When we say a body in space is X light years away from us(say 5000), how do we know that it took 5000 years for this light to reach us?
And, how accurately can we measure this figure?
In: 5
Scientists use [Redshifting](https://en.wikipedia.org/wiki/Redshift) / [Blueshifting](https://en.wikipedia.org/wiki/Redshift#Blueshift) – It’s basically [“The Doppler Effect”](https://en.wikipedia.org/wiki/Doppler_effect) but for light.
When an ambulance drives past you, you hear the pitch of the sirens get higher as it gets closer, and they get lower pitched as they get farther away from you.
Light does the same thing, but it shifts in color – redshifting or blueshifting.
Scientists can tell how old light is based on what color it is – the longer it’s been travelling, the more it will be shifted in color.
When we look at the light from the big bang, the beginning of the universe, we can see that it is massively red-shifted, so much so that it is actually no longer in the visible spectrum, and is far into the infrared.
It’s like how you can tell how far away an ambulance is based on the sound of the siren. We can tell how far away the light is based on the shift. We know how far the light went, now we just have to figure out how long that would take – light moves at light speed so we can easily calculate a ballpark figure – which gets us the “Universe is ~14 billion years old” we’ve all gotten to know. [Which is still up for debate today.](https://www.usatoday.com/story/news/nation/2023/07/14/universe-may-older-than-thought-study-shows/70411343007/)
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