4 ms·

But that 20m year old light is in our past light cone; we can say it is in our past. If it is not in our light cone, we cannot communicate any temporal info so

But that 20m year old light is in our past light cone; we can say it is in our past. If it is not in our light cone, we cannot communicate any temporal info so we cannot say if it’s now, past or future in reference to us as observer.

grumpopotamus 19d ago
I think "something that happened 20M years ago" is still wrong. One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. In other words, if we went back 20M years on Earth we could not meaningfully say the black hole event is happening "right now".
- anonzzzies 19d ago
  Yes, but we can say it happened in our past. But agreed, we cannot go back 20m years and say it’s simultaneous with the events here. If GP meant that, then agreed.
- ViktorRay 19d ago
  Wait then what about events that happen on Pluto? Pluto is around five light hours away from us. If an astronaut goes to Pluto and films herself doing activity A and then broadcasts that back to Earth, it would take around five hours for the broadcast to reach us. So you’re saying that it is not correct to say her recorded activity happened five hours ago?
  - philipov 19d ago
    The idea driving this conclusion is called the Relativity of Simultaneity - There is no absolute notion that two events happened "at the same time". You want to say that a distant event occurred at the same time as a nearby event that occurred 5 hours ago. However, a distant third party observing both events may not agree. This becomes painfully apparent when dealing with black holes. Person A falling into a black hole passes through the event horizon normally and reaches the singularity in finite time. However, Person B observing from outside never sees Person A pass through the event horizon. From Person B's perspective, it doesn't make sense to ask what happens simultaneously with Person A reaching the singularity - it takes infinite time just to reach the event horizon. Even asking the question becomes nonsense. https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
    - NamlchakKhandro 19d ago
      A quantum entangled data stream of video between Pluto and earth would mean your now observing it in real time
      - skirmish 19d ago
        Quantum entanglement does not enable FTL communication: [1]. [1] https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
  - herbstein 19d ago
    If we imagine a third observer in a space craft traveling close to he speed of light, it's possible to construct the scenario in such a way that the pilot of the space craft would observe the two events happening at the same moment. Time is relative
    - ViktorRay 19d ago
      Ah yes that’s indeed true. But from the Earth’s reference frame wasn’t the Pluto activity I mentioned earlier simultaneous with what happened 5 hours ago on Earth? So couldn’t you say from Earth’s reference frame this black hole’s activity happened simultaneously with events 20 million years ago?
      - philipov 19d ago
        As an approximation it can be fine to speak that way in practical matters, but it's dangerous to go drawing any deep conclusions while doing so. Approximations shouldn't be allowed to go outside their operational parameters.
        ViktorRay 19d ago
        But it’s not an approximation right? In our reference frame it is the truth.
        philipov 19d ago
        The approximation is the belief that things can happen simultaneously. It is approximating along the fact that we only need to concern ourselves with a single reference frame. I think the difference in the scenario you proposed is that we can conceive of a practical situation in which someone on Pluto sends a message and waits for a reply. We can then say that they are going to have to wait at least 10 hours to receive a reply. This has meaning. But when the distances become too great the urge is to resist believing that things have an existence beyond the interactions of particles. There is no practical reality to that object beyond our ability to observe it when the light reaches us.
- tomrod 19d ago
  It's a semantic saying that misses what people typically mean when we speak about simultaneous events. Someone sends an email or speaks on a voice call on the other side of world and a receiver still gets that note at the speed of light, there is slight latency. It's still completely reasonable to measure that latency as we can effectuate improvements to our machines and networks when it increases. The same as if someone wrote a letter in the 1860s and the horses were sick, causing a delay. Simply because information was received by us, the common understanding is that the notification was sent previously, because that is how cause and effect work.
  - philipov 19d ago
    It's not semantics, it's an approximation. Events that happen on the same planet are not relativistically distant, so such approximations are mostly fine in our daily lives; the same as how Newtonian gravity is fine to use in many situations. But today we're talking about astrophysics and a proper understanding of Relativity matters.
- dotancohen 19d ago
  You are getting downvotes, but you are correct. People are voting by intuition, not by physics. Relativity is not intuitive.
  - andrewflnr 19d ago
    No, I took a physics class that covered relativity of simultaneity and that's how I know this is BS.
- andrewflnr 18d ago
  > One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. No. Special relativity says that simultaneity is relative for things moving at different velocities. The classic example is a fast-moving train and a platform disagreeing on simultaneity, even when they're right next to each other. Distance, and light travel time, has nothing to do with it; that's a separate (and mostly less interesting) problem. Relativity absolutely lets you calculate what was happening simultaneously with a given past event within your own reference frame (i.e. other things moving at roughly the same velocity you are). In fact, you can think of that process, figuring out a consistent version of events after you've gathered all the data, as being exactly what it means to "observe" something in relativity. This is as opposed to "seeing it", which is what happens the exact moment the photons hit your eyes/sensors.

grumpopotamus 19d ago

I think "something that happened 20M years ago" is still wrong. One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. In other words, if we went back 20M years on Earth we could not meaningfully say the black hole event is happening "right now".

anonzzzies 19d ago
Yes, but we can say it happened in our past. But agreed, we cannot go back 20m years and say it’s simultaneous with the events here. If GP meant that, then agreed.
ViktorRay 19d ago
Wait then what about events that happen on Pluto? Pluto is around five light hours away from us. If an astronaut goes to Pluto and films herself doing activity A and then broadcasts that back to Earth, it would take around five hours for the broadcast to reach us. So you’re saying that it is not correct to say her recorded activity happened five hours ago?
- philipov 19d ago
  The idea driving this conclusion is called the Relativity of Simultaneity - There is no absolute notion that two events happened "at the same time". You want to say that a distant event occurred at the same time as a nearby event that occurred 5 hours ago. However, a distant third party observing both events may not agree. This becomes painfully apparent when dealing with black holes. Person A falling into a black hole passes through the event horizon normally and reaches the singularity in finite time. However, Person B observing from outside never sees Person A pass through the event horizon. From Person B's perspective, it doesn't make sense to ask what happens simultaneously with Person A reaching the singularity - it takes infinite time just to reach the event horizon. Even asking the question becomes nonsense. https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
  - NamlchakKhandro 19d ago
    A quantum entangled data stream of video between Pluto and earth would mean your now observing it in real time
    - skirmish 19d ago
      Quantum entanglement does not enable FTL communication: [1]. [1] https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
- herbstein 19d ago
  If we imagine a third observer in a space craft traveling close to he speed of light, it's possible to construct the scenario in such a way that the pilot of the space craft would observe the two events happening at the same moment. Time is relative
  - ViktorRay 19d ago
    Ah yes that’s indeed true. But from the Earth’s reference frame wasn’t the Pluto activity I mentioned earlier simultaneous with what happened 5 hours ago on Earth? So couldn’t you say from Earth’s reference frame this black hole’s activity happened simultaneously with events 20 million years ago?
    - philipov 19d ago
      As an approximation it can be fine to speak that way in practical matters, but it's dangerous to go drawing any deep conclusions while doing so. Approximations shouldn't be allowed to go outside their operational parameters.
      - ViktorRay 19d ago
        But it’s not an approximation right? In our reference frame it is the truth.
        philipov 19d ago
        The approximation is the belief that things can happen simultaneously. It is approximating along the fact that we only need to concern ourselves with a single reference frame. I think the difference in the scenario you proposed is that we can conceive of a practical situation in which someone on Pluto sends a message and waits for a reply. We can then say that they are going to have to wait at least 10 hours to receive a reply. This has meaning. But when the distances become too great the urge is to resist believing that things have an existence beyond the interactions of particles. There is no practical reality to that object beyond our ability to observe it when the light reaches us.
tomrod 19d ago
It's a semantic saying that misses what people typically mean when we speak about simultaneous events. Someone sends an email or speaks on a voice call on the other side of world and a receiver still gets that note at the speed of light, there is slight latency. It's still completely reasonable to measure that latency as we can effectuate improvements to our machines and networks when it increases. The same as if someone wrote a letter in the 1860s and the horses were sick, causing a delay. Simply because information was received by us, the common understanding is that the notification was sent previously, because that is how cause and effect work.
- philipov 19d ago
  It's not semantics, it's an approximation. Events that happen on the same planet are not relativistically distant, so such approximations are mostly fine in our daily lives; the same as how Newtonian gravity is fine to use in many situations. But today we're talking about astrophysics and a proper understanding of Relativity matters.
dotancohen 19d ago
You are getting downvotes, but you are correct. People are voting by intuition, not by physics. Relativity is not intuitive.
- andrewflnr 19d ago
  No, I took a physics class that covered relativity of simultaneity and that's how I know this is BS.
andrewflnr 18d ago
> One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. No. Special relativity says that simultaneity is relative for things moving at different velocities. The classic example is a fast-moving train and a platform disagreeing on simultaneity, even when they're right next to each other. Distance, and light travel time, has nothing to do with it; that's a separate (and mostly less interesting) problem. Relativity absolutely lets you calculate what was happening simultaneously with a given past event within your own reference frame (i.e. other things moving at roughly the same velocity you are). In fact, you can think of that process, figuring out a consistent version of events after you've gathered all the data, as being exactly what it means to "observe" something in relativity. This is as opposed to "seeing it", which is what happens the exact moment the photons hit your eyes/sensors.