I was wondering, can intelligent species (for example) 100 lightyears away from earth construct a telescope to see minute objects (people, buildings) on earth? Does this mean the telescope has to be very large, or is there a limit that no matter how large your telescope you cannot see things any bigger?
Thanks.
TRS.

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I think the biggest problem is the same we have with ground based scopes, the atmospheric distortions. Without any such distortion to worry about, I believe it is simply a matter of gathering enough light to make an image on the timescale you desire.

But let's face it, we have a hard time making out our own people with LEO satellites.

Then would it be possible to "beam" something in one direction, and have it "reflect" back? I know Radar does something similar; but can something carry information with it? And also, something far more focused than Radar or Ladar?

Just an idea.

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That's a very good question.

You are limited by two things: (1) atmospheric distortion, and (2) maximum achievable telescope size.

In principle if #1 was no problem, and if you could build a sufficiently large telescope, yes you could image small details on earth from 100 light years away.

Atmospheric distortions can be mostly cancelled via imaging adaptive optics. There is additional ongoing work to allow this for visible light (not just infrared), and over broader angles (not just narrow angles). However even our technology can significantly achieve this, and more progress is being made. We assume advanced aliens capable of building a sufficiently large telescope to image small details on earth from 100 l.y would be far more advanced.

What about telescope size limits? This is a two part problem (1) The largest possible unitary telescope and (2) The largest possible optical interferometer

As Keck and the discussed Overwhelmingly Large telescope (OWL) have shown, it's possible to build extremely large unitary telescopes, maybe 100 meters for OWL. http://www.eso.org/projects/owl/ If an advanced alien race could build a 1000 meter optical telescope, it could resolve 10 times more than OWL. Unfortunately that's not nearly enough to resolve small objects on earth from 100 l.y.

The other approach is using an optical interferometer, which essentially constructs a synthetic telescope from small "outrigger" mirrors placed far from the central telescope. There's much active work in this area, but optical interferometers are in use today.

However you need an *imaging* optical interferometer which is more difficult. Also you need a huge one. I don't know what the theoretical limits are, but the math is very simple:

To determine telescope diameter needed to resolve (say) 0.5 meters from 100 light years, we use the formula for angular resolution:

a = 250000 x W / d, where:
a = angular resolution in arc seconds
W = wavelength in meters
d = telescope diameter in meters

And linear resolution:

s = tan (a) x d, where:
s = linear resolution in units determined by d
a = angular resolution in degrees
d = distance to object

These show to resolve 0.5 meters from 100 light years would require a telescope 1 trillion meters (1 billion km) in diameter. That's obviously not possible for a unitary telescope, but maybe it would be using a free flying space-based interferometer. The US is planning on a small version for the upcoming Terrestrial Planet Finder (TPF). http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm

I don't know if any fundamental physics problems would preclude an imaging optical interferometer with a 1 billion km baseline. Certainly it would be an engineering challenge, even for an advanced race. However in theory if you could build it, you could see small details on earth from 100 light years.

With an optical interferometer you might be able to resolve small details on distant planet in theory, but you would still need very large telescopes (or a lot of them) to gather the light. Very few photons reflected off an object on Earth would reach a distant solar system, let alone a telescope in that system. So the images would be very dim.

Perhaps once the distant observers had determined the rotation rate of the Earth they could build up a detailed picture of our planet over a period of days, months or years; but moving objects (such as clouds, or vehicles) would not show up.

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Would the images be distorted by our atmosphere as they tried to peer through it?

I'm not sure- but I thought there was a limit to resolution even if there was no atmosphere, I mean radio telescope have a finite resolution and they don't have to worry about the atmosphere.

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The original post asked if there was a resolution limit, assuming a distant alien race could build a sufficiently large optical telescope.

As my above post shows, there is no theoretical limit to resolution, rather only a practical limit based on what current engineering can build.

Also adaptive optics works both ways -- either looking toward space from earth's surface, or looking toward earth's surface from space. Optical spy satellites likely use adaptive optics to cancel atmospheric distortion. In fact most of the original work on adaptive optics was military based.

Therefore in theory a distant alien race with a sufficiently large telescope using adaptive optics could look through the earth's atmosphere with minimal distortion, similar to a spy satellite in low earth orbit.

However as eburacum45 pointed out, having a sufficiently long baseline isn't by itself sufficient. If you want to image a moving object, you must also have the necessary light gathering to build the image in much less than 1 second. That is even more challenging, but in theory it's just an engineering problem not a fundamental insurmountable problem like the speed of light.

It's actually possible to calculate the required telescope surface area to build an image in, say, 1/4 second, assuming you know the target object luminous flux. However the math is more complex, and I don't have the equations at hand. But needless to say, it would be huge.

The above uses technology we understand. Although we currently can't even approach building such a device, we can envision doing it given enough technology.

Even further beyond this is focusing light without using optics. It doesn't require new physics, like exceeding the speed of light would. One way is a gravitational lens. Assuming an extremely advanced alien race could precisely do this, it would avoid physical constraints of physical optics. Such a mechanism in theory could capture light over many billions of square km and build a hyper resolution image in less than a second.

You'd also need to eliminate the sun's glare. Our technology can do that for extrasolar planets of a magnitude we can image. However whether it's possible for imaging the earth from 100 light years, I don't know. When extended to that scale, there are sometimes fundamental limits you don't normally consider.

There might also be a fundamental limit regarding sky background brightness, but I don't know.

But excepting unknowns like the above two items, in theory our physics allows building a sufficiently large optical telescope to image the earth in high resolution (maybe equal to an optical spy satellite) from 100 light years.

Thanks for all the answers!

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Not to hijack the thread but...
Say against all odd's they would be able to see us...

Wouldn't the sun wash out our image because its so mutch brighter?

I ask this because if you look at an object next to a bright light at a distance you can't or can barly see it... compared to if your close you can see it better because if the light.

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To the original question...

No, they could not see us, light from us can not be seen untill a long time later.

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As mentioned previously, the "glare" from the adjacent star is obviously a problem. However there are various solutions to that. The planned Terrestrial Planet Finder faces that same issue but NASA feels there are adequate solutions. It's likely an advanced alien race capable of building a gigantic space telescope could also solve that.

http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm

Yeah, they can see us. They watch us, all the time.

All the time they are watching us...

Every move we make, they watch.

Watching us.

WATCHING US!

ALL THE TIME THEY ARE WATCHING US!

AHHHHHHHHHHHHHH!


Ahem, sorry...

Carry on.

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you have raised a very nice question, sometime i feals the human brain here "how it is intelligent", but our sights have some barriers of the long length viewing, but may be the ETs can see a clear image from 100kms. !

If they were technologically advanced enough theoretically its possible. as Joema pointed out if you build a telescope with a large enough lightgather surface you could. Its just a matter of how fast and how well you can capture the light.
Few things that no matter how well you build the telescope or photon gathering device you cannot avoid.

1st Signal degridation. You can't really acount for the photons that never make it. (photons that get absorbed by intermediate gas clouds, or redirected by a high mass object)

2nd) The observer cannot be any farther than 5 billion light years away. Simply put if they point there telescope here and looked at us right now. Our star would not be there. It has not formed yet

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Close your blinds.

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Lets say us humans built one of those very lage telescope (this is starting with the idea that we will be able to do this in lets say 500y (if where stil around ).

And we say lets look at an other species and how they live... how would we know where to look?

Just point at random star with a high change of planet formation?
or some other means?

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GCS/S d(+) s+:+ a--- C++(+++)>$ W+++>$ L>+ M+>++ w++ P+>++ tv@ PS b+ DI+ G e-> h! r-- !z+

~Jorge Schrauwen

Well quite frankly yes. Right now there are planetary hunters that are looking for large jupter planets. Untill we find way to detect earth like planets, we will just have to point are stars that meet a certain cryteria. Most likely spectral class, region of stability, uni-solar systems , and the most unfortunate limitation distance from us

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Since there are so many galaxies in the universe does any one mind if I claim one? I need a place to put my stuff.

By the time it's possible to build such a huge device, the location of small, rocky earth-like extrasolar planets will be well known. Much smaller devices will be able to detect them and characterize things like atmospheric composition, temperature, etc. So when the huge telescope became available, you'd already have long known exactly where to look.

In fact NASA is planning such a device called the Terrestrial Planet Finder. If and when completed, we'll know how many such planets exist around closer stars, and where they are. http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm

TPF is currently planned for between 2014 and 2020.

I might live to see it launched then

__________________
GCS/S d(+) s+:+ a--- C++(+++)>$ W+++>$ L>+ M+>++ w++ P+>++ tv@ PS b+ DI+ G e-> h! r-- !z+

~Jorge Schrauwen