Science Questions

If aliens could see us what would they see?

Mon, 3rd Oct 2016

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Question

Omkar Chandra asked:

My question is "if aliens are watching us, from millions of light years away, with their advanced telescope or any device, won't they be looking at earth in its early form filled with prehistoric livings like dinosaurs?"

 

 

Answer

We put Omkar's question to Judith Croston... Dinosaur

Judith - Well the answer to that is yes. So the questioner is right. If we think about aliens that were say living in the Andromeda galaxy. So the Andromeda galaxy is our nearest galaxy and it is a few million light years away. And so what that means is that any signal that those aliens could measure, it would have to travel at the speed of light to get there. So it would have taken some number of millions of years to get there so what they would be seeing would be prehistoric creatures wandering around on the Earth.

Chris - Doesnít that presuppose that the light hasnít spread out so far that they wouldnít be able to see a dinosaur?

Judith - There are a couple of problems with that. One of them is that, obviously, the sorts of telescopes we have are nothing like good enough to be able to see the prehistoric creatures on planets in the Andromeda galaxy - so thatís one problem.

And so when we think about trying to hunt for aliens, we try to think about the sorts of radio signals, we think about technological society. So the earth has been only sending out signals that are likely to be detectable for sort of a hundred years so that's one thing and thatís if we think about aliens in a different galaxy.

But I think perhaps what the questioner has missed, or at least isnít in the question, is whether the aliens would have to millions of light years away. Because, in fact, we know of plenty of planets that are a lot closer than that. There are actually, I think, about seventy five known planets within fifty light years. So if there were aliens on one of those planets, then maybe they would be listening in to signals we were sending out, fifty light years away they might be listening to the Beatles on the BBC or something.

Chris - Theyíve got reasonable taste then?

Judith - We don't really know. This light travel time thing is a really important thing to take into account if youíre trying to figure out what we can see from alien civilizations and what they might be watching.

Chris - So in summary: the light is spreading out, itís spreading out at the speed of light and, in theory, they could pick up that light, they could interpret what that light came from and the would, effectively, be seeing prehistoric Earth, if they were that far away. But they may be much closer and they may even be watching Neighbours?

Judith - Yes, absolutely.

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It is not possible to see any detail on planets around the nearest star let alone millions of light years away its not a matter of super human alien technology but basic physics syhprum, Thu, 29th Sep 2016



Syhprum; could you say a bit more about the physics involved?

Omkar; I suspect that if such viewing were possible, you would be right, the aliens would see Earth as it was when the light left.  Undoubtedly, such advanced beings would not make the mistake of overlooking the millions of years of evolution which they would have to take into account. Bill S, Thu, 29th Sep 2016



Syhprum; could you say a bit more about the physics involved?

Omkar; I suspect that if such viewing were possible, you would be right, the aliens would see Earth as it was when the light left.  Undoubtedly, such advanced beings would not make the mistake of overlooking the millions of years of evolution which they would have to take into account.


You're right about the time lapse. But how would they know what route evolution would take? They wouldn't know whether the race would be smart enough to survive or whether it would wreck its habitat. Semaphore, Thu, 29th Sep 2016



They would have no way of knowing, but being aware of the time difference, they would probably assume that some sort of change had taken place.  Bill S, Fri, 30th Sep 2016


To work out what minimum size of telescope would be needed, we can apply the Dawes limit.
To detect a large 10m dinosaur from 1 million light-years away with a diffraction-limited optical telescope would require a telescope about 50,000km across.

Lets just ignore clouds, atmospheric distortion and the fact that your average dinosaur was more the size of a chicken than a T-Rex!
(Let alone multiple millions of light years...).

My rough calculations (please check!):
Dinosaur length: 10m

c: 3.00E+08 m/s
1 year: 3155760 seconds
1 light-year: 9.47E+14 m

Angular resolution required for detection: 1.06E-14 radians (small angle approximation)

yellow light: 5.80E-07 m wavelength
Minimum telescope diameter: 5.49E+07m, or 54910 km (from the Dawes Limit)
evan_au, Fri, 30th Sep 2016

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