9 ms·

Because of relativity, you can have locations A and B in spacetime and a frame of reference such that A happens before B and a different frame such that B happe

by ikeboy 6y ago

Because of relativity, you can have locations A and B in spacetime and a frame of reference such that A happens before B and a different frame such that B happens before A. So, if you're at A, you can travel faster than the speed of light and arrive before B happens. Then, after B happens, you can travel back to before A happens.

komali2 6y ago
Wait whaaaat? This is the first I've heard of this. I've a vague understanding of causality. I thought the arrow of Time moved in one direction only? How can A come before B come before A from, from one observer?
- ikeboy 6y ago
  It can't, if that observer always moves slower than the speed of light in all reference frames.
- 6gvONxR4sf7o 6y ago
  If events A and B are close to each other in space time (light from one can have arrived at the other by then), then the ordering is fixed. If they are far from each other (light won't get from one to another by then), then time gets weirder. The notion of what is simultaneous to what gets weird. If you and I are moving relative to each other, we're going to disagree on which distant events happen at the same time. If that happens, then whether something's happening now might change depending on reference frame. The ordering can be t(A) = t(B) or t(A) != t(B). Since no frame is privileged, we can also change the other way around, so t(A) != t(B) can be t(A) < t(B) or t(A) > t(B) depending on reference frame. It's only flexible like that for things that could be simultaneous in some frame, which at least in special relativity means light from one can't have gotten to another yet. I never took general relativity, so who knows how it works in that setting. https://en.wikipedia.org/wiki/Relativity_of_simultaneity#Thought_experiments https://en.wikipedia.org/wiki/Relativity_of_simultaneity#Tho...
  - Causality1 6y ago
    Relativity of simultaneity was definitely a wild concept for me in physics class. The idea that a near-c train is longer than a tunnel to its passengers and hence is never fully inside the tunnel to them but is, after relativistic compression, shorter than the tunnel and hence at one moment entirely inside it to a resting observer blew my mind.
- avmich 6y ago
  Novikov explains it approximately this way. Suppose you have a wormhole between points A and B. So from A to B - and from B to A - you can go by two different roads, one is pretty long (say, 1 light month), and another is very short (negligible distance). Next, suppose B end swiftly rotates around point A, so fast that time in B goes slower than time in A. This means clock in B is going slower than in A, and over 20 years in A only 10 years will pass by clock in B. That can be confirmed by observation - telescopes in A will see slower clock in B, telescopes in B will see faster clock in A (it's B going around A, not vice versa). That is, if we're talking about telescopes, looking through "regular" space. Since A and B are also connected by the wormhole, looking through wormhole will show us that clocks are synchronized. Over the wormhole, A and B are not moving, so clocks don't deviate from one another. We can stop B flying around A now. Now suppose B clock has accumulated 10 years of difference from A clock - again, looking via "regular" space. We're leaving point A, flying to point B over the "regular" space and spending 5 years - way slower than speed of light, so our clock is practically synchronous to A clock. At the start of flight, A clock show 20 years, and B clock is 10 years, and by the end of flight A clock is 25 years and B clock is 15 years. After arriving at B, we jump into wormhole and get back to A, when A clock shows the same as B clock - that is, 15 years. According to A clock, we left at 20 years and came back at 15 years. Time travel.
  - AaronFriel 6y ago
    This is the scenario the author Stephen Baxter used in the book Timelike Infinity. Wormhole technology on a ship sent in a relativistic journey circling the solar system, used to bootstrap humanity and influence it in the past. There is a catch, this sort of time travel like many others only allows one to go back as far as the creation of the wormhole. Sort of like in Primer, it isn't a general form of time travel. You can take the wormhole forward, or from the other end backwards, but not prior to the time it was created.
  - Acinyx 6y ago
    Nice explanation, this made more sense to me than most of the explanations I have heard about this before. But, wouldn't this just constrain the possible features of a wormhole? The issue is fixed if traveling/observing through the wormhole has the same time dilation: Both through telescope as through the wormhole, the clocks can be observed to go slower. After 20 years at A, both the telescope as observation through the wormhole show 10 years have passed at B. If we don't stop B from spinning: The regular space traveler takes 5 years, as observed from A, to reach B, so reaches B at A:25 years, B:12,5 years. If they leave immediately after arrival, they'll end up again at A at A:30y, B:15y. Otoh, the wormhole travel leaves at A:20y ends up at B at 10y and gets back to A immediately: A still at 20y and B at 10y. Mixed travel:regular travel starts at A:20, B:10y, reaches B at A:25y, B:12,5 and jumps back through the wormhole at the same time as seen from A and B.
- Dylan16807 6y ago
  So let's make this assumption explicit: A and B are far apart in distance, and the time between them (in seconds) is smaller than the distance (in light seconds). This allows different observers to reasonably disagree about which came first. Basic relativity. Alice sees A happen before B. Bob sees B happen before A. Everyone will agree that once they saw one event, it was too late to reach the other event before it happened, even at the speed of light. But faster than light? Then you could get there before it happens. > How can A come before B come before A from, from one observer? An observer wouldn't see that. What they might see is someone exiting a wormhole before they entered it. Let's say Alice observes A, then wormholes over to cause B. And Bob sees B, then wormholes over to cause A. Anyone watching from the outside will see a logical-seeming series of events: Alice and Bob exit wormholes and each cause an event. A "different" Alice and Bob watch the events and enter wormholes. Some observers will see Alice exit before entering. Some will see Bob exit before entering. Some will see both. That's obviously weird. But they'll see nothing weird about A and B. A and B are perfectly normal.
  - Acinyx 6y ago
    But what makes this a paradox/time traveling? We are talking about observers seeing wrong order (leaving the wormhole at point B before entering at point A), but that's because they are observing everything at a delay or something like that, but I don't see any time travel happening in this example or any order of flying through wormholes that I can come up with.
    - Dylan16807 6y ago
      Alice sees A then causes B. Bob sees B then causes A. That means that when you analyze the whole system, A causes A. There is a time loop. You could easily make this into a paradox, too. What if Bob goes and prevents A instead? What if events A and B are actually the birth of Alice and Bob, and after they leave the wormhole they land on the other planet and become each other's parents?
  - iso1631 6y ago
    > This allows different observers to reasonably disagree about which came first. Basic relativity. that happens all the time. Lets set aside the Earth being curved for a moment - I'll use real cities but these could be asteroids floating in intergalactic space at the same distance. I'm in London, I see an event happen in London, Moscow, New York and LA all at the same time, at 12:00:00.500 UTC according to me. London-Moscow 9ms London-NY 66ms London-LA 103ms Lets imagine they are in a straight line too and all in the same reference pane. The events actually occurred in the follow order, at the following milliseconds past 12:00:00 UTC, according to our NTP synced clocks. LA: 397ms NY: 434ms Moscow: 491ms London: 500ms From LA's point of view they would see LA: 397ms NY: 471ms London: 603ms Moscow: 603ms From Moscow's POV Moscow: 491ms London: 509ms NY: 509ms LA: 509ms From NY's POV LA: 434ms NY: 434ms London: 566ms Moscow: 566ms So LA thinks LA was first, NY was second, London and Moscow co-timed third. NY thinks LA and NY were co-timed first, then London and Moscow co-timed third. Moscow thinks Moscow was first, everywhere else was second. London thinks everywhere was first So we disagree. So what. Now sure, if your wormholes are in different reference frames you can get confusions, but why would a wormhole with two ends fixed in the same reference frame and neither end accelerating allow passing information backwards in time? I know relativity says there is no universal clock everyone agrees on, but isn't that only applicable for points in different reference planes (and even then you can surely adjust if you know your relative velocity to the universal point)? Back to cosmic scales. Point A, B, C, D, all 10 light years apart, same reference frame Point A event happens at t+0 B sees at t+10, C and t+20, D at t+30. D also has a wormhole to A, so sees the event in the wormhole at t+0, and causes a separate event (light up a sign saying "A event just happend") C would see that D is flagging the event at t+10, but have to wait until t+20 to see it. Why is that bad? How would D get a signal back to A before t+0. I guess a spaceship in a different reference frame (say v=0.99c) at D could see (old fashioned looking out the window) the "event just happened" message, then use a separate wormhole to send a message to a spaceship at A travelling in the same reference frame as spaceship D. Spaceship A could then light up a message, which planet A could see, but would planet A see that before t+0 on their local clock?
Acinyx 6y ago
I don't fully understand; > if you're at A, you can travel faster than the speed of light and arrive before B happens But if we assume A and B happen "at the same time" and your frame of reference is closer to A than B right? As the light showing B happening is still traveling to the observer while A has already arrived. If that's the case, > Then, after B happens, you can travel back to before A happens. Doesn't make sense, If the following happens: > Traveler starts at A, jumps the wormhole to point B and immediately jumps back to A, arriving 10 seconds after leaving A. The observer would see: > Traveler starts at A, arrives back at A + 10 seconds and a while later, they are seen exiting and entering the wormhole at B while also being visible at A (Assuming they didn't do anything after exiting there)
- ben_w 6y ago
  These kinds of discussion only make sense to me with images rather than words, but: > But if we assume A and B happen "at the same time" Is something you can’t do in relativity. If you see A happen before B, at the same time as B, or after B, depends on your motion relative to A and B. https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

by ikeboy 6y ago

komali2 6y ago

Wait whaaaat? This is the first I've heard of this. I've a vague understanding of causality. I thought the arrow of Time moved in one direction only? How can A come before B come before A from, from one observer?

ikeboy 6y ago
It can't, if that observer always moves slower than the speed of light in all reference frames.
6gvONxR4sf7o 6y ago
If events A and B are close to each other in space time (light from one can have arrived at the other by then), then the ordering is fixed. If they are far from each other (light won't get from one to another by then), then time gets weirder. The notion of what is simultaneous to what gets weird. If you and I are moving relative to each other, we're going to disagree on which distant events happen at the same time. If that happens, then whether something's happening now might change depending on reference frame. The ordering can be t(A) = t(B) or t(A) != t(B). Since no frame is privileged, we can also change the other way around, so t(A) != t(B) can be t(A) < t(B) or t(A) > t(B) depending on reference frame. It's only flexible like that for things that could be simultaneous in some frame, which at least in special relativity means light from one can't have gotten to another yet. I never took general relativity, so who knows how it works in that setting. https://en.wikipedia.org/wiki/Relativity_of_simultaneity#Thought_experiments https://en.wikipedia.org/wiki/Relativity_of_simultaneity#Tho...
- Causality1 6y ago
  Relativity of simultaneity was definitely a wild concept for me in physics class. The idea that a near-c train is longer than a tunnel to its passengers and hence is never fully inside the tunnel to them but is, after relativistic compression, shorter than the tunnel and hence at one moment entirely inside it to a resting observer blew my mind.
avmich 6y ago
Novikov explains it approximately this way. Suppose you have a wormhole between points A and B. So from A to B - and from B to A - you can go by two different roads, one is pretty long (say, 1 light month), and another is very short (negligible distance). Next, suppose B end swiftly rotates around point A, so fast that time in B goes slower than time in A. This means clock in B is going slower than in A, and over 20 years in A only 10 years will pass by clock in B. That can be confirmed by observation - telescopes in A will see slower clock in B, telescopes in B will see faster clock in A (it's B going around A, not vice versa). That is, if we're talking about telescopes, looking through "regular" space. Since A and B are also connected by the wormhole, looking through wormhole will show us that clocks are synchronized. Over the wormhole, A and B are not moving, so clocks don't deviate from one another. We can stop B flying around A now. Now suppose B clock has accumulated 10 years of difference from A clock - again, looking via "regular" space. We're leaving point A, flying to point B over the "regular" space and spending 5 years - way slower than speed of light, so our clock is practically synchronous to A clock. At the start of flight, A clock show 20 years, and B clock is 10 years, and by the end of flight A clock is 25 years and B clock is 15 years. After arriving at B, we jump into wormhole and get back to A, when A clock shows the same as B clock - that is, 15 years. According to A clock, we left at 20 years and came back at 15 years. Time travel.
- AaronFriel 6y ago
  This is the scenario the author Stephen Baxter used in the book Timelike Infinity. Wormhole technology on a ship sent in a relativistic journey circling the solar system, used to bootstrap humanity and influence it in the past. There is a catch, this sort of time travel like many others only allows one to go back as far as the creation of the wormhole. Sort of like in Primer, it isn't a general form of time travel. You can take the wormhole forward, or from the other end backwards, but not prior to the time it was created.
- Acinyx 6y ago
  Nice explanation, this made more sense to me than most of the explanations I have heard about this before. But, wouldn't this just constrain the possible features of a wormhole? The issue is fixed if traveling/observing through the wormhole has the same time dilation: Both through telescope as through the wormhole, the clocks can be observed to go slower. After 20 years at A, both the telescope as observation through the wormhole show 10 years have passed at B. If we don't stop B from spinning: The regular space traveler takes 5 years, as observed from A, to reach B, so reaches B at A:25 years, B:12,5 years. If they leave immediately after arrival, they'll end up again at A at A:30y, B:15y. Otoh, the wormhole travel leaves at A:20y ends up at B at 10y and gets back to A immediately: A still at 20y and B at 10y. Mixed travel:regular travel starts at A:20, B:10y, reaches B at A:25y, B:12,5 and jumps back through the wormhole at the same time as seen from A and B.
Dylan16807 6y ago
So let's make this assumption explicit: A and B are far apart in distance, and the time between them (in seconds) is smaller than the distance (in light seconds). This allows different observers to reasonably disagree about which came first. Basic relativity. Alice sees A happen before B. Bob sees B happen before A. Everyone will agree that once they saw one event, it was too late to reach the other event before it happened, even at the speed of light. But faster than light? Then you could get there before it happens. > How can A come before B come before A from, from one observer? An observer wouldn't see that. What they might see is someone exiting a wormhole before they entered it. Let's say Alice observes A, then wormholes over to cause B. And Bob sees B, then wormholes over to cause A. Anyone watching from the outside will see a logical-seeming series of events: Alice and Bob exit wormholes and each cause an event. A "different" Alice and Bob watch the events and enter wormholes. Some observers will see Alice exit before entering. Some will see Bob exit before entering. Some will see both. That's obviously weird. But they'll see nothing weird about A and B. A and B are perfectly normal.
- Acinyx 6y ago
  But what makes this a paradox/time traveling? We are talking about observers seeing wrong order (leaving the wormhole at point B before entering at point A), but that's because they are observing everything at a delay or something like that, but I don't see any time travel happening in this example or any order of flying through wormholes that I can come up with.
  - Dylan16807 6y ago
    Alice sees A then causes B. Bob sees B then causes A. That means that when you analyze the whole system, A causes A. There is a time loop. You could easily make this into a paradox, too. What if Bob goes and prevents A instead? What if events A and B are actually the birth of Alice and Bob, and after they leave the wormhole they land on the other planet and become each other's parents?
- iso1631 6y ago
  > This allows different observers to reasonably disagree about which came first. Basic relativity. that happens all the time. Lets set aside the Earth being curved for a moment - I'll use real cities but these could be asteroids floating in intergalactic space at the same distance. I'm in London, I see an event happen in London, Moscow, New York and LA all at the same time, at 12:00:00.500 UTC according to me. London-Moscow 9ms London-NY 66ms London-LA 103ms Lets imagine they are in a straight line too and all in the same reference pane. The events actually occurred in the follow order, at the following milliseconds past 12:00:00 UTC, according to our NTP synced clocks. LA: 397ms NY: 434ms Moscow: 491ms London: 500ms From LA's point of view they would see LA: 397ms NY: 471ms London: 603ms Moscow: 603ms From Moscow's POV Moscow: 491ms London: 509ms NY: 509ms LA: 509ms From NY's POV LA: 434ms NY: 434ms London: 566ms Moscow: 566ms So LA thinks LA was first, NY was second, London and Moscow co-timed third. NY thinks LA and NY were co-timed first, then London and Moscow co-timed third. Moscow thinks Moscow was first, everywhere else was second. London thinks everywhere was first So we disagree. So what. Now sure, if your wormholes are in different reference frames you can get confusions, but why would a wormhole with two ends fixed in the same reference frame and neither end accelerating allow passing information backwards in time? I know relativity says there is no universal clock everyone agrees on, but isn't that only applicable for points in different reference planes (and even then you can surely adjust if you know your relative velocity to the universal point)? Back to cosmic scales. Point A, B, C, D, all 10 light years apart, same reference frame Point A event happens at t+0 B sees at t+10, C and t+20, D at t+30. D also has a wormhole to A, so sees the event in the wormhole at t+0, and causes a separate event (light up a sign saying "A event just happend") C would see that D is flagging the event at t+10, but have to wait until t+20 to see it. Why is that bad? How would D get a signal back to A before t+0. I guess a spaceship in a different reference frame (say v=0.99c) at D could see (old fashioned looking out the window) the "event just happened" message, then use a separate wormhole to send a message to a spaceship at A travelling in the same reference frame as spaceship D. Spaceship A could then light up a message, which planet A could see, but would planet A see that before t+0 on their local clock?

Acinyx 6y ago

I don't fully understand; > if you're at A, you can travel faster than the speed of light and arrive before B happens But if we assume A and B happen "at the same time" and your frame of reference is closer to A than B right? As the light showing B happening is still traveling to the observer while A has already arrived. If that's the case, > Then, after B happens, you can travel back to before A happens. Doesn't make sense, If the following happens: > Traveler starts at A, jumps the wormhole to point B and immediately jumps back to A, arriving 10 seconds after leaving A. The observer would see: > Traveler starts at A, arrives back at A + 10 seconds and a while later, they are seen exiting and entering the wormhole at B while also being visible at A (Assuming they didn't do anything after exiting there)

ben_w 6y ago
These kinds of discussion only make sense to me with images rather than words, but: > But if we assume A and B happen "at the same time" Is something you can’t do in relativity. If you see A happen before B, at the same time as B, or after B, depends on your motion relative to A and B. https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity