A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Would aliens looking at us from 100 light-years away see earth as it was a century ago?

September 18, 2015

Dear Cecil:

If aliens on a planet 100 light-years away had a really strong telescope with super zoom, could they look at earth and see life as it was 100 years ago? I know they can travel at warp 90 and have cloaking devices so why should they bother when they can get here in five seconds? But say they didn't. Please answer. I've wondered about this for 55 years, and I don't have as much time left as when I was 11.

Cecil replies:

Well, I wouldn’t waste any more of it on science fiction, Ted. Answer: no.

I’m wondering if you’ve seen the old Star Trek episode “The Squire of Gothos,” because you’re not too far from its premise. In the show, the Enterprise crew stumbles on a planet inhabited by a nutty alien called Trelane, whose roughly Napoleonic-era taste in clothing and decor is based on what he somehow believes are up-to-date observations of earth, some 900 light-years away. Trelane also talks like an English country squire circa 1800 (or at least the actor tries to), so I guess he’s supposed to be really good at reading lips through his telescope. Whatever the case, it’s vintage Star Trek: endearing, superficially plausible, but basically nonsense when examined close up.

Here in reality, telescopes are imperfect instruments subject to the constraints of physical existence. The planet-bound variety must contend with clouds, haze, dust, atmospheric distortion, and vibration. Even instruments in orbit like the Hubble Space Telescope must gather light that’s passed through trillions of miles of cosmic dust and debris.

You say: I know, but surely advanced civilizations with super technology will figure out a way to deal with dust.

Ain’t that easy, bubba. Here’s why.

Even assuming a clear path between an alien’s telescope and us, the laws of physics put a cap on how much detail a distant observer can see. One indication of this is the diffraction limit, which effectively tells us the distance from which a telescope of a given diameter can distinguish between two objects a given distance apart. This limit is a function of the wavelength of the light conveying the distant image to your eye; shorter wavelengths (as in ultraviolet light) allow finer resolution.

For example, if a Hubble-type telescope were anchored on earth and atmospheric interference were nonexistent, the smallest feature it could resolve on the moon would be about 250 feet across. Given the moon’s brightness, additional camera trickery could be employed to essentially double the resolution, meaning objects 125 feet across could be distinguished.

To resolve a human-scale object, the Hubble would have to be within 5,360 miles. From where I sit (Chicago), that’s about the distance to Rio de Janeiro.

No problem, you say. I’ll build a bigger telescope.

Fine. Let’s suppose (a) the aliens only need to resolve down to 100 feet, enough to track human activity at a gross level (large structures, aircraft carriers, Donald Trump), and (b) they’ve parked their telescope just outside where Pluto’s orbit comes closest to the sun.

If it uses visible light, the telescope would have to be 46 miles wide to see details down to 100 feet, ignoring atmospheric haze. Citizens of the Alpha Centauri system, 4.37 light-years distant, would need a visible-light telescope 428,000 miles wide. If we were to switch strictly to UV light to economize, that would reduce the size to a not much more practical 214,000 miles.

Can advanced technology get around this problem? Up to a point. A technique called optical interferometry takes what an array of small, widely-spaced telescopes sees and combines it into a single image, in effect sampling what a larger telescope would capture. An array of four one-meter telescopes can achieve the resolving power of a single 330-meter telescope. The current record holder, the Very Large Telescope array in Chile, uses eight connected telescopes to such effect that they could distinguish between the left and right headlights on a car parked on the moon.

But the moon’s only about 1.3 light-seconds away. Optical interferometry is designed for use at much greater distances. It doesn’t produce direct images — at extreme ranges, the telescopes simply don’t capture enough photons. Instead, the technology takes precise measurements of the target using the relative handful of photons it does collect, and a computer synthesizes the data into the best visual approximation it can.

The resulting images, while scientifically interesting, aren't much to look at — typically fuzzy blobs. Interferometry works best with bright objects such as stars, which produce lots of photons; nonluminous bodies such as planets aren’t so cooperative. One now-canceled NASA planet-hunting project, the Space Interferometry Mission, would have probed for distant earth-size planets, but wouldn't have been able to resolve more than a tiny light dot. No surface detail would have been visible.

Given the march of progress, no doubt someday we’ll see detail about heavenly bodies 100 light-years distant that by today’s standards will seem astonishing. But making out the furtive scrabblings of dim creatures such as ourselves? Sorry, friend. Won’t happen.

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