# Would it be possible to see into the past

derek yau
if you were on a planet a 1000 light years away and had a really good telescope, would you be able to see a 1000 years into Earth's past? Is it as straight forward as this or what problems might this present?

## Answers and Replies

TheAntiRelative
In the same frame of reference, I'm thinkin that's all you're going to see... Add some acceleration and/or travel between the two and it might get a little stranger...

Edit: well other than some gravitational considerations that might deviate the path of the light etc lol

mitchellmckain
Theoretically yes. But in relativity 1000 years ago on Earth could be considered as part of that planet's (1000 ly away) present. The following diagram might help explain.
Code:
[FONT=Courier New]
\   future      /                          ^
\           /                            |  time
\       /                              |
present      \   /    present                     |
(absolute        \/     (also known as the          +---------------------->
elsewhere)      /\        absolute elsewhere)      |                     space
/   \                                |
/       \                              |
/           \                            |
/    past       \                          |
[/FONT]
The point is that if a place and time cannot be causally connected to the here and now then it cannot really be called the past, and might more properly be called the present. Afterall what part of this absolute elsewhere is simultaneous with your present time is relative, that is it depends entirely upon what inertial frame you are in.
The true past and future cannot be simultaneous with your present regardless of your inertial frame and it is causally connected to your present moment.

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Science Advisor
mitchellmckain said:
Theoretically yes. But in relativity 1000 years ago on Earth could be considered as part of that planet's (1000 ly away) present. The following diagram might help explain.
Code:
[FONT=Courier New]
\   future    /                                 ^
\           /                                   |  time
\       /                                     |
present   \   /    present                         |
(absolute    \/     (also known as the          +---------------------->
elsewhere) /\        absolute elsewhere)      |                     space
/   \                                       |
/       \                                     |
/           \                                   |
/    past     \                                 |
[/FONT]
The point is that if a place and time cannot be causally connected to the here and now then it cannot really be called the past, and might more properly be called the present. Afterall what part of this absolute elsewhere is simultaneous with your present time is relative, that is it depends entirely upon what inertial frame you are in.
The true past and future cannot be simultaneous with your present regardless of your inertial frame and it is causally connected to your present moment.
I basically agree with what you're saying, but I don't think the "absolute elsewhere" is usually referred to as "the present", although it's true that for any two events with a spacelike separation, there is some reference frame where the two events happen at the same time-coordinate. But there are also plenty of reference frames where they don't, and the question seems to presuppose we're using a reference frame where the event of someone receiving the light from an event on Earth happened 1000 years after the actual event. Anyway, there would be a lightlike separation between the event on Earth and the event of a distant observer seeing it, not a spacelike separation.

mitchellmckain
No it is not usually referred to as the present, therefore I gave my justification for using the term present. You could use the term absolute present as opposed to relative present if you like. A light-like separation is less than a second away from a space like separation. Our use of the word present is seldom that precise. In other words close enough.

Anyway, my point was simply that in the example given it really isn't like seeing into the past at all.

Now that said, I would like to add a comment in response to the original post that I forgot to make last time. Being able to see things with any detail at distances like 1000 light years requires long exposure photography in order to collect sufficient photons. To be able to see events on the surface of the Earth would take an awful long exposure, so that you would only be able to see the somewhat stable features of the Earth's surface like the geography and maybe a few buildings. And then there is the problem of dealing with the rotation of the Earth and the changing weather, during such long exposures. In conclusion, I think that not much of what you would call human history could be seen.

Science Advisor
mitchellmckain said:
No it is not usually referred to as the present, therefore I gave my justification for using the term present. You could use the term absolute present as opposed to relative present if you like. A light-like separation is less than a second away from a space like separation.
But the difference is crucial, because if there is a light-like separation, then all frames will agree on which event is in the past and which is in the future. And obviously for you to see an event at all, there must be a light-like separation between you and that event, and a moment later there is a timelike separation (you go inside that event's future light cone). So I don't think it makes sense to argue that you aren't seeing into the past when you see distant events--you are, in an absolute sense.

derek yau
Thanks for the response guys, good point about the telescope part of the question too.

mitchellmckain
JesseM said:
But the difference is crucial, because if there is a light-like separation, then all frames will agree on which event is in the past and which is in the future. And obviously for you to see an event at all, there must be a light-like separation between you and that event, and a moment later there is a timelike separation (you go inside that event's future light cone). So I don't think it makes sense to argue that you aren't seeing into the past when you see distant events--you are, in an absolute sense.

Well, yes of course. If you insist on being absolute about everything. But, then everything we see is in the past. Which is precise my point. It is essentially no different than anything else that we see everyday.

chronon
mitchellmckain said:
Being able to see things with any detail at distances like 1000 light years requires long exposure photography in order to collect sufficient photons. To be able to see events on the surface of the Earth would take an awful long exposure, so that you would only be able to see the somewhat stable features of the Earth's surface like the geography and maybe a few buildings.
To resolve something the size of a building at 1000 light years will need a telescope about the size of the Earth's orbit. I would think light gathering power wouldn't be so much of a worry.

mitchellmckain
When you say light gathering power would not be a worry I assume you mean by comparison to the size of the telescope? But the solution is the same. Instead of using a large telescope you can use many small ones over the same distances or simple wait for the planet to travel through a significant portion of its orbit. I believe that we now use interference techniques for computer aided image ehancement to a higher resolution so that you don't actually need a telescope of that size.

yogi
In theory - but not in practice - we can view the past - find a distant planet with a highly reflective surface (one having an ocean) from which the photons from Earth are redirected back to Earth - all that is needed is a way of sorting out those photons that originated from the Earth's surface from all other photons that strike the same surface - and process them to recover the original image.

In essence the past is forever preserved ---until the individual photons are absorbed by striking a non-reflective surface - the information that comprises the Earth's history is stored awaiting detection - but would it really change human behavior to know the reality of the past - how many Zealots would forsake their beliefs if it were revealed that their convictions were founded upon falsehoods - not one.

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