3 ms·

When we notice it exploding it would have actually exploded 600 years prior.

SAI_Peregrinus 9y ago
That depends on which frame of reference you pick, of course.
didgeoridoo 9y ago
What is the right way to think about this? If we observe a supernova 600LY away, do we say that event is happening "now" from our frame of reference? Or should we think of it as happening "600 years ago", and the light from the event is only now reaching us? If you think of causality itself moving at the speed of light (which of course it does), and think in light cones rather than referring to a nonexistent universal frame of reference with an authoritative clock, the "now" language seems more appropriate. Intuitively, it also feels quite wrong — but I attribute this more to a failure of intuition than of language.
- Santosh83 9y ago
  It is happening in our NOW and that's all that matters from a cosmic perspective, since no signal can travel faster than light. In that sense there's only the NOW, even when you're observing your friend a few meters away. We might say that physics itself travels at light speed.
  - didgeoridoo 9y ago
    I think even that undersells it. Existence itself moves at light speed.
- andrewla 9y ago
  No, this is not a good way to think about relativity and its implications. When we talk about time in the sense of things happening "now" or "in the past" or "after", we have to think about reference frame, _not_ location. Reference frames are inherently global, as opposed to an "event", which encodes both location and time relative to all possible inertial reference frames. The typical metaphor is imagine that you infiltrate space with a three-dimensional grid of clocks that are kept a fixed distance from each other (say by a rigid rod). Those clocks are not moving relative to each other, and it is trivial to synchronize them, because the distance between them is fixed -- fire a light pulse to your neighbor with the current time, and you neighbor will know when you sent the symbol by subtracting off the time it takes light to travel the distance. This grid, that covers all of space, represents a single reference frame. So if Betelgeuse had in fact exploded six hundred years ago, then the clock grid in Earth's inertial frame would have recorded the event of the Battle of Orewin Bridge on Earth at the same time as the event of the beginning of the Betelgeusian supernova in the Betelgeuse system. The complexity of this comes in the fact that the inertial frame travelling towards Betelgeuse from Earth at some significant fraction of the speed of light would, with its clock grid, measured Orewin Bridge well before the supernova's start. That's independent of the amount of time it would take for those two clocks to communicate with each other -- we can almost imagine a scientist who finally downloads the logs for all the clocks in a given reference frame collating the data. What we _can_ say, though, is that once the event of someone on Earth seeing the supernova occurs, then that event is strictly _after_ the supernova -- no inertial reference frame will ever see that event, "observation of supernova", occur before the "initiation of supernova".
- uf 9y ago
  Think of it this way: If we knew that star is going to go nova within the next 100 years, could we (science-fiction hat and faster-than-light travel on) send help? No, because it already went nova a long time ago.

That depends on which frame of reference you pick, of course.

What is the right way to think about this? If we observe a supernova 600LY away, do we say that event is happening "now" from our frame of reference? Or should we think of it as happening "600 years ago", and the light from the event is only now reaching us? If you think of causality itself moving at the speed of light (which of course it does), and think in light cones rather than referring to a nonexistent universal frame of reference with an authoritative clock, the "now" language seems more appropriate. Intuitively, it also feels quite wrong — but I attribute this more to a failure of intuition than of language.

Santosh83 9y ago
It is happening in our NOW and that's all that matters from a cosmic perspective, since no signal can travel faster than light. In that sense there's only the NOW, even when you're observing your friend a few meters away. We might say that physics itself travels at light speed.
- didgeoridoo 9y ago
  I think even that undersells it. Existence itself moves at light speed.
andrewla 9y ago
No, this is not a good way to think about relativity and its implications. When we talk about time in the sense of things happening "now" or "in the past" or "after", we have to think about reference frame, _not_ location. Reference frames are inherently global, as opposed to an "event", which encodes both location and time relative to all possible inertial reference frames. The typical metaphor is imagine that you infiltrate space with a three-dimensional grid of clocks that are kept a fixed distance from each other (say by a rigid rod). Those clocks are not moving relative to each other, and it is trivial to synchronize them, because the distance between them is fixed -- fire a light pulse to your neighbor with the current time, and you neighbor will know when you sent the symbol by subtracting off the time it takes light to travel the distance. This grid, that covers all of space, represents a single reference frame. So if Betelgeuse had in fact exploded six hundred years ago, then the clock grid in Earth's inertial frame would have recorded the event of the Battle of Orewin Bridge on Earth at the same time as the event of the beginning of the Betelgeusian supernova in the Betelgeuse system. The complexity of this comes in the fact that the inertial frame travelling towards Betelgeuse from Earth at some significant fraction of the speed of light would, with its clock grid, measured Orewin Bridge well before the supernova's start. That's independent of the amount of time it would take for those two clocks to communicate with each other -- we can almost imagine a scientist who finally downloads the logs for all the clocks in a given reference frame collating the data. What we _can_ say, though, is that once the event of someone on Earth seeing the supernova occurs, then that event is strictly _after_ the supernova -- no inertial reference frame will ever see that event, "observation of supernova", occur before the "initiation of supernova".
uf 9y ago
Think of it this way: If we knew that star is going to go nova within the next 100 years, could we (science-fiction hat and faster-than-light travel on) send help? No, because it already went nova a long time ago.