7 ms·

If the star is 120 million light-years away, does that mean this explosion actually happened 120 million years ago?

by freeslave 5y ago

pc86 5y ago
Yes
- ehsankia 5y ago
  Would the fact that space between here and there is stretching during that time impact that?
  - raegis 5y ago
    The light has traveled for 120 million years, but the galaxy today is further away because the universe has been expanding the entire time. So the distance between the position of the object 120 million years ago, and Earth today, is 120 million light years. I think :)
    - sailingparrot 5y ago
      It's not just objects getting more and more distant with time, it's spacetime itself stretching at every point. So, during it's 120M years journey, the photon had to travel the initial distance + new distance added by expansion rate. So when the photons we are receiving now left that star, that star was closer than 120M light years away. And that star is now further than 120M light years.
- shpx 5y ago
  , assuming that the speed of light is the same in both directions, which we can’t prove. https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light https://youtu.be/pTn6Ewhb27k https://youtu.be/pTn6Ewhb27k
asxd 5y ago
Always blows my mind that because light travels through space, you can literally see into the past. Imagine, way way down the line, eventually meeting a civilization from a far away planet, and they could potentially show you actual photographs of Pangaea.
- xwdv 5y ago
  More than Pangea… if they mastered the ability to resolve vast distances they could show us the herds of dinosaurs roaming the world, and then their total extinction, a Timelapse of the rise of human civilization, ancient Egypt and the Roman Empire, if only we could truly meet a civilization way down the line, or perhaps they would come to meet us, given their obsession.
  - JoeAltmaier 5y ago
    Perhaps if there is a large reflector in space somewhere, that light could be passing back by us today! If we had a large enough collector, we could create an image.
    - spaetzleesser 5y ago
      In theory black holes could do that. Some photons will slingshot around it and go straight back.
      - wumpus 5y ago
        We haven't seen a photon ring yet, and when we do, there won't be that much information in them. Perhaps after technology has advanced another 1000X...
  - asxd 5y ago
    It's cool to think that first contact between two space-faring civilizations could potentially involve a mutual exchange of detailed historical imagery that neither civilization could have captured on their own.
tegeek 5y ago
From a philosophical point of view, an event happens at the point when observed by an observer. For us, Humans on earth, this supernova happens now and not 120 million years ago.
- LegitShady 5y ago
  I think even philosophy can withstand knowing the speed of light and incorporating it into the framework of 'when things happened' so that it agrees with our understanding of the universe instead of not.
- awb 5y ago
  I don’t think that’s right. If a tree falls in a forest and we stumbled upon it today, I don’t think philosophy says that the tree fell today. I think it says we found a fallen tree today. The supernova didn’t happen today, we found evidence of it today.
  - CamperBob2 5y ago
    Minkowski spacetime confuses matters, though. From the point of view of the photon we observe, no time elapses between its emission at the location being observed and its arrival at the telescope image sensor. It's as if the photon were born in exactly the right place and time to be observed by us, at that very instant. So what we see is arguably happening in real time, regardless of distance.
    - deleted 5y ago
      [deleted]
    - andreareina 5y ago
      Ehh. The measurement is happening in real time but we have well-defined ways to say when the event happened, and that's 120MY ago (given the reference frame where Earth is motionless).
      - mr_mitm 5y ago
aaaaaaaaaaab 5y ago
It depends on the observers reference frame. https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
- andreareina 5y ago
  Unless otherwise stated the reference frame is generally taken to be the one where the observer is stationary

If the star is 120 million light-years away, does that mean this explosion actually happened 120 million years ago?

pc86 5y ago

Yes

ehsankia 5y ago
Would the fact that space between here and there is stretching during that time impact that?
- raegis 5y ago
  The light has traveled for 120 million years, but the galaxy today is further away because the universe has been expanding the entire time. So the distance between the position of the object 120 million years ago, and Earth today, is 120 million light years. I think :)
  - sailingparrot 5y ago
    It's not just objects getting more and more distant with time, it's spacetime itself stretching at every point. So, during it's 120M years journey, the photon had to travel the initial distance + new distance added by expansion rate. So when the photons we are receiving now left that star, that star was closer than 120M light years away. And that star is now further than 120M light years.
shpx 5y ago
, assuming that the speed of light is the same in both directions, which we can’t prove. https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light https://youtu.be/pTn6Ewhb27k https://youtu.be/pTn6Ewhb27k

asxd 5y ago

Always blows my mind that because light travels through space, you can literally see into the past. Imagine, way way down the line, eventually meeting a civilization from a far away planet, and they could potentially show you actual photographs of Pangaea.

xwdv 5y ago
More than Pangea… if they mastered the ability to resolve vast distances they could show us the herds of dinosaurs roaming the world, and then their total extinction, a Timelapse of the rise of human civilization, ancient Egypt and the Roman Empire, if only we could truly meet a civilization way down the line, or perhaps they would come to meet us, given their obsession.
- JoeAltmaier 5y ago
  Perhaps if there is a large reflector in space somewhere, that light could be passing back by us today! If we had a large enough collector, we could create an image.
  - spaetzleesser 5y ago
    In theory black holes could do that. Some photons will slingshot around it and go straight back.
    - wumpus 5y ago
      We haven't seen a photon ring yet, and when we do, there won't be that much information in them. Perhaps after technology has advanced another 1000X...
- asxd 5y ago
  It's cool to think that first contact between two space-faring civilizations could potentially involve a mutual exchange of detailed historical imagery that neither civilization could have captured on their own.

tegeek 5y ago

From a philosophical point of view, an event happens at the point when observed by an observer. For us, Humans on earth, this supernova happens now and not 120 million years ago.

LegitShady 5y ago
I think even philosophy can withstand knowing the speed of light and incorporating it into the framework of 'when things happened' so that it agrees with our understanding of the universe instead of not.
awb 5y ago
I don’t think that’s right. If a tree falls in a forest and we stumbled upon it today, I don’t think philosophy says that the tree fell today. I think it says we found a fallen tree today. The supernova didn’t happen today, we found evidence of it today.
- CamperBob2 5y ago
  Minkowski spacetime confuses matters, though. From the point of view of the photon we observe, no time elapses between its emission at the location being observed and its arrival at the telescope image sensor. It's as if the photon were born in exactly the right place and time to be observed by us, at that very instant. So what we see is arguably happening in real time, regardless of distance.
  - deleted 5y ago
    [deleted]
  - andreareina 5y ago
    Ehh. The measurement is happening in real time but we have well-defined ways to say when the event happened, and that's 120MY ago (given the reference frame where Earth is motionless).
    - mr_mitm 5y ago

aaaaaaaaaaab 5y ago

It depends on the observers reference frame. https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

andreareina 5y ago
Unless otherwise stated the reference frame is generally taken to be the one where the observer is stationary