[thedoc (OP)]

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?"

   
Asked by Omkar Chandra


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[chapter podcast=1001476 track=16.10.04/Naked_Scientists_Show_16.10.04_1005749.mp3]  ...or Listen to the Answer[/chapter] or [download as MP3]

[syhprum]

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

[Bill S]

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; 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.

[Semaphore]

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; 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.

[Bill S]

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.

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. 

[evan_au]

if aliens are watching us, from millions of light years away, with their advanced telescope

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)