The Hubble telescope captured an image 230 million light years away. It was an image of two galaxies colliding 230 million years ago.
If I had a space ship that contained FTL technology and traveled 230 million light years into that direction, my understanding is that it would look completely different from the image that was captured by Hubble.
**I guess my question is… how do we get that image from 230 MILLION years ago only just now? My brain wants to view it as a physical photo that flew over here because I just can’t wrap my head around how an old image could reach us.**
** FYI: I read through [this ELI5 thread on light years](https://www.reddit.com/r/explainlikeimfive/comments/2igdlf/eli5how_light_years_work/) and I still don’t really get it. Maybe I’m just stupid :/
In: Physics
> how do we get that image from 230 MILLION years ago only just now?
Light moves at a finite speed, so it takes time to arrive.
Maybe you have already seen fireworks or lighting from far away (like a few miles). You might have noticed that you hear the bang some time after you see it. That’s because sound is slower than light. If you were blind, you would perceive the bang only a few second late.
The same thing happens with light, except that we are observing stuff that is ridiculously far (trillion trillions miles), so it takes millions of years for light to reach us.
Throw a rock in a pond. Ripples radiate out from the point the rock hit the water.
When the first ripple hits you, you see a representation of what the water looked like when the ripple first left the starting point. Jump straight to the source and you see the water actually looks quite different. So much time has passed it may not even be making ripples anymore.
We look at an image of a star 100,000 light years away. Then we hop on a ship with FTL. When we arrive at the star we may instead find a black hole or a supernova or a white dwarf, indicating the star died.
You kind of answered your question already.
The speed of light is 2,999*10^6 m/s or something like that, so that is the speed it travels. Always when you see light it is from the past time. Coz it is bound by the speed of light.
Same for the speed of sound 300ish m/s, when you hear it the aggregator for the sound already happened a relatively long time before your perception of it.
Light takes time to reach us. The light that is getting to us today is old, in this case 230 millions of years old.
The image that we see is 230 million years old, if we move in the opposite direction from the light coming from the collision at exactly same speed, the image will stay constant and never change. However, if we look at the collision after travelling 20 million light years in the direction of the collision at the speed of light, the image that we see will be 210 million years old and not 250.
Feel free to correct me as I’m not a 100% sure.
Thinking of it as a photo flying over to us is actually just fine.
Imagine you got a letter with a picture of someone’s wedding. If you were to just instantly appear there the very moment you got the letter… They would not still be pos posing for the photo. Depending on how long it took the message to get to you would determine what going on when you arrive.
Have you ever looked at yourself via a webcam, perhaps during a videoconference? Did you notice the lag between you moving and the image on your screen making the same movement? If not try it, then come back.
Back? good. Now the image is delayed because the light hitting the camera gets turned into electrical impulses, and then they are processed in the camera into bits encapsulated into the USB protocol that feeds them to your PC, then your PC spends time understanding whats coming in by USB, turning it into image data through your camera driver code, and then feeding it to your monitor where the light from the screen travel to your eyes. All those steps take time, and so you see whatever you were doing a second previously, not what you are doing right now.
Now, instead of cameras and processors and software, just imagine that you are looking at a mirror half a light second away (forget for a moment that it would be too tiny an image to see, or that you’d have to be doing this in space because the curvature of the earth would get in the way). The light reflected off your face travels half a light second to the mirror, gets reflected and travels back half a light second distance to your eyes. What you see is identical to what you see with the web cam, 1 second out of sync with the gestures you were making. This time instead of slow electronics causing the lag, its just the physical distance between you and the mirror, because light takes time to travel over distances.
Now move the mirror further off, say 30 light years. Now it takes 30 years for the light reflecting from your face to rech the mirror and another 30 years for that light to travel all the way back to your eyes. (again this is a magic mirror, theres no practical way to actually see your reflection from so far away). So if you did this experiment as a teenager, you wouldnt see that reflection for 60 years! You as an elderly retired person would come back to the lab, and finally see the teenage image of yourself smiling and waving at you. You’d be seeing yourself as you were when the light set out on its journey, not how you are now when it finally arrives back again.
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