8 ms·

Gravitational Lensing to Observe Ancient Earth

140 points by pkrein 14y ago

zafka 14y ago
This is sweet. I can imagine this being woven into some great stories.
grecy 14y ago
Ha, that was the plot line of Paycheck [1], but seeing the future, not the past. [1]http://www.imdb.com/title/tt0338337/ http://www.imdb.com/title/tt0338337/
- DanBC 14y ago
  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 http://www.gregegan.net/BIBLIOGRAPHY/Bibliography.html)
guylhem 14y ago
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 14y ago
  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.
- pkrein 14y ago
  the photons "reflected" back would appear at specific locations around the edge of the MACHO, so you'd see expect to see something with Sun-like spectra at that radial location. lots of image processing would be required to un-warp the collected image.
fsiefken 14y ago
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 http://en.wikipedia.org/wiki/The_Light_of_Other_Days
- geuis 14y ago
  Good book. I still have the paperback on my shelf.
- shill 14y ago
  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.
- salgernon 14y ago
  This book has come up several times in several recent (unrelated) hacker news threads. I find the prescience of the privacy concerns (and how people react to never ever having privacy again) relevant to today's world of drones and oversharing via social media. And of course it's just got that good Clark science tone.
- comex 14y ago
  This is really off topic, but once - I think I was 13 or 14 - I found a book just lying around somewhere at what passed for my summer camp. I read the whole thing; I guess I must have tried to return it, but I don't remember that part. The book was interesting enough that I wanted to read it again at some point, but I had long since forgotten its name. This is that book. I guess I could have found the name of the book by actively asking around, but still - I love the coincidence. Thank you.
aphyr 14y ago
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 14y ago
  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 http://en.wikipedia.org/wiki/Aperture_synthesis
  - mikemoka 14y ago
    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 14y ago
      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.
- privong 14y ago
  To be technical, you could still see an object that is smaller to than your resolving power (this happens essentially every time we observe individual stars). The issue is what is called the "confusion limit" - the point at which you can't be confident that what appears to be a source is not just the summed emission from a bunch of fainter background objects. But you are right that increased resolution is key. The higher resolution helps you beat the confusion limit by working to resolve the faint background sources into individual objects, so you can separate them. So, if you wanted to be certain that what you were seeing was the refracted light from Earth, you would need the high resolution to be sure that what you were seeing was actually that reflection, and not just a superposition of fainter background sources. More on the confusion limit: http://coolwiki.ipac.caltech.edu/index.php/Confusion http://coolwiki.ipac.caltech.edu/index.php/Confusion
clavalle 14y ago
We'd be moving relative to a black hole so the 180 degrees assumption might be off.
DeepDuh 14y ago
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 14y ago
  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 14y ago
    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 14y ago
      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".
  - privong 14y ago
    Technically Sgr A* is a component of the Sgr A region. There is also Sgr A East and Sgr A West, in addition to Sgr A*.
- wasnotwas 14y ago
  As I have understood, "Black Hole" is something of a misnomer, as the singularity is in fact an intensely compact ball of matter/energy radiating x rays, etc. Matter releases energy as light in various wavelengths, including x, gamma, etc, but once inside the heart of the "hole" it is unable to emit ANYTHING. The net effect, however, is a surrounding veil of electromagnetic spectral emissions. Matter/energy can't escape once inside, but on the way in it puts on a hell of a show.
ISL 14y ago
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_vs_angular_resolution.svg http://en.wikipedia.org/wiki/File:Diffraction_limit_diameter...
- andrewflnr 14y ago
  I don't know if it's harder to get the light to where earth is going to be. There's still exactly one angle that will work, and we would have to trust to luck either way to send us the right photons. It's not like anyone is aiming.
- guelo 14y ago
  Actually it makes the problem easier because the angle is less than 180 degrees which means the lensing happens further away from the blackhole with less noise. By my quick googling and back o' the napkin calculations using MACHO-96-BLG-5, we, the Sun and the Milky Way, have traveled about 24 light years in 6,000 years giving an angle of 179.5 degrees! OK, that didn't change much.
- andyjohnson0 14y ago
  "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 14y ago
    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.
hobbyist 14y ago
Read only access!! ROFL
oofabz 14y ago
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.jpg%3Fw%3D720%26h%3D418 http://historyofourworld.files.wordpress.com/2009/11/lw-iii.... http://www.krobarch.com/images/winners/2012/entry3353.jpg http://www.krobarch.com/images/winners/2012/entry3353.jpg
- ianstormtaylor 14y ago
  And some interesting reads on his blog: http://lebbeuswoods.wordpress.com/2012/02/17/zahas-aquatic-center/ http://lebbeuswoods.wordpress.com/2012/02/17/zahas-aquatic-c...
malbs 14y ago
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!
johnpmayer 14y ago
Pictures seems far-fetched, but what about a radio signal of an old broadcast?
dantillberg 14y ago
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.
andr3w321 14y ago
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.
- ebildsten 14y ago
  The data is not the same - they vary greatly on spatial resolution and latency. Better than 10cm resolution with effectively 0s latency (in the drone case), and ~5,000-12,000km resolution (assuming you can get back only a couple of pixels from a MACHO) with yrs of latency are two very very different things.
- potatolicious 14y ago
  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.
  - teamonkey 14y ago
    Azimov explored this idea in his short story The Dead Past http://en.wikipedia.org/wiki/The_Dead_Past http://en.wikipedia.org/wiki/The_Dead_Past
  - ComputerGuru 14y ago
    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.
yakiv 14y ago
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 14y ago
  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-gravitational-lensing-account-for-einsteins-cross http://physics.stackexchange.com/questions/14056/how-does-gr...
spullara 14y ago
I was always hoping there was just a big mirror out there that we could look at and see the past.
hakaaaaak 14y ago
Posting obligitory link to: http://en.wikipedia.org/wiki/Time_viewer http://en.wikipedia.org/wiki/Time_viewer Of course someone should mention Father François Brune and his Chronovisor: http://en.wikipedia.org/wiki/Chronovisor http://en.wikipedia.org/wiki/Chronovisor But none of that is real.
ars 14y ago
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.
ck2 14y ago
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 14y ago
  > 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/AAAAAAAABLw/SRpE1WvNqC4/s1600/500px-World_line.svg.png http://4.bp.blogspot.com/--VBwEngltGc/Tzniy1imbII/AAAAAAAABL...
  - 98709870987 14y ago
    >>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 14y ago
      > 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 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]
andyjohnson0 14y ago
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 http://pastebin.com/f341e113e [2] http://www.amazon.co.uk/Axiomatic-Greg-Egan/dp/0575081740 http://www.amazon.co.uk/Axiomatic-Greg-Egan/dp/0575081740

140 points by pkrein 14y ago

zafka 14y ago

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

grecy 14y ago

Ha, that was the plot line of Paycheck [1], but seeing the future, not the past. [1]http://www.imdb.com/title/tt0338337/ http://www.imdb.com/title/tt0338337/

DanBC 14y ago
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 http://www.gregegan.net/BIBLIOGRAPHY/Bibliography.html)

guylhem 14y ago

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 14y ago
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.
pkrein 14y ago
the photons "reflected" back would appear at specific locations around the edge of the MACHO, so you'd see expect to see something with Sun-like spectra at that radial location. lots of image processing would be required to un-warp the collected image.

fsiefken 14y ago

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

geuis 14y ago
Good book. I still have the paperback on my shelf.
shill 14y ago
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.
salgernon 14y ago
This book has come up several times in several recent (unrelated) hacker news threads. I find the prescience of the privacy concerns (and how people react to never ever having privacy again) relevant to today's world of drones and oversharing via social media. And of course it's just got that good Clark science tone.
comex 14y ago
This is really off topic, but once - I think I was 13 or 14 - I found a book just lying around somewhere at what passed for my summer camp. I read the whole thing; I guess I must have tried to return it, but I don't remember that part. The book was interesting enough that I wanted to read it again at some point, but I had long since forgotten its name. This is that book. I guess I could have found the name of the book by actively asking around, but still - I love the coincidence. Thank you.

aphyr 14y ago

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 14y ago
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 http://en.wikipedia.org/wiki/Aperture_synthesis
- mikemoka 14y ago
  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 14y ago
    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.
privong 14y ago
To be technical, you could still see an object that is smaller to than your resolving power (this happens essentially every time we observe individual stars). The issue is what is called the "confusion limit" - the point at which you can't be confident that what appears to be a source is not just the summed emission from a bunch of fainter background objects. But you are right that increased resolution is key. The higher resolution helps you beat the confusion limit by working to resolve the faint background sources into individual objects, so you can separate them. So, if you wanted to be certain that what you were seeing was the refracted light from Earth, you would need the high resolution to be sure that what you were seeing was actually that reflection, and not just a superposition of fainter background sources. More on the confusion limit: http://coolwiki.ipac.caltech.edu/index.php/Confusion http://coolwiki.ipac.caltech.edu/index.php/Confusion

clavalle 14y ago

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

DeepDuh 14y ago

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 14y ago
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 14y ago
  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 14y ago
    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".
- privong 14y ago
  Technically Sgr A* is a component of the Sgr A region. There is also Sgr A East and Sgr A West, in addition to Sgr A*.
wasnotwas 14y ago
As I have understood, "Black Hole" is something of a misnomer, as the singularity is in fact an intensely compact ball of matter/energy radiating x rays, etc. Matter releases energy as light in various wavelengths, including x, gamma, etc, but once inside the heart of the "hole" it is unable to emit ANYTHING. The net effect, however, is a surrounding veil of electromagnetic spectral emissions. Matter/energy can't escape once inside, but on the way in it puts on a hell of a show.

ISL 14y ago

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_vs_angular_resolution.svg http://en.wikipedia.org/wiki/File:Diffraction_limit_diameter...

andrewflnr 14y ago
I don't know if it's harder to get the light to where earth is going to be. There's still exactly one angle that will work, and we would have to trust to luck either way to send us the right photons. It's not like anyone is aiming.
guelo 14y ago
Actually it makes the problem easier because the angle is less than 180 degrees which means the lensing happens further away from the blackhole with less noise. By my quick googling and back o' the napkin calculations using MACHO-96-BLG-5, we, the Sun and the Milky Way, have traveled about 24 light years in 6,000 years giving an angle of 179.5 degrees! OK, that didn't change much.
andyjohnson0 14y ago
"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 14y ago
  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.

hobbyist 14y ago

Read only access!! ROFL

oofabz 14y ago

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.jpg%3Fw%3D720%26h%3D418 http://historyofourworld.files.wordpress.com/2009/11/lw-iii.... http://www.krobarch.com/images/winners/2012/entry3353.jpg http://www.krobarch.com/images/winners/2012/entry3353.jpg

ianstormtaylor 14y ago
And some interesting reads on his blog: http://lebbeuswoods.wordpress.com/2012/02/17/zahas-aquatic-center/ http://lebbeuswoods.wordpress.com/2012/02/17/zahas-aquatic-c...

malbs 14y ago

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!

johnpmayer 14y ago

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

dantillberg 14y ago

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.

andr3w321 14y ago

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.

ebildsten 14y ago
The data is not the same - they vary greatly on spatial resolution and latency. Better than 10cm resolution with effectively 0s latency (in the drone case), and ~5,000-12,000km resolution (assuming you can get back only a couple of pixels from a MACHO) with yrs of latency are two very very different things.
potatolicious 14y ago
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.
- teamonkey 14y ago
  Azimov explored this idea in his short story The Dead Past http://en.wikipedia.org/wiki/The_Dead_Past http://en.wikipedia.org/wiki/The_Dead_Past
- ComputerGuru 14y ago
  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.

yakiv 14y ago

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 14y ago
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-gravitational-lensing-account-for-einsteins-cross http://physics.stackexchange.com/questions/14056/how-does-gr...

spullara 14y ago

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

hakaaaaak 14y ago

Posting obligitory link to: http://en.wikipedia.org/wiki/Time_viewer http://en.wikipedia.org/wiki/Time_viewer Of course someone should mention Father François Brune and his Chronovisor: http://en.wikipedia.org/wiki/Chronovisor http://en.wikipedia.org/wiki/Chronovisor But none of that is real.

ars 14y ago

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.

ck2 14y ago

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 14y ago
> 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/AAAAAAAABLw/SRpE1WvNqC4/s1600/500px-World_line.svg.png http://4.bp.blogspot.com/--VBwEngltGc/Tzniy1imbII/AAAAAAAABL...
- 98709870987 14y ago
  >>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 14y ago
    > 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 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]

andyjohnson0 14y ago

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