How old are images that we see on APOD some million years?

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itwasnoteasy
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How old are images that we see on APOD some million years?

Post by itwasnoteasy » Tue Aug 28, 2012 4:49 am

Hello All,

First of all please forgive my ignorance here. I am new to astronomy and I ponder about things in ever expanding space.

Whenever I see a picture on APOD, which describes something like this pic is spanned across 50,000 light years horizontally and 30,000 light years vertically and is some 4 million light years away from our planet earth it always puts a question mark in front of me.
Is this picture 4 million years old??
Because it will take 4 million years for light to travel from that place to earth or whichever spaceship is taking that picture.
This would mean as of today that could be a non-existent entity as well isn't it?

All Astronomers, please enlighten my ignorance.

Thanks this is my first post on this site.

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Chris Peterson
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Re: How old are images that we see on APOD some million year

Post by Chris Peterson » Tue Aug 28, 2012 2:43 pm

itwasnoteasy wrote:Whenever I see a picture on APOD, which describes something like this pic is spanned across 50,000 light years horizontally and 30,000 light years vertically and is some 4 million light years away from our planet earth it always puts a question mark in front of me.
Is this picture 4 million years old??
Because it will take 4 million years for light to travel from that place to earth or whichever spaceship is taking that picture.
This would mean as of today that could be a non-existent entity as well isn't it?
The Universe is made of spacetime. What that means is that space and time are fundamentally tied together (in part because of the finite speed of light). A consequence of that is it makes the whole idea of "now" rather tricky.

When we observe an object 4 million light years away, we are seeing light that took 4 million years to reach us. In some sense, that means there is a kind of "now" that means the object isn't the same as it was when it produced the light we are seeing. But from a practical sense, this has little importance. Every possible effect that the object can have on the Universe around it is limited to the speed of light. So it is almost always best to think of "now" for any object as the moment of observation, not the moment when its light was emitted.

We usually only concern ourselves with the time of flight of the photons when we are looking over cosmological distances (much greater than 4 million ly) and are using the information we receive to tell us something about how the Universe looked when it was younger.
Chris

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itwasnoteasy
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Re: How old are images that we see on APOD some million year

Post by itwasnoteasy » Tue Aug 28, 2012 6:21 pm

Exactly, this is what even I felt. Though "now" Universe may be different but all what matters is what's being affected by the old stuff "now".
Bing bang predicted to be occurred 13.4 billion years old but if a heat wave from the initial bang is felt "now" would mean the wave was racing against space and time to reach that tiny point in big bang which might be few micro mm away from it but took 13.4 billion years to show the effect or reach that tiny point in bang (earth or the observation point from which the initial bang was detected.)

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Chris Peterson
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Re: How old are images that we see on APOD some million year

Post by Chris Peterson » Tue Aug 28, 2012 6:45 pm

itwasnoteasy wrote:Exactly, this is what even I felt. Though "now" Universe may be different but all what matters is what's being affected by the old stuff "now".
Bing bang predicted to be occurred 13.4 billion years old but if a heat wave from the initial bang is felt "now" would mean the wave was racing against space and time to reach that tiny point in big bang which might be few micro mm away from it but took 13.4 billion years to show the effect or reach that tiny point in bang (earth or the observation point from which the initial bang was detected.)
Essentially, the microwave background is the heat wave we currently feel from the Big Bang.
Chris

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Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com