[thedoc (OP)]

david tompkins  asked the Naked Scientists:
   
The light we see from distant stars varies from star to star due to how far they are from earth. So say we see a stars light that is a million light years from earth then we are seeing the light from a million years ago. Is it the same for that star that they are seeing light from us a million years ago? If so is there anyway that we could see mirror the light coming towards us to see earth from a million years ago?

I hope that made sense.

I just recently found the podcast and im working my way through all the episodes and im loving it.
 

   

What do you think?

[alancalverd]

In principle yes. In practice a mirror would not be the best device to use.

The television images we get from earth survey satellites show the earth as it was maybe a second ago, and a telescope on Mars would show us what we were doing about 30 minutes ago.

However the idea has no merit since the time it would take more than a million years to get the device in place a million lightyears away. So you might as well take photographs now, and wait.

[Bored chemist]

"If we put a mirror millions of light years away and reflected earth, could we see what earth looked like millions of years ago? "
Yes, except that we would have had to put the mirror there a million years ago- and we didn't.

The interesting idea is if we find something that was already there, and acted like a mirror

[PmbPhy]

david tompkins  asked the Naked Scientists:
   
The light we see from distant stars varies from star to star due to how far they are from earth. So say we see a stars light that is a million light years from earth then we are seeing the light from a million years ago. Is it the same for that star that they are seeing light from us a million years ago? If so is there anyway that we could see mirror the light coming towards us to see earth from a million years ago?

I hope that made sense.

I just recently found the podcast and im working my way through all the episodes and im loving it.
 


What do you think?

If it was possible to create a stable wormhole so that we can place the mirror there in a short time then in principle it would be possible, but impossible in practice.  The amount of light that was emitted from the Earth and sent back would be far too small to be able to see it. You'd have to make a mirror so large that it'd be impossible to build in practice.

By the way. If you placed the mirror a million light years away then you'd be looking back two million years in the past because it would take two years for light to make a round trip from the Earth to the mirror and back.

[Zer0]

Read all the answers to the question posted, nice.
👍(good)
Also aware of the fact that no new replies have been posted since at least 120 days, but still couldn't resist from commenting...What a Fantastic Question!
👌(great)
This is for David Tompkins.
👏 (applause)

[raf21]

I would think that the mirror would show us what's happening today,  a million years from now. .....

[Kryptid]

The resolution of the reflection might not be very good. A million light-years is a lot of room for photons to spread out. Is there any way to calculate what the maximum possible resolution would be (assuming one photon per pixel)?

[raf21]

kryptid, interesting question,  but i lack the intelligence to answer it. the question somewhat relates to an ongoing discussion I'm having on another board.  I don't know how to invite people to other boards or forums,  I'm pretty new at this.  perhaps, if interested,  you can find it somehow.  it's a discussion about light and does it have weight and several branching but related lines of talk pertaining to gravity and such. perhaps you'd like to check it out if you can find it.

[Kryptid]

Let's see what I can figure out.

The peak wavelength of the Sun's emission is about 0.5 micrometers. The following equation tells how much energy each photon has (in electronvolts):

E = (hc)/γ
E = ((4.135667 x 10^-15)(2.9979 x 10¹⁴)/0.5
E = 1.2396/0.5
E = 2.4792633 eV

This is equal to 3.972 x 10^-19 joules per photon. The solar constant is 1,360.8 watts per square meter. Since watts are simply joules per second, we can now calculate the approximate number of visible photons reaching the Earth:

(1,360.8 )/(3.972 x 10^-19) = 342,598,187,311 photons per square meter per second

Given that this is the photon flux from the Sun at 1 AU, we need to find what that flux would be at 1,000,000 light-years. Photon flux falls off with the square of the distance, so the flux at 2 AU would be one-fourth what it is at 1 AU. 1,000,000 light-years is the same as 6.324108 x 10¹⁰ AU. So...

Flux = (1/d²)(342,598,187,311)
Flux = 1/((6.324108 x 10¹⁰)²)(342,598,187,311)
Flux = 1/(3.9994342 x 10²¹)(342,598,187,311)
Flux = (2.5 x 10^-22)(342,598,187,311)
Flux = 8.566166 x 10^-11 photons per square meter per second

In order for us to see the reflection, the light has to travel another million light-years to get back to us, so...

Flux = (1/d²)(8.566166 x 10^-11)
Flux = 1/((6.324108 x 10¹⁰)²)(8.566166 x 10^-11)
Flux = 1/(3.9994342 x 10²¹)(8.566166 x 10^-11)
Flux = (2.5 x 10^-22)(8.566166 x 10^-11)
Flux = 2.14154 x 10^-32 photons per square meter per second

This implies you'd need a detector 4.6695336 x 10³¹ square meters (4.6695336 x 10²⁵ square kilometers) in area in order to detect one photon per second from that mirror. Does that sound right? Feel free to check my calculations and assumptions.

[raf21]

kryptid, I would definitely like for you to weigh in on the other conversation,  the formula you just posted indicates you could contribute greatly too the discussion.   I'd like to invite you to weigh in please.
https://www.thenakedscientists.com/forum/index.php?topic=73315.new;topicseen#new

I would greatly value your input