12:39 PM   #113
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Originally Posted by Thermal View Post
...

But here is my (I think) basic conundrum:

Alice and Bob would not experience any distortion if they were traveling together. Time would pass the same for them, although much slower relative to Thermal on Earth.

The problem comes when distance increases between them. To me, that implies that vast differences in space create literal separate realities, that because of limitations of the speed of light, cannot be experienced concurrently by different observers, but can only be observed at a distance. That drops a kind of stone wall with the magic IM/telephones, which is why the OP question drops out re causality. Do I have that +/- right?

Again with "distortion".

That implies there's something "undistorted". No.

Reality for Alice is reality. Reality for Bob is reality. Neither is a distortion. They are just different.


Also noting: the point about distance increasing between them (or decreasing) ... that just means they are not at rest with respect to each other. They are not comoving.

That in turn means their time is not the same, it can't be (for light speed to be the same for them).

It's not about the distance itself. It's not about whether that's a large distance or a small distance. It's just about that distance changing (so they are not at rest with respect to each other).


I might be on an observatory on the Moon.

Seven (!) rockets could zoom past me in seven different directions and speeds. The dangerous pilots could pass me at almost the same time (according to me) with just a gnats wings space between them all; none of that stops them having seven different clock rates (according to me, and to each other).


(Now over vast distances, it's true that "now" becomes kind of meaningless. Alice and Bob might both be drifting in deep space, in voids between different galaxies. They could technically be at rest with respect to each other, their clocks could be ticking at the same rate. But when even light would take many thousands/millions/... of years to get from one to another, what does that even mean? However, that's not really the topic of this thread. ETA: further reading - "light cone".)


--------------------------
ETA2: different scenario to illustrate the "distance" thing ...



Alice and Carol are comoving (at rest with each other) so their clocks will agree. Technically, because of the different angle the Alice-Bob speed won't quite be the same as the Carol-Bob speed, so for Bob the Alice and Carol clocks will be different. (But for the purpose of a thread like this they'd be treated as the same.)

Main point: the Alice-Bob distance being long and the Carol-Bob distance being short, isn't the point.

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Old 21st November 2022, 12:44 PM   #114
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Post #112, after all that then, your videos will add nothing, sorry.

On board the ships clocks run normally at 1 second per second (and video recorders run normally at 24 frames per second). No need to record that, all you'll get is 8 minutes of tape showing a timer get to 8 minutes.

(Actually I still think there's way too much detail missing for the experiment to make sense, but I don't think it helps to keep banging on about it.)

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Old 21st November 2022, 01:46 PM   #115
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Originally Posted by pzkpfw View Post
You just say "video" and "compare" but leave out too much detail in between for it to be much use. You are assuming it'll (they'll?) record what you expect.

How many videos are there?
How do you start and stop them?
How do you know the starts and stops are synchronised?
How do you manage the accelerations needed for Alice and Bob to leave and come back? (Are they part of the running of the experiment?)
How do you account for the acceleration effects?
If their time runs at different rates, how do you actually run the videos side by side?
Are they just recording their own clocks, or also the clocks of someone else?
How?
And if so: how do you account for the light travel delay in seeing someone else's clock?
...
I think it's all moot unless we are assuming that there is an absolute time that governs the internal clock of a video camera.

The camera filming Alice will operate on Alice's time and the camera filming Bob will operate on Bob's time. So putting the videos side by side will tell you nothing about time dilation.
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Old 21st November 2022, 03:03 PM   #116
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So why don't we give it enough detail to be explicit? I will chose an inertial frame in which Alice is stationary. There is a clock stationary in that frame too, located 2.45 light seconds to the left of Alice.

All clocks are synchronised at t=0s in Alice's frame.

At t=0 Alice and Bob are at x=0 and the clock is at x=-2.649. The velocity of Alice, her clock and the second clock are v=0. The velocity of Bob and his clock are v=0.6641c.

So the question is, when Bob comes adjacent to the clock which is stationary in Alice's frame, is the time the same as his clock, or different?




And we can graph this as:

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Old 21st November 2022, 04:15 PM   #117
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The answer is that when Bob comes adjacent to the clock that is synchronised to Alice's clock and comoving with Alice then that clock will read 4s and Bob's clock will read 3s.

This often seems the wrong way around for many people. But if we have the clock comoving with Bob then when Alice comes adjacent to it, it will read 4s and hers will read 3s.

This seems to me to be the nub of the problem. In relativity there is no objective "simultaneous" so if we had instantaneous or near instantaneous communication then whose "instantaneous" would it be?

Would we see the egg still boiling when we check at three minutes, or would we see it having finished boiling a minute ago?


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Old 21st November 2022, 10:24 PM   #118
Lithrael
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That is buck wild. Thanks.
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Old 22nd November 2022, 08:03 AM   #119
Myriad
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You're riding a train towards a platform that's a light-year long. All along the platform are clocks that are synchronized in the reference frame in which the platform is stationary.

Suppose the train is moving at a significant fraction of c. The Lorentz factor corresponding to your velocity will be some number greater than 1. Let's say it's 2 (meaning the train's velocity relative to the platform is .866c). That means you will observe the length of the platform being only half a light year, and you will observe the clocks along the platform all running at only half speed.

Note that you will also observe the clocks along the platform are not synchronized in your reference frame. You will measure your trip from one end of the platform to the other using your own on-board clock as taking (length of the platform/relative speed of the platform) =.5 / .866 = .577 of a year. The first and last platform clocks as you pass them will read an interval of (length of the platform/relative speed of the train) = 1 / .866 = 1.155 years. In other words, you will observe them recording twice as much elapsed time even though they're running at half the speed. How to account for this discrepancy? Because you observe the platform clocks as not being synchronized with one another.

These relationships hold up for any velocity below c. So we can imagine the train running at such a speed that in the reference frame stationary with respect to you, riding the train, the trip from end to end of the platform only takes a minute. Can you break the laws of causality somehow with this? For instance, can you read a message posted at the near end of the platform, then repeat it to someone at the far end of the platform a light year away a minute later, thus transmitting information faster than light? No, because in your stationary-train reference frame the platform must be a little less than a light-second long (so you're not conveying the information more than a light-second in a second) and in the stationary-platform reference frame your elapsed time from one end of the platform to the other must still be a little longer than a year (so you're not conveying the information more than a light-year in a year).

Now, suppose we leave known physical possibility behind and accelerate your train all the way up to c. What does the mathematics of the Lorentz transformation say will happen when you reach the platform? Calculating the Lorentz factor gamma in the usual way ends up dividing by zero, but we can easily calculate its reciprocal and use that; it goes to zero. So in your train-stationary frame the length of the platform is now zero, and the clocks on the platform aren't moving at all. (Similarly, in the platform-stationary frame the length of the train is zero and the train's clocks aren't moving at all, but we don't need to focus on that.) You pass all the platform's clocks simultaneously, with them all superimposed on one another but reading all different times (remember how those clocks were never synchronized with one another when you were moving relative to them). You are seeing all different times all at once in one place for one moment. Essentially, your train-stationary frame is perpendicular to platform time.

Can you use that seeming observational superpower to break any laws of physics that you're not already breaking by having accelerated to c in the first place? Well, it's not clear. You can "see" information from the platform's entire history in one moment, but can you emit any signal at some particular platform-time so as to carry information back in platform-time? It doesn't seem plausible. All the numbers are zeros and infinities (for instance, you have zero time in your own frame to record any observation or make any decision regarding what to try to communicate). From the platform's frame it appears whatever information you're trying to emit, you must "always" be emitting that information.

So, what does the math of the Lorentz factor say should happen if we get the train going faster than c? Well, the trend suggests that the platform should keep getting shorter and the platform clocks should keep getting slower (as well as farther out of sync). But the platform can't get shorter than zero and the clocks can't go slower than stopped. Or can they?

What happens in the calculation is that if v>c the Lorentz factor becomes an imaginary number. There are various interpretations of what this implies, the most common being that it's confirming that velocities exceeding c are impossible, which we already knew. But if we disregard that, extrapolating from the behavior at velocities below and approaching c, it appears we have to interpret "length shorter than 0" as the platform having a negative length and "slower than stopped" as the platform clocks moving in reverse. Reading the imaginary Lorentz factors as negative numbers.

So from the ftl train, what you observe would be that you (somehow!) arrive at the far end of the platform first, and the platform's clocks are running backwards.

From the platform's point of view, you're still traveling in the "normal" direction of motion, near end to far end, just faster than light. So for instance they would observe you pass the near end of the platform, then arrive at the far end of the platform two months later (at 6x light speed) -- though it would take another year of platform time to confirm your arrival at the far end, back to a person at the near end. They might think of giving you a message as you pass, to carry to the far end at faster than light, but there's a problem: when you on the train receive that message, in your timeline you've already passed the far end. You can't carry a message that direction; you're going the other way. But what you can do is worse, from a causality violation point of view: you can deliver them message from the far end, from when you passed it in the platform's future.

Here's a scenario that demonstrates a causality violation directly. Your 6c train is scheduled to pass the platform at a certain time. At the expected time, it's observed passing the near end, and at the same platform time, the tracks in the center of the platform are blown up. (There's no need to pass any time-critical messages along the platform for this. Everything is scheduled in advance.) The train cannot possibly continue to the far end. But for you on the train, you reach the far end first, regardless of the condition of the tracks in the middle of the platform which in your timeline you haven't reached yet. So is the train ever observed at the platform's far end or not? Does the plan to destroy the tracks somehow prevent you from beginning the trip at all? At this point we're in the realm of conventional SF time travel questions.
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Last edited by Myriad; 22nd November 2022 at 08:05 AM.
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Old 22nd November 2022, 02:34 PM   #120
Lithrael
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If I was writing the sci-fi Iíd have the train arrive (because the trainís reality had it complete the journey) and also have the tracks explode while the train is on its journey (because the stationís reality had the tracks go first.) Then Iíd just have to decide if that splits off two realities that donít interact, or two that do, or if thereís just a weird effects sequence where the train both arrives and is derailed and everybody is very cross about the inconvenience this creates.

You get off your train and are informed that in fact you and the rest of the train got mangled in a 6c trainwreck a month ago! Well if that doesnít just take the biscuit. But at least theyíve had time to clean the smears up.

Yeah, the apparent arbitrary nature of even your story choices at that point make it more obvious that, absent some humongous new facts about reality, these situations are not just impossible but also incoherent. Like, the possibility of arbitrary action + causality + ftl = break law of conservation? Or I guess to make it simpler you could just have it be impossible to carry out any such actions (ie the plan to blow the tracks makes the journey impossible.)
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