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.
[quote="itwasnoteasy"]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?[/quote]
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.