A question for Astronomers?

Question

If light leaving the earth travels out into the universe, does that mean theoretically, in millions of years time, people on a distant planet with a a yet to be invented super telescope could see US walking around on earth as we are now?

Answer 1

1) In a million years, the light would be seen a million light-years away. At that distance, the resolution needed to see individuals walking about would be so much that they'd need telescopes the size of a solar system.

2) light does degrade (space is not a perfect vacuum) and the information they'd get would also carry lots of interference. For example, the photons that leave you (if they see you, it is because they capture photons emitted by your body) must travel through the atmosphere. Air turbulence distorts the wave front, restricting the resolution to 0.5" (half a second of arc = 1/7200 of a degree).

This does not sound like a lot of distortion, but at the edge of the atmosphere (let us say 100 km -- there is still some atmosphere well above that), the best resolution possible is a quarter of a metre (10 inches): you cannot see details finer than 10 inches. From the Moon, the rest of the atmosphere would increase this to more than a foot.

Over a million light-years, there would be interstellar gas and dust, and other "stuff" depending of the direction you pick. So the resolution (and the quality of other information) would continue to degrade with distance. Even the solar-system-sized telescope would not be able to pick out much surface details.

By the way, a million light years is not very far. The nearest full-fledged spiral galaxy is over 2.4 million light-years away.

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In summary: no.

However, whatever information they do get (if they are located a million light-years away) would be about what our place looked like a million years before they see it. So, in a million years, the light they get would be of the Earth you and I share. They would not have the joy of identifying neither you nor any of our YA friends.

Answer 2

From Alpha Centauri, you'd need a telescope objective 50 million kilometers in diameter to resolve a human in the solar system under conditions of high signal to noise ratio (high contrast with the background).

Answer 3

Yes, but. Practically imaginable engineering will probably limit us to seeing one square mile pixels on planets within a 1000 light year radius (much better for ones that are closer, of course). Now, that is still millions of planets. But probably none where people are walking around.

You can calculate this for yourself by simply counting the number of photons reflected from a planet. Per square meter that's about 100W worth of light. Green light has approx. 1.6eV of energy. 1J of photons is therefor 10^19 photons. 1 W of photons are 10^19 photons per second. So 100W of photons give off 10^21 photons per second. From a km^2, you can get 10^27 photons per second and roughly 4*10^30 in an hours time.

How many of those photons can we catch at a distance? Depends on the aperture of our telescope and our distance. So the total area these photons hit is 2*pi*R^2, where R is the distance. Of that area we can capture only a tiny fraction which is proportional to the aperture of our instrument: A=pi*ra^2. So the flux Phi of photons we will see is obviously:

Phi = pi*ra^2/2*pi*R^2 = (ra/R)^2/2

So obviously, you need to make ra/R on the order of sqrt(4*10^30), i.e. the ratio of instriment aperture and distance has to be on the order of 2*10^15 to see one photon per square kilometer per hour. 1 light year is pretty exactly 10^16m. So that would be sqrt(2)*5m or approx. 7m of aperture.

So a 7m aperture telescope will get you one square km pixels in one hour with a signal to noise ratio of one from a distance of a light year.

Not too bad. Now blow that up by a factor of a thousand and add in a few details (like year long observation times etc.) and you can imagine what we can see. Quite a lot! Maybe not enough to see people walking around, but enough to draw city maps of alien dwellings on planets near us.

Answer 4

That's what that means.

It means on the moon, they'd see the earth as it was about 1 1/3 seconds ago. The sun sees Earth as it was about 8 1/3 minutes ago. And on a planet orbiting one of the Centauri system, they're seeing us as we were about 4.3 years ago.

Answer 5

Yes. You can do computations based on the angular size of the image to see how big such a telescope would have to be (see "Airy disc"); we're talking about an instrument roughtly the size of the solar system.

Answer 6

A million lightyear's distance is an awfully long way.

The image of a person walking around would be dimmed and distorted by various effects. I suspect that weak gravitational lensing alone over those distances would be enough to destroy the coherence of the image.

Answer 7

Kind of, but that telescope would have to be impossibly large to show such small detail so far away. Much bigger than a planet.

Answer 8

You got it. It would take a mighty fine telescope, of course. But they would see us walking about in what would then be the past.

Answer 9

If the resolution of the super telescope is high enough.they will see us. But that is only science fiction

Answer 10

Basically.