FTL communication

[rtxoff]

You can skip the third observer only if the receiver on Mars has a very high velocity relative to Earth.

Well there is somewhere a flaw in the logic now.

[N_las]

Well there is somewhere a flaw in the logic now.

I don't see it.

[ZetaX]

Well there is somewhere a flaw in the logic now.

Point out the flaw or stop making these unscientific claims. It is getting annoying to watch this utter lack of scientificness in your posts. You are applying wishful thinking at an extreme level.

[rtxoff]

Well either superluminal communication is time travel or not. It really does not matter if there is a 3rd observer or not. It's simple logic. You can't have both of them in the mentioned scenario.

[N_las]

Well either superluminal communication is time travel or not. It really does not matter if there is a 3rd observer or not. It's simple logic. You can't have both of them in the mentioned scenario.

What are you talking about? If the receiver on mars is at rest relative to the sender on earth, then an instant ping-back will reach back to Earth after 0ms. If the receiver on mars is at motion relative to earth, then the ping-back will reach earth after 5ms or -5ms, or after 10 Years, or at your mothers birthday, it all depends on its velocity.

[Alchemist]

I think when speaking of high-velocity observers there might be something to do with one effect that doesn't even requires FTL. Twin paradox. Particularly, how time-coordinates of observable object change for an observer that changes his frame of reference by accelerating.

Message sent between objects with high relative velocities is probably subject to this very effect.

Edited September 27, 2014 by Alchemist

[ZetaX]

No. You can mathematically prove, assuming the axioms of special relativity, that there is no way to send information to your own past.

[magnemoe]

Imagine you have two completely seperate events happening: Event1 on Earth and Event2 on Mars. Let's assume that earth and mars are stationary, and at a distance of 15 light-minutes (in our reference frame).

If we observe Event1 here on earth and 15 minutes later we observe Event2 on Mars, we can say: "From our point of view, it seems that both events happend at the same time. We observe Event2 15 minutes later, but that is just the travel time of the signal."

Now imagine a 3rd observer that passes Event2 at a near light speed (relative to Mars, Earth, our reference frame). He observes Event2, and he is really close to mars, so the signal delay between Event2 and him is very very small. He might think: "Event2 is happening right now".

If you ignore special relativity, you might think that he will observe Event1 15 minutes after that, and Earth is at a 15 light-minute distance. But depending on his velocity he can see a comletely different distance, and a completely different time intervall before observing Event1.

I havent done the math, but lets assume he sees a distance of 10 light-minutes, and a time intervall of 5 minutes. So from his point of view, even if he takes the signal delay into account, Event1 happed 5 minutes before Event2.

Depending on his velocity, he might see the opposite, that Event1 happend 5 minutes after Event2. Thats the thing about relativity. You can't say that two distant events are happening at the same time. Even if you think they happen at the same time (taken signal delay into account), for a different observer they can happen at different times, or 2 events that happen at different times for you may happen at the same time for a different observer. The extrem case is if you would travel at the speed of light. Then ALL events seem to happen at the same time.

Only to events that happen at the same place can be "objectivly" happen at the same time, because every observer will agree on that.

Now whats happen if we use FTL signals. Lets say that communication with this machine is instant for the reference frame of the sender. If we say that Event1 was the sending of the instant com-signal, and Event2 was the receiving of the instant com-signal on mars, from our point of view both things happen at the same time.

In a reference frame of a 3rd observer, Event2 can happen before Event1. If he uses the same FTL communication technology, he may send us a picture of Event2. But if the FTL signal is instant from the reference frame of the sender, then the people on Earth will receive this message at a point in time before Event1.

So with FTL communication, there is a possible setup that allows us on Earth to receive information about Event2 before we cause Event1. If we send an instant message to mars, we can receive it back on earth before we send it.

As i understand the observer 3 traveling from mars to earth at close to light speed will see event 2 first, then event 1 some time later, say 3 minutes later, first he is moving towards event 1 so its signal will meet him halfway time also pass slower for him so by his clock its not 7.5 minutes but 3)

Now if he had passed mars at the event time he could see both events in any order as light from event 2 need to catch up with him while he moving fast towards event 1

Relativity only modify the length of the time delay by an factor. The intercept would work just the same if you traveled in a plane close to the speed of sound and had two sound waves one catching up with you and the other meeting with you.

[magnemoe]

No. You can mathematically prove, assuming the axioms of special relativity, that there is no way to send information to your own past.

Neither can you with faster than light communication, the only thing you break is the relative reference frame idea as you can get an signal before you should be able too like dodging a laser beam coming towards you but you could not use faster than light communication to sent betting information back before the race started.

[N_las]

As i understand the observer 3 traveling from mars to earth at close to light speed will see event 2 first, then event 1 some time later, say 3 minutes later, first he is moving towards event 1 so its signal will meet him halfway time also pass slower for him so by his clock its not 7.5 minutes but 3)

Now if he had passed mars at the event time he could see both events in any order as light from event 2 need to catch up with him while he moving fast towards event 1

Relativity only modify the length of the time delay by an factor. The intercept would work just the same if you traveled in a plane close to the speed of sound and had two sound waves one catching up with you and the other meeting with you.

If you take into account, that the 3rd observer changes position and intercepts messages halfway, than you make everything needlesly complex.

For the sake of my example, don't consider the 3rd observer traveling from mars to earth. He just crosses Mars position at exactly the same moment of Event2. He then relays his observation of Event2 with his FTL com-device back to earth. While he does that, he is still at position Mars.

EDIT: Imagine the 3rd observer isn't actualy an moving observer that changes positon, but just a moving reference frame. Imagine you are a god at the center of the galaxy, and you can measure everything in your reference frame. You can see everything happening without any signal delays because you are omniscient.

If you, as this god-observer would be not moving in relation to mars and earth, then you would see Event1 and Event2 happening at the same time (there is no signal delay, because you are omniscient, you just KNOW that this is happening at this moment from your frame of reference). But if you have a velocity in relation to Mars and Earth, then you would see Event1 and Event2 at different times.

EDIT2: Another way to think about this. Imagine the universe (everything that has happend, is happening and will happen) is a loaf of bread. A single slice of bread is a single moment. An event is a raisin. you can cut a slice out of the bread, so that the slice contains two specific raisins. You would think therefore, that both raisins (events) happend at the same moment.

But different observers cut the bread at different angles, depending on their velocity. So the slice of bread of a moving observer would just contain one of the two raisins. He has to cut a few more times, to get to a slice that contains the second raisin. So for him, both raisins are in different slices. The events happen at different moments.

Edited September 27, 2014 by N_las

[ZetaX]

Neither can you with faster than light communication, the only thing you break is the relative reference frame idea as you can get an signal before you should be able too like dodging a laser beam coming towards you but you could not use faster than light communication to sent betting information back before the race started.

N_las already gave a proof that FTL transmissions cause time travel.

[rtxoff]

What are you talking about? If the receiver on mars is at rest relative to the sender on earth, then an instant ping-back will reach back to Earth after 0ms. If the receiver on mars is at motion relative to earth, then the ping-back will reach earth after 5ms or -5ms, or after 10 Years, or at your mothers birthday, it all depends on its velocity.

Why should it? You just said the ping back is instant. Why should the velocity between sender and reciever matter? It will always be instant. However the controls of the rover will be more or less inert because for the rover time will run faster or slower.

[N_las]

Why should it? You just said the ping back is instant. Why should the velocity between sender and reciever matter? It will always be instant. However the controls of the rover will be more or less inert because for the rover time will run faster or slower.

EDIT2: Another way to think about this. Imagine the universe (everything that has happend, is happening and will happen) is a loaf of bread. A single slice of bread is a single moment. An event is a raisin. you can cut a slice out of the bread, so that the slice contains two specific raisins. You would think therefore, that both raisins (events) happend at the same moment.

But different observers cut the bread at different angles, depending on their velocity. So the slice of bread of a moving observer would just contain one of the two raisins. He has to cut a few more times, to get to a slice that contains the second raisin. So for him, both raisins are in different slices. The events happen at different moments.

It comes all down to: "What is instant". I assume by instant you mean: The reaction to an action happens at a different position, but at the same moment.

But what is happening at the same moment is dependent on the velocity of the observer. For an observer at the speed of light (say, a photon that travels from the andromeda galaxy into my eye), its creation and its destruction happen at the same moment. For me, these moments are millions of years apart. There are even reference frames in which the photons destruction happens before its creation, but because of the speed-of-light limit, there is no possible way for anyone to influence the photons creation after receiving information about its destruction.

So, the whole concept of "instant" has to be tied to a single reference frame to be meaningful.

If two people communicate via instant-messaging, is it instant from the reference frame of person1, or is is instant from the reference frame of person2? There has to be some rules.

It doesn't matter if you say that it is always instant from the reference frame of the sender, or it is always instant for the reference frame of the receiver, or always instant for the "average" reference frame between them... all situation allow for time travel, as long as the observers are in different reference frames (moving in relation to each other).

Only two people in the same reference frame could be communicating back and forth without time travel, because they would agree about "what is instant". But then a 3rd moving observer could break causality by taking part in the conversation.

You have to invent another definition of "instant" to go around this problem.

Edit: It doesn't have to be instant to allow for time travel, its just easier to visualize that way. But every communication faster than light will allow time travel.

Edit2: Ninja'd by ZetaX

Edited September 27, 2014 by N_las

[ZetaX]

Actually, not just of "insant", but also of "FTL".

[rtxoff]

It comes all down to: "What is instant". I assume by instant you mean: The reaction to an action happens at a different position, but at the same moment.

Exactly, it's infinite speed.

But what is happening at the same moment is dependent on the velocity of the observer. For an observer at the speed of light (say, a photon that travels from the andromeda galaxy into my eye), its creation and its destruction happen at the same moment. For me, these moments are millions of years apart.

And also lightyears apart, it's not only time it's also space, that is a very important part about it.

So, the whole concept of "instant" has to be tied to a single reference frame to be meaningful.

The reference frame is a picture of the whole universe taken at the duration of the Planck length.

If two people communicate via instant-messaging, is it instant from the reference frame of person1, or is is instant from the reference frame of person2? There has to be some rules.

It doesn't matter if you say that it is always instant from the reference frame of the sender, or it is always instant for the reference frame of the receiver, or always instant for the "average" reference frame between them... all situation allow for time travel, as long as the observers are in different reference frames (moving in relation to each other).

The only problems that could arise from such a communication is that the other guy is either talking to fast like Mickey mouse or to slow like a demon.

Only two people in the same reference frame could be communicating back and forth without time travel, because they would agree about "what is instant". But then a 3rd moving observer could break causality by taking part in the conversation.

We already concluded that the 3rd observer has nothing to say at all, it's not a problem with FTL comms.

You have to invent another definition of "instant" to go around this problem.

Edit: It doesn't have to be instant to allow for time travel, its just easier to visualize that way. But every communication faster than light will allow time travel.

Edit2: Ninja'd by ZetaX

I just did.

[N_las]

And also lightyears apart, it's not only time it's also space, that is a very important part about it.

Exactly, the space is very important. If someone is in a reference frame in which Event2 happens before Event1, it is the distance that prevents a message from him to arrive before Event1. By having instant communication, you can ignore every distance, so he could send the message to a moment on Earth before Event1.

We already concluded that the 3rd observer has nothing to say at all, it's not a problem with FTL comms.

You have concluded that. And it is wrong.

IF the receiver on Mars is moving in relation to the sender, then the 3rd observer is not important to show the paradox.

BUT IF the receiver on Mars is NOT moving, then the 3rd observer is important to visualize the paradox.

The reference frame is a picture of the whole universe taken at the duration of the Planck length.

What does that even mean? How can a picture be a reference frame. How can a length be a duration. Are you talking about the plank time? What has the "exposure time" of a picture of the universe has to do with a reference frame?

You are not right, you aren't even wrong.

Have you read my explanation with the loaf of bread? If I understand you correctly, by "picture of the universe', you mean a slice of that bread? And by "duration of planck time" you mean a very thin slice of bread?

That doesn't change the fact that observers at different velocities will cut their slices at different angles. The "picture of the universe" you are talking about is different for everyone, depending on their velocities.

The "picture of the universe" from someone on Earth would entail Event1 and Event2 in a single picture. The 3rd moving observer would have to take several pictures, and both events would be in different pictures. His picture of Event2 could entail a moment on earth before Event1. If he uses this picture for his instant-transmission, than we will have time-travel.

Edited September 27, 2014 by N_las

[rtxoff]

Well obviously there are no words to describe it, i don't know if there is a better way. Yes i guess i made a mistake by saying Planck length instead of Planck time. If you could take a picture of the whole universe with an exposure time of the planck time you would have a moment in the universe that is absolutely same for all entities in it. Now think of you would be able to send 1bit of information from every point in the universe to any point you like in just that moment. You could establish FTL communication without creating a paradox.

[ZetaX]

The concept of taking a picture of the entire universe "at the same time" (i.e. within one Planck time, to use your words) does not make sense: there is no valid notion of "at the same time", not even for "within the same second".

[N_las]

If you could take a picture of the whole universe with an exposure time of the planck time you would have a moment in the universe that is absolutely same for all entities in it..

That is wrong. If you would take this picture, it may entail a supernova in a distant galaxy.

If I take the picture, at the exact same position as you, at the exact same moment, but I am traveling at a certain velocity, then it may not entail that supernova, but show the star as it was during its birth.

If ZetaX is traveling in the opposite direction, and he makes the picture, it may show a black hole.

On these pictures, we all see the star at different stages in his life. What's happening at the same moment, what's "instant", that depends on the observer.

Edited September 27, 2014 by N_las

[andrewas]

The reference frame is a picture of the whole universe taken at the duration of the Planck length.

I think that you're trying to define a preferred reference frame here, in which you can define two events as absolutely instantaneous, but there is no such reference frame. All frames are equally correct.

[WestAir]

I don't understand how one can send a message "instantaneously" when your reference frame shows a completely different Universe than another persons reference frame. Can the word "instantaneous" even be used here? I thought "instantaneous" meant travel at C. It is infinite velocity, is it not?

Edited September 27, 2014 by WestAir

[ZetaX]

No, c is not infinite. It is around 300Mm/s, which is obviously finite.

[WestAir]

No, c is not infinite. It is around 300Mm/s, which is obviously finite.

I think the word you're looking for is "time travel", not instantaneous. If in my reference frame the Earth hasn't been created yet, and you send me a message saying "Hi", you've sent that message back in time. Instantaneous would be a message that reaches me when I see you speaking into your radio from my reference frame, which just so happens to coincide with C.

[rtxoff]

I don't understand how one can send a message "instantaneously" when your reference frame shows a completely different Universe than another persons reference frame?

Welcome to the club nobody of us understands that, we are hypothesizing and trying to find an answer for that.

[ZetaX]

Where am _I_ looking for such a word¿ All I did was elaborating why gpisic's statements are not even well-defined.

Anyway: It was already demonstrated that instantaneous implies time travel, and the convers eis trivially true, too. So both are equally unrealistic.