4 ms·

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 ye

by 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.

by fny 7y ago

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.