# Eli5: How do we know how far light traveled to reach us?

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When we say something is 1000 light years away, or 100,000 light years away, how do we know how far light has traveled to reach us? How do we differentiate the distance between all the different sources of light we can see so accurately?

In: Physics

Space itself is always expanding, and this also makes the light itself expand. This translates to a change in the color of the light. We know the general colors coming from each star, so we can compute the change from that and get a time. It’s a process called redshift.

It’s actually kind of complicated. We see the light and can measure its intensity, but how do we know whether this is something dim and close by or bright and far away? The answer is the “cosmic distance ladder,” which astronomers have built up over time. The physical properties of certain astronomical objects ensure they will have a certain brightness that we know. These are “standard candles.” By looking in the vicinity of these standard candles for other hints about distance from Earth, we have built methods for measuring things even farther away.

That depends on how far away something is.

For particularly close objects, we can triangulate and guess the difference by how much its position shifts when viewed from two different points. This is called stellar parallax. It’s essentially the same as crossing the street or climbing a hill to get a slightly different look at a distant object.

For farther away objects, we can guess how far it is by determining what kind of object it is versus how bright it is. For instance, we can look at a distant star and tell from the light reaching us that it is a yellow dwarf like our own sun. Then we can guess from how bright it is (or how dim it is, said a different way) how far away it must be. This is essentially the same as guessing whether someone holding a flashlight is close to you or far away based on how bright or how dim the flashlight seems to you.

For more distant galaxies, we cannot do this by measuring individual stars but we can measure other events, like supernovae (exploding stars) on the same principle.

Finally, for very distant objects, we can use the redshift. Because of cosmic inflation (a whole extra ELI5), the farther away in the universe objects are from us, the faster they are travelling away from us. This causes a noticeable downward shift in the spectrum of light reaching us from those objects, and so we can guess how far away they are based on how redshifted the light is. This is essentially the same as how you can tell whether a loud truck or an emergency vehicle with a siren is travelling towards you or away from you because of how the sound changes as it passes you.