B How would light work on a mirror very far away?

[splatcube]

TL;DR Summary

How light works at long distances via reflective objects

Hello!! I'm still in school and not great with physics, but I'd really like to get into it. All of it seems so awesome and my dad is a partical physicist so I can usually ask him about any questions I have but I had one today he couldn't answer right away, he said he'd think about it and get back to me but I know he'll probably forget so might just ask here instead, haha.

If we put a massive mirror/reflective surface in space about 5 light years away (for example) and looked through it, what would we see if the light has not yet reached us? And what would we see after five years? And 10? From a telescope, of course.

Maybe it's a dumb question, sorry if it is!!

If anyone could give me an answer I'd really appreciate it

 

[splatcube]

Hello!! I'm still in school and not great with physics, but I'd really like to get into it. All of it seems so awesome and my dad is a partical physicist so I can usually ask him about any questions I have but I had one today he couldn't answer right away, he said he'd think about it and get back to me but I know he'll probably forget so might just ask here instead, haha.

If we put a massive mirror/reflective surface in space about 5 light years away (for example) and looked through it, what would we see if the light has not yet reached us? And what would we see after five years? And 10? From a telescope, of course.

Maybe it's a dumb question, sorry if it is!!

If anyone could give me an answer I'd really appreciate it

 

[sophiecentaur]

TL;DR Summary: How light works at long distances via reflective objects

Maybe it's a dumb question

Welcome to PF. It's not a dumb question but I won't tell you the answer just like that. I can help you to work it out. Imagine a short pulse of light.

First, what is a light year?
How far will the pulse travel to get to the mirror and how long will it take?

and looked through it,

Where would you be when looking"through" (= "at" )the mirror (i.e. how far away from the mirror)? I assume back on Earth. How far would the reflected pulse take to get b ack to Earth?

That's two journeys altogether. How long will that take for the return journey?

 

[andrewkirk]

Roughly: For the first five years we will not see the mirror. After five years we will see the mirror reflecting the image of ourselves ten years earlier.

To be more precise, think about it like this.

Say we can contact intelligent beings that live near that point 5 light years away, and we send them a message on 1/1/2020 asking them to set up the mirror.
Say it takes them three years to build the mirror, and they start as soon as they get the message, which will be on 1/1/25.
Then the first time we see anything different happening in our telescopes pointed at them will be on 1/1/30, when we will see them starting to build the mirror (which they actually started 5 years earlier, on 1/1/25).
For the next three years 1/1/30 - 1/1/33 we will see them building the mirror and we may or may not catch glimpses of ourselves if any parts of mirror become visible during that time. To keep it simple, assume they cover the mirror with a veil until it's finished, so we can't see any reflections during building.
On 1/1/33 we see the unveiling of the mirror from 1/1/28, and from 1/1/33 onwards we see a reflection of ourselves that is always ten years delayed.

 

[anuttarasammyak]

You would see 2015 Earth in the mirror if it was constructed before 2020. Ten years later the image will be also ten years later i.e., that of 2025.

 

[snorkack]

For example, consider what happens if there is a reflective dust cloud near a supernova.

 

[phinds]

Roughly: For the first five years we will not see the mirror. After five years we will see the mirror reflecting the image of ourselves ten years earlier.

To be more precise, think about it like this.

So much for @sophiecentaur trying to get him to think about it for himself.

 

[sophiecentaur]

So much for @sophiecentaur trying to get him to think about it for himself.

Let him that is without sin cast the first stone.

But point taken. But this is harder to figure out than one might think.

Roughly: For the first five years we will not see the mirror.

We would never actually see a perfect mirror. We could only conclude there was a mirror when we can see the flash after an appropriate delay.

@splatcube didya make any sense of all that. Never stop asking (yourself first) questions. Come back for second helpings soon.

 

[Baluncore]

The experiment would take too long for any laboratory session, or academic degree.

As a quicker demonstration, it could be done in a 100-metre-long coaxial cable, with a directional coupler, and a pair of signal generators, all in less than about one microsecond.
The cable would have a one way transit time of T≈450 ns. Watch the down signal from the far-end generator. Send a forward signal up the line, that shorts or opens the coax at the far-end. The far-end then becomes a reflector, so reflects your up signal, back down to you. You see the change at 2T, when the far-end generator disappears, to be replaced by a reflection of your near-end generator from 2T previously.

 

[phinds]

Let him that is without sin cast the first stone.

I always make it a point to try to help people see how to solve problems for themselves. It annoys me when people just spoon-feed the answer to a question.

 

[snorkack]

The experiment would take too long for any laboratory session, or academic degree.

We don´t need to experiment. Supernovae and novae occur. Reflective dust clouds exist, including near supernovae and novae.

 

[sophiecentaur]

We don´t need to experiment. Supernovae and novae occur. Reflective dust clouds exist, including near supernovae and novae.

An observation is equally good as an "experiment". There are many different methods to find the speed of light and they vary in accuracy. This Video is 16m long and is worth checking out. It starts describing (Römer's method) how the difference in distance between Jupiter and Earth is enough to give a measurable time that a moon of Jupiter appears eclipsed in its orbit. You have your distance and your time so you can use c =d/t. His version of c was something like 70% of the accepted value because they couldn't measure the Earth's orbital distance accurately.

BTW, your thought experiment with the mirror at 5LY distance would require a very powerful light source and a massive mirror. You might as well do it on the ground with a mirror, laser, a [Edit: FAST] light switch and a fast oscilloscope.