4 ms·

No, in our reference frame it took 26,000 years for the light to travel to us so that we could see it. We measure that the event occurred 26,000 light years awa

by zeven7 7y ago

No, in our reference frame it took 26,000 years for the light to travel to us so that we could see it. We measure that the event occurred 26,000 light years away so we calculate that it occurred 26,000 years ago. There are other reference frames where it also happened 26,000 years ago but that are much closer and saw it occur earlier. There are also reference frames where it happened 26,000 years ago but they haven't observed it yet because they are much further away. And as OP mentioned, there are reference frames where it occurred 5 million years ago yet it still hasn't even been observed.

fny 7y ago
I clearly don't have a grasp of reference frames. Can someone elucidate? The event occurred is 26,000 light years away, we date it as having occurred 26,000 years ago. How is it that a location that's closer, say 10,000 light years, would have dated the event to 26,000 ago rather than 10,000 light years?
- antognini 7y ago
  The closer location would have received the signal 16,000 years earlier than us here on Earth. They would still say that it happened 10,000 years before they observed it, but they would still say it happened in year 24,000 BC.
  - mirimir 7y ago
    > ... but they would still say it happened in year 24,000 BC. I can't imagine that they would say that! I'm sure that you mean the equivalent of 24,000 BC in our reference frame, after Lorentz transformation. Not that they would likely even know about us, or manage the transformation. This is bloody confusing.
    - Khoth 7y ago
      Assuming these people 10000 lightyears away from the black hole are also on a planet, their reference frame is almost the same as ours - the relative speed of different planets is very small compared to the speed of light. So they'll agree on when 24000BC is.
      - mirimir 7y ago
        Why would they have a Christ? Or know when ours was supposedly born?
- CuriouslyC 7y ago
  It isn't just about location. Someone moving between both sources near the speed of light would have the event occurring a much shorter time ago.
- ohhhwell 7y ago
  The key in that case is that the other reference frame that from our reference frame will measure a different distance to the black hole. It might be 25000 light years away from them, but we would not be 25000 light years away (but longer I believe) in their reference frame. This can happen for instance with extreme differences in relative velocities. Someone else can probably show an example with the math, but a key concept here is the that distances also change when you move real fast or are in a deep gravity well.
- DiogenesKynikos 7y ago
  The only interval that is constant in Special Relativity is the spacetime interval. It's a distance measure. In 3D space, we normally say that distance is given by ds^2 = dx^2 + dy^2 + dz^2. In Special Relativity, you define a quantity called the spacetime interval: ds^2 = -(c*dt)^2 + dx^2 + dy^2 + dz^2. You treat time as a 4th dimension, and take it into account when calculating distances. Notice that this interval can be negative (the metric is "semi-Riemannian"). "Events" (locations in 4D spacetime) can only be causally connected (one can influence the other) if the spacetime interval separating them is negative (this is equivalent to saying that no information can ever propagate faster than light). The key is that all non-accelerating observers agree on the spacetime interval between any two events. The spacetime interval between two events is the only solid distance you can give. The time interval by itself depends on the observer's reference frame (you've probably heard of time dilation). So too does the spatial distance (length contraction). But time dilation and length contraction behave in such a way that the spacetime interval remains constant. The long and the short of it is that time intervals are observer-dependent.
baddox 7y ago
Are you saying that the only reference frame where the event happened "now" is the reference frame of the black hole itself? I'm quite familiar with relativity of simultaneity, and that would lead me to conclude that from my reference point, two things happened simultaneously: the black hole flared, and the clock read 11am on May 13th, 2019. Given this, I would think we would describe that in English as "the black hole flared at 11am on May 13th, 2019."
- zeven7 7y ago
  What do you mean by "now"? Which clock are you talking about? A clock here on Earth? Clocks here on Earth currently read May 13th, 2019. And you can conclude that we observed the event on May 13th, 2019. We believe the black hole is approx. 26,000 light years away. Thus we know the light took 26,000 years to reach us. So we say the event took place around 23,981 BC. We have 3 events here: A. A clock on Earth reading May 13th, 2019 B. Light from the black hole flare reaching Earth C. The black hole flaring We can say that A and B occurred simultaneously in all reference frames (they occurred at the same location and at the same time). C did not occur simultaneously with A or B in any reference frame (except perhaps, debatably, that of something moving at the speed of light).
  - dpratt 7y ago
    > Clocks here on Earth currently read May 13th, 2019. Are you sure that we are in the same reference frame? I’m willing to accept that you started a fairly short journey at some appreciable fraction of C, turned around and gave just recently arrived back home, but I’m going to need some proof.
    - zeven7 7y ago
      Hahah, oops. I assumed the date given by parent was today's date and didn't think about it much. I guess I should have said August 13, 2019. My bad.
  - deleted 7y ago
    [deleted]

by zeven7 7y ago

fny 7y ago

I clearly don't have a grasp of reference frames. Can someone elucidate? The event occurred is 26,000 light years away, we date it as having occurred 26,000 years ago. How is it that a location that's closer, say 10,000 light years, would have dated the event to 26,000 ago rather than 10,000 light years?

antognini 7y ago
The closer location would have received the signal 16,000 years earlier than us here on Earth. They would still say that it happened 10,000 years before they observed it, but they would still say it happened in year 24,000 BC.
- mirimir 7y ago
  > ... but they would still say it happened in year 24,000 BC. I can't imagine that they would say that! I'm sure that you mean the equivalent of 24,000 BC in our reference frame, after Lorentz transformation. Not that they would likely even know about us, or manage the transformation. This is bloody confusing.
  - Khoth 7y ago
    Assuming these people 10000 lightyears away from the black hole are also on a planet, their reference frame is almost the same as ours - the relative speed of different planets is very small compared to the speed of light. So they'll agree on when 24000BC is.
    - mirimir 7y ago
      Why would they have a Christ? Or know when ours was supposedly born?
CuriouslyC 7y ago
It isn't just about location. Someone moving between both sources near the speed of light would have the event occurring a much shorter time ago.
ohhhwell 7y ago
The key in that case is that the other reference frame that from our reference frame will measure a different distance to the black hole. It might be 25000 light years away from them, but we would not be 25000 light years away (but longer I believe) in their reference frame. This can happen for instance with extreme differences in relative velocities. Someone else can probably show an example with the math, but a key concept here is the that distances also change when you move real fast or are in a deep gravity well.
DiogenesKynikos 7y ago
The only interval that is constant in Special Relativity is the spacetime interval. It's a distance measure. In 3D space, we normally say that distance is given by ds^2 = dx^2 + dy^2 + dz^2. In Special Relativity, you define a quantity called the spacetime interval: ds^2 = -(c*dt)^2 + dx^2 + dy^2 + dz^2. You treat time as a 4th dimension, and take it into account when calculating distances. Notice that this interval can be negative (the metric is "semi-Riemannian"). "Events" (locations in 4D spacetime) can only be causally connected (one can influence the other) if the spacetime interval separating them is negative (this is equivalent to saying that no information can ever propagate faster than light). The key is that all non-accelerating observers agree on the spacetime interval between any two events. The spacetime interval between two events is the only solid distance you can give. The time interval by itself depends on the observer's reference frame (you've probably heard of time dilation). So too does the spatial distance (length contraction). But time dilation and length contraction behave in such a way that the spacetime interval remains constant. The long and the short of it is that time intervals are observer-dependent.

baddox 7y ago

Are you saying that the only reference frame where the event happened "now" is the reference frame of the black hole itself? I'm quite familiar with relativity of simultaneity, and that would lead me to conclude that from my reference point, two things happened simultaneously: the black hole flared, and the clock read 11am on May 13th, 2019. Given this, I would think we would describe that in English as "the black hole flared at 11am on May 13th, 2019."

zeven7 7y ago
What do you mean by "now"? Which clock are you talking about? A clock here on Earth? Clocks here on Earth currently read May 13th, 2019. And you can conclude that we observed the event on May 13th, 2019. We believe the black hole is approx. 26,000 light years away. Thus we know the light took 26,000 years to reach us. So we say the event took place around 23,981 BC. We have 3 events here: A. A clock on Earth reading May 13th, 2019 B. Light from the black hole flare reaching Earth C. The black hole flaring We can say that A and B occurred simultaneously in all reference frames (they occurred at the same location and at the same time). C did not occur simultaneously with A or B in any reference frame (except perhaps, debatably, that of something moving at the speed of light).
- dpratt 7y ago
  > Clocks here on Earth currently read May 13th, 2019. Are you sure that we are in the same reference frame? I’m willing to accept that you started a fairly short journey at some appreciable fraction of C, turned around and gave just recently arrived back home, but I’m going to need some proof.
  - zeven7 7y ago
    Hahah, oops. I assumed the date given by parent was today's date and didn't think about it much. I guess I should have said August 13, 2019. My bad.
- deleted 7y ago
  [deleted]