Paradox? (split from Is FTL actually possible?)

[Genady]

1 hour ago, TheVat said:

they do not lead to causal paradoxa.

How come?

Let's say events A and B are spacelike separated. Then there are observers moving with a normal, i.e., < c, speed for whom event B occurs before event A. If A is sending a signal and B is its receiving, then for these observers, the signal is received before it is sent. Isn't it a causal paradox?  

[swansont]

1 hour ago, Genady said:

How come?

Let's say events A and B are spacelike separated. Then there are observers moving with a normal, i.e., < c, speed for whom event B occurs before event A. If A is sending a signal and B is its receiving, then for these observers, the signal is received before it is sent. Isn't it a causal paradox?  

The argument presented is that under GR no signal is sent backward in time. The paradox is only present in SR

[Genady]

1 hour ago, swansont said:

The argument presented is that under GR no signal is sent backward in time. The paradox is only present in SR

In the paradox I've described, no signal is sent backward in time.

[swansont]

18 minutes ago, Genady said:

In the paradox I've described, no signal is sent backward in time.

Where is the causality violation?

[Genady]

1 minute ago, swansont said:

Where is the causality violation?

Thy signal is received before it is sent. 

[swansont]

1 minute ago, Genady said:

Thy signal is received before it is sent. 

Not according to the person who received the signal.

[Genady]

1 minute ago, swansont said:

Not according to the person who received the signal.

If that person received the signal while moving, yes, according to this person as well.

[swansont]

58 minutes ago, Genady said:

If that person received the signal while moving, yes, according to this person as well.

How so? 

Relativity of simultaneity says that two independent events can have different orders, but that’s because the signal travels at c. Getting the signal you describe only involves one event, and in no case does it arrive before it was sent.

[Genady]

Let's say some signal moves with twice the speed of light in observer A's frame. It is sent, the event S, at time t_s=0 from the origin x_s=0. At the time t_r=1 it arrives at the distance coordinate x_r=2, where a detector blows a bomb. This is event R. 

Observer B moves with the speed v=3/5 c along the x axis, with their x axes and origins coincide. In the B's frame, the event S occurs at t_s'=0, x_s'=0. The event R in his frame occurs at 

t_r' = gamma*(t_r - v*x_r/c²) = 5/4 * (1 - 6/5) = -1/4

i.e., 1/4 before the event S.

Edited April 8, 2023 by Genady
some detector, not necessarily photo

[swansont]

1 hour ago, Genady said:

Let's say some signal moves with twice the speed of light in observer A's frame

You didn’t specify this before. Let’s not add new things.

Regardless, no matter the speed, it takes time for the signal to get from source to target.

[Genady]

2 minutes ago, swansont said:

You didn’t specify this before. Let’s not add new things.

Regardless, no matter the speed, it takes time for the signal to get from source to target.

Yes, it took time 1 in the A's frame: the signal left the source at time t_s=0 and arrived at the target at time t_r=1.

[swansont]

55 minutes ago, Genady said:

Yes, it took time 1 in the A's frame: the signal left the source at time t_s=0 and arrived at the target at time t_r=1.

And if that signal went to any other observer, it also took time. 

[Genady]

6 minutes ago, swansont said:

And if that signal went to any other observer, it also took time. 

The Lorentz transformation gives times of the same two events in another observer's frame knowing their times and coordinates in the first observer's frame.

[MigL]

Let me see if I understand this correctly.

There is already a causality violation

3 hours ago, Genady said:

Let's say some signal moves with twice the speed of light in observer A's frame.

and you wish to know if causality is violated ???

Edited April 9, 2023 by MigL

[Genady]

23 minutes ago, MigL said:

Let me see if I understand this correctly.

There is already a causality violation

and you wish to know if causality is violated ???

No, I don't. I know that causality is violated. Assuming that a 'signal' moves faster than light leads to violation of causality. This is my whole point. See the OP here: https://www.scienceforums.net/topic/130520-paradox-split-from-is-ftl-actually-possible/

 

[MigL]

I see.

I do believe TheVat's comment which started this thread was referencing Sabine Hossenfelder's video, and her explanatiom of the effect of co-moving frames on direction of time.
This effect is absent in SR.
Co-moving distances and times are closely related to proper distances and times.
Her argument, that communicatng with Andromeda with a FTL transmission, then having the Andromedans communicate back to you, also FTL, so you receive the communication before you sent the first, would not be possible as the second communication must also originate with you and be sent to Andromeda.

Watch the video posted by Moontanman in the other thread, starting at about 16 min to 20 min for S Hossenfelder's explanation.

 

Edited April 9, 2023 by MigL

[Genady]

7 minutes ago, MigL said:

I see.

I do believe TheVat's comment which started this thread was referencing Sabine Hossenfelder's video, and her explanatiom of the effect of co-moving frames on direction of time.
This effect is absent in SR.
Co-moving distances and times are closely related to proper distances and times.
Her argument, that communicatng with Andromeda with a FTL transmission, then having the Andromedans communicate back to you, also FTL, so you receive the communication before you sent the first, would not be possible as the second communication must also originate with you and be sent to Andromeda.

 

Yes, you're right. That was how it started. However, I did not discuss her claims as I did not see the video. (Perhaps this led to this thread being split.) I wanted to point out that causality violation appears in SR as a result of FTL also without a signal being sent back in time.

[dimreepr]

Is a paradox actually possible, outside of our brain?

[swansont]

9 hours ago, Genady said:

No, I don't. I know that causality is violated. Assuming that a 'signal' moves faster than light leads to violation of causality. This is my whole point. See the OP here: https://www.scienceforums.net/topic/130520-paradox-split-from-is-ftl-actually-possible/

 

But that discusses a different scenario, in which SR can’t be applied. One of the points was that we are always in a state where you must use GR. So this is moot; we already know causality is violated in a FTL situation where SR applies. It doesn’t rebut the GR scenario.

 

8 hours ago, Genady said:

Yes, you're right. That was how it started. However, I did not discuss her claims as I did not see the video. (Perhaps this led to this thread being split.)

Yes, and it underscores the requirement that a summary needs to be included when someone posts a video.

[Genady]

25 minutes ago, dimreepr said:

Is a paradox actually possible, outside of our brain?

If you write it down, it exists on paper, which is outside of our brain.

[dimreepr]

24 minutes ago, Genady said:

If you write it down, it exists on paper, which is outside of our brain.

Does that mean it's possible? 

[Genady]

1 hour ago, dimreepr said:

Does that mean it's possible? 

Yes, it does.

[TheVat]

11 hours ago, MigL said:

I do believe TheVat's comment which started this thread was referencing Sabine Hossenfelder's video,

No, I quoted a clip from her blog.

http://backreaction.blogspot.com/2015/06/does-faster-than-light-travel-lead-to.html

Not clear on why this led to a thread split, given that the question of paradox seems pertinent to FTL travel.  

[Moontanman]

Why is causality violation such a deal breaker? Let's set up a real world example:

Assume we have a "phone" that allows for communication with the past but only between the point in time that the "phone" is created and the present. An asteroid strikes the ocean off the east coast of North America resulting in a wave that wipes out much of the east coast from the Appalachian Mountains to the sea coast.  Millions of lives are lost and property destruction is off the charts in cost. A simple "phone" call allows for the evacuation of the affected area and no lives are lost. 

Does this break causality? Does the universe "break"? Or is this no different than from warning people of an approaching hurricane and taking measures to avoid loss of human lives?  

[Genady]

23 minutes ago, Moontanman said:

Does this break causality?

Yes, it does. The electrons in your "phone", in the past, start moving without a physical cause.