3 ms·

This is...exactly right. You could say "it's already happened" as a first approximation but this isn't really true because the statement implicitly assumes the

This is...exactly right. You could say "it's already happened" as a first approximation but this isn't really true because the statement implicitly assumes there is some kind of universal "now" that can be used as a reference point for timing things. No such universal "now" exists. That's why the question "What's happening right now on Mars?" makes no physical sense. "Now" is a function of both when and where you're standing. The question "What's happening right now on the other side of my living room?" also doesn't make any physical sense. But the difference between reality and our perception of it is less easily detectable in that case.

charcircuit 5y ago
There is a universal now. Just because you are in a different location it doesn't change what now is. We have clocks all over the earth synced close to each other depsite being located in different places. While physically it may be impossible to perfectly sync clocks in theory we could have theoretically synced clocks.
- danans 5y ago
  I don't think this is true. General relativity says that time slows down under greater gravitational forces. This has been experimentally proven. Therefore clocks at different elevations run at different speeds.
  - charcircuit 5y ago
    >Therefore clocks at different elevations run at different speeds. If clock A runs at 1x speed and clock B runs at 2x speed. Now will refer to time t for clock A and t / 2 for clock B.
    - danans 5y ago
      You can refer to it differently, but time progresses at different rates at the different places. So in your example, physical processes (i.e rusting, aging) at place B happen 2X as fast as those at place A.
      - charcircuit 5y ago
        >but time progresses at different rates at the different places. What's the problem? We can still have a now defined as I mentioned above. t1=t2=0 at the start. Now when t1=x for place A will refer to when t2=2*t1 at place B.
        random314 5y ago
        As per Einstein's theory of relativity, there is no universal now https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity Also, from https://en.wikipedia.org/wiki/Spacetime https://en.wikipedia.org/wiki/Spacetime "the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity); and the linear additivity of velocities no longer holds true."
        charcircuit 5y ago
        >the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity) I'm not saying to use different frames of reference. I am saying to use a single universal frame of reference.
        random314 5y ago
        There is a frame of reference where the merger hasn't happened yet. Let's use that as a universal frame of reference.
  - dreamcompiler 5y ago
    Yes. In addition, Special relativity says clocks moving faster through space tick slower than clocks that move more slowly. So let's say you're a GPS satellite. GR says your clock ticks too fast because you're farther from the center of the earth's gravity well than the wristwatches of people on the surface. But SR says your clock ticks slower than a wristwatch on the surface because you're whizzing through space at ~15,000 km/hour. Which is right? Both. And they do not exactly cancel each other out (in general). So both GR and SR have to be taken into account to make GPS satellites produce accurate locations and times.
- random314 5y ago
  As per einstein's 1905 paper, there is no universal now. What is universal is the space time distance between 2 events. https://en.m.wikipedia.org/wiki/Spacetime https://en.m.wikipedia.org/wiki/Spacetime
andrewflnr 5y ago
Incorrect. Relativistic differences in simultaneity between observers have nothing to do with light travel time between them and everything to do with different velocities. The other side your living room, and even the other side of the galaxy, is moving close enough to the same speed that simultaneity is perfectly well defined between them. So we can confidently assign times in the past to events based on what we see in the present, in the knowledge that they will be consistent with times assigned by other observers in different locations at the same speed. The only wrinkle in the scenario of a distant galaxy is cosmic expansion inducing a different reference frame by causing motion relative to Earth. However, if the redshift isn't more than a few percent then the time dilation won't be either, so we can be pretty sure it will be in the past by any definition that isn't moving a fair fraction of c relative to us. Don't confuse light travel time with relative simultaneity. Relativity does weird things to time but that's not one of them.
- narag 5y ago
  Don't forget they're talking about two black holes merging, so there are actually extreme relativistic effects to consider.
- palmtree3000 5y ago
  "Incorrect" is way too strong here. I agree that a reference frame that has about the same velocity as the stuff you care about is in some sense privileged. But the fact that we haven't seen the merger happen yet means that it's outside our light cone, which means there exists a reference frame where the merger hasn't happened yet. that is to say, t_{this_coversation} < t_{merger}, in that frame.
  - andrewflnr 5y ago
    It is not too strong. GP specifically claimed that simultaneity depends on "where you're standing". This is wrong in all reference frames. As for timing of distant events, GP didn't make any reference to other reference frames at arbitrary velocity, only different positions. In our reference frame, which they were implicitly talking about, the merger (assuming the prediction is correct) was in the past, unambiguously.
DavidSJ 5y ago
There's always the CMBR rest frame.

charcircuit 5y ago

There is a universal now. Just because you are in a different location it doesn't change what now is. We have clocks all over the earth synced close to each other depsite being located in different places. While physically it may be impossible to perfectly sync clocks in theory we could have theoretically synced clocks.

danans 5y ago
I don't think this is true. General relativity says that time slows down under greater gravitational forces. This has been experimentally proven. Therefore clocks at different elevations run at different speeds.
- charcircuit 5y ago
  >Therefore clocks at different elevations run at different speeds. If clock A runs at 1x speed and clock B runs at 2x speed. Now will refer to time t for clock A and t / 2 for clock B.
  - danans 5y ago
    You can refer to it differently, but time progresses at different rates at the different places. So in your example, physical processes (i.e rusting, aging) at place B happen 2X as fast as those at place A.
    - charcircuit 5y ago
      >but time progresses at different rates at the different places. What's the problem? We can still have a now defined as I mentioned above. t1=t2=0 at the start. Now when t1=x for place A will refer to when t2=2*t1 at place B.
      - random314 5y ago
        As per Einstein's theory of relativity, there is no universal now https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity Also, from https://en.wikipedia.org/wiki/Spacetime https://en.wikipedia.org/wiki/Spacetime "the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity); and the linear additivity of velocities no longer holds true."
        charcircuit 5y ago
        >the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity) I'm not saying to use different frames of reference. I am saying to use a single universal frame of reference.
        random314 5y ago
        There is a frame of reference where the merger hasn't happened yet. Let's use that as a universal frame of reference.
- dreamcompiler 5y ago
  Yes. In addition, Special relativity says clocks moving faster through space tick slower than clocks that move more slowly. So let's say you're a GPS satellite. GR says your clock ticks too fast because you're farther from the center of the earth's gravity well than the wristwatches of people on the surface. But SR says your clock ticks slower than a wristwatch on the surface because you're whizzing through space at ~15,000 km/hour. Which is right? Both. And they do not exactly cancel each other out (in general). So both GR and SR have to be taken into account to make GPS satellites produce accurate locations and times.
random314 5y ago
As per einstein's 1905 paper, there is no universal now. What is universal is the space time distance between 2 events. https://en.m.wikipedia.org/wiki/Spacetime https://en.m.wikipedia.org/wiki/Spacetime

andrewflnr 5y ago

Incorrect. Relativistic differences in simultaneity between observers have nothing to do with light travel time between them and everything to do with different velocities. The other side your living room, and even the other side of the galaxy, is moving close enough to the same speed that simultaneity is perfectly well defined between them. So we can confidently assign times in the past to events based on what we see in the present, in the knowledge that they will be consistent with times assigned by other observers in different locations at the same speed. The only wrinkle in the scenario of a distant galaxy is cosmic expansion inducing a different reference frame by causing motion relative to Earth. However, if the redshift isn't more than a few percent then the time dilation won't be either, so we can be pretty sure it will be in the past by any definition that isn't moving a fair fraction of c relative to us. Don't confuse light travel time with relative simultaneity. Relativity does weird things to time but that's not one of them.

narag 5y ago
Don't forget they're talking about two black holes merging, so there are actually extreme relativistic effects to consider.
palmtree3000 5y ago
"Incorrect" is way too strong here. I agree that a reference frame that has about the same velocity as the stuff you care about is in some sense privileged. But the fact that we haven't seen the merger happen yet means that it's outside our light cone, which means there exists a reference frame where the merger hasn't happened yet. that is to say, t_{this_coversation} < t_{merger}, in that frame.
- andrewflnr 5y ago
  It is not too strong. GP specifically claimed that simultaneity depends on "where you're standing". This is wrong in all reference frames. As for timing of distant events, GP didn't make any reference to other reference frames at arbitrary velocity, only different positions. In our reference frame, which they were implicitly talking about, the merger (assuming the prediction is correct) was in the past, unambiguously.

DavidSJ 5y ago

There's always the CMBR rest frame.