Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: looking back

Learn more physics!

Most recent answer: 10/22/2007
Q:
If an object is 50 million light years away, and we can see it in the night sky, aren’t we actually looking back in time and seeing how the object appeared 50 million light years ago since it took the light we are seeing at that moment 50 million light years to get here?
How do we know that object didn’t disappear 25 million light years ago?
- Bob H
Crestwood IL
A:
Yes, we are indeed looking back in time.
The only way we can figure out what’s been happening more recently to the object (e.g. star) is by comparison with other objects like it. Those comparisons tell us what sort of life-cycle stars go through. Whatever might happen, however, ’disappearance’ isn’t really an option, since stars contain some conserved quantities (energy, baryon number...) which will persist even if the star burns out or blows up.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.