4 ms·

> Take the case of two people on Earth, one stationary and one travelling past in a car. They each have a different reference frame, and a different set events

by awb 5y ago

> Take the case of two people on Earth, one stationary and one travelling past in a car. They each have a different reference frame, and a different set events which make up their present. Now imagine that there is an alien government in Andromeda debating a possible invasion of Earth. In the reference frame of the person on Earth who is moving, the debate is still going on. But, in the reference frame of the stationary person, it is a day or so later in Andromeda, and the invasion fleet is already on it's way! I’m pretty sure this is wrong (or misleading) on multiple levels: 1) An event in Andromeda occurs at single moment, it’s our observation of the event that is warped. So there aren’t two realities, one where they’re still debating and one where they aren’t. In both realities the debate has concluded, it’s just our observation of the event takes place at different reference times. 2) The car would need to be moving at a non-insignificant fraction of the speed of light to have an observation dilution of a day or more. It’s physically impossible for humans to move that fast with our current technology. https://www.quora.com/Does-time-slow-down-in-a-moving-vehicle#:~:text=According%20to%20the%20theory%20of,to%20the%20speed%20of%20light%20 https://www.quora.com/Does-time-slow-down-in-a-moving-vehicl...! You’d have to be traveling at 0.86c for an entire day to be observing 1 day slower than a stationary person.

Jeff_Brown 5y ago
The reason the scenario posits an invasion from so far away is that that's about how much distance it takes for a small speed difference here to translate into a day long time difference there.
- awb 5y ago
  You could imagine an event happening a day later from a traveling car, but that’s about it. Two people can’t actually observe an event a day later from the same reference point (Earth), and naturally the event occurs at a singular time at Andromeda’s reference point. Same with any point between Earth and Andromeda. So, if you can’t observe it at different times from the same reference point did it actually happen at different times? From the Wikipedia page on the thought experiment, check out Criticisms: https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument#Criticisms https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argu... > The interpretations of relativity used in the Rietdijk–Putnam argument and the Andromeda paradox are not universally accepted. > Howard Stein[5] and Steven F. Savitt[6] note that in relativity the present is a local concept that cannot be extended to global hyperplanes. > Furthermore, N. David Mermin[7] states: That no inherent meaning can be assigned to the simultaneity of distant events is the single most important lesson to be learned from relativity. — David Mermin, It’s About Time
  - goblin89 5y ago
    A philosophical idea may not have meaningful implications within the scope of physics, but that doesn’t make it useless. Rietdijk–Putnam argument, for example, appears to sort of elevate the idea of differing planes of simultaneity, taken from special relativity, to an idea of a four-dimensional universe. It’s a curious thought experiment and I wonder how it reconciles with a model where the universe is considered to be 4D with the fourth dimension being time. My intuition doesn’t work all that well beyond three dimension so I can’t tell if those are essentially the same or not. It’s not falsifiable, but neither is string theory.
- Retric 5y ago
  That’s not how it works. But even beyond that you have the math wrong. Andromeda galaxy is only 2.5 million light-years from Earth. To get a 1 day difference in time dilation across 2.5 million years would take 1 in 913 million difference which someone traveling at even 100MPH (45m/s) isn’t going to see. (1 - (45^2/((3x10^8)^2))^0.5 simply isn’t enough.
  - penteract 5y ago
    I did the calculations from https://en.wikipedia.org/wiki/Special_relativity#Lorentz_transformation_and_its_inverse https://en.wikipedia.org/wiki/Special_relativity#Lorentz_tra... and it comes to ~0.25 years if the person in the car is moving towards Andromeda at 30m/s. γ is indeed very close to 1, so the humans' watches would still agree closely after 2.5 million years, but the humans would be a quarter of a light year apart at that point, so they need to disagree about something to get the same observed speed of light from Andromeda.
    - deleted 5y ago
      [deleted]
    - Retric 5y ago
      Direction of travel was undefined in the example, just a car on the earth. Suppose they where traveling 30m/s away from andromeda vs towards it suddenly the timing switches. Chose a specific vector and both observers exactly agree with the events timing. Essentially the only thing they disagree about is what Andromeda’s location and velocity is. (Edited this several times, but it all simplified to choice of vectors.) But even that’s assuming spaceships rather than “one stationary and one travelling past in a car” the guy in a car is never going to end up light years distant from the stationary person.
- feoren 5y ago
  So as I'm driving, the debate is going on, but as soon as I slam on my brakes, it's already happened! They're already on their way! Oh but now the squirrel is out of the road and I'm late so I speed way back up and the debate hasn't even started yet now. No. The only thing that is "happening right now" on Andromeda from my point of view is the causality wave (light) that is hitting me right now. It's nonsense to talk about anything happening "right now" if its causality wave hasn't reached me yet.
bena 5y ago
> I’m pretty sure this is wrong (or misleading) on multiple levels: > 1) An event in Andromeda occurs at single moment, it’s our observation of the event that is warped. So there aren’t two realities, one where they’re still debating and one where they aren’t. In both realities the debate has concluded, it’s just our observation of the event takes place at different reference times. Yeah, they're making the mistake of assuming a minute is a minute. It's closer to the difference in angular motion vs linear motion. If you are closer to the center, you move very little linearly. If you are closer to the outer edge, you move very little angularly. Now you look at the linear distance as time passed for the object and the angular distance as the amount of "space-time" an object has gone. The amount of "space-time" is constant. It's a bit of a wonky model, but it's probably the simplest one that encapsulates the concept. So just because they spent 3 days debating, you spent 5 days watching, and Bob only spent 1 day watching; that doesn't mean Bob is 2 days ahead of Andromeda and 4 days ahead of you. Since we are all also traveling through space at different speeds, we've all traveled the same amount of space-time, so we're all seeing the same things. As for our perception, that more depends on our relative positions from each other rather than speed. Because nothing is faster than light, we can never experience an event before we see it.
Diggsey 5y ago
Actually no, the parent comment is correct: When you undergo an acceleration, it corresponds to a hyperbolic rotation of the spacetime around you (from your frame of reference). When the acceleration is small, the rotation is also very, very small; however at very large distances, even a small rotation can have a large effect. In the example, that "large effect" is the alien debate moving in time forward or backwards a day (again, from the accelerated reference frame). This has nothing to do with the observation delay, it's just an artefact of what it means for two far apart events to happen "simultaneously": simultaneity is a relative property, so two things that happen simultaneously in one reference frame might not happen simultaneously in another. The reason this doesn't introduce a paradox is because without a specific reference frame, events only have a partial order: when you compare two events, there are three possible results: 1) Event A is in the past light-cone of event B. 2) Event B is in the past light-cone of event A. 3) Neither event is in the past light-cone of the other. For 1+2 this means that the time between the events is greater than the distance between the events, whereas for 3 it means the reverse. For 1+2, the ordering of the events is the same for any reference frame. For 3, there will always be some frame of reference where both events appear to happen at exactly the same time, so nothing can be said about their order without choosing a reference frame.
- awb 5y ago
  I don’t think the interpretation you describe is widely held. Check out the criticisms of the paradox: https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument#Criticisms https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argu...
  - Diggsey 5y ago
    That it has critics does not imply it's not widely held. Regardless, that's just a dispute about terminology: sure we could say that we should only talk about events being "simultaneous" if they happen in exactly the same place - but in that case it's not a particularly useful term anymore. It's just an argument about how we apply words derived from newtonian intuitions to a relativistic context. Mathematically, for a given reference frame we can draw a space-time diagram, and certain events will be above the horizontal line. We can draw the same space-time diagram for a different reference frame and have those same events be below the horizontal line. Whether that horizontal line has any actual physical relevance doesn't really matter IMO, the maths says that events move in relation to the line when the frame of reference accelerates, and that's what the "paradox" is illustrating. Having said that, if the "one way speed of light" exists (https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light) and even if it's not the same in all directions, then this "plane of simultaneity" is definitely a physical thing, since you can send a beam of light to a target, measure how long it takes to return, and using the speed of light in that direction you can work out exactly when that beam hit the target in your local time - ie. you've found two events to be simultaneous from your frame of reference, that are not in the same location.
irthomasthomas 5y ago
I once read a physicist on this, in a newspaper, who said "if I didn't believe in free-will, then I wouldn't bother getting up in the morning." I thought that was cute. I think the term free-will has clouded the issue a bit. Forget about free-will. Scientists, instead, normally talk about Determinism vs indeterminism, or nondeterminism in CS. These have a more precise definition than that other thing. The universe is deterministic if the outcome of events are completely determined by previous events. Or the universe is nondeterministic if the outcome of events are not certain, but follow a random distribution. As to the precise details of the speeds and distances etc, it is a thought experiment, swap the car for a rocket traveling past at 0.86c, if you like. Or instead of aliens deliberating all day, maybe they are more efficient than we are, and they make their decision instantly by flipping a digital coin powered by the perfectly nondeterministic quantum fluctuations of the vacuum.[0] For earth rocket man, there is a moment before the quantum dice roll when the decision is indeterminable, it's completely random. But at the same moment in the stationary frame, the fleet is already on it's way. Unless you wish to argue that a stationary reference frame is somehow more valid than a moving one, you have to accept that in the moving frame, the event of the dice roll has still not occurred and should be random, but how can it be random, if it has already happened in the stationary frame. Hence the paradox. I hope that cleared things up some? [0] The vacuum has an api: https://qrng.anu.edu.au/
- afiori 5y ago
  > For earth rocket man, there is a moment before the quantum dice roll when the decision is indeterminable Assuming earth rocket man's speed does not change, if he quickly alternates between 0c and 0.99c he could flip flop between "determinate" and "indeterminate". Anyway for all system on earth the decision event would be in the relativistic present with no causality order https://en.wikipedia.org/wiki/Minkowski_space#Causal_structure https://en.wikipedia.org/wiki/Minkowski_space#Causal_structu... Part of the point of relativity is that t1<t2 is not a good test of "comes before"

by awb 5y ago

Jeff_Brown 5y ago

The reason the scenario posits an invasion from so far away is that that's about how much distance it takes for a small speed difference here to translate into a day long time difference there.

awb 5y ago
You could imagine an event happening a day later from a traveling car, but that’s about it. Two people can’t actually observe an event a day later from the same reference point (Earth), and naturally the event occurs at a singular time at Andromeda’s reference point. Same with any point between Earth and Andromeda. So, if you can’t observe it at different times from the same reference point did it actually happen at different times? From the Wikipedia page on the thought experiment, check out Criticisms: https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument#Criticisms https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argu... > The interpretations of relativity used in the Rietdijk–Putnam argument and the Andromeda paradox are not universally accepted. > Howard Stein[5] and Steven F. Savitt[6] note that in relativity the present is a local concept that cannot be extended to global hyperplanes. > Furthermore, N. David Mermin[7] states: That no inherent meaning can be assigned to the simultaneity of distant events is the single most important lesson to be learned from relativity. — David Mermin, It’s About Time
- goblin89 5y ago
  A philosophical idea may not have meaningful implications within the scope of physics, but that doesn’t make it useless. Rietdijk–Putnam argument, for example, appears to sort of elevate the idea of differing planes of simultaneity, taken from special relativity, to an idea of a four-dimensional universe. It’s a curious thought experiment and I wonder how it reconciles with a model where the universe is considered to be 4D with the fourth dimension being time. My intuition doesn’t work all that well beyond three dimension so I can’t tell if those are essentially the same or not. It’s not falsifiable, but neither is string theory.
Retric 5y ago
That’s not how it works. But even beyond that you have the math wrong. Andromeda galaxy is only 2.5 million light-years from Earth. To get a 1 day difference in time dilation across 2.5 million years would take 1 in 913 million difference which someone traveling at even 100MPH (45m/s) isn’t going to see. (1 - (45^2/((3x10^8)^2))^0.5 simply isn’t enough.
- penteract 5y ago
  I did the calculations from https://en.wikipedia.org/wiki/Special_relativity#Lorentz_transformation_and_its_inverse https://en.wikipedia.org/wiki/Special_relativity#Lorentz_tra... and it comes to ~0.25 years if the person in the car is moving towards Andromeda at 30m/s. γ is indeed very close to 1, so the humans' watches would still agree closely after 2.5 million years, but the humans would be a quarter of a light year apart at that point, so they need to disagree about something to get the same observed speed of light from Andromeda.
  - deleted 5y ago
    [deleted]
  - Retric 5y ago
    Direction of travel was undefined in the example, just a car on the earth. Suppose they where traveling 30m/s away from andromeda vs towards it suddenly the timing switches. Chose a specific vector and both observers exactly agree with the events timing. Essentially the only thing they disagree about is what Andromeda’s location and velocity is. (Edited this several times, but it all simplified to choice of vectors.) But even that’s assuming spaceships rather than “one stationary and one travelling past in a car” the guy in a car is never going to end up light years distant from the stationary person.
feoren 5y ago
So as I'm driving, the debate is going on, but as soon as I slam on my brakes, it's already happened! They're already on their way! Oh but now the squirrel is out of the road and I'm late so I speed way back up and the debate hasn't even started yet now. No. The only thing that is "happening right now" on Andromeda from my point of view is the causality wave (light) that is hitting me right now. It's nonsense to talk about anything happening "right now" if its causality wave hasn't reached me yet.

bena 5y ago

> I’m pretty sure this is wrong (or misleading) on multiple levels: > 1) An event in Andromeda occurs at single moment, it’s our observation of the event that is warped. So there aren’t two realities, one where they’re still debating and one where they aren’t. In both realities the debate has concluded, it’s just our observation of the event takes place at different reference times. Yeah, they're making the mistake of assuming a minute is a minute. It's closer to the difference in angular motion vs linear motion. If you are closer to the center, you move very little linearly. If you are closer to the outer edge, you move very little angularly. Now you look at the linear distance as time passed for the object and the angular distance as the amount of "space-time" an object has gone. The amount of "space-time" is constant. It's a bit of a wonky model, but it's probably the simplest one that encapsulates the concept. So just because they spent 3 days debating, you spent 5 days watching, and Bob only spent 1 day watching; that doesn't mean Bob is 2 days ahead of Andromeda and 4 days ahead of you. Since we are all also traveling through space at different speeds, we've all traveled the same amount of space-time, so we're all seeing the same things. As for our perception, that more depends on our relative positions from each other rather than speed. Because nothing is faster than light, we can never experience an event before we see it.

Diggsey 5y ago

Actually no, the parent comment is correct: When you undergo an acceleration, it corresponds to a hyperbolic rotation of the spacetime around you (from your frame of reference). When the acceleration is small, the rotation is also very, very small; however at very large distances, even a small rotation can have a large effect. In the example, that "large effect" is the alien debate moving in time forward or backwards a day (again, from the accelerated reference frame). This has nothing to do with the observation delay, it's just an artefact of what it means for two far apart events to happen "simultaneously": simultaneity is a relative property, so two things that happen simultaneously in one reference frame might not happen simultaneously in another. The reason this doesn't introduce a paradox is because without a specific reference frame, events only have a partial order: when you compare two events, there are three possible results: 1) Event A is in the past light-cone of event B. 2) Event B is in the past light-cone of event A. 3) Neither event is in the past light-cone of the other. For 1+2 this means that the time between the events is greater than the distance between the events, whereas for 3 it means the reverse. For 1+2, the ordering of the events is the same for any reference frame. For 3, there will always be some frame of reference where both events appear to happen at exactly the same time, so nothing can be said about their order without choosing a reference frame.

awb 5y ago
I don’t think the interpretation you describe is widely held. Check out the criticisms of the paradox: https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument#Criticisms https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argu...
- Diggsey 5y ago
  That it has critics does not imply it's not widely held. Regardless, that's just a dispute about terminology: sure we could say that we should only talk about events being "simultaneous" if they happen in exactly the same place - but in that case it's not a particularly useful term anymore. It's just an argument about how we apply words derived from newtonian intuitions to a relativistic context. Mathematically, for a given reference frame we can draw a space-time diagram, and certain events will be above the horizontal line. We can draw the same space-time diagram for a different reference frame and have those same events be below the horizontal line. Whether that horizontal line has any actual physical relevance doesn't really matter IMO, the maths says that events move in relation to the line when the frame of reference accelerates, and that's what the "paradox" is illustrating. Having said that, if the "one way speed of light" exists (https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light) and even if it's not the same in all directions, then this "plane of simultaneity" is definitely a physical thing, since you can send a beam of light to a target, measure how long it takes to return, and using the speed of light in that direction you can work out exactly when that beam hit the target in your local time - ie. you've found two events to be simultaneous from your frame of reference, that are not in the same location.

irthomasthomas 5y ago

I once read a physicist on this, in a newspaper, who said "if I didn't believe in free-will, then I wouldn't bother getting up in the morning." I thought that was cute. I think the term free-will has clouded the issue a bit. Forget about free-will. Scientists, instead, normally talk about Determinism vs indeterminism, or nondeterminism in CS. These have a more precise definition than that other thing. The universe is deterministic if the outcome of events are completely determined by previous events. Or the universe is nondeterministic if the outcome of events are not certain, but follow a random distribution. As to the precise details of the speeds and distances etc, it is a thought experiment, swap the car for a rocket traveling past at 0.86c, if you like. Or instead of aliens deliberating all day, maybe they are more efficient than we are, and they make their decision instantly by flipping a digital coin powered by the perfectly nondeterministic quantum fluctuations of the vacuum.[0] For earth rocket man, there is a moment before the quantum dice roll when the decision is indeterminable, it's completely random. But at the same moment in the stationary frame, the fleet is already on it's way. Unless you wish to argue that a stationary reference frame is somehow more valid than a moving one, you have to accept that in the moving frame, the event of the dice roll has still not occurred and should be random, but how can it be random, if it has already happened in the stationary frame. Hence the paradox. I hope that cleared things up some? [0] The vacuum has an api: https://qrng.anu.edu.au/

afiori 5y ago
> For earth rocket man, there is a moment before the quantum dice roll when the decision is indeterminable Assuming earth rocket man's speed does not change, if he quickly alternates between 0c and 0.99c he could flip flop between "determinate" and "indeterminate". Anyway for all system on earth the decision event would be in the relativistic present with no causality order https://en.wikipedia.org/wiki/Minkowski_space#Causal_structure https://en.wikipedia.org/wiki/Minkowski_space#Causal_structu... Part of the point of relativity is that t1<t2 is not a good test of "comes before"