Gravitational Lensing to Observe Ancient Earth | Hacker News Reader

Gravitational Lensing to Observe Ancient Earth

rein.pk

137 pointspinko45 comments

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.

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.

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

> 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).

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...

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.)

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.

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.

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

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

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. :)

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.

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)

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

> Small amounts of time travel happen all the time.

What are you referring to here?

Time is relative.

Yes, there are many ways in which cutting edge science seems indistinguishable from limitless magic. As a scientist though, this perspective requires ignoring that nearly all my time requires attempts to overcome physical limitations. Sure, these things may be possible, but they require overcoming many currently impossible limits.

As long it is logically possible, we can make it happen.

But you won't violate the laws of thermodynamic no matter how hard you try.

Never say never. There's speculation of a multi-verse, and in some universes, nature's laws are random and different from this one. There could be a universe where the laws of thermodynamics is different, and there may be a way to tap into that universe.

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.

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.

For the digital-network analogue, see pingfs: http://www.shysecurity.com/posts/PingFS

http://en.wikipedia.org/wiki/Delay_line_memory

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...

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

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".

This was one of the first works I read that changed my perspective to focus on the human elements in science fiction as opposed to the bells and whistles.

>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.

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

http://www.reddit.com/r/askscience/comments/2y93cz/if_we_wer...

(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.

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.

Godel Escher Bach has a chapter riffing on the acoustics version of this idea: http://www.facstaff.bucknell.edu/jvt002/BrainMind/Readings/P.... 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).

Reminds me of this http://newsoffice.mit.edu/2014/algorithm-recovers-speech-fro...

> This might be possible if these photons left a holographic imprint on matter which is still present

How would that happen? Holographic imprints aren't left on just any old material; you need special film to make a hologram.

There was an X-Files episode with this idea. Something Jesus said ("Talitha cumi") was recorded in a piece of pottery.

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?

For one thing we might be able to measure the polar ice caps -- I don't know how far back our current data go, but I don't think they go back to 1950. Maybe spectronomy (?) could tell us something about the proportion of greenhouse gases.

If the telescopes had adequate resolution, we might be able to measure the size of cities over time. Images from 1950 forward might give us data on the progress of city growth and electrification by observing the spread of night-time illumination.

It's fantastic to imagine similar data going back centuries or millenia. It would be remarkable to chart the growth of Rome, London, Delhi, Beijing over time, or perhaps gather demographic data on the spread of the "barbarians". Or track the agricultural and demographic history of Egypt by watching the spread of cultivated land around the Nile delta. Or gather data on the history of trade routes like the Silk Road by observing the growth of towns along them. We might track the history of the Black Plague by observing the shrinkage of cultivated land, and the pause in the growth of urban centers.

We might learn a lot of climate science by gathering data on the explosion of Krakatoa and the subsequent effects on the polar ice caps.

We might gather data on the emergence and spread of agriculture from pre-history by observing transformation of forest land into farmland, and the appearance of greenery in places supportable only by irrigation. You might tell the story of the Tigris and the Euphrates that way.

Of course, all of that depends on spectacular levels of resolution. And those would have to grow as we looked back further in time, because the lensed light would appear that much further away. I suppose most of what I've listed here could easily be outside any plausible resolution. I guess I imagine that some of climate science stuff might be dreamt of.

But what a thing to think of!

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!

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.

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.

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

> 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

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

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

This article from berkeley is super relevant, it was published just a few days ago: https://newscenter.berkeley.edu/2015/03/05/distant-supernova...

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

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?

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

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

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