posted on Sep, 21 2014 @ 02:30 AM
If this sort of thing were ever to be possible to any practical degree, it'd have to involve us seeing our reflection within the gravitational
lens of a black hole. Black holes are capable of causing light to be trapped in orbit around itself (this occurs at the event horizon, the point at
which the escape velocity of an object matches the speed of light) so they're easily capable of causing light to make a complete U-turn back to a
point source. This comes with the added benefit that black holes produce no confounding light of their own; but the whole thing already has a number
of catches that make it... rather unlikely.
The first thing is that neither a black hole, nor any other system capable of gravitational lensing, would really need to send the light on a
180-degree back towards its source. Our black hole would have to be sending our light on an intercept course towards our future location: and
since the earth is in motion, that specific point and the angle to it will change depending on what specific point in Earth's past that you're
The second, related, thing is that a black hole capable of gravitationally lensing our image back towards us would have to be located at a very
specific distance from us. For light from the Jurassic to reach our eyes today, the light must have spent about 150 million to 200 million years
moving through space. Halve that number to account for the two-way journey, and what you get is that black holes 75 million to 100 million light-years
away from us today are the only ones capable of having ever sent us an image of our own Jurassic.
The third thing is that in any situation where light from earth makes a turn towards us, light from the sun will be doing so as well. Earth's light
would be drowned out by Sol's; so instead of trying to observe the earth directly, we would really have to be trying to observe the dip in Sol's
light as the earth transits across its surface relative to the black hole. (This technique is the one we've used so far to find extrasolar earth-like
But read that last paragraph carefully; we'd have to have a transit occur in space-time between the earth, the sun, and the black hole. Those ain't
betting odds, to put it mildly...
... at least not for any specific time point. I should admit here that at longer, more flexible scales, it might not be impossible per se to find such
a black hole, one capable of receiving our light and then sending it back toward a place where we can observe it millions of years later.
The galaxy itself is only 100,000 light years wide, so there do exist plenty of black holes within the correct radius for such events as "the time of
the dinosaurs". Also, the way the earth and the sun are oriented, there are times when the earth and the sun line up along a line pointing
perpendicular to the plane of the galaxy. A signal returning to us from a black hole lying outside our galaxy would have less confounding light to
deal with than one within our galaxy (assuming that there are such things as rogue black holes passing through empty inter-galactic space).
That all said, any would-be time-astronomer would face insurmountable problems.
First, cosmic dust gathers itself around black holes. It's simply unavoidable. This dust will scatter any light rays passing through it, clouding the
face of any gravitational mirror; and in a situation like this demanding extreme sensitivity, it's unlikely we'd be able to see ourselves in any
black hole's face.
Second, at the scales we'd be talking about here, the light would have to exit the galaxy and move many a distance times longer than the diameter of
our galaxy itself (at least if we want to see events happening in the deep past, like the dino-age and the formation of the earth). What this means is
that the sun's light will invariably be itself drowned out by the light of the rest of the galaxy; if the sun's not visible, then a transit between
the earth and the sun won't be either.
But what if we had a good enough telescope? That's the third thing. Its short answer: we won't be able to do it without some serious breakthroughs
in physics. Light itself doesn't have enough resolution for that.
The long answer is that light is quantized in a way that makes it extremely unlikely that we will ever be able to resolve objects in a meaningful way
at that distance. (Readable explanation of photon quantization next: skip ahead if you want.) When light hits certain substances, they become
electrically excited. This electrical excitation produces an electric field, and these electric fields can be measured as a way to measure the light
hitting the object. If you take a laser, and shine it on such objects, you can then measure the field of electrical excitation across the area where
the laser's light hits the object. This field looks like a smooth continuous space, just like how the laserbeam looks like a smooth, continuous
circle; but as you turn down the laser's power lower and lower (past the point where a human can see) you eventually reach a point where the
excitation field no longer looks smooth. It's grainy; the electrical field shows up in little blips here and there, like raindrops in a puddle: in
discrete chunks. Stated in a general way, this observation means really that light is fundamentally similar to rain, in that both are made up of tiny,
uniform parts, which are simply more or less frequent as the intensity of the rain or the light changes; and the name of a raindrop of light is a
So, in order for us to get any useful information about our planet's deep past, we would need a continuous stream of these photons. One photon
wouldn't do it, because one photon alone could have come from anywhere. It could be random. And since the sun's light is not like a laser's, where
all the photons are oriented in the same direction, a photon stream naturally coming from earth towards the right kind of black hole... such a stream
of photons is inherently going to scatter with distance, to the point that at the scales we're talking about, it's extremely unlikely that any more
than a single photon would be bent so perfectly as to reach us, even if there existed a black hole capable of doing so, and even
if we assumed that any photons made it to us in the first place. The stream would simply be too dissipated at that point.
That all said, this is the sort of thing that could make an interesting setting for a sci-fi book, or movie, or game; a planet whose sun orbits a
distant black hole. If the black hole were sufficiently dust-free, the planet would likely be able to see its own twisted time-distant reflection in
the black-hole's gravitational mirror. Information sent via laser beam toward the black hole at the proper direction could be bent around back
towards receivers on the planet. And if the receivers and broadcast lasers were linked, they could potentially use the laser loop as an
information-storage mechanism for some giant computer. (Except that the computer could be easily disrupted by an asteroid accidentally flying through
the loop. Or an astrobird pooping on the receivers.)