7 ms·

I am constantly reading that information cannot travel faster than light, and I accept each of the explanations as to why the various methods for attempting to

I am constantly reading that information cannot travel faster than light, and I accept each of the explanations as to why the various methods for attempting to send information faster than light do not work. What I do not understand is whether sending information faster than light would cause a paradox of any sort. It may be that it happens to be impossible. But is it necessarily impossible?

Confusion 15y ago
Information can kill (indirectly, for instance by driving someone to commit murder or suicide) and as such it is the grandfather paradox from the article all over again.
llimllib 15y ago
This page looks to answer your question with a fairly comprehensive "we're not sure": http://www.desy.de/user/projects/Physics/Relativity/SpeedOfLight/FTL.html#18 http://www.desy.de/user/projects/Physics/Relativity/SpeedOfL... (I can't vouch for its accuracy. It seems that the grandfather paradox is the biggest problem with FTL communication)
- raganwald 15y ago
  The suggestion seems to be that FTL communication is equivalent to sending information back in time. But the explanation on this page asks me to make massive leaps. I can only imagine it is not meant to explain, but rather as a reminder for people who already understand the explanation.
martincmartin 15y ago
But is it necessarily impossible? Yes, because of time dilation. In particular, suppose that, in some reference frame, you can send a message from location A to location B faster than light. In that reference frame, it arrives at B after it leaves A. Then, in some other reference frame (moving at less than the speed of light relative to the first), it arrives at B before it leaves A. So, we can reverse that: Let's say I (in Boston) want to send you (in Toronto) a message that you'll receive before I send it. Then I simply figure out a reference frame where the message would travel forward in time but faster than light. Then I use the faster-than-light communication in that reference frame to send it to you. If you now do the same -- choose a 3rd reference frame, where the message from Toronto to Boston travels forward in time but faster than light, but where on the Earth's surface it travels backward in time -- you can now send your reply before I send my message. Then I can arrange a simple grandmother paradox. e.g. I plan to send a message to you at 2pm, but if I get your reply before 2pm, I don't actually send it. EDIT: In particular, if one observer measures an event as happening at position-time (x, y, z, t), and another observer is moving at speed u along the x axis, that observer will see: x' = (x - ut) / sqrt(1 - u^2/c^2) y' = y z' = z t' = (t - ux/c^2) / sqrt(1 - u^2/c^2) (This is the Lorentz transformation.) Suppose the first observer can send info faster than light. Suppose she sends it along the x axis at a speed of 2c. You can repeat the derivation with any kc where k > 1. If the sending event is at position-time (0, 0, 0, 0), and it takes one time unit to get there, then the receiving event in her frame is (2c, 0, 0, 1). What does the other observer see? Let's suppose they're moving more than half the speed of light, say 3/4 the speed of light. The sending event is the origin, and the origin maps to the origin, so that's easy. For the receiving event: x' = (x - ut) / sqrt(1 - u^2/c^2) = (2c - (3/4 c) * 1) / sqrt(1 - (3/4 c)^2/c^2) y' = y = 0 z' = z = 0 t' = (t - ux/c^2) / sqrt(1 - u^2/c^2) = (1 - (3/4 c) * 2 * c/c^2) / sqrt(1 - (3/4 c)^2 / c^2) = (1 - 3/4 * 2) / sqrt(1 - (3/4)^2). In the formula for t', note that the first term, 1 - 3/4 * 2, is negative. So, if I can send messages at a speed of k * c, then an observer moving with a speed greater than c/k in the same direction sees the message arrive before it leaves. That description is very easy to reverse.
- raganwald 15y ago
  Time dilation: http://en.wikipedia.org/wiki/Time_dilation http://en.wikipedia.org/wiki/Time_dilation Ok, so let's take two twins, Raj and Ragan. Raj is orbiting a massive cosmological object, Ragan is on the surface. Ragan's time appears to travel slower than Raj's time. Does FTL communication between Ragan and Raj ever create a situation where messages can travel backwards in time? Does bringing an observer into it help?
  - martincmartin 15y ago
    As you were posting this, I edited my post to fill in more details. You've got me re-reading my special relativity texts, which I haven't touched in 20 years, and for which I thank you. :)
- Groxx 15y ago
  I've seen that explanation over and over, but I don't see why there's a problem with something arriving at B before B sees it leave A. We have the same situation with faster-than-sound travel, but it doesn't appear to be impossible. Granted, it's not impossible by that proof because we have faster means of communication (you can detect something before its sound reaches you) (or maybe because it's slower than light), but that's not a problem with faster-than-light communication either. By hypothetically accepting that there's a FTL communication method, didn't you just accept that there's something faster than light? Wouldn't that imply there's something we don't know about time dilation (if FTL comm is achieved), given that it defines such a thing as impossible? It's a circular argument, it's impossible by this proof because this proof relies on it being impossible. So arguing a hypothesis which rejects that requirement with this proof is ignoring the hypothetical universe that was constructed, and it's not a proof at all.
  - bermanoid 15y ago
    ...I don't see why there's a problem with something arriving at B before B sees it leave A. The problem is that if something arrives at B before B sees it leave A, then there is a way to arrange for a message to be sent from B to A, and a response back from A to B, such that the response arrives before the message was sent. This is a consequence of special relativity, which allows any spacelike vector to be transformed into any spacelike vector (and any timelike into any other timelike) by a subluminal (less than light speed) change in observer velocity. If you can do any spacelike (FTL) transmission of data, then you can do every spacelike transmission of data if you care to go through the trouble (which is not to say it would be easy, just that physics doesn't prohibit it). Combining spacelike vectors lets you move back in time, so causality is screwed beyond belief.
pygy_ 15y ago
I'm not a physicist, I'm just repeating something I read somewhere (I cant even remember where). As far as I know, general relativity relies on the existence of a finite speed upper bound. It appears that this bound is equal to the speed of light, but any higher speed would do.
- raganwald 15y ago
  I understand that, although my own explanation is more along the lines that c represents a fundamental constant describing the geometry of space, and that the reason no physical object can travel faster than light is that the speed of light represents the shortest distance in space-time between any two points.
deleted 15y ago
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chulan 15y ago
Quantum entanglement seems to allow instantaneous info transfer over any distance. Any Quantum Physicists on here?
- Alex3917 15y ago
  Some people like to point out that technically the information isn't traveling through space, but to me this seems like a rather academic distinction. This is the relevant wikipedia article: http://en.wikipedia.org/wiki/Bells_theorem http://en.wikipedia.org/wiki/Bells_theorem See also: http://forum-network.org/lecture/how-hippies-saved-physics http://forum-network.org/lecture/how-hippies-saved-physics
- rdtsc 15y ago
  Not _info_ transfer but it allows instantaneous collapse of the entangled state into known states after the measurement is done on a entangled system. Here is how I remember it (and I am just a comp sci nerd, not a physicist so please correct me): * you have 2 qubits Q1 and Q2. You entangle them. Now you have an entangled system Q1Q2. * You separate Q1 and Q2 in space. Let's say you put Q2 on a spaceship and blast it into space, but leave Q1 in the lab on earth. * After some time you measure Q1 to get its value. At that point the entanglement of Q1Q2 collapses. You now know the value of Q1 that you just measured and at the same time Q2 is forced to a known value too. * At _appears_ as if you could send information this way but you cannot. Think about it. You don't know what value you'll measure in the lab for Q1. So you can't force Q2 to be a certain value either. * Let's think about it another way -- suppose you tell the spaceship operator that if Q2 collapses to |0> then they should turn the spaceship immediately around and head back home and if it collapses to |1> they should arm their weapons and prepare for an alien attack. Now you are on earth in control of Q1, and you want to force Q2 to be measured to |1> because you know the aliens are coming. There is nothing you can do to Q1 to force Q2 to be measured as |1>.
  - aquark 15y ago
    OK, but can you detect in the spaceship when Q2 collapses? (I know no quantum physics beyond a few articles0 If so why can't you simply modulate the signal on top of a series of collapsing entangled pairs. Basically morse code with the timing of the collapses, you don't care what value they collapse too.
deleted 15y ago
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jerf 15y ago
It's difficult to explain some of the other things mentioned in this thread without going to math, like why FTL = time travel. It should also be mentioned that the reason why time travel is a problem is not merely that you can send a signal backwards in time, but that the signal may be used to then prevent the sending of the signal. Moreover, it isn't necessarily the case that this requires a human; you can get something like an asteroid going around the loop and knocking itself off the path that led it around the CTC in the first place, so having this sort of paradox in the universe is genuinely bad and does not require human involvement. But as to FTL, let me horrifically simplify. Imagine space as a 2D grid of points. Vertical is time (up is the future), left-right is 1D of space. Take every point, and connect it to the one directly above it, the one above and right, and the one above and left, with a directed link. You are allowed to move through spacetime along those lines. You can see your past and future lightcones on this graph, if you look. Light always travels diagonally on this graph. Normal travel in this horrifically degenerate spacetime is travel that can be expressed on those links. FTL is somehow figuring out how to bypass the graph structure of space, which the shape of that space doesn't permit. Space is literally shaped so that FTL isn't really possible. Reality is of course substantially more complicated than this, and this model is useless for any purpose beyond trying to show what I mean by space being shaped such that FTL isn't possible. This also sort of demonstrates another thing, which is that ultimately, any point that you can't reach is, from the point of view of where you are, really the same. They are all equally simply "unreachable". So you if you create an "unreachable" drive that lets you get there, you can basically just use it twice to jump far away from where you are now, then from there just jump directly into your past. Also, there is no compelling reason to think you would have a drive that would only let you go "one light year"; it would open the entire universe to you. In General Relativity, this still basically holds true, though it requires more explanations about how there isn't one true reference frame, etc, etc, but FTL really is time travel and it really does just shoot physics and causality full of holes.
- bermanoid 15y ago
  It should also be mentioned that the reason why time travel is a problem is not merely that you can send a signal backwards in time, but that the signal may be used to then prevent the sending of the signal. This is not necessarily a problem - in a quantum mechanical formulation, it's simple enough to resolve (for instance) a grandfather paradox by looking at quantum interference around causal loops (almost 100% analogous to the way interference on a violin string will restrict stable modes to certain frequencies, interference around a causal loop will only allow stable self-sustaining situations to survive, so any grandfather paradox would self-interfere and not be allowed). The main difference is that with closed timelike curves the quantum effects can get a lot stronger than without them, even affecting very large objects.
phlux 15y ago
>I am constantly reading that information cannot travel faster than light (I am not sure if this analogy works - if it is flawed, please help me fix it until it is correct) Lets assume that you are on a computer connected to another machine that is at the exact furthest opposite possible on the globe, via fiber. The remote machine will display a question, which you have never seen before and do not know the answer to, but you can read the question and then the answer to the question will be displayed. Is it impossible that you could not derive an answer to the question fast than the answer could be displayed? Thus, information (your derived answer) arriving in your head faster than the information arriving on screen as delivered at the speed of light? My assumption is that information can be derived/created/transferred/understood faster than linear light travel as the processing of thoughts in our minds is semaphoric/symbolic and parrallel - light transmission is serial.
ignifero 15y ago
Yes because it would break Causality, you would have the broken cup along with the intact one. You may ask "why have causality"? Well, it's your pick, either FTL, or Causality. btw, a great book about time (and time loops) is Igor Novikov's "River of time"

Confusion 15y ago

Information can kill (indirectly, for instance by driving someone to commit murder or suicide) and as such it is the grandfather paradox from the article all over again.

llimllib 15y ago

This page looks to answer your question with a fairly comprehensive "we're not sure": http://www.desy.de/user/projects/Physics/Relativity/SpeedOfLight/FTL.html#18 http://www.desy.de/user/projects/Physics/Relativity/SpeedOfL... (I can't vouch for its accuracy. It seems that the grandfather paradox is the biggest problem with FTL communication)

raganwald 15y ago
The suggestion seems to be that FTL communication is equivalent to sending information back in time. But the explanation on this page asks me to make massive leaps. I can only imagine it is not meant to explain, but rather as a reminder for people who already understand the explanation.

martincmartin 15y ago

But is it necessarily impossible? Yes, because of time dilation. In particular, suppose that, in some reference frame, you can send a message from location A to location B faster than light. In that reference frame, it arrives at B after it leaves A. Then, in some other reference frame (moving at less than the speed of light relative to the first), it arrives at B before it leaves A. So, we can reverse that: Let's say I (in Boston) want to send you (in Toronto) a message that you'll receive before I send it. Then I simply figure out a reference frame where the message would travel forward in time but faster than light. Then I use the faster-than-light communication in that reference frame to send it to you. If you now do the same -- choose a 3rd reference frame, where the message from Toronto to Boston travels forward in time but faster than light, but where on the Earth's surface it travels backward in time -- you can now send your reply before I send my message. Then I can arrange a simple grandmother paradox. e.g. I plan to send a message to you at 2pm, but if I get your reply before 2pm, I don't actually send it. EDIT: In particular, if one observer measures an event as happening at position-time (x, y, z, t), and another observer is moving at speed u along the x axis, that observer will see: x' = (x - ut) / sqrt(1 - u^2/c^2) y' = y z' = z t' = (t - ux/c^2) / sqrt(1 - u^2/c^2) (This is the Lorentz transformation.) Suppose the first observer can send info faster than light. Suppose she sends it along the x axis at a speed of 2c. You can repeat the derivation with any kc where k > 1. If the sending event is at position-time (0, 0, 0, 0), and it takes one time unit to get there, then the receiving event in her frame is (2c, 0, 0, 1). What does the other observer see? Let's suppose they're moving more than half the speed of light, say 3/4 the speed of light. The sending event is the origin, and the origin maps to the origin, so that's easy. For the receiving event: x' = (x - ut) / sqrt(1 - u^2/c^2) = (2c - (3/4 c) * 1) / sqrt(1 - (3/4 c)^2/c^2) y' = y = 0 z' = z = 0 t' = (t - ux/c^2) / sqrt(1 - u^2/c^2) = (1 - (3/4 c) * 2 * c/c^2) / sqrt(1 - (3/4 c)^2 / c^2) = (1 - 3/4 * 2) / sqrt(1 - (3/4)^2). In the formula for t', note that the first term, 1 - 3/4 * 2, is negative. So, if I can send messages at a speed of k * c, then an observer moving with a speed greater than c/k in the same direction sees the message arrive before it leaves. That description is very easy to reverse.

raganwald 15y ago
Time dilation: http://en.wikipedia.org/wiki/Time_dilation http://en.wikipedia.org/wiki/Time_dilation Ok, so let's take two twins, Raj and Ragan. Raj is orbiting a massive cosmological object, Ragan is on the surface. Ragan's time appears to travel slower than Raj's time. Does FTL communication between Ragan and Raj ever create a situation where messages can travel backwards in time? Does bringing an observer into it help?
- martincmartin 15y ago
  As you were posting this, I edited my post to fill in more details. You've got me re-reading my special relativity texts, which I haven't touched in 20 years, and for which I thank you. :)
Groxx 15y ago
I've seen that explanation over and over, but I don't see why there's a problem with something arriving at B before B sees it leave A. We have the same situation with faster-than-sound travel, but it doesn't appear to be impossible. Granted, it's not impossible by that proof because we have faster means of communication (you can detect something before its sound reaches you) (or maybe because it's slower than light), but that's not a problem with faster-than-light communication either. By hypothetically accepting that there's a FTL communication method, didn't you just accept that there's something faster than light? Wouldn't that imply there's something we don't know about time dilation (if FTL comm is achieved), given that it defines such a thing as impossible? It's a circular argument, it's impossible by this proof because this proof relies on it being impossible. So arguing a hypothesis which rejects that requirement with this proof is ignoring the hypothetical universe that was constructed, and it's not a proof at all.
- bermanoid 15y ago
  ...I don't see why there's a problem with something arriving at B before B sees it leave A. The problem is that if something arrives at B before B sees it leave A, then there is a way to arrange for a message to be sent from B to A, and a response back from A to B, such that the response arrives before the message was sent. This is a consequence of special relativity, which allows any spacelike vector to be transformed into any spacelike vector (and any timelike into any other timelike) by a subluminal (less than light speed) change in observer velocity. If you can do any spacelike (FTL) transmission of data, then you can do every spacelike transmission of data if you care to go through the trouble (which is not to say it would be easy, just that physics doesn't prohibit it). Combining spacelike vectors lets you move back in time, so causality is screwed beyond belief.

pygy_ 15y ago

I'm not a physicist, I'm just repeating something I read somewhere (I cant even remember where). As far as I know, general relativity relies on the existence of a finite speed upper bound. It appears that this bound is equal to the speed of light, but any higher speed would do.

raganwald 15y ago
I understand that, although my own explanation is more along the lines that c represents a fundamental constant describing the geometry of space, and that the reason no physical object can travel faster than light is that the speed of light represents the shortest distance in space-time between any two points.

deleted 15y ago

[deleted]

chulan 15y ago

Quantum entanglement seems to allow instantaneous info transfer over any distance. Any Quantum Physicists on here?

Alex3917 15y ago
Some people like to point out that technically the information isn't traveling through space, but to me this seems like a rather academic distinction. This is the relevant wikipedia article: http://en.wikipedia.org/wiki/Bells_theorem http://en.wikipedia.org/wiki/Bells_theorem See also: http://forum-network.org/lecture/how-hippies-saved-physics http://forum-network.org/lecture/how-hippies-saved-physics
rdtsc 15y ago
Not _info_ transfer but it allows instantaneous collapse of the entangled state into known states after the measurement is done on a entangled system. Here is how I remember it (and I am just a comp sci nerd, not a physicist so please correct me): * you have 2 qubits Q1 and Q2. You entangle them. Now you have an entangled system Q1Q2. * You separate Q1 and Q2 in space. Let's say you put Q2 on a spaceship and blast it into space, but leave Q1 in the lab on earth. * After some time you measure Q1 to get its value. At that point the entanglement of Q1Q2 collapses. You now know the value of Q1 that you just measured and at the same time Q2 is forced to a known value too. * At _appears_ as if you could send information this way but you cannot. Think about it. You don't know what value you'll measure in the lab for Q1. So you can't force Q2 to be a certain value either. * Let's think about it another way -- suppose you tell the spaceship operator that if Q2 collapses to |0> then they should turn the spaceship immediately around and head back home and if it collapses to |1> they should arm their weapons and prepare for an alien attack. Now you are on earth in control of Q1, and you want to force Q2 to be measured to |1> because you know the aliens are coming. There is nothing you can do to Q1 to force Q2 to be measured as |1>.
- aquark 15y ago
  OK, but can you detect in the spaceship when Q2 collapses? (I know no quantum physics beyond a few articles0 If so why can't you simply modulate the signal on top of a series of collapsing entangled pairs. Basically morse code with the timing of the collapses, you don't care what value they collapse too.

deleted 15y ago

[deleted]

jerf 15y ago

It's difficult to explain some of the other things mentioned in this thread without going to math, like why FTL = time travel. It should also be mentioned that the reason why time travel is a problem is not merely that you can send a signal backwards in time, but that the signal may be used to then prevent the sending of the signal. Moreover, it isn't necessarily the case that this requires a human; you can get something like an asteroid going around the loop and knocking itself off the path that led it around the CTC in the first place, so having this sort of paradox in the universe is genuinely bad and does not require human involvement. But as to FTL, let me horrifically simplify. Imagine space as a 2D grid of points. Vertical is time (up is the future), left-right is 1D of space. Take every point, and connect it to the one directly above it, the one above and right, and the one above and left, with a directed link. You are allowed to move through spacetime along those lines. You can see your past and future lightcones on this graph, if you look. Light always travels diagonally on this graph. Normal travel in this horrifically degenerate spacetime is travel that can be expressed on those links. FTL is somehow figuring out how to bypass the graph structure of space, which the shape of that space doesn't permit. Space is literally shaped so that FTL isn't really possible. Reality is of course substantially more complicated than this, and this model is useless for any purpose beyond trying to show what I mean by space being shaped such that FTL isn't possible. This also sort of demonstrates another thing, which is that ultimately, any point that you can't reach is, from the point of view of where you are, really the same. They are all equally simply "unreachable". So you if you create an "unreachable" drive that lets you get there, you can basically just use it twice to jump far away from where you are now, then from there just jump directly into your past. Also, there is no compelling reason to think you would have a drive that would only let you go "one light year"; it would open the entire universe to you. In General Relativity, this still basically holds true, though it requires more explanations about how there isn't one true reference frame, etc, etc, but FTL really is time travel and it really does just shoot physics and causality full of holes.

bermanoid 15y ago
It should also be mentioned that the reason why time travel is a problem is not merely that you can send a signal backwards in time, but that the signal may be used to then prevent the sending of the signal. This is not necessarily a problem - in a quantum mechanical formulation, it's simple enough to resolve (for instance) a grandfather paradox by looking at quantum interference around causal loops (almost 100% analogous to the way interference on a violin string will restrict stable modes to certain frequencies, interference around a causal loop will only allow stable self-sustaining situations to survive, so any grandfather paradox would self-interfere and not be allowed). The main difference is that with closed timelike curves the quantum effects can get a lot stronger than without them, even affecting very large objects.

phlux 15y ago

>I am constantly reading that information cannot travel faster than light (I am not sure if this analogy works - if it is flawed, please help me fix it until it is correct) Lets assume that you are on a computer connected to another machine that is at the exact furthest opposite possible on the globe, via fiber. The remote machine will display a question, which you have never seen before and do not know the answer to, but you can read the question and then the answer to the question will be displayed. Is it impossible that you could not derive an answer to the question fast than the answer could be displayed? Thus, information (your derived answer) arriving in your head faster than the information arriving on screen as delivered at the speed of light? My assumption is that information can be derived/created/transferred/understood faster than linear light travel as the processing of thoughts in our minds is semaphoric/symbolic and parrallel - light transmission is serial.

ignifero 15y ago

Yes because it would break Causality, you would have the broken cup along with the intact one. You may ask "why have causality"? Well, it's your pick, either FTL, or Causality. btw, a great book about time (and time loops) is Igor Novikov's "River of time"