So I heard that, possibly, two super massive black holes could be merging by the end of this or next year. My question is this, and essentially trying to get my head around, if it takes x amount of time for light to reach us, has this event already happened? And we are just seeing it in its past?
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If we’re detecting a black hole merger it’s through their gravitational wave signal, and gravitational waves travel at… wait for it… the speed of light. So does light. For various reasons there can be a delay, but assuming the merger produced a visual signal it would arrive within hours or days of the gravitational signal.
Yes it already happened in the past, and yes we’re just seeing evidence of it now.
Yes, we are viewing “the past.”
The fun bit is when you realize that *everything* you observe is *necessarily* already in the past, just by different degrees.
In the case of interstellar events, the distance is so great that the age of the event is dramatic by the time the light we use to observe it reaches us.
When astronomers say the black holes are going to mere at some time in the future, what they mean is that the merge will be observable at some time in the future. From our earthly perspective, it is simply that we don’t have a compelling reason to distibguish that the event happened X years ago and we will be observing it Y years from now. It’s an excessive mouthful and everyone serriously studying the event already knows that such a caveat is being assumed.
Many people have experienced this. A very similar thing is happening when you hear echoes. If you shout and hear your echo a few seconds later, that is ‘your voice’ from ‘the past’.
Well, the same thing, not the echo part, but the delay part happens with light. Light travels at a very fast speed but not an infinite speed. So light from any event takes time to reach your eyes. NOTHING you see with your eyes is happening at the time that you see it. Everything is delayed, it is just that for most human stuff, we cannot notice the delay.
Since stars are really far away, the delay is measurable.
If a tree falls in the forest and there is no one to hear it, was there really a sound?
You are right it already happened a really long time ago. We just say it’s happening now because it’s only now that we can see the effects of it.
So im late to this one but I figure I’ll add my bit all the same.
The answer is both yes and no. Yes, because you are exactly right about light and time over distance.
No because of relativity. If you and I both have clocks set to the exact same time but I fly off into space on a rocket at near the speed of light for a year and return. Our clocks will be dramatically different. Yours will have advanced much more than my own. If while we carried out this experiment while noting the time at which significant events occured we would report different times that they occured, neither of us would be wrong since the time would be relative to our own perspective. For this reason saying an event has already happened isn’t quite true. This is because we often describe these events as occuring “relative to our own reference frame”
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