
Gravitational Lensing to Observe Ancient Earth - pinko
http://rein.pk/gravitational-lensing-to-observe-ancient-earth/
======
ComputerGuru
Reminds me of one of my favorite books (well, not really. The premise was
incredible and the writing was impeccable but it was part one of a trilogy
that never materialized): _Pastwatch: The Redemption of Christopher Columbus_

It was basically a brilliant alternative historical fiction novel based around
the premise of being able to rewatch the past.

~~~
hawkice
I consider it the best science fiction book for people who are interested in
historical fiction and speculative historical fiction. Longest list of
citations for a fiction book I've ever seen. And as you say, excellent
premise.

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rcthompson
So it's possible for a beam of light to wrap around a black hole and intersect
its own path. This must mean that light doesn't follow Kepler orbits[1] in
general relativity?

[1] i.e. ellipse, parabola, or hyperbola:
[http://en.wikipedia.org/wiki/Kepler_orbit](http://en.wikipedia.org/wiki/Kepler_orbit)

~~~
pdonis
_> This must mean that light doesn't follow Kepler orbits[1] in general
relativity?_

That's right. This is true even in the weak field limit: the orbits of planets
in the Solar System are not exact ellipses when GR is taken into account. This
is called "perihelion precession", and was observed with Mercury even in the
19th century; it was one of the "classic tests" that Einstein calculated to
satisfy himself that GR was correct. With modern observations I believe this
effect has been observed for at least all of the inner planets (Mercury,
Venus, Earth, and Mars).

~~~
rcthompson
I see, so GR causes all orbits to precess, and actually nothing follows
classical Kepler orbits exactly. Even the simple two-body problem doesn't
result in elliptical orbits.

[http://en.wikipedia.org/wiki/Two-
body_problem_in_general_rel...](http://en.wikipedia.org/wiki/Two-
body_problem_in_general_relativity)

~~~
pdonis
Yes. (Strictly speaking, a perfectly circular orbit would qualify as a
"classical Kepler orbit", and is still possible in GR, but this is an
idealized case.)

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sixQuarks
This is what's so crazy about technology. Whatever we can imagine, we can make
happen, no matter how crazy it sounds. Our descendants 1,000 years from now
will probably be able to observe everything we are doing today. If you asked
me how yesterday, I wouldn't have a clue. But today, this article shows me one
way to do it. Imagine the other possibilities yet to be discovered.

~~~
jcoffland
This is a common view point which kind of irritates me. It leads to a lot of
false assumptions and gullibility with regards to pop science. What about time
travel, teleportation or travel to the nearest star system? All things well
with in reach of our imagination and far from our ability.

~~~
k__
I don't think it is impossible per se, but we put our time and resources in
other things, so it won't happen.

~~~
jcoffland
We will see none of these things in our lifetime nor is it likely to happen in
the next 1000 years. I challenge you to read up on what it would take to reach
Proxima Centauri. Small amounts of time travel happen all the time.
Significant forward time travel is physically possible, as is interstellar
travel, but unobtainable by humans. Pop science has tried to make us think
teleportation is possible through quantum entanglement but that has been
proven false. I would love it if we could do any of these things but it's just
not reasonable to think it will happen.

I believe this sort of gullibility stems from not really understanding what we
have achieved. If our current achievements seem like magic then anything seems
possible. Mankind's achievements are impressive to mankind but we're also a
bunch of self-aggrandizing egotists. Hopefully no one else is watching.

[http://earthsky.org/space/alpha-centauri-travel-
time](http://earthsky.org/space/alpha-centauri-travel-time)

~~~
meric
If you travel close enough to c it wouldn't take very long to the astronaut to
get to Proxima Centauri. It's _very_ far from our capability but you're saying
it won't happen in the next thousand years? It sounds like someone in 1200 A.D
saying it's impossible for humanity calculate PI to the 100,000th digit within
the next 1000 years.

I'm forward time travelling, and have traveled over twenty years in my life
time, at a rate of 1 second per second and that's significant enough for me.
:)

~~~
quanticle
There's a big difference between "it's possible" and "it will happen". I mean,
look at air travel for example. Supersonic air travel has been possible since
the '70s. But it doesn't happen because it's not cost effective, and the
benefit from it never outweighed the cost.

I think it's possible that we _can_ to send a human consciousness (physical
astronaut, uploaded mind, etc.) to Proxima Centauri in a thousand years. But
_will_ we? "It would be amazing", as it turns out, isn't actually a very
strong justification for doing anything.

~~~
porsupah
Doesn't happen now, because the sole craft was eventually retired. (Rather
spitefully, too, with BA refusing to cede any to Virgin)

There is, nonetheless, Skylon under development, though dreadfully
underfunded. If it actually works as expected, SABRE will be a quite
remarkable innovation. So far, it's all looking good, including the
particularly gnarly issue of cooling the incoming air in extremely little
time.

[http://en.wikipedia.org/wiki/SABRE_(rocket_engine)](http://en.wikipedia.org/wiki/SABRE_\(rocket_engine\))

~~~
jcoffland
re: SABRE, there's no air in space.

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mehwoot
Is there any calculation on what resolution would be even theoretically
possible? There are only a finite number of photons being emitted by the
surface of the earth and so at a certain distance there wouldn't be enough to
show detail past some level.

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3327
I've spent a good amount of engineering hours doing research on this thought
when I had it in the shower a few years back. My thought was a little
different in that : " perhaps we could beam a laser with encoded information
on a loop journey, that is meant to return to earth in X million years."

Such a trajectory might also depend on the position of earth and other objects
and have a very narrow windows. As if watching a pond on a windy day and the
wind eases off for a second or two yielding a glimpse of your own reflection
and then to pick up again leaving a blurry pond.

~~~
gue5t
For the digital-network analogue, see pingfs:
[http://www.shysecurity.com/posts/PingFS](http://www.shysecurity.com/posts/PingFS)

~~~
tbrownaw
[http://en.wikipedia.org/wiki/Delay_line_memory](http://en.wikipedia.org/wiki/Delay_line_memory)

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scarygliders
This reminded me of a short story I read, called Light Of Other Days, written
by Bob Shaw.

Instead of black holes or other massive objects, he used a concept called
"slow glass" \- glass in which light takes years to pass.

Obviously, though, you'd had to have panes of slow glass in front of the
scenes from life 6000 years ago...

~~~
jcoffland
"Light of Other Days" is a great book but it was written by Stephen Baxter and
Arthur C. Clarke.

~~~
jcoffland
Wait, sorry. There are two books with similar titles. "The Light of Other
Days" is by Stephen Baxter and Arthur C. Clarke. "Light of Other Days" is
indeed by Bob Shaw. I have not read the latter. The former deals with a new
technology which allows people to see in to the past via microscopic
"wormholes".

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Intermernet
>In 2002 Daniel Holz and John Wheeler investigated this idea and ran all the
calculations in their paper Retro-MACHOs: Pi in the Sky?

That's one of the best paper names I've come across.

From the introduction:

>The bending power of a black hole is not limited, however, to small angles
but reaches π and odd multiples of π. Illuminated by a powerful point source
of light, the black hole will therefore shine back with a series of concentric
rings (we call this retrolensing).

Does anyone know if this sort of research is on the agenda for the James Webb
Space Telescope? Our space imaging has progressed dramatically in the last
couple of decades, so it wouldn't surprise me to see many more MACHOs (maybe
even Retro-MACHOs!) being discovered, in the same way our understanding of the
vast number of galaxies in the universe was increased by the Hubble Space
Telescope.

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bsurmanski
Coincidentally, something like this was asked on reddit.com/r/askscience
recently.

[http://www.reddit.com/r/askscience/comments/2y93cz/if_we_wer...](http://www.reddit.com/r/askscience/comments/2y93cz/if_we_were_far_enough_from_earth_could_we_see_the/)

(if we were far enough from the earth, could we see the dinosaurs)

One comment claimed that the lens would be the limiting factor. If using
traditional glass lens, they calculated that it would need a diameter of 4
light years to be able to see the dinosaurs. They furthermore claimed that a
lens of this size would gravitationally collapse, and thus would not be
feasible.

I don't believe gravitational lensing was mentioned as of my reading.

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intrasight
I've a different idea. Rather than try to recover old photons that left earth
long ago, can recover and collate the remnants of these photons. This might be
possible if these photons left a holographic imprint on matter which is still
present – buildings, old trees, rock formations. I envision that in the future
there will be a field of "photonic archaeology" where scientists will
reconstruct past images from these trace holographic signatures. Same thing
could theoretically be done for acoustics.

~~~
davmre
_Godel Escher Bach_ has a chapter riffing on the acoustics version of this
idea:
[http://www.facstaff.bucknell.edu/jvt002/BrainMind/Readings/P...](http://www.facstaff.bucknell.edu/jvt002/BrainMind/Readings/Prelude.pdf).
Though the discussion there is about recovering sound waves that are _still
propagating_ in the air, which seems orders of magnitude more difficult than
recovering sound from impressions on solid objects (which already feels likely
impossible in most interesting cases).

~~~
adamio
Reminds me of this [http://newsoffice.mit.edu/2014/algorithm-recovers-speech-
fro...](http://newsoffice.mit.edu/2014/algorithm-recovers-speech-from-
vibrations-0804)

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jimmcslim
Assuming we could overcome the technological/scientific/physical limitations
and we did have a 'realtime' video feed from a Google Earth-like perspective
of events happening 65 years ago (picking the final example)... I wonder what
the societal impacts of that would be?

~~~
EGreg
I would first of all want to check if any religion's historical claims are
true.

That would help a lot of people with their beliefs!

~~~
Intermernet
Using the GP's figure of 65 years ago, you could check out Elron's claims.
Scientology was founded in 1952. Dianetics was going crazy for a few years
before that.

Other than that we need to find some MACHOs between ~700 and ~1800 lightyears
away in order to check the claims of the more popular religions.

But, considering the data would be coming from the scientific (and in
particular the atrophysics) community, it would be ignored by those who found
it inconvenient.

Religion and Astronomy have a very checkered past.

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dghughes
Imagine if everything in the Universe was just the light from our solar system
refracted billions of ways making it seem like there were galaxies but really
we are all alone.

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jkot
There is a lot of exiting stuff: observing Einstein cross visually, GAI
mapping mission, MACHO detection... Perhaps we could start with that...

~~~
privong
> There is a lot of exiting stuff: observing Einstein cross visually, GAI
> mapping mission, MACHO detection... Perhaps we could start with that...

I am not sure what you mean. We have had optical observations of at least one
Einsten cross since the late 80s / early 90s[0, 1]. Signs of MACHOs were
detected in 1995[2]. I don't know what a "GAI mapping mission" is, so I cannot
comment on that.

[0]
[http://adsabs.harvard.edu/abs/1993BCFHT..29...22H](http://adsabs.harvard.edu/abs/1993BCFHT..29...22H)

[1] [http://arxiv.org/abs/astro-ph/9512103](http://arxiv.org/abs/astro-
ph/9512103)

[2]
[http://www.aps.org/publications/apsnews/199507/macho.cfm](http://www.aps.org/publications/apsnews/199507/macho.cfm)

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mulligan
This article from berkeley is super relevant, it was published just a few days
ago: [https://newscenter.berkeley.edu/2015/03/05/distant-
supernova...](https://newscenter.berkeley.edu/2015/03/05/distant-supernova-
split-four-ways-by-gravitational-lens/)

We can see a supernova 4 different times using gravitational lensing.

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cake93
How would the calculations play out for near-earth MACHOs and radio
frequencies instead of visible light? I can imagine that the construction of a
suitable antenna is more feasible than for a visible-light telescope?

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chrischen
I am not a lawyer, but maybe if multiple MACHOs in the right positions could
make this work.

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nether
Would we ever be able to collect enough photons to form any kind of image
though?

