Stars we see are actually their past?

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Discussion Overview

The discussion revolves around the concept of observing celestial objects and the implications of light travel time, particularly how the stars and other astronomical phenomena we see are representations of their past states. Participants explore the nature of light travel from distant stars and the potential for those stars to have changed or ceased to exist by the time their light reaches Earth.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether seeing a star that is 15 billion light years away means we are observing it as it was 15 billion years ago, raising the possibility that it may no longer exist.
  • Another participant agrees that we see celestial objects in the past and emphasizes the complexity of defining "now" for distant objects.
  • A participant mentions a recent supernova in Centaurus A, noting that the light from that event took 12 million years to reach Earth, indicating it occurred 12 million years ago.
  • Discussion includes a remark about the speed of gravity propagation, explaining that if the Sun were to vanish, Earth would continue to orbit it for another 8 minutes due to the time it takes for gravitational effects to be felt.
  • Another participant points out that even nearby objects are seen in the past, with light travel times being measurable in nanoseconds for short distances.

Areas of Agreement / Disagreement

Participants generally agree that we see celestial objects as they were in the past, but there is no consensus on the implications of this for understanding the current state of those objects. The discussion includes both agreement on the nature of light travel and differing views on the significance of these observations.

Contextual Notes

Some statements rely on assumptions about the definitions of time and distance in cosmology, and there are unresolved aspects regarding the implications of light travel time on our understanding of distant celestial phenomena.

Esas Shakeel
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One questions that I am really confused with!
If the sun is 8 light minutes away, this means that whenever we see the Sun, we are actually looking at how the Sun looked like 8 minutes ago if I am not wrong.
If we look at a star that is let's say 15 billion light years away from the earth, is it even possible for us to say that the star we see could have died because all we are seeing is the past? (how it looked like 15 billion years ago?)
Im sorry if you think its a stupid question but I am curious about it
 
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It's a perfectly reasonable question and one that I think comes to all of us when we first start finding out about this stuff. Yep, we're seeing them in the past and for all we know they are "now" long dead and gone. I put "now" in quotes because "now" for something 15 billion light years away is a non-trivial topic.

And by the way, the universe is less that 15 billion years old, so your number is a bit too large.
 
hi there

welcome to PF :smile:

yup, everything we look at out in space, we are seeing it in the past

There was a new supernova ( an exploding star) discovered in a galaxy called Centaurus A ( NGC5128) just a few days ago
it took around 12 million years for the light of that explosion to reach Earth ... ie. it happened 12 million years ago !Dave
 
@davenn I see you're a day late (and probably a dollar short) :smile:

Well, OK, maybe only a few seconds late
 
phinds said:
@davenn I see you're a day late (and probably a dollar short) :smile:

Well, OK, maybe only a few seconds late

what are you referring to ??
 
davenn said:
what are you referring to ??
Just a smart-ass remark about the fact that I beat you to the reply by a few seconds. Come on, try to keep up here :smile:
 
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phinds said:
Just a smart-ass remark about the fact that I beat you to the reply by a few seconds. Come on, try to keep up here :smile:

it took time to google NGC5128 to find its distance :wink::rolleyes:
 
davenn said:
it took time to google NGC5128 to find its distance :wink::rolleyes:
See, that's what you get for using facts in your answer instead of just making s*** up the way I do :smile:
 
@Esas Shakeel, sorry Dave and I have hijacked your thread with our foolishness. PF is generally a serious science forum but occasionally a few of us get carried away.
 
  • #10
Esas Shakeel said:
... all we are seeing is the past? ...
One thing that might interest you is that the oldest light we now can 'see' comes from a time when the Universe was much less than even 1 million years old.
At this very early time no stars or galaxies had even formed yet.
https://en.wikipedia.org/wiki/Cosmic_microwave_background
 
  • #11
Not only the light we see but also the gravity pulling on us is from the past. Because gravity propagates at the speed of light, if the sun suddenly vanished, the Earth would continue orbiting where the sun was for another 8 minutes before flying off into space at a tangent.
 
  • #12
..."all we are seeing is the past?"...

Something we see a foot away from us is already about 1 nanosecond in the past. A mountain peak or volcanic plume 50 miles away is about...25 milliseconds in the past.

c roughly 1 ft/nS
 
  • #13
Incredible response from you guys! Thanks a lot! Everything makes much more sense now
And @phinds haha, I don't mind, Its good to be carried away every once in a while I think :)
 
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