kamenjar

26 minutes ago, swansont said:

Which is not how causality works.

From Wikipedia:

Quote

In Einstein's theory of special relativity, causality means that an effect can not occur from a cause that is not in the back (past) light cone of that event. Similarly, a cause can not have an effect outside its front (future) light cone. These restrictions are consistent with the grounded belief (or assumption) that causal influences cannot travel faster than the speed of light and/or backwards in time.

Here we already have an assumption (a thought experiment) that the event of using superluminal communication violates this principle. This is how the universe works and I accept that. There is a reason of course why Einstein's predictions have been proven to be correct - because they are. We have not figured out and probably never will figure out how to communicate/travel FTL. However, I just prompted this discussion by my lack of understanding of what is it about it that makes FTL communication thought experiments claim that  FTL communication "sends events to the future". I guess I have my answer now. Well if future is defined by the light cones, then of course it invalidates causality as it is defined! What I'm saying, I guess, is that it seems to me that if we talk FTL communication/travel we can figure out if alternative ways to define "future" and "simultaneity" in some coherent way are possible. I was hoping bright minds on this forum could help.

I am just using a thought experiment that violates this principle, but I don't see how it violates logic.

I also realize that this topic is in the wrong forum

2 hours ago, MigL said:

I throw a baseball at you faster than the speed of light.
Because light takes a certain amount of time to reach my eyes, you have already been hit, and have fallen on the ground, even though I still see you standing. You are, then, no longer where I aimed the baseball.
So you didn't get hit by the baseball.
But then you wouldn't have fallen.
So you did get hit.
And on. and on, and on...

It is a paradox, like time travel, which yields results at odds with each other, where there should only be one result for causality to hold.
( as Swansont has already explained )

 

First of all, you have no proof that I existed at the time when you threw the superluminal ball. You just saw me standing and assumed that I existed.
The only proof or disproof of my existence that you can have is in the future, when looking at t0=time when you threw the ball.
Therefore you can not make any assumptions about hitting me in the first place, until you either get the light form an event of me falling, or light from an event of me dodging your ball.
I don't see a paradox there.

1 hour ago, Janus said:

Here's how the causality reversal could happen...

And thanks Janus for the detailed description. I'll try to comprehend it better,  but in my (inverted) mind, the diagram looks flawed. In the first line, it shows both and earth AC at year 3000, which is fine -- you can choose any way you want to synchronize clocks -- But do you mean that in the first row a) GPS-like -- AC is seeing light from earth that is 4 years old (form 2996 celebrations) and the same way for Earth (At time of earth label 3000 it is celebrating year 3000 and seeing celebrations form Proxima of year 2996), or b) is set to Earth's light flow reference (frame) Earth sees AC's celebrations of year 2992 and AC sees celebrations of year 3000 from earth.

Now again, you can choose any convention you want, but in case of:

a) Why wouldn't the leading ship celebrate the year 3000 at the "same time" (now "same time" is a problem to define for sure), and set its label to 3000, so it celebrates at the same time as planets? If years are "synchronized" (ala GPS), why would any of the ship's clocks read other than 3000 GPS time (like we do on Earth).

b) The time at AC is set wrong because if the leading ship is seeing the light from earth from 2998.3, there is just no way for AC to see light from 3000 "at the same time" that the leading ship sees the old light from Earth. It would seem that AC sees the light from earth arrive before the ship does, but the ship is inbetween Earth and AC.

Now if we were to set the time labels accordingly, does everything still make sense?

2 hours ago, fiveworlds said:

Where do you get that from?

My bad. I wanted to say that you "see" (with your eyes) two events (lifetimes) leading out of one single (real) event. If a superluminal ship was to appear in vicinity of earth (but actually emit light the whole time while traveling) and stop to earth's orbit. We would see two things:

1) Trail of the ship going backwards towards their origin.

2) Ship standing in Earh's orbit.

Does that make sense?

2 hours ago, fiveworlds said:

1-Earth, 2-Proxima, 3-Ship, 4-Universe....

You mean the ship is not in between Earth and Proxima? The article didn't seem to say that.

7 minutes ago, swansont said:

You are only supposed to have one outcome of an event. With FTL you could have more than one outcome.

You have the call to Proxima. Let's say that call causes some event, which can only happen of there is a call. Then the callers on Earth get the signal not to place the call. They don't place it. And yet, the event has happened, even though the cause of it is absent.

Please read my reply more carefully. You are failing to see that I pointed out (at least per how I perceive the order of events) that the relativistic ship simply cannot affect either of the two events (call being placed, and call being received), because they already did happen in the past of the event of the ship receiving them,

On 8/1/2018 at 5:14 AM, swansont said:

... FTL breaks causality...

I've read this over a couple of times and I'm failing to see the point, at least in this amazingly well written article.

It says for example, "...they could call Earth before Earth placed the call. They could even tell Earth "hey, don't make that call to Proxima Centauri we just saw you make." Then what?"

What makes the relativistic ship crew believe that they could affect the order of these two FTL events when they received them at light speed. Earth could just answer "We made that call years ago, and Proxima already prepared for the event months ago, and they are ready. Didn't you hear our FTL comms?". If the ship could have received the FTL communication, it would have received it first, then "see" the call arrive at Proxima, then "see" the call originate from earth.

Let's even consider that there's no instant communication, but Earth sends an FTL ship to inform Proxima. Say, that ship travels at faster than light, rather than whatever one may call "infinity" or "instantly" (I suspect that there could be issues with what "infinity" means when you consider relativity in FTL environment). The relativistic ship would "see" the FTL ship first, out of nowhere, then it will "see" it traveling backwards towards Earth AND forward towards Proxima at the same time! The ship would be "seen" to arrive at Proxima before the event of ship launch is "seen" to happen on Earth. Even then, I'm failing to see any problems. It almost sounds to me like the Einstein's train problem/paradox where simultaneity is relative.

The tachyonic anititelephone paradox has been bothering me for a while, but my knowledge of physics is limited so I could not comprehend it. I would really appreciate if someone could explain (his without Minkowski or complex formulas).

Thanks