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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 activit

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 3d 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
herbstein 3d 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 3d 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 3d 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 3d ago
      But it’s not an approximation right? In our reference frame it is the truth.
      - philipov 3d 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.
        xyzzy_plugh 3d ago
        I get what you're saying but you're also being incredibly dismissive to the common understanding which is still correct, otherwise most astrophysics wouldn't work, because it wouldn't be possible to predict where another planet will be at a later time. It's possible to have two alarm clocks go off at the same instant on two planets, though obviously each will observe the other going off later, after a delay. It's obviously fair to say they happened at the same time in this scenario.

philipov 3d 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

herbstein 3d 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 3d 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 3d 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 3d ago
    But it’s not an approximation right? In our reference frame it is the truth.
    - philipov 3d 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.
      - xyzzy_plugh 3d ago
        I get what you're saying but you're also being incredibly dismissive to the common understanding which is still correct, otherwise most astrophysics wouldn't work, because it wouldn't be possible to predict where another planet will be at a later time. It's possible to have two alarm clocks go off at the same instant on two planets, though obviously each will observe the other going off later, after a delay. It's obviously fair to say they happened at the same time in this scenario.