# FTL implies Backward Time Effect

• stglyde
In summary: I call such a line a "line of simultaneity".The kind of FTL message that's the easiest to draw is one that moves at infinite speed in the inertial coordinate system in which the emitter is at rest. When someone on Earth sends such a message, you draw it as a horizontal line. When someone on the spaceship sends it, you draw it as a line parallel to the blue simultaneity line (the one marked x'). (If the idea of infinite speed bothers you, just tilt the line a little bit, so that it now represents ludicrous speed instead of infinite speed).Now, a horizontal line representing a message sent from a small x coordinate (
stglyde
Any faster than light signal can create backward time effect by means of some frames seeing the signal go to the past of the origin.

Is there any illustration or website that illustrates this clearly?

I'd like to know if the return path carrying the signal needs to travel faster than light or mere superluminal signalling is enough.

For example. Supposed we sent a faster than light signal to pluto. Then it was sent back at similarly superluminal velocity. Would it reach the origin before it was sent out.. or do you have to carry the signal in a superluminal carrier like spaceship for this backward time effect to manifest at the origin?

stglyde said:
For example. Supposed we sent a faster than light signal to pluto. Then it was sent back at similarly superluminal velocity. Would it reach the origin before it was sent out.. or do you have to carry the signal in a superluminal carrier like spaceship for this backward time effect to manifest at the origin?

You need a subluminal spaceship submitting the signal in its own rest frame with superluminal velocity.

DrStupid said:
You need a subluminal spaceship submitting the signal in its own rest frame with superluminal velocity.

For example. Earth sent a superluminal signal to a station in Pluto. If the station there sent another superluminal signal back to earth. Would it arrive in Earth past? Is a spaceship back to Earth really necessary? Please elaborate the pathways. Thanks.

stglyde said:
For example. Earth sent a superluminal signal to a station in Pluto. If the station there sent another superluminal signal back to earth. Would it arrive in Earth past?

Of course not.

stglyde said:
Is a spaceship back to Earth really necessary?

In fact you need two spaceships: One that transmits the signal from Earth and another that receives it at Pluto and vice versa for the way back. If the signal velocity is superluminal in the rest system of the ships it can be negative in the rest system of Earth or Pluto and a negative velocity leads to a negative time of flight.

stglyde said:
Is a spaceship back to Earth really necessary? Please elaborate the pathways. Thanks.
The spaceship needs to be moving away from Earth. This is very easy to see in a spacetime diagram. I'm too lazy to draw one, so I'll just use this one from Wikipedia to explain my point.

Think of the black axes as describing the coordinates assigned by an observer on Earth. Think of the blue line marked ct' as the world line of the spaceship. The other blue line represents a set of events that the observer on the spaceship would assign the same time coordinate. In other words, those are events that he considers simultaneous. Yet another way of saying it: That's a slice of spacetime that he thinks of as "space". I call such a line a "line of simultaneity".

The kind of FTL message that's the easiest to draw is one that moves at infinite speed in the inertial coordinate system in which the emitter is at rest. When someone on Earth sends such a message, you draw it as a horizontal line. When someone on the spaceship sends it, you draw it as a line parallel to the blue simultaneity line (the one marked x'). (If the idea of infinite speed bothers you, just tilt the line a little bit, so that it now represents ludicrous speed instead of infinite speed).

Now, a horizontal line representing a message sent from a small x coordinate (Earth) to a large one (spaceship) will cross one of the spaceship's simultaneity lines (a line parallel to the blue line marked x'). So the sending event and the receiving event are not simultaneous. In the diagram, the sending event is above the simultaneity line, and the receiving event is below it. So the sending event has the larger time coordinate in the inertial coordinate system in which the spaceship is at rest. From their point of view, they got a message from the future.

If they reply to it immediately, the reply message will be a line parallel to (or almost parallel to) the blue simultaneity line, and will therefore arrive at Earth before the message they replied to was sent.

Last edited:
Fredrik said:
The spaceship needs to be moving away from Earth. This is very easy to see in a spacetime diagram. I'm too lazy to draw one, so I'll just use this one from Wikipedia to explain my point.

Think of the black axes as describing the coordinates assigned by an observer on Earth. Think of the blue line marked ct' as the world line of the spaceship. The other blue line represents a set of events that the observer on the spaceship would assign the same time coordinate. In other words, those are events that he considers simultaneous. Yet another way of saying it: That's a slice of spacetime that he thinks of as "space". I call such a line a "line of simultaneity".

The kind of FTL message that's the easiest to draw is one that moves at infinite speed in the inertial coordinate system in which the emitter is at rest. When someone on Earth sends such a message, you draw it as a horizontal line. When someone on the spaceship sends it, you draw it as a line parallel to the blue simultaneity line (the one marked x'). (If the idea of infinite speed bothers you, just tilt the line a little bit, so that it now represents ludicrous speed instead of infinite speed).

Now, a horizontal line representing a message sent from a small x coordinate (Earth) to a large one (spaceship) will cross one of the spaceship's simultaneity lines (a line parallel to the blue line marked x'). So the sending event and the receiving event are not simultaneous. In the diagram, the sending event is above the simultaneity line, and the receiving event is below it. So the sending event has the larger time coordinate in the inertial coordinate system in which the spaceship is at rest. From their point of view, they got a message from the future.

If they reply to it immediately, the reply message will be a line parallel to (or almost parallel to) the blue simultaneity line, and will therefore arrive at Earth before the message they replied to was sent.

How come there are particles that can travel faster than light?

Thanks Fredrik and others for the tips. I found a web site:

http://sheol.org/throopw/tachyon-pistols.html

Consider a duel with tachyon pistols. Two duelists, A and B, are to stand back to back, then start out at 0.866 lightspeed for 8 seconds, turn, and fire. Tachyon pistol rounds move so fast, they are instantaneous for all practical purposes.

So, the duelists both set out --- at 0.866 lightspeed each relative to the other, so that the time dilation factor is 2 between them. Duelist A counts off 8 lightseconds, turns, and fires. Now, according to A (since in relativity all inertial frames are equally valid) B's the one who's moving, so B's clock is ticking at half-speed. Thus, the tachyon round hits B in the back as B's clock ticks 4 seconds.

Now B (according to relativity) has every right to consider A as moving, and thus, A is the one with the slowed clock. So, as B is hit in the back at tick 4, in outrage at A's firing before 8 seconds are up, B manages to turn and fire before being overcome by his fatal wound. And since in B's frame of reference it's A's clock that ticks slow, B's round hits A, striking A dead instantly, at A's second tick; a full six seconds before A fired the original round. A classic grandfather paradox.

So the requirements to get a signal back in time is if the sender and emitter should be moving near light speed to take advantage of time dilation?

Fredrik. What is the speed of the spacetime in your example with respect to light? Is it in the speed comparible to our space shuttle now or more impractical like 1/3 the speed of light?

So you mean to say we can't do any backward in time superluminal experiment even if the neutrino were proven to be superluminal... like we need a signal faster like twice the speed of light?

Last edited by a moderator:
stglyde said:
So the requirements to get a signal back in time is if the sender and emitter should be moving near light speed to take advantage of time dilation?

The minimum speed u of sender and emitter depends on the signal velocity v in their rest frame: u·v > c²

Fredrik said:
The spaceship needs to be moving away from Earth. This is very easy to see in a spacetime diagram. I'm too lazy to draw one, so I'll just use this one from Wikipedia to explain my point.

Think of the black axes as describing the coordinates assigned by an observer on Earth. Think of the blue line marked ct' as the world line of the spaceship. The other blue line represents a set of events that the observer on the spaceship would assign the same time coordinate. In other words, those are events that he considers simultaneous. Yet another way of saying it: That's a slice of spacetime that he thinks of as "space". I call such a line a "line of simultaneity".

The kind of FTL message that's the easiest to draw is one that moves at infinite speed in the inertial coordinate system in which the emitter is at rest. When someone on Earth sends such a message, you draw it as a horizontal line. When someone on the spaceship sends it, you draw it as a line parallel to the blue simultaneity line (the one marked x'). (If the idea of infinite speed bothers you, just tilt the line a little bit, so that it now represents ludicrous speed instead of infinite speed).

Now, a horizontal line representing a message sent from a small x coordinate (Earth) to a large one (spaceship) will cross one of the spaceship's simultaneity lines (a line parallel to the blue line marked x'). So the sending event and the receiving event are not simultaneous. In the diagram, the sending event is above the simultaneity line, and the receiving event is below it. So the sending event has the larger time coordinate in the inertial coordinate system in which the spaceship is at rest. From their point of view, they got a message from the future.

If they reply to it immediately, the reply message will be a line parallel to (or almost parallel to) the blue simultaneity line, and will therefore arrive at Earth before the message they replied to was sent.

Why are the spaceships necessary in the first place? Is it to experience time dilation. I can't totally understand the above. My background of backtime time signalling is based on the following:

http://sheol.org/throopw/tachyon-pistols.html

Consider a duel with tachyon pistols. Two duelists, A and B, are to stand back to back, then start out at 0.866 lightspeed for 8 seconds, turn, and fire. Tachyon pistol rounds move so fast, they are instantaneous for all practical purposes.

So, the duelists both set out --- at 0.866 lightspeed each relative to the other, so that the time dilation factor is 2 between them. Duelist A counts off 8 lightseconds, turns, and fires. Now, according to A (since in relativity all inertial frames are equally valid) B's the one who's moving, so B's clock is ticking at half-speed. Thus, the tachyon round hits B in the back as B's clock ticks 4 seconds.

Now B (according to relativity) has every right to consider A as moving, and thus, A is the one with the slowed clock. So, as B is hit in the back at tick 4, in outrage at A's firing before 8 seconds are up, B manages to turn and fire before being overcome by his fatal wound. And since in B's frame of reference it's A's clock that ticks slow, B's round hits A, striking A dead instantly, at A's second tick; a full six seconds before A fired the original round. A classic grandfather paradox.

Now what is its analogy to your example? In the case of the dualists. They don't need a third party spaceship. Maybe we need it in the Earth-Pluto example because the two planets are not moving significantly with respect to each other so you need spaceships that are at least half the speed of light in addition to the Earth and pluto?

Last edited by a moderator:
GNI1 said:
How come there are particles that can travel faster than light?
There aren't. At least none that we know of.

stglyde said:
So the requirements to get a signal back in time is if the sender and emitter should be moving near light speed to take advantage of time dilation?
What they're taking advantage of is the tilting of the simultaneity lines, which is of course related to time dilation, but not quite the same thing.

stglyde said:
Fredrik. What is the speed of the spacetime in your example with respect to light? Is it in the speed comparible to our space shuttle now or more impractical like 1/3 the speed of light?
The speed we need depends on how far away the shuttle is. If it's really far away, then it doesn't need to move really fast.

stglyde said:
So you mean to say we can't do any backward in time superluminal experiment even if the neutrino were proven to be superluminal... like we need a signal faster like twice the speed of light?
No, we could get the same effect with messages moving at any speed >c. I chose almost infinite speed only because it makes it easier to draw the diagram.

stglyde said:
Why are the spaceships necessary in the first place?
To tilt the simultaneity lines so that an event on the spaceship is simultaneous with an earlier event on Earth.

stglyde said:
In the case of the dualists. They don't need a third party spaceship.
That's because they're assumed to be walking at speed 0.866c.

Fredrik said:
There aren't. At least none that we know of.

What they're taking advantage of is the tilting of the simultaneity lines, which is of course related to time dilation, but not quite the same thing.

The speed we need depends on how far away the shuttle is. If it's really far away, then it doesn't need to move really fast.

No, we could get the same effect with messages moving at any speed >c. I chose almost infinite speed only because it makes it easier to draw the diagram.

You said "we could get the same effect with messages moving at any speed >c". Now if the neutrinos as reported current were really superluminal. How can you design an experiment setup where the neutrinos can be sent backward in time?

stglyde said:
You said "we could get the same effect with messages moving at any speed >c". Now if the neutrinos as reported current were really superluminal. How can you design an experiment setup where the neutrinos can be sent backward in time?
If you send them to someone who's already moving away from us at a speed that's extremely close to c, then the guy on the receiving end will assign an earlier time coordinate to the event where he receives the message than to the event where it was sent from Earth. So in this sense, he would consider it a "message from the future".

If you want to send a message to yourself in the past, you need a more complicated setup, and...uh, it looks like you also need the reply messages to travel with a much higher speed than the original message. Looks like I was wrong to suggest that you can get all of the same effects with a speed only slightly higher than c.

Fredrik said:
If you send them to someone who's already moving away from us at a speed that's extremely close to c, then the guy on the receiving end will assign an earlier time coordinate to the event where he receives the message than to the event where it was sent from Earth. So in this sense, he would consider it a "message from the future".

If you want to send a message to yourself in the past, you need a more complicated setup, and...uh, it looks like you also need the reply messages to travel with a much higher speed than the original message. Looks like I was wrong to suggest that you can get all of the same effects with a speed only slightly higher than c.

But practically. Can we even design spaceships that can move even half the speed of light? What is the most practical maximum speed that our spaceships can attain in the next hundreds or thousands of years based on physics as we know now. If we can never attend even half the speed of light. Then there may be things in the world like superluminal neutrinos yet causality never violated because we can never design the experiments to practical usage even in principle. Right?

I would say that's accurate. Not sure what speeds (relative to Earth) that are realistic with present-day technology, but it certainly isn't anywhere near c/2.

Anyway, I would be very surprised if it turns out that these neutrinos are really moving faster than c. I haven't even bothered to read the news about it or the very long thread about it, because it seems so likely that they just did something wrong.

## 1. What is FTL and how does it relate to time travel?

FTL stands for Faster Than Light and it refers to any object or information that can travel faster than the speed of light. According to Einstein's theory of relativity, the speed of light is the fastest possible speed in the universe. Therefore, if something can travel faster than light, it would also be able to travel back in time, hence the idea of FTL implying a backward time effect.

## 2. Is FTL travel or communication possible?

Currently, there is no scientific evidence or technology that supports the possibility of FTL travel or communication. The laws of physics, specifically the theory of relativity, have been extensively tested and it is widely accepted that the speed of light is the ultimate speed limit in the universe. However, some theories, such as wormholes and warp drives, suggest that it may be possible to bend space-time in a way that allows for FTL travel.

## 3. What are the potential consequences of FTL travel or communication?

If FTL travel or communication were possible, it would have significant consequences on our understanding of the laws of physics and the nature of the universe. It could also have implications for causality, as it would allow for events to occur before their causes. This could potentially lead to paradoxes and disrupt the fabric of time and space as we know it.

## 4. Are there any experiments or studies that have been conducted on FTL and its effects on time?

No, there have been no experiments or studies that have been able to confirm the existence of FTL travel or communication, let alone its effects on time. The concept of FTL is currently purely theoretical and there is no scientific evidence to support it.

## 5. How does the concept of FTL and backward time effect impact our understanding of time travel?

The idea of FTL implying a backward time effect is just one of many theories on time travel. It suggests that FTL could be a potential method for traveling back in time, but it is not the only one. Other theories, such as the grandfather paradox and the many-worlds interpretation, offer different explanations for how time travel could occur. Ultimately, the concept of FTL and backward time effect is still purely hypothetical and more research and evidence is needed to fully understand the possibilities of time travel.

• Special and General Relativity
Replies
65
Views
4K
• Special and General Relativity
Replies
10
Views
2K
• Special and General Relativity
Replies
19
Views
2K
• Special and General Relativity
Replies
74
Views
6K
• Special and General Relativity
Replies
44
Views
3K
• Special and General Relativity
Replies
13
Views
1K
• Special and General Relativity
Replies
13
Views
1K
• Science Fiction and Fantasy Media
Replies
4
Views
1K
• Special and General Relativity
Replies
6
Views
1K
• Special and General Relativity
Replies
1
Views
870