7 ms·

> There are other reference frames in which this event occurred ... 5 million years ago. For me, that was a surprising bit in your comment. I don't understand

by sago 7y ago

> There are other reference frames in which this event occurred ... 5 million years ago. For me, that was a surprising bit in your comment. I don't understand how it can be true (5 million years in the future, sure, but the past?). Can you give an example or explain?

antognini 7y ago
Yes, if you had a reference frame that was moving away from the SMBH with a velocity very close to the speed of light. It would take the light an extremely long time to catch up to the observer in that reference frame and so it would appear that the event occurred an extremely long time ago.
- sago 7y ago
  Thank you. Make sense with a bit more of my thought!
- taneq 7y ago
  How would time dilation play into this? That observer would see the SMBH as further away at the time of the event, but they’d experience time at a slower rate. Would the extra time ever ‘beat’ the extra distance?
  - anchpop 7y ago
    from the black hole's perspective, the observer would be travelling slowly in time. But from the observer's perspective, the black hole would be travelling slowly in time (since the observer sees themselves as being at rest, and the black hole as shooting away from them).
  - antognini 7y ago
    The observer moving away from the SMBH would see the event proceed more slowly. The light curve would therefore be stretched out in time --- the source would get brighter more slowly and would get dimmer more slowly. This is actually an important effect that needs to be accounted for in supernova observations in distant galaxies since time dilation is non-negligible.
    - taneq 7y ago
      Thanks, but I was actually wondering more about the apparent time of the flare (say, peak brightness since as you say the duration changes) based on the observer's speed. Say we're both on Earth 26,000 years ago. You stay on Earth and I shoot off at C/2 in a direction away from the SMBH. You wait 26,000 years and then see the flare. How long do I wait in my own reference frame before I see the flare? My guess intuitively is that time dilation and the extra distance cancel out and I still wait exactly 26,000 years (no matter how fast I go), but I'd have to dig out my old physics textbook to figure it out properly. Also, what happens if instead I shoot off towards the SMBH? Or at 90 degrees? I'd obviously see the flare sooner in my reference frame, but how much? It seems like there's a general rule that, by moving, you can decrease the time until you see events (at least those communicated by light) but you can never increase it.
      - roywiggins 7y ago
        If you (somehow) keep accelerating in one direction, there are events you will never observe. It's called a Rindler horizon. Your world-line traces a hyperbola... which will always outpace photons from certain spacetime locations. https://youtu.be/7cj6oiFDEXc https://youtu.be/7cj6oiFDEXc
      - scatters 7y ago
        The Lorentz factor here is 1/sqrt(3/4), around 15% over unity. So the elapsed time for you to see the flare is 2/1.15 * 26ky, or 45 ky. In general, for any λ multiple of c, the elapsed time multiple is sqrt(1 - λ^2) / (1 - λ), which reduces to 1 + λ in the λ << 1 case. You can try plotting it for λ from -1 (c directly towards the event) to +1 (c directly away from the event) to get a feel for how it behaves.
        taneq 7y ago
        That's great, thanks! For anyone else who wanted to see the graph, here's mine: https://www.desmos.com/calculator/kihnh5ohrv https://www.desmos.com/calculator/kihnh5ohrv
  - Rapzid 7y ago
    I believe that because the speed of light is the same in any inertial frame reference.. If you were two light minutes away from the event in the SMBHs inertial frame at the event and traveling toward the earth at a speed sufficient to reach it at the same time as the light from the event.. You would arrive at the earth at the same time as the light but only 2 minutes will have passed in your frame of reference?
    - shkkmo 7y ago
      > but only 2 minutes will have passed in your frame of reference? Only 2 minutes would have passed for the SMBH in your frame of reference (but 52k years would have passed for earth in your frame of reference)
- waynecochran 7y ago
  But if you existed long ago wouldn't have time moved slower since you would have been closer to the SMBH and everything else (i.e., universe would be more dense). So according to general relativity wouldn't it also be right to say it happened less than 26,000 years ago from that reference frame?
im3w1l 7y ago
Since light from that event has reached us, it is undisputably in the past. There is no reference frame in which it happens in the future.
- mirimir 7y ago
  Huh? It hasn't happened yet in reference frames around the Large Magellanic Cloud, right?
  - zeven7 7y ago
    It already happened for those reference frames, they just haven't been able to observe it yet. For any such beings living there, they don't know about the event yet. But when they witness it, they will measure and calculate that the event did occur before today. The only reference frame that could see it and then measure and calculate it as having happened after today would be one travelling faster than c relative to us.
    - mirimir 7y ago
      OK, I get it. Thanks.
  - im3w1l 7y ago
    A reference frame does not have a notion of position. By past I meant the "past from the perspective of someone on Earth right now, possibly traveling at near speed of light in some direction".
deleted 7y ago
[deleted]

by sago 7y ago

antognini 7y ago

Yes, if you had a reference frame that was moving away from the SMBH with a velocity very close to the speed of light. It would take the light an extremely long time to catch up to the observer in that reference frame and so it would appear that the event occurred an extremely long time ago.

sago 7y ago
Thank you. Make sense with a bit more of my thought!
taneq 7y ago
How would time dilation play into this? That observer would see the SMBH as further away at the time of the event, but they’d experience time at a slower rate. Would the extra time ever ‘beat’ the extra distance?
- anchpop 7y ago
  from the black hole's perspective, the observer would be travelling slowly in time. But from the observer's perspective, the black hole would be travelling slowly in time (since the observer sees themselves as being at rest, and the black hole as shooting away from them).
- antognini 7y ago
  The observer moving away from the SMBH would see the event proceed more slowly. The light curve would therefore be stretched out in time --- the source would get brighter more slowly and would get dimmer more slowly. This is actually an important effect that needs to be accounted for in supernova observations in distant galaxies since time dilation is non-negligible.
  - taneq 7y ago
    Thanks, but I was actually wondering more about the apparent time of the flare (say, peak brightness since as you say the duration changes) based on the observer's speed. Say we're both on Earth 26,000 years ago. You stay on Earth and I shoot off at C/2 in a direction away from the SMBH. You wait 26,000 years and then see the flare. How long do I wait in my own reference frame before I see the flare? My guess intuitively is that time dilation and the extra distance cancel out and I still wait exactly 26,000 years (no matter how fast I go), but I'd have to dig out my old physics textbook to figure it out properly. Also, what happens if instead I shoot off towards the SMBH? Or at 90 degrees? I'd obviously see the flare sooner in my reference frame, but how much? It seems like there's a general rule that, by moving, you can decrease the time until you see events (at least those communicated by light) but you can never increase it.
    - roywiggins 7y ago
      If you (somehow) keep accelerating in one direction, there are events you will never observe. It's called a Rindler horizon. Your world-line traces a hyperbola... which will always outpace photons from certain spacetime locations. https://youtu.be/7cj6oiFDEXc https://youtu.be/7cj6oiFDEXc
    - scatters 7y ago
      The Lorentz factor here is 1/sqrt(3/4), around 15% over unity. So the elapsed time for you to see the flare is 2/1.15 * 26ky, or 45 ky. In general, for any λ multiple of c, the elapsed time multiple is sqrt(1 - λ^2) / (1 - λ), which reduces to 1 + λ in the λ << 1 case. You can try plotting it for λ from -1 (c directly towards the event) to +1 (c directly away from the event) to get a feel for how it behaves.
      - taneq 7y ago
        That's great, thanks! For anyone else who wanted to see the graph, here's mine: https://www.desmos.com/calculator/kihnh5ohrv https://www.desmos.com/calculator/kihnh5ohrv
- Rapzid 7y ago
  I believe that because the speed of light is the same in any inertial frame reference.. If you were two light minutes away from the event in the SMBHs inertial frame at the event and traveling toward the earth at a speed sufficient to reach it at the same time as the light from the event.. You would arrive at the earth at the same time as the light but only 2 minutes will have passed in your frame of reference?
  - shkkmo 7y ago
    > but only 2 minutes will have passed in your frame of reference? Only 2 minutes would have passed for the SMBH in your frame of reference (but 52k years would have passed for earth in your frame of reference)
waynecochran 7y ago
But if you existed long ago wouldn't have time moved slower since you would have been closer to the SMBH and everything else (i.e., universe would be more dense). So according to general relativity wouldn't it also be right to say it happened less than 26,000 years ago from that reference frame?

im3w1l 7y ago

Since light from that event has reached us, it is undisputably in the past. There is no reference frame in which it happens in the future.

mirimir 7y ago
Huh? It hasn't happened yet in reference frames around the Large Magellanic Cloud, right?
- zeven7 7y ago
  It already happened for those reference frames, they just haven't been able to observe it yet. For any such beings living there, they don't know about the event yet. But when they witness it, they will measure and calculate that the event did occur before today. The only reference frame that could see it and then measure and calculate it as having happened after today would be one travelling faster than c relative to us.
  - mirimir 7y ago
    OK, I get it. Thanks.
- im3w1l 7y ago
  A reference frame does not have a notion of position. By past I meant the "past from the perspective of someone on Earth right now, possibly traveling at near speed of light in some direction".

deleted 7y ago

[deleted]