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Video lecture for this chapter
Even though astronomers cannot do controlled experiments and they are confined to observing the universe from locations near the Earth, the universe gives us a vast number of different phenomena to observe. Many of these things cannot be reproduced in Earth laboratories. There are gas clouds in such a rarefied state that they give off radiation not seen on Earth. Some objects are so dense that their gravitational fields bend light so much that it is prevented from leaving the object! Many things that are unlikely or impossible on Earth are routinely observed in the cosmos. Many of the scientific theories in other fields make predictions of what would happen under very extreme circumstances. Sometimes those extreme circumstances are the only situations distinguishing two or more contradictory theories. Unfortunately, the scientists of those other disciplines cannot test their ``wild'' ideas---is it hogwash or reality? Astronomy allows those theories to be tested. Very subtle and easily missed but crucial processes may be missed by observers focussing on the Earth, but the astronomer can see those processes magnified to easily noticeable levels in some other celestial object.
In addition you will see later that the light coming from far-away objects in all parts of the universe tells us about the laws of physics (the rules of nature governing how physical things interact with each other) there. Astronomers find that the laws of physics discovered here on the Earth are the same throughout the cosmos. The fact that nature makes nearly an infinite variety of things from the same types of material we have here on the Earth and has those things interact with each other in so many different ways using the same rules we see followed here on the Earth is awe-inspiring.
Now back to the long term evolution side of the coin. We actually have a time machine! Not the H.G. Wells variety or G. Roddenberry's Guardian of Forever but something much simpler due to the large distances and finite speed of light (300,000 kilometers/second!). It takes time for radiation from a celestial object to reach the Earth. Therefore, when you examine an object at a large distance from us, you see it as it was. The farther away the object is, the longer it took the radiation to reach the Earth, and the further back in time you observe it. The Sun is 150 million kilometers from us, so you see the Sun as it was 8-1/3 minutes ago. The farthest object you can see without a telescope is the Andromeda galaxy about 2.8×1019 kilometers from the Earth, so you see it as it was almost 3 million years ago. Recall that a light year is how far light travels in one year (about 9.46×1012 kilometers). Therefore, the Andromeda galaxy is almost 3 million light years away from us. (The speed of light is the key in the relationship between space and time, a fact used by Albert Einstein in developing his Relativity theories that are described in another chapter.)
To study the evolution of long-lived objects like stars (with lifetimes of millions to billions of years) or galaxies, astronomers observe the objects of interest at different distances from the Earth so they are seen at different epochs. Therefore, the objects are seen at various different ages or evolutionary stages. Since light from remote objects can take millions to billions of years to reach the Earth, astronomers find out about the laws of physics at different times. What they find is that the universe has used the same laws of physics throughout its 14-billion year lifetime (and presumably will continue using those same rules). Pretty amazing!
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last updated: November 26, 2021