4 ms·

Can you or someone else "explain to me like I'm five" why faster-than-light travel violates causality?

SerLava 10y ago
To travel faster than light, you'd have to skip places in space instead of moving through them continuously. So you'd be constantly appearing out of nowhere. Other reasons.
euyyn 10y ago
It's one of a bunch of mathematical conclusions of Special Relativity (the most famous one being that objects at rest have a "kinetic" energy of mc^2). I'll try to explain it simply, without the math: Imagine a friend and you, moving very fast with respect to each other. You measure your friend's clock as advancing slowly (due to time dilation), on account of the fact that he's moving with respect to you. But your friend measures _your_ clock as advancing more slowly than _his_, because motion is relative. The way to reconcile this mathematically is to give up on absolute simultaneity of distant events. In other words, when two things happen in different places, at the same time according to your clock, they happened at different times according to your friend's clock. This is analogous to how you measure your friend to be in different places in different times (because he's moving with respect to you), but he measures himself to be stationary, always in the same place. To recap: If comparing things at different moments, "being in the same position or not" is relative to the observer. If comparing things at different places, "happening at the same time or not" is relative to the observer. That's how causality comes in. If your friend tosses a very fast ball at you, and the moment you receive the ball you start towards him faster-than-c, you'll arrive where he is before he tossed it (if you went fast enough). That's what the equations say. Now the breaking of absolute simultaneity allows naturally for disagreements on what of two things happened first, assuming they happened far enough from each other that light couldn't cover the distance in time. But by putting faster-than-c velocities in the equations, we have your friend and you disagreeing on what happened first (him tossing the ball, or you arriving where he is?) in what (to him) is the same place. And it's not just a confusion of the mind: all your measuring and recording artifacts will agree with you, and all of his will agree with him. The mathematical conclusion is that if you can send information faster-than-c, you can send it back in time and break causality.
- mariodiana 10y ago
  I want to make sure I understand "time dilation," as you describe it. I have no formal background in science or math. So, please bear with me. For the sake of making the math easy, I'm going to talk about traveling faster than the speed of light, but only for the sake of argument. You'll see why. I understand it this way. Assume I were traveling away from my friend at twice the speed of light. My time of departure is 12:00:00. After 1 second (my clock) I have traveled 2 light-seconds. Light reflected off the face of my clock now travels towards my friend, reading 12:00:01. We'll call this view of my clock a "snapshot." From my friend's point of view, that snapshot reaches him at 12:00:03. Yes? Now, we allow for 1 additional second, my time. I have traveled in total 4 light-seconds. My 12:00:02 snapshot now travels towards my friend. It reaches him at 12:00:06, his time. Yes? So, from my friend's point of view, my time has seemed to slow down. Is that what you mean by time dilation?
  - zo1 10y ago
    Try not to think of time-dilation in relation to "light travelling". Time dilation just means that time passes more slowly for you due to your speed relative to an observer. So imagine you have an identical twin, and you leave on a Journey to a nearby solar system at say 0.9x the speed of light. Your twin stays on earth and for arguments sake you can say he is "stationary" in comparison to you. When you come back, he'll be much more "aged" than you. You had a clock on-board your craft, and it measured your trip as say "2 years". Likewise, your twin also had a clock and it measured your trip as having lasted "20 years". The clocks are not wrong. And trust me, this isn't some science-fiction type thing we're discussing. GPS satellites orbiting the Earth carry very accurate atomic-clocks, and they need to take time-dilation into account in order to compensate for the drift in values, simply due to the accuracy necessary for GPS. Just had a look on the wiki page, and the drift is approximately 38 microseconds per day. When I first encountered this concept in high-school randomly, I was quite shocked at how "non-complicated" the equation for it is. https://en.wikipedia.org/wiki/Time_dilation#Simple_inference_of_time_dilation_due_to_relative_velocity https://en.wikipedia.org/wiki/Time_dilation#Simple_inference... https://en.wikipedia.org/wiki/Global_Positioning_System https://en.wikipedia.org/wiki/Global_Positioning_System
talmand 10y ago
My understanding of it is that it all goes to crap when outside observers manage to see the outcome of your event before they see the event started. Therefore, to be safe, keep your faster-than-light travel a secret.
lisper 10y ago
It would allow you to send information backwards in time.
jandrese 10y ago
Effectively, the speed of light is the rate at which Cause and Effect propagate through the universe. The speed of light isn't an intrinsic property of light, it just so happens that light travels that fast and it was the first thing we were able to measure that was traveling at the limit. If you were traveling at FTL velocities, you would effectively get there before you left.
- mjevans 10y ago
  What if you arrived at some point after you actually left? (Not the other data of you leaving arriving at that point) Granted I believe that doing so would probably involve bending space/time so that your actual position was in the new frame.
  - krapp 10y ago
    You're still getting there at a point in time earlier than the universe permits. This is essentially the same as traveling backwards in time, and violating causality by allowing effect to precede cause.
- mariodiana 10y ago
  > If you were traveling at FTL velocities, you would effectively get there before you left. To be more exact, aren't you saying that you would seem to get there before you left, from the point of view of someone not traveling with you who is observing you from some particular vantage point?
  - kirykl 10y ago
    would the observer's time slow down enough to run negative (backwards) from the POV of those traveling FTL ?

To travel faster than light, you'd have to skip places in space instead of moving through them continuously. So you'd be constantly appearing out of nowhere. Other reasons.

euyyn 10y ago

It's one of a bunch of mathematical conclusions of Special Relativity (the most famous one being that objects at rest have a "kinetic" energy of mc^2). I'll try to explain it simply, without the math: Imagine a friend and you, moving very fast with respect to each other. You measure your friend's clock as advancing slowly (due to time dilation), on account of the fact that he's moving with respect to you. But your friend measures _your_ clock as advancing more slowly than _his_, because motion is relative. The way to reconcile this mathematically is to give up on absolute simultaneity of distant events. In other words, when two things happen in different places, at the same time according to your clock, they happened at different times according to your friend's clock. This is analogous to how you measure your friend to be in different places in different times (because he's moving with respect to you), but he measures himself to be stationary, always in the same place. To recap: If comparing things at different moments, "being in the same position or not" is relative to the observer. If comparing things at different places, "happening at the same time or not" is relative to the observer. That's how causality comes in. If your friend tosses a very fast ball at you, and the moment you receive the ball you start towards him faster-than-c, you'll arrive where he is before he tossed it (if you went fast enough). That's what the equations say. Now the breaking of absolute simultaneity allows naturally for disagreements on what of two things happened first, assuming they happened far enough from each other that light couldn't cover the distance in time. But by putting faster-than-c velocities in the equations, we have your friend and you disagreeing on what happened first (him tossing the ball, or you arriving where he is?) in what (to him) is the same place. And it's not just a confusion of the mind: all your measuring and recording artifacts will agree with you, and all of his will agree with him. The mathematical conclusion is that if you can send information faster-than-c, you can send it back in time and break causality.

mariodiana 10y ago
I want to make sure I understand "time dilation," as you describe it. I have no formal background in science or math. So, please bear with me. For the sake of making the math easy, I'm going to talk about traveling faster than the speed of light, but only for the sake of argument. You'll see why. I understand it this way. Assume I were traveling away from my friend at twice the speed of light. My time of departure is 12:00:00. After 1 second (my clock) I have traveled 2 light-seconds. Light reflected off the face of my clock now travels towards my friend, reading 12:00:01. We'll call this view of my clock a "snapshot." From my friend's point of view, that snapshot reaches him at 12:00:03. Yes? Now, we allow for 1 additional second, my time. I have traveled in total 4 light-seconds. My 12:00:02 snapshot now travels towards my friend. It reaches him at 12:00:06, his time. Yes? So, from my friend's point of view, my time has seemed to slow down. Is that what you mean by time dilation?
- zo1 10y ago
  Try not to think of time-dilation in relation to "light travelling". Time dilation just means that time passes more slowly for you due to your speed relative to an observer. So imagine you have an identical twin, and you leave on a Journey to a nearby solar system at say 0.9x the speed of light. Your twin stays on earth and for arguments sake you can say he is "stationary" in comparison to you. When you come back, he'll be much more "aged" than you. You had a clock on-board your craft, and it measured your trip as say "2 years". Likewise, your twin also had a clock and it measured your trip as having lasted "20 years". The clocks are not wrong. And trust me, this isn't some science-fiction type thing we're discussing. GPS satellites orbiting the Earth carry very accurate atomic-clocks, and they need to take time-dilation into account in order to compensate for the drift in values, simply due to the accuracy necessary for GPS. Just had a look on the wiki page, and the drift is approximately 38 microseconds per day. When I first encountered this concept in high-school randomly, I was quite shocked at how "non-complicated" the equation for it is. https://en.wikipedia.org/wiki/Time_dilation#Simple_inference_of_time_dilation_due_to_relative_velocity https://en.wikipedia.org/wiki/Time_dilation#Simple_inference... https://en.wikipedia.org/wiki/Global_Positioning_System https://en.wikipedia.org/wiki/Global_Positioning_System

talmand 10y ago

My understanding of it is that it all goes to crap when outside observers manage to see the outcome of your event before they see the event started. Therefore, to be safe, keep your faster-than-light travel a secret.

lisper 10y ago

It would allow you to send information backwards in time.

jandrese 10y ago

Effectively, the speed of light is the rate at which Cause and Effect propagate through the universe. The speed of light isn't an intrinsic property of light, it just so happens that light travels that fast and it was the first thing we were able to measure that was traveling at the limit. If you were traveling at FTL velocities, you would effectively get there before you left.

mjevans 10y ago
What if you arrived at some point after you actually left? (Not the other data of you leaving arriving at that point) Granted I believe that doing so would probably involve bending space/time so that your actual position was in the new frame.
- krapp 10y ago
  You're still getting there at a point in time earlier than the universe permits. This is essentially the same as traveling backwards in time, and violating causality by allowing effect to precede cause.
mariodiana 10y ago
> If you were traveling at FTL velocities, you would effectively get there before you left. To be more exact, aren't you saying that you would seem to get there before you left, from the point of view of someone not traveling with you who is observing you from some particular vantage point?
- kirykl 10y ago
  would the observer's time slow down enough to run negative (backwards) from the POV of those traveling FTL ?