Looking into the past from really far away

Factual Questions
1

Looking at distant stars through a powerful telescope is rather like looking into the past. What is seen when we look at star that is, say, 10,000 light years away is really what that star looked like 10,000 years ago, because the light by which we are able to see it has been that long in reaching us.

So, let’s say that I build a super powerful teleporter that is capable of teleporting anything or anyone anywhere in the universe instantly. I then use that teleporter to instantly transport myself to a small class M planet called Bethselamin some 2,000ly distant. Don’t ask me how I know it’s there, it just is. The people there are very nice, but get a receipt when you go to the bathroom. Now let’s say that once there, I have access to a ridiculously massive orbiting telescope – 10km in diameter and capable of collecting enough light that I could see Caesar’s dandruff back on Earth. The question is: Would I be able to see Caesar’s dandruff back on Earth? Since all light being emitted from Earth would effectively be 2,000 years old by the time it arrives at Bethselamin, would I essentially be witnessing the past as if it were happening right then? Could I watch the Illyrian revolt? Watch the Temple of Concordia being built? Because that would be kind of cool.

2

Well, of course. It works the same in both directions. What is it that makes you doubt it?

3

Faster-than-light transport poses certain theoretical challenges. But if you perfect it, it would certainly be cool.

4

It’s not so much that I doubt it as it is the whole idea of effectively being able to see into the past without having to fiddle with time, which gets notoriously crotchety when it thinks you’re trying to mess with its head. Not, however, just seeing into the past, but the idea of being able to magnify something to such a degree that you can see an entire civilization that’s long since dead in such detail. I mean, I know that reflected light is still light, but the idea that it remains coherent enough at such great distances to make out fine details like people and things is what I’m having trouble with.

Subspace, man. It’s the wave of the future.

5

Well, there’s nothing from theoretical standpoint which prevents one from forming an image to any arbitrary magnification using perfect optics and if your subject doesn’t move. The biggest problem (again, assuming perfect optics) would be getting enough light to actually see the image. At those distances, the amount of light you’d capture from an area the size of, say, a person’s head is maybe a few photons a day, depending on how large your primary aperture is. To form a clear image of Caesar’s noggin would take an exposure of weeks or even months–and I doubt he’s going to stand still long enough.

6

He did say the aperture would be 10 kilometers wide. I doubt even that would give a great picture from so far away, but I’d expect more than a few photons per day to hit the mirror.

7

I read that as 10 m. Still, I think light-gathering power is going to be a major issue.

8

Let’s see:

How big is a fleck of dandruff at 2000LY?
2000LY = 1.89 X 10[sup]16[/sup] km
A 1mm bit of dandruff is 1 X 10[sup]-6[/sup] km in diameter.
sin(theta) = 1 X 10[sup]-6[/sup]/1.89 X 10[sup]16[/sup] = 5.29X 10[sup]-23[/sup]
So theta, the angular size of the dandruff is 1.09 X 10[sup]-17[/sup] arcseconds

What’s the resolution of your 10km scope?
At 550 nm, arcseconds of angular resolution αR = 138/ Diameter of mirror in mm
10km = 10 million mm, so αR = 1.38X10[sup]-5[/sup] arcseconds.

That mirror’s not nearly big enough.

How big would the mirror have to be?
Well, subbing into the angular resolution equation:
1.09 X 10[sup]-17[/sup] = 138/D
D = 1.27 X 10[sup]19[/sup] mm.
So you’d need a mirror 1.27 X 10[sup]13[/sup] km in diameter to see a speck of dandruff on earth, from 2000 LY.
That’s a mirror 1.34 light years in diameter.

Even if you had such a mirror, your resolution would probably get screwed up by same atmospheric turbulence that degrades the performance of spy satellites.

9

You can’t scale things infinitely. Eventually, if your lens (or mirrr, or whatever) gets too big you end up not actually being able to use it for imaging – you start hitting the lens at sych an angle that the light doesn’t get refracted in, but bounces off due to frazing angle reflection. You get absorption of surface waves, and other weird things start to happen.

There are always dodges, of course (they’ve actually beaten the diffraction limit now with super “active element” lenses that manage to grab the energy formerly lost to evanescent waves and the like, but don’t count on using that here), but I think that even if you ysed scads of optical elements out ion space to gather the light from an area as big as that light year and coherently combined it to get an image, you still wouldn’t get anything close to what you;'re looking for.

10

I kicked around the idea of this for a Star Trek fanfic a while back. Picture a company (Memtek, say) that sets up a huge antenna array in a star system 400 light years from Earth (call it Delta Omega, arbitrarily), aimed toward it, receiving the faint signals from the beginning of the broadcast era and marketing them in the TNG/DS9/Voyager era. These signals, greatly cleaned up, are put onto the standard subspace communications networks (or whatever passes for the 24th-century Internet). One particular rebroadcast station, state-of-the-art at the time, gets built at the edge of Federation space in anticipation of a new wave of colonial expansion in that direction. Trouble is, that way happens to be Cardassian space, which previously was thought to be un- or at most sparsely-populated. The Nova station (as it is known) gets destroyed in the early weeks of the Federation/Cardassian War, just a month into its operating life.

Some forty-five years later, the Delta-marooned USS Voyager moves into a position which allows them, for the first time, a direct line-of-sight to where the Nova station was. Tuning and aligning the deflector array precisely allows them to receive the signal and decode it, separating out the 1024 channels of entertainment, sports, news, political and military traffic. Tom Paris, established as a fan of the hokey sci-fi serial “Captain Proton”, seeks out the captured 20th-century television broadcasts, fascinated by them, earning the contempt of B’Ellana Torres (who is already in a state of annoyance after she decoded some of the old Starfleet military traffic and learned that the Cardassians had been noticed and designated “Alien Contact 3481-Alpha” but no other information had been established, which irritated her because if contact had been established properly, the war and the Maquis and the Caretaker and basically her whole life up to that point could have been far different and far better):

“Are you still watching that stuff?” she asked impatiently.
“Shh,” he hissed. “I don’t want to miss this 1936 Olympics broadcast - it’s live.”
She frowned. “It’s not live. It was picked up by the Memtek corporation array 400 years after it was live, filtered, and put on the repertory channel until the Nova rebroadcast it and Tuvok filtered and decoded it yesterday.”
“Don’t spoil my fun. Don’t you have some Ensign to torture?”
She allowed herself the first half-smile of the day. “What do you think I’m doing now?”

Along with the other mind-bending elements is when Torres points out that the Memtek’s gigantic antenna array is positioned with one side facing the Delta Omega star (drawing energy from it) and the other facing Earth, so that it receives the light from 400 years earlier, and 400 years later, when the light from Delta Omega works it way back to Earth, the star will seem to blink out of existence, eclipsed in the name of entertainment.

And a month later when the Nova signal is abruptly cut off, the Voyager sadly resumes its years-long trip home, hoping for a time when they can receive news from home when it is still new and not the blurred static ghosts of the past.

11

Yes, a lightyear is way too big to expect reasonable behavior. A simpler approach might be to find a nice cluster of stars at about the right distance, and use its natural gravitational lensing to examine Caesar’s hair
There’s some talk of using this approach with the sun as a lens:

12

But that’s the point. Transporting somewhere faster than light is fiddling with time, and is therefore (as far as we know) impossible.

Travelling faster than light would mean moving outside of your future light cone, which is essentially exactly the same as travelling back in time. (In other words, moving from point A to point B, 10 light years away, instantly, is exactly the same as moving to point B at the speed of light, and then travelling back in time 10 years.)

From the Wiki link: “Elsewhere, an integral part of light cones, is the region of spacetime outside the light cone at a given event (a point in spacetime). Events that are elsewhere from each other are mutually unobservable, and cannot be causally connected.”

(Emphasis added)

13

Colophon - I won’t claim to be an expert on special relativity or anything, but I have trouble accepting this. That C is the “speed limit of the universe” is a given only if you accept that we know everything about the way the universe works. Now, I know we’ve got a pretty good handle on things in a sort of general sense, but I think it’s a little premature to be speaking in absolutes, so I prefer to think of C as being the “speed limit of the universe – as far as we know, anyway.” Besides, I seem to recall something about the discovery of some infinitesimal particle some years back that was found to be capable of traveling just a wee bit faster than light. I wish I could find the article, but slowing it down just long enough to measure it apparently involved a massive tank of heavily salinated water or somesuch. But that’s neither here nor there. My OP is working under the assumption that it’s not only possible to travel faster than light, it’s also high in Vitamin Awesome. My own argument, at least as far as your point is concerned, is that what you observe from a distance is not what is currently going on over there, and if you were to instantly transport yourself there, to the very spot you were observing, you would not see what you were observing before being transported. Instead, you would be in the midst of what’s currently going on. There’s no fiddling with time here; you may be able to observe the past by viewing it from a great distance and at high magnification, but if you could transport yourself instantly to the place you are currently observing, you’d arrive at whatever is there and going on presently. It only seems like you moved forward in time, when in reality you were simply travelling faster than the visual information contained in the light you observed.

As for the rest:

Okay, observing a flake of dandruff was just my ridiculous setup. The idea still stands though: Could one, given a large enough and powerful enough lens, presuming perfect, digitally assisted optics and considerable light-gathering capabilities, and with optimal astronomical conditions, observe some kind of coherent, detailed imagery from the past from some considerable distance away? I realize there is a limit to the distance and scale of optics at which one could still feasibly observe the goings on on a distant planet, so feel free to play with the distances and optics until a reasonable combination can be reached at which it would be possible to observe, with some reasonable level of clarity, the historical events of Earth from a relatively distant extrasolar planet.

14

Another problem no one has mentioned yet, is the angle of viewing.

You only have 50/50 odds that the event you want to watch is in the visible hemisphere to begin with. Even then, the odds are pretty good that you’re either looking almost straight down, or at some other bizarre angle. If you’re lucky enough to be watching from a sufficiently gentle angle, then there are either going to be some buildings in the way, or you won’t have that good view for more than an hour or two.

For some reality-based examples, of what I mean, just try our own satellite imagine on Google Earth or Yahoo Maps or something, and imagine how many bad shots they had to go through till they found the ones you’re looking at.

15

This is one of the things I love about the New York Sun’s circa 1835 “Moon Hoax”. That’s the one where they claimed John Herschel was using a super-telescope to spy on the moon people, and they reported the goings-on in great detail, with drawings and everything. But nobody seems to have observed that most of that lunar surface would’ve been seen from directly above, or pretty close to it. You could conceivably see things at the limb edge-on, but you wouldn’t be able to see things from a “normal” perspective.
Edgar Allen Poe, criticizing the reports, showed that, even with the outrageous magnification the Sun CLAIMED to have they wouldn’t have been able to see the details they reported. (And this was before Airy’s paper on the Airy disc/resolution, so they couldn’t make that objection.)

16

You could almost do the same thing with radio waves and listen to past live broadcasts.
Say you sent out a satellite from earth to a point 1 light year away from earth. The satellites job is to receive a radio siganl and immediately retransmit (or reflect) it back towards earth.
On earth you could be listening to live broadcasts that occured 2 years ago.

Working on the same premise could you send out a massive mirror into space a “light days” distance away (26 billion kilometers), use a powerful earthbased telescope, and observe the earth 2 days old? Would be handy in solving murders as long as you watched the replay within 48 hours and it occured outside on a non-cloudy day facing the mirror.

17

It’d all be fun and games until the RIAA sued you for breaching copyright by rebroadcasting their content.

18

Not to mention problems if you don’t have the express written consent of Major League Baseball.

19

Rebroadcast of these Gladiatorial Games without the express written consent of Flavian Amphitheater Broadcasting wioll result in fines of not more than 10,000 talents of gold or five years in the galleys.

20

Ahh, Bull! Everyone knows that the ‘surface of last copyright’ is stable at the life of the author plus 70 lightyears from earth. Anything beyond that is in the public domain.