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# FTL thought experiment

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During this grueling time of self quarantining I've been trying to practice a little mental gymnastics everyday and I think I've come up with a small paradox. Well for me anyway.

Let's say we have two stars 10 light years apart, star A and B, you have a space ship with 'magical technology" that allows you travel 5 light years in an instant. So you get in your space ship at Star A instantly you are 5 LYs from both stars, I'll call that point C, looking back at star A you see light that is 5 years in star A's past but the light from star B is 5 years in star A's future. The reverse would also be true. Star B's light you see is 5 years in star B's past but still in 5 years in star A's future so you would be 5 years in both star's past and future... right?

Am I wrong or right or neither?

Crap I should have put this in speculations...

Edited by Moontanman
wrong place
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45 minutes ago, Moontanman said:

Am I wrong or right or neither?

Yes, no and it depends. Problem is that when instant travel or instant exchange of information is introduced into accepted theories of relativity the theories does not predict what is supposed to happen. I think the thought experiment is interesting as a way to see how and why problems arise.

As far as I can tell: Lets remove the magic from the post it and assume that A, B and C are not moving relative to one another. Then your description seems to match what observers, stationary at A, B and C, would be calculating using mainstream theories. Observations from C, using a telescope to look at a synchronised clocks located at the other locations (A and B) seems to fit your description.

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When you arrive at point C, you will see the same light coming from both stars as someone who never moved from point C; Light that left both stars 5 yrs ago.  You see both stars as they were 5 yrs ago.

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56 minutes ago, Moontanman said:

During this grueling time of self quarantining I've been trying to practice a little mental gymnastics everyday and I think I've come up with a small paradox. Well for me anyway.

Let's say we have two stars 10 light years apart, star A and B, you have a space ship with 'magical technology" that allows you travel 5 light years in an instant. So you get in your space ship at Star A instantly you are 5 LYs from both stars, I'll call that point C, looking back at star A you see light that is 5 years in star A's past but the light from star B is 5 years in star A's future. The reverse would also be true. Star B's light you see is 5 years in star B's past but still in 5 years in star A's future so you would be 5 years in both star's past and future... right?

Am I wrong or right or neither?

Crap I should have put this in speculations...

Even without much time (or energy) to think, it sounds about right to me. And the reason is that you're going to a point outside the light cone of both departure points. That is known technically as "outside the causal cone." The future light cone of event "departure from A" is seen as the future of "departure from A" by all inertial observers. Analogously respect to the past. But areas outside, like the point event you're proposing, are neither past nor future. Some inertial observers will see them as future of either A or B; others will see it as past.

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1 hour ago, Moontanman said:

Crap I should have put this in speculations...

That's not for me to decide but as long as you don't claim that the "magic" exists I guess several aspects of the thought experiment fits in the mainstream sections.

One way I think that paradoxes could arise in your experiment is due to relativity of simultaneity. If we had observers in relative movement of one another and they also could instantaneously jump 5 lightyears (not possible of course) that seem to allow time travel paradoxes. For instance combining Andromeda Paradox* and (impossible) instant travel and relative movement would allow observers to jump and arrive at different times and interact with, and cause events that are not allowed in theories of relativity.

Edited by Ghideon
clarified
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2 hours ago, Moontanman said:

During this grueling time of self quarantining I've been trying to practice a little mental gymnastics everyday and I think I've come up with a small paradox. Well for me anyway.

Let's say we have two stars 10 light years apart, star A and B, you have a space ship with 'magical technology" that allows you travel 5 light years in an instant. So you get in your space ship at Star A instantly you are 5 LYs from both stars, I'll call that point C, looking back at star A you see light that is 5 years in star A's past but the light from star B is 5 years in star A's future. The reverse would also be true. Star B's light you see is 5 years in star B's past but still in 5 years in star A's future so you would be 5 years in both star's past and future... right?

Am I wrong or right or neither?

Crap I should have put this in speculations...

Why do you need “magical technology” for this scenario? Seems like you can just ask this question about a point midway between two stars 10 LY apart.

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4 hours ago, swansont said:

Why do you need “magical technology” for this scenario? Seems like you can just ask this question about a point midway between two stars 10 LY apart.

The point I was trying to make was did the spacecraft travel in time due to traveling faster than light and which star was it traveling in time relation too...

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22 minutes ago, Moontanman said:

The point I was trying to make was did the spacecraft travel in time due to traveling faster than light and which star was it traveling in time relation too...

Neither.

You moved (impossibly) on the "grid", but as described you simply did it all in the same frame . Points A, C, and B are all in the same frame and you moved spatially only in that frame. You would have travelled forward or backward in time in some other frames, but not in that one.

So you travelled in time relative to neither star.

Edited by J.C.MacSwell
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42 minutes ago, Moontanman said:

The point I was trying to make was did the spacecraft travel in time due to traveling faster than light and which star was it traveling in time relation too...

If you break the laws of physics, you can’t use the laws of physics to tell you what happens.

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24 minutes ago, swansont said:

If you break the laws of physics, you can’t use the laws of physics to tell you what happens.

For thought experiments you can break some, and maintain others, even sometimes without running into a logical contradiction.

Not that reality would agree with the conclusions...

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17 hours ago, J.C.MacSwell said:

You moved (impossibly) on the "grid", but as described you simply did it all in the same frame . Points A, C, and B are all in the same frame and you moved spatially only in that frame. You would have travelled forward or backward in time in some other frames, but not in that one.

This relates also to our other discussion regarding wormholes.
All spatial points are separated by time also; the person next to you ( assuming social distancing ) is about 6.6 ns in the past.
If you travel 'faster than light' through those two meters, you travel faster than causality, and are in effect travelling into the 'future'.
( to be clear, the 'future' of the person standing 2 m away from you, not your own 'future' )

The effect is more clear if you draw light cones.

Edited by MigL
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43 minutes ago, MigL said:

This relates also to our other discussion regarding wormholes.
All spatial points are separated by time also; the person next to you ( assuming social distancing ) is about 6.6 ns in the past.
If you travel 'faster than light' through those two meters, you travel faster than causality, and are in effect travelling into the 'future'.
( to be clear, the 'future' of the person standing 2 m away from you, not your own 'future' )

The effect is more clear if you draw light cones.

I don't think that is correct. Assuming you are stationary wrt each other you will agree on simultaneity and what is the present in that frame, even if spatially separated. The lag due to the limitations of lightspeed doesn't change that agreement.

If you view a star that's stationary wrt you 5 light years away you see it 5 years in the past (and they you), but you both agree on what the present is (despite your inability to see the other's current present for another 5 years)

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posted by mistake, I'm not in this thread.

Edited by studiot
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53 minutes ago, J.C.MacSwell said:

If you view a star that's stationary wrt you 5 light years away you see it 5 years in the past (and they you), but you both agree on what the present is (despite your inability to see the other's current present for another 5 years)

And a 'wormhole, or any other means of FTL motion ( however improbable, just for discussion sake ) allows you to break causality.
IOW 'see' things before they 'happen'.

You are trying to apply physical rules to an un-physical ( FTL travel ) situation.
I'm only telling you what that un-physical situation seems to imply.

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2 hours ago, MigL said:

And a 'wormhole, or any other means of FTL motion ( however improbable, just for discussion sake ) allows you to break causality.
IOW 'see' things before they 'happen'.

You are trying to apply physical rules to an un-physical ( FTL travel ) situation.
I'm only telling you what that un-physical situation seems to imply.

I disagree. J.C.MacSwell has merely described the situation that others set up, in real terms like frames of reference, without any assumption about how that situation was arrived at. No false claims were made. Your idea of "traveling into the future of a person" doesn't make sense to me in any of the frames mentioned (which events are you comparing, and in which frame? If the events are in the same place (star B), it seems to describe only events in a mutual present).

I don't see any paradoxes mentioned yet, but one could be built from the situation. In the frame of reference of an object moving slower than c in the direction from A to B, the ship arrives at B before it leaves A, which I don't think is itself a paradox, but can lead to one.

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4 hours ago, MigL said:

You are trying to apply physical rules to an un-physical ( FTL travel ) situation.
I'm only telling you what that un-physical situation seems to imply.

That post was with regard to your seeming to claim that "All spatial points are separated by time also".

I don't think standard physics would agree with that.

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5 hours ago, md65536 said:

Your idea of "traveling into the future of a person" doesn't make sense to me in any of the frames mentioned (which events are you comparing, and in which frame?

and

4 hours ago, J.C.MacSwell said:

That post was with regard to your seeming to claim that "All spatial points are separated by time also".

I could have worded that better, JC, but very well, I'll describe a situation, and then you and md can try and rationalize it.

You guys are both aliens on a planet P, 100 LY away, looking at the Earth through a powerful telescope.
You see the Earth of 1920 through that telescope.
You step through a 'wormhole, where time is local at both ends, and instantly come to Earth.
On Earth, you read a History book that outlines the rise of A Hitler, WW2, the rise of Communism, the Korean and Vietnam wars and the election of a game show host to the US Presidency; then you step back through the wormhole and instantaneously go back to planet P.
You look through your telescope at the Earth again, and, for the next hundred years, you see the rise of A Hitler, WW2, the rise of Communism, the Korean and Vietnam wars and the election of a game show host to the US Presidency.
What are you seeing ?

Edited by MigL
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1 hour ago, MigL said:

You guys are both aliens on a planet P, 100 LY away, looking at the Earth through a powerful telescope.You see the Earth of 1920 through that telescope.

You step through a 'wormhole, where time is local at both ends, and instantly come to Earth.
On Earth, you read a History book that outlines the rise of A Hitler, WW2, the rise of Communism, the Korean and Vietnam wars and the election of a game show host to the US Presidency; then you step back through the wormhole and instantaneously go back to planet P.
You look through your telescope at the Earth again, and, for the next hundred years, you see the rise of A Hitler, WW2, the rise of Communism, the Korean and Vietnam wars and the election of a game show host to the US Presidency.
What are you seeing ?

This doesn't sound like a scientific argument. Besides, you haven't described anything that you're seeing that is at all different than if you'd never stepped through the wormhole. You're seeing light from about a hundred years of Earth's worldline arrive at planet P.

Why describe it as a story? Why not speak of events and light cones, etc? Why not use defined scientific terms? Your story doesn't help at all explain the meaning of "[you] are in effect travelling into the 'future' [...] of the person standing 2 m away from you". I doubt you could explain the meaning in that, because it seems meaningless.

Can you point out a specific error in what J.C.MacSwell wrote? You dismissed it, but I don't see any error in it.

Edited by md65536
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3 hours ago, MigL said:

and

I could have worded that better, JC, but very well, I'll describe a situation, and then you and md can try and rationalize it.

You guys are both aliens on a planet P, 100 LY away, looking at the Earth through a powerful telescope.
You see the Earth of 1920 through that telescope.
You step through a 'wormhole, where time is local at both ends, and instantly come to Earth.
On Earth, you read a History book that outlines the rise of A Hitler, WW2, the rise of Communism, the Korean and Vietnam wars and the election of a game show host to the US Presidency; then you step back through the wormhole and instantaneously go back to planet P.
You look through your telescope at the Earth again, and, for the next hundred years, you see the rise of A Hitler, WW2, the rise of Communism, the Korean and Vietnam wars and the election of a game show host to the US Presidency.
What are you seeing ?

What I see:

Assuming my planet is stationary wrt Earth, just displaced by 100 ly distance:

I see what I would consider an impossible (wormhole hypotheses  aside) FTL spatial displacement of 100 ly, both to Earth and later back to my planet, but steady time lapse in my frame, which I share with Earth.

WRT some other frames I would travel forward and then backward in time, or backward and then forward in time, depending on how the frames are moving wrt mine,

What I don't see in what you described, but could using instantaneous travel:

If I could actually do what is suggested then I could presumably ratchet myself back or forward in time in my planet/Earth frame, say arrive back before I even left, by switching to and making use of other frames and doing the same instantaneous travel. I will not have done that in what you described, but with similar extra steps using the rules of physics as we know it and the extra "superpower" of instantaneous travel in any frame I'm in...then I could. I could go back and kill my grandfather, or even duplicate myself multiple times over and form JC ARMY.

If only allowed instantaneous communication I would need you and a few others, spatially separated on different frames, to go into business as Galactica  Stock Tip Inc.

Edited by J.C.MacSwell
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On 10/31/2020 at 9:19 AM, Moontanman said:

I think I've come up with a small paradox. Well for me anyway.

Let's say we have two stars 10 light years apart, star A and B, you have a space ship with 'magical technology" that allows you travel 5 light years in an instant.

After thinking about this, I think this is not a paradox. Instead of "space ship" you could have said "tachyon". You haven't described the ship doing anything that would break causality or do anything paradoxical. If by "you travel" you mean literally a person, then you can come up with something paradoxical, but it's not a paradox yet.

(Edit: As for the rest of what you wrote after the above, I don't think it's right. Especially anything that adds a paradox, the paradox seems to come from an incorrect description.)

Relativity doesn't specifically disallow nor predict the possibility of faster-than-light particles. Or wormholes for that matter. You can't accelerate something massive to the speed of light or faster, but if something's traveling faster than light already, it doesn't break anything. You can't use FTL to transmit information, because then you could break causality, but you haven't described any information transmitted, or anything else that would be a paradox.

I think we'd all (anyone here talking about relativity) agree that if something doesn't agree with "common sense" or intuition, that doesn't make it a paradox. But then, when you describe something that "common sense" says disagrees with relativity, such as something traveling faster than light, we tend to jump to the conclusion that it does, along with any assumptions needed (eg. I might assume you're talking about accelerating a person from rest). There's a difference between a common sense paradox like this, and an actual paradox that's not theoretically possible.

Edited by md65536
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I think I'm guilty of introducing an element of confusion here. You can set up Moontanman's though experiment with pretty much everything at rest. With an equidistant point C waiting for the signals to arrive, as Swansont said, I think. Then someone pointed out the relevance of question 'how do you define synchronicity between departures?' or something to that effect. I think that's key, because if C is at rest and planet A and B synchronize their clocks so as to depart at the same inertial time, there is no paradox. Paradoxes would appear if observers passed through C with different constant velocities, because in that case they would record A in the past of B or viceversa depending on the relative motion.

I'm trying very hard not to be sloppy and not saying anything already covered. but I will have to look into this in more detail later.

Well, paradoxes... You know what I mean.

2 minutes ago, joigus said:

Then someone pointed out the relevance of question 'how do you define synchronicity between departures?'

Yeah, I think it was Ghideon when mentioning 'relativity of simultaneity'.

So the scenario in the simplest possible case is like Janus describes, I think. Everything is quite symmetrical and there is no paradox, but A and B must sync their clocks, and C musn't move with respect to them.

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6 hours ago, md65536 said:

You can't use FTL to transmit information, because then you could break causality, but you haven't described any information transmitted, or anything else that would be a paradox.

An observer who teleported instantly would have information from their original location that would not be available to an observer at point C for 5 hours. No specific information was listed, but that's because of the vague description of the scenario. It certainly exists.

As you say, you can certainly build a paradox (or causality-violating scenario) from this. You could envision the observer at A seeing a display describing the status of something. They teleport, knowing that condition plaid has just been put into effect at A. C should not know this for 5 years.

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On 10/31/2020 at 11:19 AM, Moontanman said:

During this grueling time of self quarantining I've been trying to practice a little mental gymnastics everyday and I think I've come up with a small paradox. Well for me anyway.

Let's say we have two stars 10 light years apart, star A and B, you have a space ship with 'magical technology" that allows you travel 5 light years in an instant. So you get in your space ship at Star A instantly you are 5 LYs from both stars, I'll call that point C, looking back at star A you see light that is 5 years in star A's past but the light from star B is 5 years in star A's future. The reverse would also be true. Star B's light you see is 5 years in star B's past but still in 5 years in star A's future so you would be 5 years in both star's past and future... right?

Am I wrong or right or neither?

Crap I should have put this in speculations...

Okay I haven't actually read the whole thread because I got caught up in trying to understand the post as presented. This is the confusing part.

On 10/31/2020 at 11:19 AM, Moontanman said:

I'll call that point C, looking back at star A you see light that is 5 years in star A's past but the light from star B is 5 years in star A's future.

If at point C you are 5 years in star A's past and you are presumably between Point A and point B it is seemingly a paradox. But if you are seeing light five ly's in A's past and B is five years in A's future it seems more like an equal radius than a paradox.

But, you start of by saying

On 10/31/2020 at 11:19 AM, Moontanman said:

Let's say we have two stars 10 light years apart, star A and B

Then you say

On 10/31/2020 at 11:19 AM, Moontanman said:

the light from star B is 5 years in star A's future

Is it 10 light years, or five? Hmm, seems more like a thought exercise, than a paradox... Personally, this depth of thought has placed me in danger of drowning in possibilities. So, I'm gonna stop, take a nap, then come back and read the whole thread and see if there is anything that explains my confusion. 🤔😊

Didn't even get the chance to take a nap, but now I assume that any light I observed from point C is technically five light years distant from point A. But that's another discussion. Or, is it. Might be back to the equal radius thought?

Edited by jajrussel
I seem to have problems with the keyboard output matching the thought input.
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9 hours ago, md65536 said:

This doesn't sound like a scientific argument.

We agree that it isn't scientific.
Relativity allows for certain effects, but they seem to be un-physical.
FTL motion, whether through wormholes, warp drives, or magic, allows for the superluminal transfer of information.
( in my example, reading Earth's history of the last 100 years, before seeing it unfold )

This causality break is un-physical.
Pointing out the un-scientific and un-physical effects ( time travel ) of an un-physical initial cause ( FTL travel ) is kind of redundant.
If I said that a massive object travelling at the speed of light has infinite energy, would you say that it's impossible for anything to have infinite energy ?
Or would you realize that an impossible cause leads to an impossible effect, and it is the travel at c that is impossible ?

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On 10/31/2020 at 12:19 PM, Janus said:

When you arrive at point C, you will see the same light coming from both stars as someone who never moved from point C; Light that left both stars 5 yrs ago.  You see both stars as they were 5 yrs ago.

I should have read the whole thread first.

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