- It takes time for light to reach us from astronomical objects.
- Therefore, we see distant objects as they looked in the past.
- Very distant objects (galaxies) are red shifted. In fact, the further away these distant galaxies are, the more red shifted they are.
- Therefore, red shifted objects are objects as they looked in the past. Very red shifted objects look like they did in the distant past.
- So, are blue shifted objects as they will appear in the future?
The original question even had the relationship
| red | Earth | blue |
For me, the diagram also said that blue objects are not only in the future, but behind us. To me, this suggested that if you take a blue shifted object, like M31 (the Andromeda Galaxy), and looked in the opposite direction, you might find the same object red shifted, and therefore, also get to see it as it looked in the past.
OK, so this isn't how it works. If it did, I'd make a camera the knows where to look to get the blue shifted image, so for example, you could take a picture of your significant other to see if they are still sexy in 30 years. You'd more than likely get a picture of them with someone else, since their blue shifted picture of you would show you as a fat slob...
The sad fact is that you never see anything in the present - it's always in the past. Light moves at finite speed. In a nanosecond, light travels about a foot. So,
- When I'm 5 feet from my girlfriend, she looks the way she looked about 5 nanoseconds ago. When I'm 10 feet away, she looks as she did 10 nanoseconds ago.
- Therefore, the further away from her I am, the younger she looks.
- She looked better when she was younger, so...
- The further away from my girlfriend that I am, the better she looks.
I'm sadly aware that I look better from a distance as well.
So, here's how the universe really works.
- It takes time for light to reach us from astronomical objects.
- Therefore, we see distant objects as they looked in the past, rather than as they look in the present. But even if you and I are in the same room at 5 feet apart, I see you the way you looked about 5 nano-seconds ago - not really the way you look "now". You always see things as they looked in the past - even if that past wasn't very long ago.
- Very distant objects (galaxies) are red shifted, because they are moving away from us. This movement shifts the light toward the red end of the spectrum because the light waves are stretched to longer (redder) wavelengths. It turns out that, due to the expansion of the Universe, the more distant an object is from us, the faster it appears to be moving away from us, and the more red shifted it is.
- Some nearby galaxies are blue shifted because they are moving towards us (for example, M31, the Andromeda Galaxy is slightly blue shifted). It's close enough to be gravitationally bound to us, so the expansion of the Universe is more than overcome. We still see it as it looked in the past, but because it is moving towards us, the light waves are compressed to shorter (more blue) wavelengths. As galaxies go, M31 isn't that far from us, so it looks pretty much as it must look "now". M31 is 2.3 million light years from us, so we see it as it looked about 2.3 million years ago. However, for galaxies, that's not very long. It takes 200 million years for the Milky Way (our galaxy) to rotate once. So we're seeing M31 perhaps 1% of a rotation ago.
Are blue shifted stars blue?
No. Blue shifted stars have their spectrum, the entire rainbow of light, shifted to higher frequencies. Since in visible light, red is the lower frequency end of the spectrum we can see and blue is at the higher frequency end, it is customary to talk about red shifted and blue shifted to mean shifted to lower and higher frequencies. The same terminology is used with radio (which is also light) even though these frequencies are not visible to the eye.
Here's a link to a recent Scientific American article that really puts all this together, and gets many of the interesting details right.
Print it. Read it seven times. Then, next year, read it seven times again.
To navigate to it on their site, go to
In the CHANNELS bar, click on "Astronomy".
then look for the article:
Misconceptions about the Big Bang