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Q & A: seeing planets and stars now?

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Most recent answer: 08/05/2015
Hi,I have a small question about discovering the far we can discover a planet which is far by millions of light years from us??? any existing discovering machine need a very long time to reaching the planet. millions of years... so how can a telescope which use light to discover such far planets???even if we receiving the light which the planet send to us millions of years ago how can this teach us about the "current" situation on the planet??
- youssef (age 16)

You're right that we can only see distant stars and planets in the past. To figure out what's going on with them now we have to know something about the laws for how things change over time. Most stars change slowly, so most of what we see for individual stars within a few million light years resembles what's going on now. 

There's an additional complication, in that there's no single best rule for matching up events at remote locations to say which ones are at the same time. Relativity allows different good coordinate systems with different versions of simultaneity. One fairly standard one for these issues is the co-moving frame, in which the time coordinate in each place would be the time since the  Big Bang as measured by an average object in that region.

Mike W.

(published on 08/05/2015)

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