Eli5: How will NASA’s telescope be able to observe stuff happening from billions of years ago?



I saw a post on reddit saying NASA has a telescope that is able to see the creation of planets and stars from 13 billion years ago. How can that work?

Link to post: https://www.reddit.com/r/interestingasfuck/comments/ke2p2r/nasa_is_about_to_launch_a_telescope_that_can/?utm_medium=android_app&utm_source=share

In: Physics

It takes light a billion years to go a billion light years. If you’re looking at something a billion light years away, you’re seeing what it looked like a billion years ago.

Light takes time to travel. When we look at something 13 billion light-years away we are seeing what it looked like 13 billion years ago. Because 13 billion-year-old light is what we are looking at.

The light from those events has traveled for 13.8 billion years and is only reaching us now.

What they’re talking about is the light that Hubble (or any telescope really) is seeing started on its journey billions of years ago. Light has a finite speed… the speed of light. A light-year is literally the distance that light travels at this speed during one our Earth years.

So the light that we see from other parts of just our own galaxy have taken years to arrive at where the Earth is in space. But they’re considered a snapshot from the past because its light from where those stars _used to be_ when they emitted that light. In actuality, those stars have moved bagillions of miles away by now.

Because other other galaxies are thousands or millions of light years away, and most galaxies in the universe are flying apart from each other – at speeds approximating the speed of light – the light that we now, here, from those galaxies has been travelling for… millions of years.

Speed of light = 299,792,458 m / s OR 670,616,629 mph

So a light year is how far that light can travel in a year’s worth of time

1 light-year = 5,879,000,000,000 miles

So if we observe something realllyyyyy really far away, say 5,879,000,000,000 miles away from us we are actually seeing what state it was in one year ago because the light took that long to get to us to inform us of what was happening.

Think of it as an old time letter you would have grandma send from the mailbox. She wrote it December 1st, but by the time it got to her sister across the country it was now December 15th. Her sister sees it for the first time and reads it for the first time on the 15th but all of the events it talks about and tells the tales of happened on December first.

So take that same concept now multiply the distance that light has to travel to get to us by a BILLION times. That’s how we can see what happened 13 billion years ago. Those objects are actually 5,879,000,000,000,000,000,000 x 13 miles away from us 🤣🤣🤣 and it took that long for that light to make it all the way here.

Kind of incredible huh?

If you look at the sun, you are seeing it as it was 8 minutes ago, since it took the light from the sun 8 minutes to travel to earth.

Imagine light hitting your eyes as information from wherever that light came from, telling your eyes and brain what that thing looked like when it reflected off it.

That light didn’t reach you instantly, it travelled (albeit very quickly) from that thing to your retinas. As a result, what you’re seeing is technically out of date. Always.

We and almost everything else in the universe (that we’re aware of) are time travellers. We just have no way to control that travel – we move forwards, through time, at the rate local effects (gravity and whatnot) dictate.

Imagine you have a camera in one room that’s recording someone dancing. The feed from that camera is being displayed live in another room on a TV that you’re watching. Logically, you’d assume that person must be in the room dancing right now because that’s what you see on the TV right now.

Now imagine that instead of the signal going from the camera to the TV over a relatively short cable that’s a few meters long, it instead has to go through a cable that’s billions of meters long. It’s going to take a little bit of time for the signal to make it from the camera to the TV. The result is what you’re seeing on the TV actually happened a few seconds ago but you’re only just now seeing it.

That’s what’s happening when you observe distant things in space. Light travels very fast, but not instantaneously. So with enough distance, you start to get a delay between when something happens and when you can see it happening.

When you think about it, everything that we see is technically in the past. Mind you, if you are looking at something only a few feet away it only took the light you are seeing the tiniest fraction of a second to arrive but there actually is a very very very tiny delay. To add to this, there is a delay between when your eyes receive the light and when the signals carrying this information arrives in the sight centers of your brain. Add another incredibly tiny fraction of a second to actually process the signals and convert them into the image that your brain “sees” and you’re now running a few millionths of a second behind “real time”. But given how fast our brain actually reacts to events this “time delay” is negligible and we consider what we are seeing as being “immediate”.

As the distance increases, this effect slowly increases. At the distance between the Earth and the Moon you are up to a delay of around a second. Still pretty close to “real time” for most of us so we would not really notice much of a delay. The Sun is about 8.5 “light minutes” away, meaning that the light you are seeing left the surface of the Sun over 8.5 minutes ago. If the Sun exploded this very second no one would realize it for 8.5 minutes. And so on. As you get to “interstellar” distances the time it takes light to travel expands rapidly. The nearest star to the Sun is over 4 “light years” away, so if it exploded this very second we will not see the event for another 4 years.

The satellite that NASA is using is tasked with examining objects that are 13 billion light years away. What we are seeing when the satellite looks at these objects is events that happened 13 billion years in the past. It is very possible, even likely, that those stars and planets no longer even exist today, but depending upon how long they survived past the moment that we are currently observing, it may be millions of years or more before the light arrives to allow us to finally be able to see their “deaths”, even though the actual events have already happened.