You wouldn't do the calculation like this, though. You'd get a redshift of the observed photon, and then relate this to the distance between the observer and the emitter when the photon was emitted.mathman said:This is probably an elementary exercise, but my head spins trying to think about it. Assume that on Earth we see something 1 billion light years away (i.e. light took 1 billion years to reach us). To simplify, assume no relative proper motion, although the distance will change due to universe expanding. How far apart are the Earth and the source now and how far apart were they when the light was emitted ( 1 billion years ago)?
He means that the distance is increasing due to the "universe expanding."Redbelly98 said:Assuming no relative motion, the object was and still is 1 billion light-years away. Or am I missing something in your question?
cristo said:He means that the distance is increasing due to the "universe expanding."
mathman said:Does anyone know the answer to my original question?
cristo said:I answered your question, didn't I? The point is, how would you know that the light had taken a billion years to travel to you? It's not like a 100m race or something like that, where you can measure it; you need to use the redshift.
1 billion light years is a unit of measurement used in astronomy to describe the distance light travels in 1 billion years. This distance is equivalent to about 9.46 trillion kilometers or 5.88 trillion miles.
Light from objects that are 1 billion light years away has been traveling towards Earth for 1 billion years. This means that the light we see from these objects is actually from the past. Telescopes and other instruments are used to detect and amplify this light, allowing us to see objects that are extremely far away.
At a distance of 1 billion light years, we can see a variety of objects including galaxies, quasars, and other celestial bodies. These objects are typically very large and bright, making them visible even at great distances.
The distance of 1 billion light years can greatly impact our understanding of the object. Since we are seeing the object as it was 1 billion years ago, we are essentially looking back in time. This allows us to study the evolution and changes of the object over a long period of time.
Due to the vastness of the universe and the limitations of our current technology, it is highly unlikely that we will ever be able to physically reach or visit an object that is 1 billion light years away. However, through advanced technology, we can continue to study and learn about these distant objects from Earth.