
Gravitational Lensing to Observe Ancient Earth - pkrein
http://rein.pk/gravitational-lensing-to-observe-ancient-earth/
======
aphyr
The practical limitation isn't just magnitude, but resolving power.
Atmospheric disturbance makes it difficult to resolve anything smaller than a
meter from orbit, and the diffraction limit means the further are away from
the thing you want to observe, the wider your telescope needs to be. Since the
distortion effects near a black hole are near-asymptotic, the features we want
to see have _very small_ subtended angles, and we need much higher resolving
power to "see" recognizable features.

Stars? Definitely feasible. Planets? Much harder. People? Certainly not with
available optics.

~~~
pkrein
Aperture synthesis (several widely-separated telescopes looking at one object)
is often used to increase the resolving power.
<http://en.wikipedia.org/wiki/Aperture_synthesis>

~~~
mikemoka
yes but if we manage to do everything (including unwarping the earth's
reflection in the image) we would only see how our planet looked like 6 light
years ago (3 for the light to arrive to the black hole and 3 to go back) and
we won't be able to discern anything but the earth's surface, is that right?

~~~
pkrein
oh -- yeah seeing what people are doing on Earth via a black hole is totally
out of the question technologically. that's pretty much the conclusion of the
article.

------
ISL
You don't want the light reflected by 180 degrees - it needs to be reflected
where Earth is _going to be_ ; a much harder problem. For the easy case quoted
in the article of 6 light years, any collimated light would need to be
properly directed at the sub-nanoradian (milliarcsecond) level (which is
possible -- see sig and [0]).

Interesting to think about, even if it is an incredible observational
challenge.

Fun application for the Arecibo radar, if it's still running. Locking into a
modulated signal might yield the necessary SNR?

[0]
[http://en.wikipedia.org/wiki/File:Diffraction_limit_diameter...](http://en.wikipedia.org/wiki/File:Diffraction_limit_diameter_vs_angular_resolution.svg)

~~~
andyjohnson0
_"it needs to be reflected where Earth is going to be; a much harder problem"_

The relative velocity of the ancient Earth, black hole, and present earth
would also need to coincide in such a way that you'd get a continuous (or,
less desirably, periodic) stream of photons making the complete round-trip.
Such a coincidence seems pretty unlikely.

Its a nice idea though.

~~~
JoeAltmaier
Actually it seems pretty inevitable. Light is sprayed across the black hole
from Earth; some of it ends up in every possible trajectory; some ends up in
the critical 'orbit window' to be reflected back to where earth is going to
be.

------
fsiefken
You need to read 'The Light of Other Days', detailing the use of microscopic
wormhole lenses to observe ancient earth:
<http://en.wikipedia.org/wiki/The_Light_of_Other_Days>

~~~
shill
I had an idea to write a story about this when I was a kid. In my version,
planets made of pure mercury were sought by detectives for their potential
crime-solving reflections of Earth at various points in history.

------
oofabz
This Lebbeus Woods has some great artwork. I'd never heard of him before but I
found these two pieces by him:

[http://historyofourworld.files.wordpress.com/2009/11/lw-
iii....](http://historyofourworld.files.wordpress.com/2009/11/lw-
iii.jpg%3Fw%3D720%26h%3D418)

<http://www.krobarch.com/images/winners/2012/entry3353.jpg>

~~~
ianstormtaylor
And some interesting reads on his blog:
[http://lebbeuswoods.wordpress.com/2012/02/17/zahas-
aquatic-c...](http://lebbeuswoods.wordpress.com/2012/02/17/zahas-aquatic-
center/)

------
yakiv
Does anyone here know why the Einstein cross has 4 of the repeated image
instead of being a continuous repetition around the lens object? It seems to
me like you should see the magnified image as a sort of circle instead of
having a finite number of repetitions.

Is this related to the lines forming a cross you see in pictures of stars?

~~~
privong
The lines forming a cross in pictures of stars is a separate phenomenon. That
is due to diffraction from the support structures of the secondary mirror.

For a detailed answer to why the Einstein cross has 4 images, see:
[http://physics.stackexchange.com/questions/14056/how-does-
gr...](http://physics.stackexchange.com/questions/14056/how-does-
gravitational-lensing-account-for-einsteins-cross)

------
guylhem
I'm not so much into physics but based on what I understand - ok, we might get
just enough photons reflected back by MACHOs if we build a massive new
telescope.

But then how do we separate them from other photons - ie the ones not
reflected back? Do photons reflected by MACHOs have a specific signature on
their EM wave ??

~~~
ChuckMcM
That is exactly the problem. The author talks about the photons that get
turned 180 but the reality of the situation would be similar to seeing our sun
reflected in a piece of glass a few thousand light years out (no need for a
MACHO). The reflection is there but so is all the background noise, so just
like trying to look the reflection on a clear piece of glass, the light mixing
would be killer.

------
andr3w321
I find it sort of ironic that everyone here thinks this would be awesome, but
are completely opposed to hovering predator drones observing our every action
24/7. Obviously it's all a matter of who has access to the data, but
essentially the data would be the same.

~~~
potatolicious
Even disregarding the resolution problems as brought up by ebilsten, and
assuming we have some magical tech that would allow us spy-satellite-like
capabilities with this... we'd still be looking at Earth _decades_ in the
past.

I for one could not care less if historians centuries in the future want to
observe me through a black hole. Its effects on freedom and civil rights is
basically nil, unlike predator drones.

~~~
ComputerGuru
In Orson Scott Card's "Pastwatch: The Redemption of Christopher Columbus"
(awesome book, btw! I don't think he's planning on writing the remaining 2 in
the trilogy, though :() they have machines that do exactly this, letting them
monitor ancient history up to a decade ago: anyone, anywhere on the planet.
The catch is that the builders of the machine actually have access to real-
time monitoring, but pretend this isn't the case to avoid public outrage.

------
malbs
Was reading the article, excited by the prospect when I realised, we have a
hard enough time identifying planets dozens of light years away, from direct
light, that surely the reflected light from a massive object 3000 light years
away would be orders of magnitudes more difficult. Perhaps impossible.
Theoretically possible yes, but what sort of technology needs to be invented
to actually do it?

edit: turns out I should have just finished reading the article.. bigger
telescopes!

------
ars
Why not use two black holes, each bending the light approximately 90 degrees?
You don't even need a black hole - a large sun would work too.

------
andyjohnson0
This reminds me of Greg Egan's short story "The Hundred Light-Year Diary". The
text is at [1] but better still buy the book [2]

[1] <http://pastebin.com/f341e113e> [2] <http://www.amazon.co.uk/Axiomatic-
Greg-Egan/dp/0575081740>

------
hakaaaaak
Posting obligitory link to:

<http://en.wikipedia.org/wiki/Time_viewer>

Of course someone should mention Father François Brune and his Chronovisor:
<http://en.wikipedia.org/wiki/Chronovisor>

But none of that is real.

------
ck2
If we ever invented FTL travel, even for just probes, this could be done too.

Except you'd need insane levels of resolution or a whole new way to think
beyond optics to be able to see details.

I mean what if you could see that far back but the earth's resolution was only
the size of a pin head?

~~~
lutusp
> If we ever invented FTL travel, even for just probes, this could be done
> too.

If we had FTL travel, we could outrace light waves, look back, and see events
in the past. The we could race back and report what we saw. That's why it's
not possible -- it violates some basic physical ideas about causality.

Remember that the speed of light isn't just a speed limit for light waves --
it's a barrier that sorts out temporal causes and effects.

Imagine a flat plane in the time dimension called the present -- on each side
of the plane is a cone. The cone in the past encloses possible causes for
present events. The cone in the future encloses possible effects from events
in the present. Outside the past cone are events that _may not_ produce
results in the present. Outside the future cone are effects that _cannot have
been_ caused by events in the present. _The slope of the cone surfaces is the
speed of light._

Light cone picture:

[http://4.bp.blogspot.com/--VBwEngltGc/Tzniy1imbII/AAAAAAAABL...](http://4.bp.blogspot.com/--VBwEngltGc/Tzniy1imbII/AAAAAAAABLw/SRpE1WvNqC4/s1600/500px-
World_line.svg.png)

~~~
98709870987
>>If we had FTL travel, we could outrace light waves, look back, and see
events in the past. The we could race back and report what we saw. That's why
it's not possible -- it violates some basic physical ideas about causality.

You are confused. You are not actually traveling to the past therefore
causality is not a problem here.

~~~
lutusp
> You are confused.

I'm confused? Which of us is discussing FTL without understanding its physical
meaning?

> You are not actually traveling to the past ...

Locate where I made this claim.

> ... therefore causality is not a problem here.

Causality is certainly a problem. FTL is impossible in the same way that
dividing a number by zero is impossible or taking the square root of a
negative number is impossible.

Here's the simplest form of the relativistic equation that tells us the amount
of time dilation we can expect for a given velocity v:

t' = t * sqrt(1-v^2/c^2)

t = time rate at v = 0

t' = time rate at v

c = speed of light

v = frame velocity

Compute the time dilation for v > c, then get back to me.

FTL means leaving the causality light cone, conventional notions of causality,
and therefore all of physics:

<http://en.wikipedia.org/wiki/Light_cone>

Quote: "Because signals and other causal influences cannot travel faster than
light (see special relativity and spooky action at a distance), the light cone
plays an essential role in defining _the concept of causality_ \- for a given
event E, the set of events that lie on or inside the past light cone of E
would also be the set of all events that could send a signal that would have
time to reach E and influence it in some way." [emphasis added]

~~~
nessus42
_> Causality is certainly a problem. FTL is impossible in the same way that
dividing a number by zero is impossible or taking the square root of a
negative number is impossible._

This assertion is false. GR, even without FTL, allows for closed time-like
loops, which violate causality. Violations of causality are not logical
contradictions like dividing by zero. (Though, of course, there _are_
alternative mathematical systems that allow for dividing by zero. And taking
the square root of a negative number is surely not impossible. You just get an
Imaginary result.) It's just that a universe in which causality is violated
all the time would be a very strange one, and consequently, it is reasonable
to assume that if there are, for instance, closed time-like loops in the
universe, they are not traversed very often.

~~~
lutusp
> GR, even without FTL, allows for closed time-like loops, which violate
> causality.

The theory may "allow" them, but this is not to say they actually exist in
reality.

<http://en.wikipedia.org/wiki/Causality_(physics)>

Quote: "In the theory of general relativity, the concept of causality is
generalized in the most straightforward way: _the effect must belong to the
future light cone of its cause_ , even if the spacetime is curved."

[http://hipercom.inria.fr/~jacquet/retro1/crazy-science-
corne...](http://hipercom.inria.fr/~jacquet/retro1/crazy-science-
corner/causality.html)

Quote: "Weird space-time geometry distortion within general relativity can
result in causality violation, but there is no evidence that they are
physically possible."

~~~
nessus42
_> Quote: "Weird space-time geometry distortion within general relativity can
result in causality violation, but there is no evidence that they are
physically possible."_

And there's no evidence that they are not possible. With no evidence either
way, one must remain agnostic.

On the other hand, we _do_ have evidence that closed time-like loops are
possible: GR has never been observed to be wrong, and GR predicts their
existence under the right circumstances.

It may certainly be the case, that even though closed time-like loops are
_possible_ , they do not actually exist in our universe and never will.
Paradoxical universes are ruled out by logic alone, and so the space of
possible universes without closed time-like loops (or least without humans
traversing them) may outnumber the space of possible universes with time-like
loops in them, allowing us to make a probabilistic argument against their
occurrence in our universe. My transfinite number theory is rusty, however, so
YMMV.

~~~
lutusp
> And there's no evidence that they are not possible.

In science, the default precept toward ideas without evidence is the _null
hypothesis_ \-- they are presumed to be false. This is why Bigfoot is not
accepted as a scientific theory -- until it is observed, it doesn't exist.

[http://www.null-
hypothesis.co.uk/science/item/what_is_a_null...](http://www.null-
hypothesis.co.uk/science/item/what_is_a_null_hypothesis/)

Quote: "In statistics, the only way of supporting your hypothesis is to refute
the null hypothesis. Rather than trying to prove your idea (the alternate
hypothesis) right you must show that the null hypothesis is likely to be wrong
– you have to ‘refute’ or ‘nullify’ the null hypothesis."

Saying "there's no counterevidence, therefore ..." is not a scientific
statement, because it can be said about anything not conclusively falsified,
including meadowland fairies and honest politicians.

> With no evidence either way, one must remain agnostic.

Not in science. It's nice to have an open mind, but not to the degree that
one's brains spill onto the ground.

~~~
nessus42
This is just untrue. When presented with a hugely precise and successful
theory that has never been demonstrated wrong in even the slightest degree,
that is certainly evidence (not proof, but _evidence_ ) towards whatever
claims that theory makes. If you don't understand this, then you don't know
what the word "evidence" means.

~~~
lutusp
> This is just untrue. When presented with a hugely precise and successful
> theory that has never been demonstrated wrong in even the slightest degree,
> that is certainly evidence (not proof, but evidence) towards whatever claims
> that theory makes.

Nonsense -- perfect nonsense that the history of science repeatedly falsifies.

1\. Science is empirical -- it must be. If it's not empirical, it's not
science.

2\. GR succeeded with every challenge that was put before it until it was
compared to quantum theory, at which point it failed. Your position is that,
because it had passed any number of difficult tests until then, it should have
been assumed to reflect reality in all respects.

But this is not how a scientist approaches theories and experiments. Any
aspect of a theory that has not been tested, is assumed to be false until
there is evidence to support it.

The ether theory of the 19th century was successful until it failed one
crucial test. The Phlogiston theory was successful until it failed one crucial
test. The Ptolemaic theory of orbital mechanics seemed correct until Galileo
looked through his telescope.

All scientific theories are assumed to be false until there is evidence to
support them, then, after evidence supports them, they are assumed to be
perpetually falsifiable by new evidence. An unfalsifiable theory is not a
scientific theory.

> If you don't understand this, then you don't know what the word "evidence"
> means.

Science is not law. Learn about science.

> When presented with a hugely precise and successful theory _that has never
> been demonstrated wrong in even the slightest degree_ ...

You need to learn about quantum theory, which falsifies GR. Start here:

[http://library.thinkquest.org/04apr/01330/currentphysics/con...](http://library.thinkquest.org/04apr/01330/currentphysics/conflicts.htm)

The present enthusiasm for superstring theory is that it might -- might! --
replace both GR and quantum theory, with a more comprehensive theory that
would resolve the conflict between the two earlier theories.

~~~
nessus42
_> Nonsense -- perfect nonsense that the history of science repeatedly
falsifies._

You are completely incorrect in this and everything else you said above. Well,
it _is_ true that science is empirical, but it seem that you have profound
misconceptions about what "empirical" means. One of the things that it means
is that every additional confirming piece of observed evidence for a theory
gives extra weight to the probability that the theory is correct, especially
when the theory makes correct predictions against previously unobserved
phenomena, which GR has done many times.

You also apparently don't understand that theories are not typically purported
to be facts per se, but rather they are models. Models that have greater or
lesser accuracy. Models are not expected to be perfectly accurate, which is
why they are models and not facts. Though in fundamental physics, we do expect
a level of accuracy far above what we would expect in almost any other
science.

The ether model was proven to be so inaccurate as to no longer be useful. The
limitations of Newtonian mechanics were shown by Relativity, but nonetheless,
the model is still a hugely important and useful model, and using it correctly
is the epitome of good science, despite the fact that we know that in some
sense it is "false".

Re your examples, all the examples you give of theories that were eventually
proven incorrect are arguments against a strawman, because no one here or
anywhere has claimed that any amount of empirical evidence will grant a theory
a 100% probability of correctness.

No one has asserted that GR is unfalsifiable, or that it has a 100%
probability of being correct. None of this means that we can't have an
extremely high confidence level in GR, considering all the empirical gauntlets
that it has successfully passed. And when we finally have a successful TOE, GR
is almost certainly not going to be scrapped. It will still be the extremely
useful and productive model that it has always been. We will just have to be
aware of the model's caveats, just as we must with Newtonian Mechanics, which
is also one of the most successful scientific models ever constructed.

Additionally, all this is neither here nor there for the original point, which
was about your claim that FTL is logically impossible. That is just not the
case. FTL is completely within the realm of _possibility_. It just has serious
and problematic consequences that should make us look askance when people
claim to have figured out how to make FTL drives or communication channels.
They are almost certainly mistaken, but not for the reasons that you claim.

------
dantillberg
Another possibility (perhaps more feasible?) would be for an intelligent alien
civilization to observe light / radio transmissions from Earth, record them,
and kindly play them back for us.

------
DeepDuh
I've never seen that image of our galaxy's central region, it's fascinating!
Is "Sgr A" the black hole, Sagittarius A*? If yes, why would it be bright? I
thought it's inactive.

~~~
pkrein
Sgr A is the black hole Sagittarius A* as far as I know. The black hole itself
isn't bright. As far as I understand, the swirling gas around it (outside the
event horizon) is what's sending out all the light.

What do you mean by inactive?

~~~
DeepDuh
By inactive I meant that it's not swallowing anything currently, but I guess
if it has attracted a gas cloud this is simply not true. Must be quite a
sight. Imagine there's a civilization on one of the central stars (if that's
even possible considering the bursts of radiation they must get there) - I
wonder what the night sky would look like.

~~~
privong
The activity level (in terms of how much material is falling into it) is
fairly low, such that it doesn't qualify as being an "active galactic
nucleus". However, a gas cloud (called "G2") will be passing close enough to
the supermassive black hole that it will be tidally disrupted and part of the
cloud will fall into the black hole. This will give us a chance to see
enhanced accretion, though still not of the level for it to be considered
"active".

------
grecy
Ha, that was the plot line of Paycheck [1], but seeing the future, not the
past.

[1]<http://www.imdb.com/title/tt0338337/>

~~~
DanBC
Greg Egan had a short story ('The Hundred Light Year Diary') in his collection
'Axiomatic' which dealt with people be able to send messages to the past (and
thus becoming aware of their future).

(<http://www.gregegan.net/BIBLIOGRAPHY/Bibliography.html>)

------
spullara
I was always hoping there was just a big mirror out there that we could look
at and see the past.

------
clavalle
We'd be moving relative to a black hole so the 180 degrees assumption might be
off.

------
johnpmayer
Pictures seems far-fetched, but what about a radio signal of an old broadcast?

------
zafka
This is sweet. I can imagine this being woven into some great stories.

------
hobbyist
Read only access!! ROFL

