Observing the past

[Zachary Smith]

Here's a thought for consideration: If it were somehow possible for us to travel faster than the speed of light, would it not also be possible to travel out into space and intercept light reflected from Earth at a previous period in time?

Assuming one had observational equipment of sufficient resolution couldn't we then observe and record "past" events--in effect make a data record of history? Now, clearly this observation would be limited to what could be seen from a perspective of outside and above but still, it could make some interesting study to have, say, images of the pyramids being built or even pictures of the primordial Earth with dinsoaurs migrations and such.

 

[Christopher]

It's theoretically possible, but resolution would be a factor. You'd need a truly gigantic telescope to detect Earth as anything more than a single pixel. And the farther away you get, the more interstellar extinction (absorption of light by interstellar dust/gas) you have to deal with. The interstellar medium is pretty tenuous, but like atmospheric haze, it makes things look more indistinct the farther away you get from them. So you couldn't clearly see events from very far in the past even if you had a telescope as wide as a solar system, or used a star's gravity focus. Pyramids are unlikely, dinosaurs are pretty much a non-starter. Maybe the Civil War or the Renaissance. But I wouldn't bet on it.

 

[Zachary Smith]

I wonder if it might be possible to place a number of light collectors spread far apart, maybe even with light year's distances between them (but the same distances from Earth), looking back at Earth to collect more light, as if a single giant eye light years in size. That might improve resolution as well.

 

[Christopher]

Yes, the concept of "a truly gigantic telescope" includes the idea of an array of smaller telescopes spanning a gigantic distance. Multiple telescopes acting in concert can work equivalently to a single giant telescope as wide across as the whole array. With a telescope as wide as the Solar System, you might be able to make out surface details on a distant planet.

But that wouldn't help you with the interstellar extinction problem, because that's not about resolution, it's about having stuff physically in between the telescope and the subject, absorbing light before it reaches the detector. Even the hugest of telescopes can't make out an image if the light doesn't reach it.

 

[Zachary Smith]

How much information is carried by a single photon of light? Is it the equivilant of the physical size of the photon, for example? I understand that we do not actually "see" objects but we receive information in the light reflected off of the object and that the character of the photons are changed by interacting with the environment. What I'm wondering about now is just how many photons would need to be reflected off an object to render usable information? Would a single photon reflected off an apple, for example, tell you anything about the apple or would it be like receiving a single pixel from a much larger image and you might get nothing more than, say, a red photon? Or do you need enough photons to be physically reflected off the whole surface of the apple (the visible suface, obviously--as those on the non-visible side are reflected AWAY from the observer) to render the "image". Photons are very small items, indeed and incoherent light reflects off objects at all kinds ofangles, depending on what it contacts. One wonders how "complete" of an "image" the eye really receives and how much of it is reconstructed by the brain from partial information. At the same time, since a single cell contains the entire DNA of a much more complex organism, is it possible that a single photon may carry more information than at first seems reasonable via "common sense"? Now, if a photon is merely the real world counterpart to a "pixel" and the information it carries is limited to the physical size of the photon itself and communicates what it touches, a single one isn't going to render much usuable data. It would be like having a snapshot of a photon-sized piece of a puzzle to work with.

 

[Shaw]

Well, light doesn't really carry that much information when you think about it... at least from a human eye's perspective. We require lots of photons streaming at us to gather information about our surroundings. And you just don't really have that option at great distances.

On the other hand, light carries a lot of information that the human eye doesn't usually see. For example light sources can tell you about their makeup by their spectra (which is how we get a lot of data from places we haven't been). We know what elements create what bands in spectra (like a finger print) and when we see those bands that lets us know what we are looking at. Additionally, the shifting of those bands towards red or blue can tell us if (and how fast) something is moving away or towards us.

Something else to keep in mind, any light coming from Earth (in the visible spectrum or otherwise) is going to become harder to distinguish from light from the Sun the further away you get.

As an example, at night you might be able to see a match from a few miles away with the right equipment... but put that match next to a spotlight and it gets lost. When you go out far enough, the Earth and Sun are just too close together to easily separate them.

And the easiest way to see just how close they would seem is by doing a little geometry. The Earth is about 8 light minutes away from the Sun, and you are talking about looking into the past, so your observer might be 200 light years away from both the Earth and Sun. If we were looking down on the plane of the solar system, what would be the angular difference between where the Earth is and the Sun from the observer's perspective?

Now, consider what Christopher said... assuming that your display is showing pixels at 72 dpi, and that your eye is about 24 inches away from the screen, what is the angular difference between two pixels from your eye's perspective?

That should give you a rough idea of the hurdles that historians would be facing attempting to study the past by warping out away from the Earth and looking back at it.

 

[JustAFriend]

Easier to say: SIGNAL TO NOISE RATIO

Would be hard to build an instrument half the size of the Universe....

 

[Timo]

One might wish to concentrate on chasing after information packets that are inherently easy to observe, or to decipher, or have high odds of surviving the transit.

Even if one cannot read over the shoulder of Pontius Pilatus to see what sort of a verdict he really gave and whether he washed his hands for real, one may one day intercept the television broadcasts of the first Moon landings and verify that there wasn't a Coke can in one of the scenes that the Men in Black later censored. Or one may do a spectral analysis on the Tunguska explosion plume, see for sure how hot the Sun was during the Little Ice Age, or observe how much methane there was in the atmosphere during the early Triassic period. That sort of direct observation would be of some interest, now wouldn't it?

Timo Saloniemi

 

[Christopher]

How much information is carried by a single photon of light? Is it the equivilant of the physical size of the photon, for example? I understand that we do not actually "see" objects but we receive information in the light reflected off of the object and that the character of the photons are changed by interacting with the environment. What I'm wondering about now is just how many photons would need to be reflected off an object to render usable information? Would a single photon reflected off an apple, for example, tell you anything about the apple or would it be like receiving a single pixel from a much larger image and you might get nothing more than, say, a red photon? Or do you need enough photons to be physically reflected off the whole surface of the apple (the visible suface, obviously--as those on the non-visible side are reflected AWAY from the observer) to render the "image".

A single photon has only one wavelength, and that can't tell you much about an object. To get a spectrographic reading telling you the object's composition, you need a sample of all the wavelengths it's reflecting or emitting. And no, you can't get any information about the shape of an object from a single photon emitted or reflected from a single point on its surface.

And of course there's no guarantee that a single photon could reach the detector, since it could be absorbed or scattered along the way. And statistically speaking, if your detector only reads a single-photon blip, there's no way to be sure that isn't just some random photon scattered from somewhere else in the universe, or even just a glitch in the equipment. You can never trust a single, unrepeated result, because there's always the possibility of error.

And a photon doesn't have any "size," except for its wavelength.

 

[Lindley]

One of the Hidden Frontier episodes uses this method to get direct observations of a terrorist attack. However, that was only a day or so in the past.

 

[Christopher]

^^Only a day or so makes perfect sense -- again, assuming you have a sufficiently sensitive telescope/sensor. This trick is also used in Jerry Oltion's Captain Pike novel Where Sea Meets Sky -- when an incident happens too fast for them to figure out what occurred, they warp a few light-minutes away and watch it again from a distance. Also, in my own Titan: Orion's Hounds, an astronomical phenomenon is discovered to have changed by comparing how it looks in the present using FTL subspace sensors to how it looks in the visible-light image arriving from a decade or so in the past.