5 ms·

I don't think I understand what's shocking about this. It's like saying that for somebody who watches a taping of a soccer match the result is not yet known, ye

by simias 6y ago

I don't think I understand what's shocking about this. It's like saying that for somebody who watches a taping of a soccer match the result is not yet known, yet for somebody who watched it live it's a foregone conclusion. I don't really get what's the paradox here. Or maybe more to the point: if you were watching a soccer game Earth from Proxima Centauri, you could only get the result about 4 years after the game took place on earth, so for you the issue is uncertain while for somebody on Earth "at the same moment" the result has been known for years. But of course then the whole concept of "at the same moment" is pretty ambiguous and fuzzy. I don't really get what's paradoxical here.

klodolph 6y ago
The notion of “at the same moment” is not ambiguous and fuzzy. You use clocks and synchronize them. What relativity adds to the picture is the requirement for a common reference frame, which is surprising. It creates paradoxes because our intuitive notion of “at the same moment” does not include the reference frame, so there is a conflict between our intuitive understanding of past/future (which does not change whether you are moving) and the relativistic description of past/future (where two people can disagree about whether a particular event is int the past or the future). Because of this disagreement, if you had FTL communication and wired it up so you could make a phone call to Andromeda, your phone call would arrive before or after the debate depending on your reference frame on Earth, and you could use it to send information backwards in time.
- TheCraiggers 6y ago
  Synchronize the clocks how? Last I knew, this was still impossible, unless you employ hand-waving. If I synchronize two clocks next to each other, and then move one away, they are no longer synchronized as one was moving. And we still don't have the speed of c one way, so you can't use that either.
  - drran 6y ago
    Move clocks at the exact same speed and acceleration in opposite directions, so they stay synchronized.
    - andrewbarba 6y ago
      I don’t think this works either because you can’t meaningfully agree on when light was emitted from one of the clocks. The only solution should involve a single clock but then you are always measuring 2 way and not 1 way speed.
      - klodolph 6y ago
        > I don’t think this works either because you can’t meaningfully agree on when light was emitted from one of the clocks. You can in fact meaningfully agree. You put the clocks in similar reference frames (like “standing on the surface of the Earth”), and accept some small but non-zero margin for error. The fact that a spaceship traveling around the earth does not measure light pulses emitted by these clocks as simultaneous is irrelevant—because we know that the notion of simultaneity relies on reference frame. What we do get is that anyone standing on the Earth’s surface can measure the pulses as simultaneous, within the specified margin of error. This error must necessarily account for the the fact that time progresses differently at different altitudes and latitudes, if your clocks are accurate enough. Because General Relativity is an extremely accurate theory and we have accurate measurements of the relevant physical constants & the mass of the Earth, we can account for the different reference frames and run the clocks at the correct rates so they continue to be synchronized even when they are free running (again, with the appropriate margin of error). If you are using ordinary quartz clocks, then the clocks probably have enough error that you can ignore relativistic effects. I’m assuming we want atomic clocks if we’re series about chronometry, especially considering how cheap they are these days. The idea that synchronizing clocks is somehow impossible stems from an unrealistic idea of how people measure time in the first place.
    - jerf 6y ago
      Veritasium just had a good video which discusses this: https://www.youtube.com/watch?v=pTn6Ewhb27k https://www.youtube.com/watch?v=pTn6Ewhb27k It turns out there's some circularity in your definitions of "Move clocks at the exact same speed and acceleration in opposite directions" and "synchronized"; that's basically the definition of "synchronized", which is fine from a certain point of view, but from the point of view of examining the nature of space and time themselves, not adequate.
  - klodolph 6y ago
    > Last I knew, this was still impossible, unless you employ hand-waving. I think you may be thinking of synchronization as perfect synchronization for ideal clocks. If you go down this route everything becomes impossible. Instead, think of synchronization as assurance that the maximum error is below some known bound. In general, the bounds can be made quite low.
    - thelastwave 6y ago
      This is not intended as sarcastic, but you solve the theoretical problem by making it a practical problem? If 2 clocks are out synch, but only to a small amount that doesn't affect my ability to meet someone for dinner, that doesn't mean they are in sync. I agree that if you go down this route everything becomes impossible, because at some point you're talking about Planck time, which thankfully we don't need to meetup for dinner.
      - drran 6y ago
        Two physical objects or measurements are never equal in theory. :-/
      - klodolph 6y ago
        > This is not intended as sarcastic, but you solve the theoretical problem by making it a practical problem? I think this is based on some false idea that error is somehow “not theoretical” which makes no sense. There is a rich set of about error and even an entire field dedicated to its study (statistics). The only reason we are interested in theories in the first place is because the theories correspond to the real world. If you have some theory about time which cannot be measured, or cannot explain errors, then that theory belongs in the garbage. (Think about this: if your theory cannot explain errors, but any actual experiment produces errors, then any actual experiment will demonstrate that your theory is “wrong”.) Or consider other concepts like “flat”. What does it mean for a surface to be “flat”? No matter how flat a real surface is, it will never be mathematically flat. So either we have to throw away the word “flat” and stop using it outside mathematics (which is a stupid idea), or we define “flat” to mean that a surface is within some deviation from mathematical flatness. You can’t escape error, even in theory, unless you leave the world of physics and do pure mathematics.
  - brandmeyer 6y ago
    Its not impossible. Civil time is just this, in fact. Its true that if two observers use their own local atomic standard to precisely measure elapsed time then they would have difficulty agreeing on when something happened. However, that's not how civil timekeeping works. One clock declares itself to be a master clock and broadcasts a time+position standard to everyone else. Other observers then construct their own PLL to compute a running estimate of the total phase shift between their local time standard and the one that is broadcast. In effect, received civil time has a velocity relative to your local atomic time. If every observer can label their observations with civil time then the paradox disappears and you are left with information flow that is bounded to the speed of light. A fast-moving observer has a local atomic standard of time that moves at one second per second, and a received civil time that appears to be running much faster. A peer-to-peer version of this is how we define international atomic time. Subtle variations in local gravity conspire to actually give the different clocks in the network a different local speed of time. They steer a global PLL to minimize the total squared phase error in the network. Each node in the network can make observations on the TAI timebase by adding its own clock offset to their local clock.
  - koonsolo 6y ago
    But you can calculate the difference, which happens with GPS satellites all the time. GPS is based on synced clocks. Problem is that the satellites suffer the rules of relativity. But the good news is that we know the rules of relativity, and therefore can compensate for them. I always thought relativity was purely theoretical, but it is actually practically applied in GPS sattelites.
SamBam 6y ago
I think it's still surprising, if not "paradoxical" (obviously it's not paradoxical if it's reality). The difference is: in your two examples we know there's a delay, and all you did was say that some people will know the result before others who are receiving the delayed result. What the original question is saying is that you can't ask "well, what is happening right now on Proxima Centauri?" since the very question of "now" is not specific. I think that's quite different. On Earth, with Newtonian physics, we intuitively think that "now" is shared, so we really can say "I wonder whether someone has scored a goal now," and the answer shouldn't depend on whether you're in a car or not.
- simias 6y ago
  >On Earth, with Newtonian physics, we intuitively think that "now" is shared, so we really can say "I wonder whether someone has scored a goal now," and the answer shouldn't depend on whether you're in a car or not. That's insightful, and actually it made me realize that it's probably true that nowadays we expect information to be available worldwide effectively instantly, but that obviously wasn't always the case. "Right now" we're having this discussion on Hacker News with people from around the world, but merely a few decades ago this type of simultaneity wouldn't have been possible. Nothing of what would could've done locally could've immediately influenced dozens of people around the globe. This shared "now" is effectively a modern invention.
  - gmadsen 6y ago
    I think the unusual part is that it is not a limit of communication, it is time itself. With a tape, you know that it happened in the past, for this, both are experiencing their real present time, but they don't agree with each other.
- ngvrnd 6y ago
  It's only surprising if you don't understand that this is a block universe.
ardy42 6y ago
> I don't really get what's paradoxical here. IIRC, the implication is that change predetermined and fixed, so is essentially static.
Schiphol 6y ago
Not paradoxical, but it does suggest that the future is as taped as that football match.

by simias 6y ago

klodolph 6y ago

The notion of “at the same moment” is not ambiguous and fuzzy. You use clocks and synchronize them. What relativity adds to the picture is the requirement for a common reference frame, which is surprising. It creates paradoxes because our intuitive notion of “at the same moment” does not include the reference frame, so there is a conflict between our intuitive understanding of past/future (which does not change whether you are moving) and the relativistic description of past/future (where two people can disagree about whether a particular event is int the past or the future). Because of this disagreement, if you had FTL communication and wired it up so you could make a phone call to Andromeda, your phone call would arrive before or after the debate depending on your reference frame on Earth, and you could use it to send information backwards in time.

TheCraiggers 6y ago
Synchronize the clocks how? Last I knew, this was still impossible, unless you employ hand-waving. If I synchronize two clocks next to each other, and then move one away, they are no longer synchronized as one was moving. And we still don't have the speed of c one way, so you can't use that either.
- drran 6y ago
  Move clocks at the exact same speed and acceleration in opposite directions, so they stay synchronized.
  - andrewbarba 6y ago
    I don’t think this works either because you can’t meaningfully agree on when light was emitted from one of the clocks. The only solution should involve a single clock but then you are always measuring 2 way and not 1 way speed.
    - klodolph 6y ago
      > I don’t think this works either because you can’t meaningfully agree on when light was emitted from one of the clocks. You can in fact meaningfully agree. You put the clocks in similar reference frames (like “standing on the surface of the Earth”), and accept some small but non-zero margin for error. The fact that a spaceship traveling around the earth does not measure light pulses emitted by these clocks as simultaneous is irrelevant—because we know that the notion of simultaneity relies on reference frame. What we do get is that anyone standing on the Earth’s surface can measure the pulses as simultaneous, within the specified margin of error. This error must necessarily account for the the fact that time progresses differently at different altitudes and latitudes, if your clocks are accurate enough. Because General Relativity is an extremely accurate theory and we have accurate measurements of the relevant physical constants & the mass of the Earth, we can account for the different reference frames and run the clocks at the correct rates so they continue to be synchronized even when they are free running (again, with the appropriate margin of error). If you are using ordinary quartz clocks, then the clocks probably have enough error that you can ignore relativistic effects. I’m assuming we want atomic clocks if we’re series about chronometry, especially considering how cheap they are these days. The idea that synchronizing clocks is somehow impossible stems from an unrealistic idea of how people measure time in the first place.
  - jerf 6y ago
    Veritasium just had a good video which discusses this: https://www.youtube.com/watch?v=pTn6Ewhb27k https://www.youtube.com/watch?v=pTn6Ewhb27k It turns out there's some circularity in your definitions of "Move clocks at the exact same speed and acceleration in opposite directions" and "synchronized"; that's basically the definition of "synchronized", which is fine from a certain point of view, but from the point of view of examining the nature of space and time themselves, not adequate.
- klodolph 6y ago
  > Last I knew, this was still impossible, unless you employ hand-waving. I think you may be thinking of synchronization as perfect synchronization for ideal clocks. If you go down this route everything becomes impossible. Instead, think of synchronization as assurance that the maximum error is below some known bound. In general, the bounds can be made quite low.
  - thelastwave 6y ago
    This is not intended as sarcastic, but you solve the theoretical problem by making it a practical problem? If 2 clocks are out synch, but only to a small amount that doesn't affect my ability to meet someone for dinner, that doesn't mean they are in sync. I agree that if you go down this route everything becomes impossible, because at some point you're talking about Planck time, which thankfully we don't need to meetup for dinner.
    - drran 6y ago
      Two physical objects or measurements are never equal in theory. :-/
    - klodolph 6y ago
      > This is not intended as sarcastic, but you solve the theoretical problem by making it a practical problem? I think this is based on some false idea that error is somehow “not theoretical” which makes no sense. There is a rich set of about error and even an entire field dedicated to its study (statistics). The only reason we are interested in theories in the first place is because the theories correspond to the real world. If you have some theory about time which cannot be measured, or cannot explain errors, then that theory belongs in the garbage. (Think about this: if your theory cannot explain errors, but any actual experiment produces errors, then any actual experiment will demonstrate that your theory is “wrong”.) Or consider other concepts like “flat”. What does it mean for a surface to be “flat”? No matter how flat a real surface is, it will never be mathematically flat. So either we have to throw away the word “flat” and stop using it outside mathematics (which is a stupid idea), or we define “flat” to mean that a surface is within some deviation from mathematical flatness. You can’t escape error, even in theory, unless you leave the world of physics and do pure mathematics.
- brandmeyer 6y ago
  Its not impossible. Civil time is just this, in fact. Its true that if two observers use their own local atomic standard to precisely measure elapsed time then they would have difficulty agreeing on when something happened. However, that's not how civil timekeeping works. One clock declares itself to be a master clock and broadcasts a time+position standard to everyone else. Other observers then construct their own PLL to compute a running estimate of the total phase shift between their local time standard and the one that is broadcast. In effect, received civil time has a velocity relative to your local atomic time. If every observer can label their observations with civil time then the paradox disappears and you are left with information flow that is bounded to the speed of light. A fast-moving observer has a local atomic standard of time that moves at one second per second, and a received civil time that appears to be running much faster. A peer-to-peer version of this is how we define international atomic time. Subtle variations in local gravity conspire to actually give the different clocks in the network a different local speed of time. They steer a global PLL to minimize the total squared phase error in the network. Each node in the network can make observations on the TAI timebase by adding its own clock offset to their local clock.
- koonsolo 6y ago
  But you can calculate the difference, which happens with GPS satellites all the time. GPS is based on synced clocks. Problem is that the satellites suffer the rules of relativity. But the good news is that we know the rules of relativity, and therefore can compensate for them. I always thought relativity was purely theoretical, but it is actually practically applied in GPS sattelites.

SamBam 6y ago

I think it's still surprising, if not "paradoxical" (obviously it's not paradoxical if it's reality). The difference is: in your two examples we know there's a delay, and all you did was say that some people will know the result before others who are receiving the delayed result. What the original question is saying is that you can't ask "well, what is happening right now on Proxima Centauri?" since the very question of "now" is not specific. I think that's quite different. On Earth, with Newtonian physics, we intuitively think that "now" is shared, so we really can say "I wonder whether someone has scored a goal now," and the answer shouldn't depend on whether you're in a car or not.

simias 6y ago
>On Earth, with Newtonian physics, we intuitively think that "now" is shared, so we really can say "I wonder whether someone has scored a goal now," and the answer shouldn't depend on whether you're in a car or not. That's insightful, and actually it made me realize that it's probably true that nowadays we expect information to be available worldwide effectively instantly, but that obviously wasn't always the case. "Right now" we're having this discussion on Hacker News with people from around the world, but merely a few decades ago this type of simultaneity wouldn't have been possible. Nothing of what would could've done locally could've immediately influenced dozens of people around the globe. This shared "now" is effectively a modern invention.
- gmadsen 6y ago
  I think the unusual part is that it is not a limit of communication, it is time itself. With a tape, you know that it happened in the past, for this, both are experiencing their real present time, but they don't agree with each other.
ngvrnd 6y ago
It's only surprising if you don't understand that this is a block universe.

ardy42 6y ago

> I don't really get what's paradoxical here. IIRC, the implication is that change predetermined and fixed, so is essentially static.

Schiphol 6y ago

Not paradoxical, but it does suggest that the future is as taped as that football match.