6 ms·

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 mov

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.
  - TheCraiggers 6y ago
    That was a very neat video, and I learned some stuff too! Thanks for posting that.
    - jerf 6y ago
      He clickbaited me but good on that one, but it delivered the goods.
      - eins1234 6y ago
        Long time Veritasium subscriber here. He actually made a great video a while ago explaining why professional Youtubers don't have much of a choice when it comes to clickbaiting with titles and thumbnails, because it's the only way to reliably get lots of impressions and views and grow their channel: https://www.youtube.com/watch?v=fHsa9DqmId8 https://www.youtube.com/watch?v=fHsa9DqmId8 It made me gain a bit of empathy for creators using these tactics on Youtube and tolerate it as long as it's not being used to promote content that's devoid of any substance, which is pretty much never the case for Veritasium videos or any of the other educational channels I subscribe to.
        jerf 6y ago
        Yes, I understand and agree. It makes me sad as well but I've made my peace with it. Nevertheless, I commented about this one in particular because when I saw the title card, I legitimately thought, even having seen many of his videos, that he couldn't possibly deliver on that. He got me; he did. I was impressed.
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.

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.
- TheCraiggers 6y ago
  That was a very neat video, and I learned some stuff too! Thanks for posting that.
  - jerf 6y ago
    He clickbaited me but good on that one, but it delivered the goods.
    - eins1234 6y ago
      Long time Veritasium subscriber here. He actually made a great video a while ago explaining why professional Youtubers don't have much of a choice when it comes to clickbaiting with titles and thumbnails, because it's the only way to reliably get lots of impressions and views and grow their channel: https://www.youtube.com/watch?v=fHsa9DqmId8 https://www.youtube.com/watch?v=fHsa9DqmId8 It made me gain a bit of empathy for creators using these tactics on Youtube and tolerate it as long as it's not being used to promote content that's devoid of any substance, which is pretty much never the case for Veritasium videos or any of the other educational channels I subscribe to.
      - jerf 6y ago
        Yes, I understand and agree. It makes me sad as well but I've made my peace with it. Nevertheless, I commented about this one in particular because when I saw the title card, I legitimately thought, even having seen many of his videos, that he couldn't possibly deliver on that. He got me; he did. I was impressed.

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.