5 ms·

Given that we're able to aim telescopes at things that are billions of light years away, I wonder how close we are to being able to see the reflection of such a

Given that we're able to aim telescopes at things that are billions of light years away, I wonder how close we are to being able to see the reflection of such a thing when it actually happens on some celestial body 3.5 million light years away.

raattgift 7y ago
You might be interested in the examples section of https://en.wikipedia.org/wiki/Light_echo https://en.wikipedia.org/wiki/Light_echo
ljf 7y ago
I remember as a child thinking I had fixed time travel (in a very limited sense) when I wondered why we didn't just stick a bunch of mirrors far far away, and then aim them back at the earth and use telescopes to look back at the dinosaurs etc. While that wouldn't work for heaps of reasons, I like your idea. I guess the issue would be discerning 'when' such a thing happened, and finding the right item to look back at, which would be a difficult challenge...
- smudgymcscmudge 7y ago
  I had the same idea. I forgot about it before I was old enough to reason about it. At the time I thought the size of the mirrors would be the biggest issue. I guess I thought faster-than-light travel was easy.
- indigo747 7y ago
  I have a distant childhood memory of a book based on this concept, in which the protagonist is projected back in time through light beams bounced off of distant mirrors. I think I stopped reading because I could tell the science was completely impossible, and ultimately just an excuse to write an historical fiction book which I didn't feel like reading. Perhaps I judged it too harshly?
- jandrese 7y ago
  Ignoring the problems with resolution, the big issue with this is you would need FTL travel to position the mirrors ahead of the light. And this in turn is one of the big reasons why FTL is probably impossible: It is in essence time travel.
- jerf 7y ago
  You need natural mirrors, because as others have pointed out, they have to already be there, you can't get ahead of the light to put them there. It has been suggested that black holes could provide this mirror, but it requires resolving not just something the size of Earth, but something much smaller, at a distance of 30 million light years +. Unfortunately you actually start hitting up against the limits of spacetime's resolution itself fairly quickly. But at least in principle, it is possible that some lucky photon has bounced off a dinosaur, and has traveled all the way out there, and all the way back to Earth, just now, that if you knew where to look in the sky, perhaps such a photon might even enter your eye now. Or perhaps there is some human out there who really has "seen" the dinosaurs. But you'd never really know.
  - klank 7y ago
    What are these "limits of spacetime's resolution" you speak of? Beyond speculation, I'm not aware of anything showing spacetime to be discrete.
    - fmihaila 7y ago
      > What are these "limits of spacetime's resolution" you speak of? > Beyond speculation, I'm not aware of anything showing spacetime to be discrete. It is a prediction of loop quantum gravity [1]: In 1988, Carlo Rovelli, Lee Smolin, and Abhay Ashtekar introduced a theory of quantum gravity called loop quantum gravity. In 1995, Rovelli and Smolin obtained a basis of states of quantum gravity, labelled by Penrose's spin networks, and using this basis they were able to show that the theory predicts that area and volume are quantized. This result indicates the existence of a discrete structure of space at very small scale. [1] https://en.wikipedia.org/wiki/Carlo_Rovelli#Loop_quantum_gravity https://en.wikipedia.org/wiki/Carlo_Rovelli#Loop_quantum_gra...
      - jerf 7y ago
        It's a prediction of other things, too. Even just sheer every-day quantum mechanics sets a limit, too; eventually you're asking to nail down a photon's origin with so much precision that you lose the frequency or something, things you don't normally think about even in the most extreme astronomy today, but become relevant when discussing bouncing photons around a black hole 30 million light years away.
- jakeogh 7y ago
  :) https://news.ycombinator.com/item?id=9161967 https://news.ycombinator.com/item?id=9161967
iamgopal 7y ago
Bit turnoff is, we couldn't even take photograph of human size on moon, things far away are mind blowing huge. Every data is blurred out.
mirimir 7y ago
We already have: Clavel et al. (2013) Echoes of multiple outbursts of Sagittarius A[star] revealed by Chandra.[0] Here's the abstract: > The relatively rapid spatial and temporal variability of the X-ray radiation from some molecular clouds near the Galactic center shows that this emission component is due to the reflection of X-rays generated by a source that was luminous in the past, most likely the central supermassive black hole, Sagittarius A[star]. Studying the evolution of the molecular cloud reflection features is therefore a key element to reconstruct Sgr A[star]'s past activity. The aim of the present work is to study this emission on small angular scales in order to characterize the source outburst on short time scales. We use Chandra high-resolution data collected from 1999 to 2011 to study the most rapid variations detected so far, those of clouds between 5' and 20' from Sgr A[star] towards positive longitudes. Our systematic spectral-imaging analysis of the reflection emission, notably of the Fe Kalpha line at 6.4 keV and its associated 4-8 keV continuum, allows us to characterize the variations down to 15" angular scale and 1-year time scale. We reveal for the first time abrupt variations of few years only and in particular a short peaked emission, with a factor of 10 increase followed by a comparable decrease, that propagates along the dense filaments of the Bridge cloud. This 2-year peaked feature contrasts with the slower 10-year linear variations we reveal in all the other molecular structures of the region. Based on column density constraints, we argue that these two different behaviors are unlikely to be due to the same illuminating event. The variations are likely due to a highly variable active phase of Sgr A[star] sometime within the past few hundred years, characterized by at least two luminous outbursts of a few-year time scale and during which the Sgr A[star] luminosity went up to at least 10^39 erg/s. 0) https://arxiv.org/abs/1307.3954 https://arxiv.org/abs/1307.3954 Edit: Older image: https://www.chandra.harvard.edu/photo/2007/gcle/ https://www.chandra.harvard.edu/photo/2007/gcle/
- FrameworkFred 7y ago
  WOWWW...that's super cool. If you could find multiple reflections of the same source [edit: after a second look, that's actually what they did which is even more amazing], you could start piecing together a sequence. But, darn it, now we're going to have to map the cosmic background light pollution. :)
  - 7y ago

raattgift 7y ago

You might be interested in the examples section of https://en.wikipedia.org/wiki/Light_echo https://en.wikipedia.org/wiki/Light_echo

ljf 7y ago

I remember as a child thinking I had fixed time travel (in a very limited sense) when I wondered why we didn't just stick a bunch of mirrors far far away, and then aim them back at the earth and use telescopes to look back at the dinosaurs etc. While that wouldn't work for heaps of reasons, I like your idea. I guess the issue would be discerning 'when' such a thing happened, and finding the right item to look back at, which would be a difficult challenge...

smudgymcscmudge 7y ago
I had the same idea. I forgot about it before I was old enough to reason about it. At the time I thought the size of the mirrors would be the biggest issue. I guess I thought faster-than-light travel was easy.
indigo747 7y ago
I have a distant childhood memory of a book based on this concept, in which the protagonist is projected back in time through light beams bounced off of distant mirrors. I think I stopped reading because I could tell the science was completely impossible, and ultimately just an excuse to write an historical fiction book which I didn't feel like reading. Perhaps I judged it too harshly?
jandrese 7y ago
Ignoring the problems with resolution, the big issue with this is you would need FTL travel to position the mirrors ahead of the light. And this in turn is one of the big reasons why FTL is probably impossible: It is in essence time travel.
jerf 7y ago
You need natural mirrors, because as others have pointed out, they have to already be there, you can't get ahead of the light to put them there. It has been suggested that black holes could provide this mirror, but it requires resolving not just something the size of Earth, but something much smaller, at a distance of 30 million light years +. Unfortunately you actually start hitting up against the limits of spacetime's resolution itself fairly quickly. But at least in principle, it is possible that some lucky photon has bounced off a dinosaur, and has traveled all the way out there, and all the way back to Earth, just now, that if you knew where to look in the sky, perhaps such a photon might even enter your eye now. Or perhaps there is some human out there who really has "seen" the dinosaurs. But you'd never really know.
- klank 7y ago
  What are these "limits of spacetime's resolution" you speak of? Beyond speculation, I'm not aware of anything showing spacetime to be discrete.
  - fmihaila 7y ago
    > What are these "limits of spacetime's resolution" you speak of? > Beyond speculation, I'm not aware of anything showing spacetime to be discrete. It is a prediction of loop quantum gravity [1]: In 1988, Carlo Rovelli, Lee Smolin, and Abhay Ashtekar introduced a theory of quantum gravity called loop quantum gravity. In 1995, Rovelli and Smolin obtained a basis of states of quantum gravity, labelled by Penrose's spin networks, and using this basis they were able to show that the theory predicts that area and volume are quantized. This result indicates the existence of a discrete structure of space at very small scale. [1] https://en.wikipedia.org/wiki/Carlo_Rovelli#Loop_quantum_gravity https://en.wikipedia.org/wiki/Carlo_Rovelli#Loop_quantum_gra...
    - jerf 7y ago
      It's a prediction of other things, too. Even just sheer every-day quantum mechanics sets a limit, too; eventually you're asking to nail down a photon's origin with so much precision that you lose the frequency or something, things you don't normally think about even in the most extreme astronomy today, but become relevant when discussing bouncing photons around a black hole 30 million light years away.
jakeogh 7y ago
:) https://news.ycombinator.com/item?id=9161967 https://news.ycombinator.com/item?id=9161967

iamgopal 7y ago

Bit turnoff is, we couldn't even take photograph of human size on moon, things far away are mind blowing huge. Every data is blurred out.

mirimir 7y ago

We already have: Clavel et al. (2013) Echoes of multiple outbursts of Sagittarius A[star] revealed by Chandra.[0] Here's the abstract: > The relatively rapid spatial and temporal variability of the X-ray radiation from some molecular clouds near the Galactic center shows that this emission component is due to the reflection of X-rays generated by a source that was luminous in the past, most likely the central supermassive black hole, Sagittarius A[star]. Studying the evolution of the molecular cloud reflection features is therefore a key element to reconstruct Sgr A[star]'s past activity. The aim of the present work is to study this emission on small angular scales in order to characterize the source outburst on short time scales. We use Chandra high-resolution data collected from 1999 to 2011 to study the most rapid variations detected so far, those of clouds between 5' and 20' from Sgr A[star] towards positive longitudes. Our systematic spectral-imaging analysis of the reflection emission, notably of the Fe Kalpha line at 6.4 keV and its associated 4-8 keV continuum, allows us to characterize the variations down to 15" angular scale and 1-year time scale. We reveal for the first time abrupt variations of few years only and in particular a short peaked emission, with a factor of 10 increase followed by a comparable decrease, that propagates along the dense filaments of the Bridge cloud. This 2-year peaked feature contrasts with the slower 10-year linear variations we reveal in all the other molecular structures of the region. Based on column density constraints, we argue that these two different behaviors are unlikely to be due to the same illuminating event. The variations are likely due to a highly variable active phase of Sgr A[star] sometime within the past few hundred years, characterized by at least two luminous outbursts of a few-year time scale and during which the Sgr A[star] luminosity went up to at least 10^39 erg/s. 0) https://arxiv.org/abs/1307.3954 https://arxiv.org/abs/1307.3954 Edit: Older image: https://www.chandra.harvard.edu/photo/2007/gcle/ https://www.chandra.harvard.edu/photo/2007/gcle/

FrameworkFred 7y ago
WOWWW...that's super cool. If you could find multiple reflections of the same source [edit: after a second look, that's actually what they did which is even more amazing], you could start piecing together a sequence. But, darn it, now we're going to have to map the cosmic background light pollution. :)
- 7y ago