eli5: How are we accurately able to study far off cosmic objects if our primary source of information is from light (that sometimes has traveled to us for 1000’s of years). Am I missing something?
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Light carries pretty much all of the information you need.
There’s the basics. We can see motion with light. We can see distance with light from a few different objects. We can also see relative size. And we can see color with light. We can also see the absence of light.
That information alone conveys a *lot* of information. By seeing motion, distance, and relative size, we can make very good estimates of how big an object is, how fast it is moving, what direction it is going, and where it will be in a *very* long time from now. And by seeing its color, we can determine how hot or cold it is. For stars, we know temperature and size directly relate to age, so we can predict when a star is going to die/go supernova with surprising accuracy.
Using the absence of light, we can detect things that don’t produce much light on there own, like a planet. If we point a telescope at a star and see that there’s a shadow over it every X number of days, we can determine that it is orbited by a planet, and we can learn a lot about that planet based on that shadow.
But it gets more complex and crazy from there.
For one, our eyes only see a small range of the light spectrum which we call visible light. But there is a wide range of other light on the spectrum (radio waves, microwaves, UV, x-rats – you name it). And by viewing that light, we can see other things we can’t normally see. We can then determine what those things are made of based on how they look in different ranges of light.
All matter absorbs and reflects different ranges of light based on what it is. This is called spectrography. A big ball of hydrogen reflects back a different spectrum of light than a big ball of oxygen. Using this, we can determine what the atmosphere of a distant object is made of, for instance.
Light from an object also changes color based on how it moves. If an object moves away from us, it gains a red hue. This is called red shift, and observing red shift in distant galaxies is how we determined that the universe is expanding.
Light also bends due to gravity. If you have a big enough source of gravity, you can use it as a lens to see even further objects with higher detail. You can also use this property to detect black holes.
There’s a lot more than just that. But light is pretty much the only way information gets transmitted in this universe, so everything we know about it has been learned through light.
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