4 ms·

If the lens curved light back toward us, could we see earth several million years ago?

wizzwizz4 2y ago
Technically? But the image would be very very very small, so we'd need a detector bigger than the solar system (guesstimate) to see it. That's to see it: I can't imagine what it would take to resolve the image. The tricks in this paper are a start.
- westurner 2y ago
  To zoom into a reflection on a lens or a water droplet? From "Hear the sounds of Earth's magnetic field from 41,000 years ago" (2024) https://news.ycombinator.com/item?id=42010159 https://news.ycombinator.com/item?id=42010159 : > [ Redshift, Doppler effect, ] > to recall Earth's magnetic field from 41,000 years ago with such a method would presumably require a reflection (41,000/2 = 20,500) light years away To see Earth in a reflection, though Age of the Earth: https://en.wikipedia.org/wiki/Age_of_Earth https://en.wikipedia.org/wiki/Age_of_Earth : > 4.54 × 10^9 years ± 1% "J1721+8842: The first Einstein zig-zag lens" (2024) https://arxiv.org/abs/2411.04177v1 https://arxiv.org/abs/2411.04177v1 What is the distance to the centroid of the (possibly vortical ?) lens effect from Earth in light years? /? J1721+8842 distance from Earth in light years - https://www.iflscience.com/first-known-double-gravitational-lens-is-a-cosmic-telescope-into-the-distant-universe-76798 https://www.iflscience.com/first-known-double-gravitational-... : > The first lens is relatively close to the source, with a distance estimated at 10.2 billion light-years. What happens is that the quasar’s light is magnified and multiplied by this massive galaxy. Two of the images are deflected in the opposite direction as they reach the second lens, another massive galaxy. The path of the light is a zig-zag between the quasar, the first lens, and then the second one, which is just 2.3 billion light-years away So, given a simplistic model with no relative motion between earth and the presumed constant location lens: Earth formation: 4.54b years ago 2.3b * 2 = 4.6b years ago 10.2b * 2 = 20.4b years ago Does it matter that our models of the solar systems typically omit that the sun is traveling through the universe (with the planets swirling now coplanarly and trailing behind), and would the relative motion of a black hole at the edge of our solar system change the paths between here and a distant reflector over time? "The helical model - our solar system is a vortex" https://youtube.com/watch?v=0jHsq36_NTU https://youtube.com/watch?v=0jHsq36_NTU
DoingIsLearning 2y ago
@Dang is there a version of /best but for comments? The thought experiment in this comment broke my mind.
- slater 2y ago
  https://news.ycombinator.com/highlights https://news.ycombinator.com/highlights
Danieru 2y ago
No, because the light requires twice the time to travel there then back. If Earth did not move relative to the lens, it would work. Sadly we move, a lot, so what was here 2x ago was something not-earth. To see earth, the lensing would been to be focused on where Earth was 2x ago. Still possible in theory, and you might even argue just as likely as a fully reflecting curve. But you'd not call it "back towards us". It would need to be "curved to where earth was".
- 0xDEAFBEAD 2y ago
  Seems like if you could retrodict the position of past lenses, and predict their effects, perhaps it would somehow be possible to send a spacecraft to a specific location in order to observe Earth's past. The idea being that a spacecraft traveling at 99% of light speed can't ordinarily catch up with light reflected by Earth. But if the light curves, and the spacecraft can travel directly towards where the light will end up (spacecraft traveling "as the crow flies"), it might be possible to catch up. Same way I might be able to catch up with Usain Bolt at a track event if he's forced to run on the track, and I'm allowed to run across the turf in the middle.
  - w0de0 2y ago
    https://en.wikipedia.org/wiki/Closed_timelike_curve https://en.wikipedia.org/wiki/Closed_timelike_curve
- deskamess 2y ago
  Would this be the case even if you were moving toward or along-side the 'reflector' (black hole/other body)? For the sake of discussion assume we are at or beyond the focal point.
dtgriscom 2y ago
I believe this could only happen around a black hole. In that case, yes: light that we emitted umpty-million years ago could be shot back at us. The problem is that there would be no focussing. At best it would be like looking at an Earth 2 x umpty-million light years away. I'm guessing that it would actually be worse, with the black hole dispersing light. (IANAastronomer, but I have opinions on any given topic...)

Technically? But the image would be very very very small, so we'd need a detector bigger than the solar system (guesstimate) to see it. That's to see it: I can't imagine what it would take to resolve the image. The tricks in this paper are a start.

westurner 2y ago
To zoom into a reflection on a lens or a water droplet? From "Hear the sounds of Earth's magnetic field from 41,000 years ago" (2024) https://news.ycombinator.com/item?id=42010159 https://news.ycombinator.com/item?id=42010159 : > [ Redshift, Doppler effect, ] > to recall Earth's magnetic field from 41,000 years ago with such a method would presumably require a reflection (41,000/2 = 20,500) light years away To see Earth in a reflection, though Age of the Earth: https://en.wikipedia.org/wiki/Age_of_Earth https://en.wikipedia.org/wiki/Age_of_Earth : > 4.54 × 10^9 years ± 1% "J1721+8842: The first Einstein zig-zag lens" (2024) https://arxiv.org/abs/2411.04177v1 https://arxiv.org/abs/2411.04177v1 What is the distance to the centroid of the (possibly vortical ?) lens effect from Earth in light years? /? J1721+8842 distance from Earth in light years - https://www.iflscience.com/first-known-double-gravitational-lens-is-a-cosmic-telescope-into-the-distant-universe-76798 https://www.iflscience.com/first-known-double-gravitational-... : > The first lens is relatively close to the source, with a distance estimated at 10.2 billion light-years. What happens is that the quasar’s light is magnified and multiplied by this massive galaxy. Two of the images are deflected in the opposite direction as they reach the second lens, another massive galaxy. The path of the light is a zig-zag between the quasar, the first lens, and then the second one, which is just 2.3 billion light-years away So, given a simplistic model with no relative motion between earth and the presumed constant location lens: Earth formation: 4.54b years ago 2.3b * 2 = 4.6b years ago 10.2b * 2 = 20.4b years ago Does it matter that our models of the solar systems typically omit that the sun is traveling through the universe (with the planets swirling now coplanarly and trailing behind), and would the relative motion of a black hole at the edge of our solar system change the paths between here and a distant reflector over time? "The helical model - our solar system is a vortex" https://youtube.com/watch?v=0jHsq36_NTU https://youtube.com/watch?v=0jHsq36_NTU

DoingIsLearning 2y ago

@Dang is there a version of /best but for comments? The thought experiment in this comment broke my mind.

slater 2y ago
https://news.ycombinator.com/highlights https://news.ycombinator.com/highlights

Danieru 2y ago

No, because the light requires twice the time to travel there then back. If Earth did not move relative to the lens, it would work. Sadly we move, a lot, so what was here 2x ago was something not-earth. To see earth, the lensing would been to be focused on where Earth was 2x ago. Still possible in theory, and you might even argue just as likely as a fully reflecting curve. But you'd not call it "back towards us". It would need to be "curved to where earth was".

0xDEAFBEAD 2y ago
Seems like if you could retrodict the position of past lenses, and predict their effects, perhaps it would somehow be possible to send a spacecraft to a specific location in order to observe Earth's past. The idea being that a spacecraft traveling at 99% of light speed can't ordinarily catch up with light reflected by Earth. But if the light curves, and the spacecraft can travel directly towards where the light will end up (spacecraft traveling "as the crow flies"), it might be possible to catch up. Same way I might be able to catch up with Usain Bolt at a track event if he's forced to run on the track, and I'm allowed to run across the turf in the middle.
- w0de0 2y ago
  https://en.wikipedia.org/wiki/Closed_timelike_curve https://en.wikipedia.org/wiki/Closed_timelike_curve
deskamess 2y ago
Would this be the case even if you were moving toward or along-side the 'reflector' (black hole/other body)? For the sake of discussion assume we are at or beyond the focal point.

dtgriscom 2y ago

I believe this could only happen around a black hole. In that case, yes: light that we emitted umpty-million years ago could be shot back at us. The problem is that there would be no focussing. At best it would be like looking at an Earth 2 x umpty-million light years away. I'm guessing that it would actually be worse, with the black hole dispersing light. (IANAastronomer, but I have opinions on any given topic...)