3 ms·

> If you watch your friend do X right in front of you, would you argue it happened yesterday? If he is right in front of me, it won't take a day for the light

> If you watch your friend do X right in front of you, would you argue it happened yesterday? If he is right in front of me, it won't take a day for the light to reach me. So, no, I wouldn't. > They're both valid interpretations. I wasn't talking about subjective interpretations. I was saying that General Relativity clearly distinguishes between the event X of something happening and the event Y of information about that event reaching an observer. X and Y will necessarily be at a timelike or lightlike distance. Therefore, unless this distance is zero, time will necessarily pass between those two events from the perspective of the observer and, in fact, from the perspective of any other observer you could possibly ask, where "observer" is more or less defined by a coordinate system (whose time axis will be the observer's worldline). This is not up for debate.

anamexis 4y ago
> Therefore, unless this distance is zero, time will necessarily pass between those two events from the perspective of the observer No, this is where I disagree. From the perspective of the observer, they are seeing what they are seeing exactly when they see it. They don't perceive any time having passed at all, because we can't perceive an event's age. What's not up for debate is how long it would take to do round-trip communication between the two places. What's also not up for debate is that is impossible to show that these far away events are not happening at the time we see them. How you interpret this is absolutely up for debate, because it's not some intrinsic physical fact.
- codethief 4y ago
  > No, this is where I disagree. From the perspective of the observer, they are seeing what they are seeing exactly when they see it. They don't perceive any time having passed at all, because we can't perceive an event's age. You are twisting my words and once again interpreting the words "observer" and "event" in a way that deviates from their standard definition in General Relativity. Yes, of course we (as beings with pairs of eyes) will perceive something happening in the sky when we actually see it happening or, more generally, when we first hear about it. This is trivial. That's not a good way to define when something happened, though, because it is highly subjective – depending on whether you received the information through light, gravitational waves, sound, or carrier pigeon, your notion of when something happens will differ greatly. In fact, given that information from the event will typically reach you in multiple ways at the same time (think of lightning & thunder) and sometimes you might focus on one, sometimes on the other, and your brain might process things at different speeds, even if we're just talking about you, this is not a great and consistent definition. Moreover, other people could not predict what times you would ascribe to certain events. This is why General Relativity does not define it that way. GR is concerned with objective, well-defined statements. Sometimes a statement might depend on the observer (this is relativity after all) but other observers will still be able to predict what that observer will say because there are clear rules to translate observer-dependent statements between observers. So subjective observer-dependent statements can still be objectively true in this sense. > What's also not up for debate is that is impossible to show that these far away events are not happening at the time we see them. This is simply not true. How do you think we know that they are actually not happening at the time we see them? Would you also say that it's impossible to prove that the events which cause thunder don't happen at the time you hear the thunder, but earlier, i.e. shortly before you see the lightning? > How you interpret this is absolutely up for debate, because it's not some intrinsic physical fact. You are of course free to interpret all the things you see in the sky or on Earth however you like. The problem is: Given the aforementioned issues with such subjective interpretations, they are essentially useless. We won't be able to agree on anything and won't be able to make reliable predictions. In any case, I was responding to OP who was saying[0] > For all intents and purposes this is happening in real time from our local frame of reference. "Frame of reference" has a precise mathematical meaning: It's a coordinate frame with coordinates that an observer would use to map out their local region of the universe, with some precise rules as to what "mapping out" means. Now this coordinate frame assigns different times to the event X of something happening and the event Y of you receiving light that was emitted at X (again, provided that the distance between the thing that's happening and you is not zero but that's essentially always the case). So from the perspective of our local frame of reference, whatever we see in the sky is not happening in real time. In hindsight, I should have maybe omitted the second half of my comment > If your friend tells you today about how they did X yesterday, would you also argue X happened today from your frame of reference? I was merely trying to hint at the fact that the definition "It happens when I perceive it" is, as discussed above, completely and utterly useless, let alone ill-defined. [0]: https://news.ycombinator.com/item?id=34937699 https://news.ycombinator.com/item?id=34937699
  - anamexis 4y ago
    I don't suspect we will get anywhere with this debate, but what I'm saying doesn't contradict any of what you're saying about General Relativity, and also I'm not suggesting the trivial fact that we observe events at the time that we observe them. > This is simply not true. How do you think we know that they are actually not happening at the time we see them? Would you also say that it's impossible to prove that the events which cause thunder don't happen at the time you hear the thunder, but earlier, i.e. shortly before you see the lightning? No, and this is exactly where the analogy between the speed of sound and speed of light breaks down, because they are fundamentally different. The speed of sound is not subject to (significant) relativistic effects. You can easily prove the speed of sound by measuring the time between the lightning flash and when you hear the thunder. There is no analog with the speed of light. I agree that this is the crux of the argument: > For all intents and purposes this is happening in real time from our local frame of reference. But you have not shown this to be false.
    - codethief 4y ago
      > > For all intents and purposes this is happening in real time from our local frame of reference. > But you have not shown this to be false. Once again you have ignored how General Relativity defines "local frame of reference" – which I have elaborated on extensively –, replaced it with your very own and, as discussed, inconsistent definition, and assert that I am wrong based on this made up definition of yours. Take gravitational lensing. Light from one event will reach you in multiple (in fact, an infinite number of) ways, at different times. What now? How do you define the time at which an event occured? Take the fact light often propagates at speeds v < c, where c is "the" speed of light, an effect that all light rays you see in the sky are subject to to some degree because they are passing through interstellar medium et cetera. (For an extreme example see slow light[0].) How do you reconcile your definition with the fact that the speed of (such) light will no longer be the speed that bounds whether two events are causally connected? [0]: https://en.wikipedia.org/wiki/Slow_light https://en.wikipedia.org/wiki/Slow_light

anamexis 4y ago

> Therefore, unless this distance is zero, time will necessarily pass between those two events from the perspective of the observer No, this is where I disagree. From the perspective of the observer, they are seeing what they are seeing exactly when they see it. They don't perceive any time having passed at all, because we can't perceive an event's age. What's not up for debate is how long it would take to do round-trip communication between the two places. What's also not up for debate is that is impossible to show that these far away events are not happening at the time we see them. How you interpret this is absolutely up for debate, because it's not some intrinsic physical fact.

codethief 4y ago
> No, this is where I disagree. From the perspective of the observer, they are seeing what they are seeing exactly when they see it. They don't perceive any time having passed at all, because we can't perceive an event's age. You are twisting my words and once again interpreting the words "observer" and "event" in a way that deviates from their standard definition in General Relativity. Yes, of course we (as beings with pairs of eyes) will perceive something happening in the sky when we actually see it happening or, more generally, when we first hear about it. This is trivial. That's not a good way to define when something happened, though, because it is highly subjective – depending on whether you received the information through light, gravitational waves, sound, or carrier pigeon, your notion of when something happens will differ greatly. In fact, given that information from the event will typically reach you in multiple ways at the same time (think of lightning & thunder) and sometimes you might focus on one, sometimes on the other, and your brain might process things at different speeds, even if we're just talking about you, this is not a great and consistent definition. Moreover, other people could not predict what times you would ascribe to certain events. This is why General Relativity does not define it that way. GR is concerned with objective, well-defined statements. Sometimes a statement might depend on the observer (this is relativity after all) but other observers will still be able to predict what that observer will say because there are clear rules to translate observer-dependent statements between observers. So subjective observer-dependent statements can still be objectively true in this sense. > What's also not up for debate is that is impossible to show that these far away events are not happening at the time we see them. This is simply not true. How do you think we know that they are actually not happening at the time we see them? Would you also say that it's impossible to prove that the events which cause thunder don't happen at the time you hear the thunder, but earlier, i.e. shortly before you see the lightning? > How you interpret this is absolutely up for debate, because it's not some intrinsic physical fact. You are of course free to interpret all the things you see in the sky or on Earth however you like. The problem is: Given the aforementioned issues with such subjective interpretations, they are essentially useless. We won't be able to agree on anything and won't be able to make reliable predictions. In any case, I was responding to OP who was saying[0] > For all intents and purposes this is happening in real time from our local frame of reference. "Frame of reference" has a precise mathematical meaning: It's a coordinate frame with coordinates that an observer would use to map out their local region of the universe, with some precise rules as to what "mapping out" means. Now this coordinate frame assigns different times to the event X of something happening and the event Y of you receiving light that was emitted at X (again, provided that the distance between the thing that's happening and you is not zero but that's essentially always the case). So from the perspective of our local frame of reference, whatever we see in the sky is not happening in real time. In hindsight, I should have maybe omitted the second half of my comment > If your friend tells you today about how they did X yesterday, would you also argue X happened today from your frame of reference? I was merely trying to hint at the fact that the definition "It happens when I perceive it" is, as discussed above, completely and utterly useless, let alone ill-defined. [0]: https://news.ycombinator.com/item?id=34937699 https://news.ycombinator.com/item?id=34937699
- anamexis 4y ago
  I don't suspect we will get anywhere with this debate, but what I'm saying doesn't contradict any of what you're saying about General Relativity, and also I'm not suggesting the trivial fact that we observe events at the time that we observe them. > This is simply not true. How do you think we know that they are actually not happening at the time we see them? Would you also say that it's impossible to prove that the events which cause thunder don't happen at the time you hear the thunder, but earlier, i.e. shortly before you see the lightning? No, and this is exactly where the analogy between the speed of sound and speed of light breaks down, because they are fundamentally different. The speed of sound is not subject to (significant) relativistic effects. You can easily prove the speed of sound by measuring the time between the lightning flash and when you hear the thunder. There is no analog with the speed of light. I agree that this is the crux of the argument: > For all intents and purposes this is happening in real time from our local frame of reference. But you have not shown this to be false.
  - codethief 4y ago
    > > For all intents and purposes this is happening in real time from our local frame of reference. > But you have not shown this to be false. Once again you have ignored how General Relativity defines "local frame of reference" – which I have elaborated on extensively –, replaced it with your very own and, as discussed, inconsistent definition, and assert that I am wrong based on this made up definition of yours. Take gravitational lensing. Light from one event will reach you in multiple (in fact, an infinite number of) ways, at different times. What now? How do you define the time at which an event occured? Take the fact light often propagates at speeds v < c, where c is "the" speed of light, an effect that all light rays you see in the sky are subject to to some degree because they are passing through interstellar medium et cetera. (For an extreme example see slow light[0].) How do you reconcile your definition with the fact that the speed of (such) light will no longer be the speed that bounds whether two events are causally connected? [0]: https://en.wikipedia.org/wiki/Slow_light https://en.wikipedia.org/wiki/Slow_light