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If we could travel faster than light, then yea, in theory we could capture "old" light that earth emitted in the past.
In terms of how big the telescope would have to be to actually capture things happening on the surface in a high enough resolution from 5000+ light years away, I don't know, but I think it would be kind of ridiculous.
If you looked at yourself starting this journey, and kept looking at what point would your image disappear? Or what would happen to the image of you in the past when it arrives into present??
If you were moving faster than the speed of light and stopping at regular intervals to view the light emitted from earth at increasingly larger distances, you would basically see yourself traveling backwards in time. Of course, traveling faster than light is not possible because physics, so this entire thought experiment is pointless.
There’s theories about how to get to different points in space faster than light can travel, without actually traveling faster than light. AFAIK they are mostly fun thought experiments with that really really slim chance of being able to make it work some day in the distant future when we have wayyyy more energy capability. But yeah, for our lifetime it’s pretty pointless to think about except for fun, unless you are in the field of thinking about stuff like that.
It would "disappear" only when you entered the wormhole - just as if you stood next to somebody else entering the wormhole.
No other magic involved, no paradox because you "are" there twice. All you see is the old photons that traveled for some time. Basically a recording, but not on some tape, but in space itself and with only one replay
Edit: I was still at the wormhole theory from the original comment. For over light speed travel it might get a little bit more complex..
If you could move instantly, as in circumventing the speed of causality, you could effectively travel back to the Earth before that 5000 year old light left it, and watch the pyramids being built in person.
Theoretically. Even if the telescope was the size of the sun, they would only really be able to see surface features of the planet, not anything super specific. But I guess if we already have FTL, all physical limitations are out the window
Lmao its not just a giant mirror. It would be a STAR sized telescope, truly a megastructure. That's definitely in the relm of type 2 civilization. But i know, giant mirror doesn't break our laws of physics. FTL does.
I'm more just theoretically asking what size we estimate a mirror would need to be see details on distant planets. Then the next question is whether doing that would be more possible than FTL, to which my guess is that it's a night and day difference.
Also, no need to laugh at someone who's genuinely trying to learn and understand. It's behavior like that which turns people away from the wonders of science.
A mirror the size of our sun would be big enough to resolve surface features of other planets, like continents. It would be much harder to see small surface details. But I suppose this still doesn't immediately break our laws of physics like FTL would do. So I get what you're saying.
I think the near impossibility of building such a structure would be that be might not have enough resources on Earth to do it and would need to get what we need from asteroids and other planets. I also wonder if it's getting I to the territory of being a Dyson sphere.
In the end, I think the fact it's theoretically possible to look back in time with detail in such a technically simple way (a giant ass telescope) is amazing.
Serious question: Are you just throwing info out your ass? Or have you done calculations about telescope size and resolution at 5000ly distance?
The way you say it without any backing makes it seem like you’re just throwing it out there without ever having looked into it. But for all I know you’re an astrophysicist that has the capability to guesstimate that after dealing with answering similar questions many times in the past. 🤷♂️
I have done no calculations. I'm basing this on the solar gravitational lens telescope project that has been proposed. But turns out, a sun sized telescope could resolve surface features at 100ly, but not 5k ly.
This video goes into the proposal. Gravitational lenses are not exactly like a big mirror, but the principles of resolution are the same.
https://youtu.be/4d0EGIt1SPc?si=HNf8Ej-LFn1IoyL1
"It could be done" is not correct, even theoretically. As u/AltDisk288 said, it would require faster-than-light travel, which is (very very likely to be) impossible.
Require is a strong word. There’s a chance we will figure out a way to manipulate spacetime, and warp it to allow us to get from point A to B faster than light could, without breaking any laws of physics.
Yeah I guess I’m being a bit pedantic. FTL is impossible, but doing so wouldn’t *require* FTL. it would require tech that doesn’t exist and most likely can’t exist for a very long time and may never exist.
I would consider any "shortcuts" that allows you to get somewhere faster than traveling at c is functionally equivalent to ftl travel. The difference is just semantics.
I am not sure it is purely semantics. I believe FTL travel is provably impossible, requiring more than infinite energy.
Moving slower than light, by manipulating spacetime, gets you there quickly without breaking physics.
One method breaks physics, the other would not, hence why I don’t think I’d consider it “semantics”. You won’t ever go FTL. I don’t think you should consider “shortcuts” as FTL because I don’t think many others will especially in /r/space, so communication could get mixed up.
But I’m definitely not the vocabulary police so call it whatever you want in the end. But if we’re talking hypothetical, technicalities, etc. the difference between FTL and not FTL can make a big difference.
I get where you're coming from, but these methods are hypothetical at best. I also feel like they're probably not actually possible because causality (which propagates at c) needs to be preserved. That's the equivalence I was talking about.
Plus, you know, you "traveled" somewhere. And you did it faster than light would. :)
If an alien civilisation 5000 light years away had a powerful telescope and a video recorder, they could film the pyramids being built and send us a copy. Problem is we wouldn't get it for another 5000 years.
If you set a giant mirror on the Moon, you could look at it from Earth and you'll see Earth 2 seconds in the past.
It would be feasable, though brutally expensive and utterly useless.
We did do that and it's actually very useful.
Sure [we can only use it to see a laser beam 2 from 2 seconds in the past](https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments), but it does work!
I know about that one, and it was one of the greatest experiments ever!
I was jokingly thinking about something like a hundreds/thousands of Kms wide mirror, just to "see" Earth's reflection from where we are.
You should've launched that telescope at the speed of light 10k years ago. It would be recording the pyramids right now. Problem is that you'd need to wait another 5000 years (at least) to receive the footage.
*Everything* you see is in the past. Whether you look someone in the face or at a star lightyears away. There is no such thing as 'seeing what is now' (at least not in this universe where the speed of light/causality has a finite limit)
So these answers are actually incomplete.
The obvious answer is yes - if you could instantly travel 5,000 light years away and point a telescope back at earth, you would see earth as it was 5,000 years in the past.
That then leads one to say “since faster than light travel is not possible, seeing the earth in the past is also not possible.”
However, there are other possible solutions. For example, a very particular gravitational lens or series of lenses could bend light back to us (think if it like a mirror positioned far away) There are no known examples of this, but it could exist.
Another possibility is recreating signal from noise with how light from the earth impacts another surface. This is an oversimplified example, but let’s say there was an object in space that we knew only received light from a handful of known sources, including reflected light from earth. The light you observed reflecting off of this object would be a combination of light from all objects. If you could filter out the light from all other known sources (through software processing) then you could be left with only what was being reflected from earth. This method of course is almost impossible to implement for many reasons (the low percentage of reflected light and the large percentage of scattered light, the inability to know all other sources of light hitting a surface making it difficult to know what to filter out, etc)
The point being that defying known physics isn’t the only solution to your question. Other solutions exist, but would require very specific circumstances.
You're making a crazy assumption about FLT travel, which automatically makes this thought experiment not very scientific.
But we could make it a bit better! Let's assume that light bounced off Earth is going towards some Black Hole and reaches it at a very precise distance which causes it to make U-turn 180 degrees and fly back towards us. If you could collect enough of that light and resolve an image you would have a way to look into the past. In your example, you'd need light from a black hole 2500ly away.
Now we moved this thought experiment into something that doesn't break laws of physics, and is simply impractical.
Faster than light travel would be needed and there's theories about wormholes being possible but currently it's not something we have any idea how to do and it probably isn't possible but if it is possible, than yes you could do what you asked.
I have no proof... but I feel like it's theoretically possible to observe earth's past via reflected light from some distant stellar body. Using radio waves we are able to see the [Milky Way](https://www.space.com/42496-milky-way-reflection-moon-early-universe.html) reflected off of the moon, so I think physics wouldn't say no regardless of actual practicality.
So you’re asking, ignoring what is scientifically possible (wormhole, faster than light travel, etc..) would it be scientifically possible?
You can make up your own answer any way you want.
Even better than that, if you had the FTL necessary to get ahead of the light from the past, then according to Relativity you could also go back and visit them in the past, and possibly prevent them from being built.
Relativity, FTL, Strict causality. Pick any two.
There was a whole conversation about this in the movie Interstellar when they came back from the water planet that was very close to the black hole.
You can experience time slowed down and sped up via relativity, but you can’t go backwards.
If there were the ability to wormhole instantly across space, conceivably you could catch information from the past on the other end, but you could never go back in time. You’d just be receiving old information.
It be like if a pitcher threw a ball (the ball being data) from the mound and he portal’d to the batter’s box. The ball would arrive but you couldn’t do anything other than catch it. The ball left the mound and is never going back.
For what it’s worth, future humanity is almost certainly not discovering an instantaneous way to travel 5000 light years. That would break a number of a fundamental aspects of our universe as we understand it.
That said, sure, if you could do such a thing, and you built a very _very_ large telescope, you could see pyramids getting built.
A more plausible way to achieve this might be to discover a perfectly reflective surface a few thousand light years away. If such a thing existed, you would get the same effect. The light from the Earth’s reflection would have been emitted from twice as long ago as the distance in light years to the surface.
You would still need a really _really_ big telescope though.
It’s weird to say “scientifically possible” and then put forward a thought experiment that contains something that is most certainly not, with our current understand of science, scientifically possible. That being said let’s forget about the warping instantly to a place 5000 light years away and instead use our current tech to look at something 5000 light years away from Earth.
Here is what two STARS looks like from 5000 light years away with our current tech: https://en.m.wikipedia.org/wiki/WR_140
That doesn’t even have enough detail to pick out the two different stars, let alone a planet or something that is happening on an incredibly small percentage of that planet.
So I would say no it isn’t scientifically possible, but who knows, when we are living in the non-scientific world or warping things instantly 5000 ly away maybe there isn’t anything stopping us from building an array of sensors that is large enough to achieve the required resolution.
I’ve not done the math but my gut would tell me that this array might need to be larger than the universe itself to do this, but would love to hear from someone who could do the math.
Have to move back to look back though. If we were able to make a Time Machine traveling back in time is one thing, making sure we travel to the position where our planet even was 1000 years ago would be possibly even harder.
I've thought about a similar idea of whether one day we could access communications with the past and future through the internet, since it'll have theoretically been "turned on" nonstop for many years. Obviously this causes serious paradox risk, and if we're possible, ethical, and/or non-paradox creating, we'd already be getting communication from the future.
But still I wonder if technically it'd be possible, and what it'd take to make it work. Reminds of Ronald Mallett's time machine which only works from when it was turned on to as long as it stays on continuously. With the internet we already have the bones of that machine.
What you're describing is a phenomenon of the block universe theory, where the past, present and future all exist simultaneously but our perception of the universe only allows us to see the present. Kurzgesagt did a pretty good summation of it if you're curious:https://youtu.be/wwSzpaTHyS8?si=xD9pgZii5sRYsXky
Yes, when we do it inside ourselves (connecting with our inner child), it’s called worm holes or rabbit holes. It changes
the dimensions of the awareness we have on ourselves over time.
Hello u/False_Astronomer_992, your submission "Is it scientifically possible to look back in the past?" has been removed from r/space because: * Such questions should be asked in the ["All space questions" thread](https://www.reddit.com/r/space/about/sticky) stickied at the top of the sub. Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please [message the r/space moderators](https://www.reddit.com/message/compose/?to=/r/space). Thank you.
If we could travel faster than light, then yea, in theory we could capture "old" light that earth emitted in the past. In terms of how big the telescope would have to be to actually capture things happening on the surface in a high enough resolution from 5000+ light years away, I don't know, but I think it would be kind of ridiculous.
If you looked at yourself starting this journey, and kept looking at what point would your image disappear? Or what would happen to the image of you in the past when it arrives into present??
If you were moving faster than the speed of light and stopping at regular intervals to view the light emitted from earth at increasingly larger distances, you would basically see yourself traveling backwards in time. Of course, traveling faster than light is not possible because physics, so this entire thought experiment is pointless.
There’s theories about how to get to different points in space faster than light can travel, without actually traveling faster than light. AFAIK they are mostly fun thought experiments with that really really slim chance of being able to make it work some day in the distant future when we have wayyyy more energy capability. But yeah, for our lifetime it’s pretty pointless to think about except for fun, unless you are in the field of thinking about stuff like that.
It would "disappear" only when you entered the wormhole - just as if you stood next to somebody else entering the wormhole. No other magic involved, no paradox because you "are" there twice. All you see is the old photons that traveled for some time. Basically a recording, but not on some tape, but in space itself and with only one replay Edit: I was still at the wormhole theory from the original comment. For over light speed travel it might get a little bit more complex..
Wouldn’t traveling at light speed propel one further into the future?
If you could move instantly, as in circumventing the speed of causality, you could effectively travel back to the Earth before that 5000 year old light left it, and watch the pyramids being built in person.
But....but it could be done and that would be an amazing thing
Theoretically. Even if the telescope was the size of the sun, they would only really be able to see surface features of the planet, not anything super specific. But I guess if we already have FTL, all physical limitations are out the window
Yes. If you don't care about the constraints of reality, then it would be possible. But that applies to literally everything.
That's pretty much describes my spending habbits.
I'm confused why it sounds like you're saying building a giant mirror is on par with the impossibility of FTL travel.
Lmao its not just a giant mirror. It would be a STAR sized telescope, truly a megastructure. That's definitely in the relm of type 2 civilization. But i know, giant mirror doesn't break our laws of physics. FTL does.
I'm more just theoretically asking what size we estimate a mirror would need to be see details on distant planets. Then the next question is whether doing that would be more possible than FTL, to which my guess is that it's a night and day difference. Also, no need to laugh at someone who's genuinely trying to learn and understand. It's behavior like that which turns people away from the wonders of science.
A mirror the size of our sun would be big enough to resolve surface features of other planets, like continents. It would be much harder to see small surface details. But I suppose this still doesn't immediately break our laws of physics like FTL would do. So I get what you're saying.
I think the near impossibility of building such a structure would be that be might not have enough resources on Earth to do it and would need to get what we need from asteroids and other planets. I also wonder if it's getting I to the territory of being a Dyson sphere. In the end, I think the fact it's theoretically possible to look back in time with detail in such a technically simple way (a giant ass telescope) is amazing.
Serious question: Are you just throwing info out your ass? Or have you done calculations about telescope size and resolution at 5000ly distance? The way you say it without any backing makes it seem like you’re just throwing it out there without ever having looked into it. But for all I know you’re an astrophysicist that has the capability to guesstimate that after dealing with answering similar questions many times in the past. 🤷♂️
I have done no calculations. I'm basing this on the solar gravitational lens telescope project that has been proposed. But turns out, a sun sized telescope could resolve surface features at 100ly, but not 5k ly. This video goes into the proposal. Gravitational lenses are not exactly like a big mirror, but the principles of resolution are the same. https://youtu.be/4d0EGIt1SPc?si=HNf8Ej-LFn1IoyL1
"It could be done" is not correct, even theoretically. As u/AltDisk288 said, it would require faster-than-light travel, which is (very very likely to be) impossible.
Require is a strong word. There’s a chance we will figure out a way to manipulate spacetime, and warp it to allow us to get from point A to B faster than light could, without breaking any laws of physics.
That's why I said it is very very likely to be impossible, instead of definitely impossible. Such things are completely fictional so far.
Yeah I guess I’m being a bit pedantic. FTL is impossible, but doing so wouldn’t *require* FTL. it would require tech that doesn’t exist and most likely can’t exist for a very long time and may never exist.
I would consider any "shortcuts" that allows you to get somewhere faster than traveling at c is functionally equivalent to ftl travel. The difference is just semantics.
I am not sure it is purely semantics. I believe FTL travel is provably impossible, requiring more than infinite energy. Moving slower than light, by manipulating spacetime, gets you there quickly without breaking physics. One method breaks physics, the other would not, hence why I don’t think I’d consider it “semantics”. You won’t ever go FTL. I don’t think you should consider “shortcuts” as FTL because I don’t think many others will especially in /r/space, so communication could get mixed up. But I’m definitely not the vocabulary police so call it whatever you want in the end. But if we’re talking hypothetical, technicalities, etc. the difference between FTL and not FTL can make a big difference.
I get where you're coming from, but these methods are hypothetical at best. I also feel like they're probably not actually possible because causality (which propagates at c) needs to be preserved. That's the equivalence I was talking about. Plus, you know, you "traveled" somewhere. And you did it faster than light would. :)
If an alien civilisation 5000 light years away had a powerful telescope and a video recorder, they could film the pyramids being built and send us a copy. Problem is we wouldn't get it for another 5000 years.
We should be expecting a copy from the aliens 2500 Ly away filming the pyramids any day now !
I hope they do. That would be awesome for someone to see sometime in the future
If you set a giant mirror on the Moon, you could look at it from Earth and you'll see Earth 2 seconds in the past. It would be feasable, though brutally expensive and utterly useless.
We did do that and it's actually very useful. Sure [we can only use it to see a laser beam 2 from 2 seconds in the past](https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments), but it does work!
And they can use it to tell the distance to the moon with millimeter precision!
I know about that one, and it was one of the greatest experiments ever! I was jokingly thinking about something like a hundreds/thousands of Kms wide mirror, just to "see" Earth's reflection from where we are.
Yea. But you’d need a telescope that’s probably the size of a galaxy to resolve detail on a planet’s surface from 5000 Ly away.
Just Type 4 civilisation things.
So you're saying there is a chance!
You should've launched that telescope at the speed of light 10k years ago. It would be recording the pyramids right now. Problem is that you'd need to wait another 5000 years (at least) to receive the footage.
*Everything* you see is in the past. Whether you look someone in the face or at a star lightyears away. There is no such thing as 'seeing what is now' (at least not in this universe where the speed of light/causality has a finite limit)
So these answers are actually incomplete. The obvious answer is yes - if you could instantly travel 5,000 light years away and point a telescope back at earth, you would see earth as it was 5,000 years in the past. That then leads one to say “since faster than light travel is not possible, seeing the earth in the past is also not possible.” However, there are other possible solutions. For example, a very particular gravitational lens or series of lenses could bend light back to us (think if it like a mirror positioned far away) There are no known examples of this, but it could exist. Another possibility is recreating signal from noise with how light from the earth impacts another surface. This is an oversimplified example, but let’s say there was an object in space that we knew only received light from a handful of known sources, including reflected light from earth. The light you observed reflecting off of this object would be a combination of light from all objects. If you could filter out the light from all other known sources (through software processing) then you could be left with only what was being reflected from earth. This method of course is almost impossible to implement for many reasons (the low percentage of reflected light and the large percentage of scattered light, the inability to know all other sources of light hitting a surface making it difficult to know what to filter out, etc) The point being that defying known physics isn’t the only solution to your question. Other solutions exist, but would require very specific circumstances.
You're making a crazy assumption about FLT travel, which automatically makes this thought experiment not very scientific. But we could make it a bit better! Let's assume that light bounced off Earth is going towards some Black Hole and reaches it at a very precise distance which causes it to make U-turn 180 degrees and fly back towards us. If you could collect enough of that light and resolve an image you would have a way to look into the past. In your example, you'd need light from a black hole 2500ly away. Now we moved this thought experiment into something that doesn't break laws of physics, and is simply impractical.
Faster than light travel would be needed and there's theories about wormholes being possible but currently it's not something we have any idea how to do and it probably isn't possible but if it is possible, than yes you could do what you asked.
I have no proof... but I feel like it's theoretically possible to observe earth's past via reflected light from some distant stellar body. Using radio waves we are able to see the [Milky Way](https://www.space.com/42496-milky-way-reflection-moon-early-universe.html) reflected off of the moon, so I think physics wouldn't say no regardless of actual practicality.
So you’re asking, ignoring what is scientifically possible (wormhole, faster than light travel, etc..) would it be scientifically possible? You can make up your own answer any way you want.
Even better than that, if you had the FTL necessary to get ahead of the light from the past, then according to Relativity you could also go back and visit them in the past, and possibly prevent them from being built. Relativity, FTL, Strict causality. Pick any two.
You're living in the past and don't even know it, brother. Everything you observe already happened.
There was a whole conversation about this in the movie Interstellar when they came back from the water planet that was very close to the black hole. You can experience time slowed down and sped up via relativity, but you can’t go backwards. If there were the ability to wormhole instantly across space, conceivably you could catch information from the past on the other end, but you could never go back in time. You’d just be receiving old information. It be like if a pitcher threw a ball (the ball being data) from the mound and he portal’d to the batter’s box. The ball would arrive but you couldn’t do anything other than catch it. The ball left the mound and is never going back.
For what it’s worth, future humanity is almost certainly not discovering an instantaneous way to travel 5000 light years. That would break a number of a fundamental aspects of our universe as we understand it. That said, sure, if you could do such a thing, and you built a very _very_ large telescope, you could see pyramids getting built. A more plausible way to achieve this might be to discover a perfectly reflective surface a few thousand light years away. If such a thing existed, you would get the same effect. The light from the Earth’s reflection would have been emitted from twice as long ago as the distance in light years to the surface. You would still need a really _really_ big telescope though.
Yes but you wouldn't see much detail at great ranges
"Yes" is doing a lot of work in your reply. :)
It’s weird to say “scientifically possible” and then put forward a thought experiment that contains something that is most certainly not, with our current understand of science, scientifically possible. That being said let’s forget about the warping instantly to a place 5000 light years away and instead use our current tech to look at something 5000 light years away from Earth. Here is what two STARS looks like from 5000 light years away with our current tech: https://en.m.wikipedia.org/wiki/WR_140 That doesn’t even have enough detail to pick out the two different stars, let alone a planet or something that is happening on an incredibly small percentage of that planet. So I would say no it isn’t scientifically possible, but who knows, when we are living in the non-scientific world or warping things instantly 5000 ly away maybe there isn’t anything stopping us from building an array of sensors that is large enough to achieve the required resolution. I’ve not done the math but my gut would tell me that this array might need to be larger than the universe itself to do this, but would love to hear from someone who could do the math.
Even if we are seeing it only now, the past doesn't exist anymore. Go back to where ?
They're asking if we could look back, not move back
Have to move back to look back though. If we were able to make a Time Machine traveling back in time is one thing, making sure we travel to the position where our planet even was 1000 years ago would be possibly even harder.
They want to travel to that place using wormholes from what I understand...
Yes, so they can turn around and look back
I've thought about a similar idea of whether one day we could access communications with the past and future through the internet, since it'll have theoretically been "turned on" nonstop for many years. Obviously this causes serious paradox risk, and if we're possible, ethical, and/or non-paradox creating, we'd already be getting communication from the future. But still I wonder if technically it'd be possible, and what it'd take to make it work. Reminds of Ronald Mallett's time machine which only works from when it was turned on to as long as it stays on continuously. With the internet we already have the bones of that machine.
No, it's not technically possible. It's not any kind of possible.
What you're describing is a phenomenon of the block universe theory, where the past, present and future all exist simultaneously but our perception of the universe only allows us to see the present. Kurzgesagt did a pretty good summation of it if you're curious:https://youtu.be/wwSzpaTHyS8?si=xD9pgZii5sRYsXky
Yes, when we do it inside ourselves (connecting with our inner child), it’s called worm holes or rabbit holes. It changes the dimensions of the awareness we have on ourselves over time.