
Light - If you view the Earth from far enough away can you observe its past? - jjchiw
http://physics.stackexchange.com/questions/11940/if-you-view-the-earth-from-far-enough-away-can-you-observe-its-past
======
ctdonath
On a related note:

A 3GHz computer (ignoring multi-core, hyperthreading, other issues) executes 3
instructions every nanosecond. Light travels 1 foot per nanosecond. Your head
is about 2 feet from your monitor. In the time it takes the light from this
post on your monitor to reach your eye, your computer performs 6 instructions.
Scale as appropriate.

When I first realized that, I wandered around in a slight daze thinking "wow,
light is slow..."

~~~
rkalla
Did not know this, love it and am also amazed that all of us are alive right
now in an era where humans are dabbling with technology that is starting to
ram right up against (perceived?) physical limitations.

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whatwhatwhat
I remember thinking of this one time as a teenager. Could we see a top down
view of the rise of man? The huge caveat is that you have some capability of
travelling faster than the light leaving earth, so that you can then look back
and absorb the rays. It makes perfectly good sense that if you _could_ travel
faster than the speed of light you could use a massive telescope to peer back
at the earth and see it's past geological events -- and I mean millions of
years even, not just something trivial -- or given a strong enough telescope
perhaps more detail could be seen. All of the information that left the earth
as light is still out there, all over the universe.

~~~
ch0wn
_All of the information that left the earth as light is still out there, all
over the universe._

A very poetic point of view. :)

As others mentioned in the thread, you don't necessarily travel yourself. If
you had a mirror far enough away, you could look into the past as well.

~~~
jterce
Right, but the mirror would have to have been placed at least as far back as
the half way point to where you want to see. If you want to see back two
million years, you would have had to place the mirror one million years ago.

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aubergene
There must be a calculable limit to the size of the event that you can see at
a given distance. There are finite number of photons which are emitted from an
event, these spread out with the inverse square law, so as your distance
increases the probability of collecting sufficient photos to reconstruct an
image of the event decreases and I would think at a distance of one lightyear
it would be guess hard to see something much dimmer than an atom bomb.

~~~
hasenj
But light travels as a wave, and my humble (and possibly mistaken)
understanding (or conjecture) is that photons are nothing but particle-like
manifestations of whatever thing light is "really" made of.

Waves travel in all directions, they get weaker over long distances, but it's
not as though some photon particles reach us while others don't. (or is it?)

I mean, if light is just photon particles, then we'd have to be really lucky
to see that many starts that are billions of light years away.

Of course, I could be totally wrong!

~~~
wtallis
Light travels as both a wave and as particles. Whether you observe light to
behave as mostly particle-like or mostly wave-like depends on the
circumstances. When you are dealing with very small intensities, you usually
observe mostly particle-like behavior (but wave-like behavior still shows up
in things like self-interference as demonstrated by the two-slit experiment).

If you think that it's unlikely for a photon from a distant star to reach us,
then you are simply vastly underestimating the number of photons involved.
Consider by analogy the process of smelling something: actual molecules need
to fly off the object, travel through the air, and impact your nasal passages
in order for you to smell it. That means that anything you smell is constantly
dispersing its mass into the atmosphere, but an object can remain pungent for
a long time without significant weight loss. Photons are vastly more numerous
than olfactory molecules, and carry incredibly small amounts of energy.

------
arctangent
One pottential difficulty in achieving this in practice would be that the
amount of information (photons) available to measure would rapidly attenuate
with distance, so you'd need to mitigate this by building larger and/or more
sensitive detectors.

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rkalla
Fascinating question and fantastic answer.

That being said, as Edgar added to Vintage's answer, if you put a mirror 13.5
light years away, and watched your reflection from earth, that would be the
same as being 27 light years away.

So if I doubled the number of mirrors (2 in orbit, 2 on earth) and halved the
distance to 6.75 light years I could accomplish the same thing.

If you take that example to it's conclusion, could I construct an
(expensive/complicated/etc.) device here on earth that had so many mirrors it
could let me look into the past at all?

The physics-answer seems "yes" so my question is "Why haven't we tried that?"
and one obvious limitation, I suppose, would be just how many mirrors you
would need.

Light travels 5,878,499,562,554 miles (5.78 trillion) a year[1].

Given that, if I just wanted to see an hour into the past, I think that means
I would need to observe earth from: 5,878,499,562,554 / 365 days / 24 hours =
671,061,594 miles away.

Or I could stick a mirror in space 335,530,797 miles away (~ 540,000,000 km)
from earth and stare at it.

Mars, at the widest distance from Earth, is 401,000,000 km away[2], which is
close enough for my purposes (I'm not picky)... so I guess if I stuck a mirror
on Mars and looked at the reflection of earth I could see something like 45
minutes in the past.

The Moon is almost exactly 1000x closer to the earth than Mars[3], so I wonder
if I used it for my mirror array instead if I could just put a station with
500 mirrors on it to accomplish the same thing.

Or build something on earth with millions of mirrors in it to accomplish the
same thing.

I would normally think something like this impossible, but I just watched a
docu on the LHC and now I wonder if even at a micro-second scale, if we have
tried building something like this and observed two points in space using a
computer and seeing if the visual data coming in is micro-seconds apart from
each other?

For example (assume I have a camera and visual-diff software sufficient for
this and that my "mirrors" have sufficient magnification capabilities to make
this seem like an easy setup), if I pointed one camera at a monitor drawing a
unique pattern 2' away from me, then point another camera at a mirror that has
bounced the image 10 miles before being displayed... I imagine, like sound,
there would be a lag in that image if we bounced it enough times.

(DOH)

It suddenly dawns on me that using this method to look into the past is
effectively the same thing as recording something with a video camera and
playing it back later... you are literally capturing the light for review at a
later date.

So as cool as this idea is, I think I just answered my own question as to why
we haven't tried to build a million-mirror-array before... cause I can buy a
video camera for $300 instead :)

[1] [http://www.universetoday.com/45047/how-far-does-light-
travel...](http://www.universetoday.com/45047/how-far-does-light-travel-in-a-
year/)

[2] [http://www.universetoday.com/14824/distance-from-earth-to-
ma...](http://www.universetoday.com/14824/distance-from-earth-to-mars/)

[3] [http://www.enotes.com/science-fact-finder/space/how-far-
moon...](http://www.enotes.com/science-fact-finder/space/how-far-moon-from-
earth)

~~~
hugh3
Since you'd need to put all this stuff in place before the event you want to
see actually occurs, wouldn't it be easier to just break out a video camera
and record it?

~~~
rkalla
hugh3, you are exactly right. I updated the original post as I was thinking
through how this amazing physics experiment could be helpful, and it dawned on
me, it wasn't.

I could just use my Canon to accomplish the same thing.

I think my brain was more caught up in the fun of walking through the thought
than accomplishing something with it.

~~~
Splines
Even so, I think it would be fascinating to interact with a device that
consisted of mirrors, even if was just to experience a delay of a millisecond
(or whatever is the minimum amount of time to perceive a delay).

~~~
chrischen
You can slow down light <http://news.bbc.co.uk/2/hi/science/nature/655518.stm>
and see the past!

~~~
Splines
Exactly. I think it'd be incredibly cool to walk in front of a thin
transparent wall with this property, run around to the other side and see
yourself walk along it.

I wonder if you could information in this fashion.

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webXL
What? Nobody has mentioned a worm hole yet? Screw mirrors to cut the distance
needed in half (although requiring the same amount of time).

How about this:

Step 1) open up a worm hole at a point along the path of light emanating from
Florida on June 16th, 2008. 2) install a video camera with a giant lens around
the same point pointing back at us 3) send the wireless ;) signal back through
the worm hole. 4) Determine if the Anthony jury is a bunch of idiots or not.

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marcamillion
This is something I have always wondered about.

Can the reverse be true too? i.e. if you are on the moon and can see what
happens 1000km away, before someone else on earth 2000km away from that event,
is that the future? Is that possible?

Can that then be extended to 27 lightyears into the future too?

~~~
tuhin
I think it is relative. Your argument considers time as a non relative entity
and the past and present as static points in a timeline. So the same "event"
is one man's past and one man's future. It depends on your space coordinates
from the coordinates of the even if this is your past or present or how back
in past.

Disclaimer: This is deduced logic and not scientific information.

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falcolas
Somewhat of a tangent, but the exploitation of this very phenomenon is one of
the things that endears the book "Battlefield Earth" to me. Despite whatever
other issues the author had, he wrote a darned good science fiction book.

~~~
Splines
I've never read Battlefield Earth, but I found The Forever War to be a really
interesting view on the effects that relativity would have on light-year-scale
war (and the expansion of the human race).

<http://en.wikipedia.org/wiki/The_Forever_War>

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csomar
We definitively can, but there is an issue: The light can be absorbed by some
substance or refracted. How do you account for that?

You can account for that by recording every atom/photon on the space and then
using some kind of miraculous processing power, to calculate how everything
goes and estimate/verify the past actions.

For the processing ability, certainly some day we'll get there. For the
possibility of getting the coordination/nature of any atom/photon in the whole
space, this will need a discovery of something faster (way too much or may be
instantaneous) than the speed of light that gives us the ability to recognize
particles.

This is not amazing. This is sick. We'll be able to watch Pharaohs with an
infinite precision. See how the earth was billions of years ago and how life
evolved.

~~~
scrrr
You want to record / analyze every atom in the universe with a machine that
exists within the universe?

~~~
roundsquare
Just for fun... it might not be impossible:

You can reduce R^2 to {x | 0<=x<=1}[1] and thus, by induction, we can reduce
R^n to {x | 0<=x<=1} for any integer n.

If I remember correctly, there are about 10^80 particles. The position of each
particle is a point in R^3 so the position of all particles is R^(3*10^80).
So, the position of every particle could be stored by the position of a
particle on a 1 meter (foot, inch, whatever) long stick.

Of course, you run into problems if space is discrete or, in any event, with
the Heisenberg uncertainty principle but you can still store a lot of
information with each particle.

Horribly impractical of course, but like I said, just for fun...

[1]<http://www.earlham.edu/~peters/writing/infapp.htm>

~~~
xyzzyz
Your argument from cardinality of sets is not relevant -- current state of
knowledge of nature of the universe prohibits building a device which carries
on with computation using real numbers. See Bekenstein bound.

Actually, if we could perform computations using real numbers (think of it as
we're back using analog computers and the universe is continuous again and not
discrete/quantum), we would for instance be able to solve NP-complete (also
#P-complete) problems in polynomial time.

Anyway, the rest of your argument is what philosophers were arguing about two,
three and even four hundreds years ago. See Wikipedia pages for "Determinism"
or "Mechanism".

------
alecbenzer
one of the commenters: "the question seems to lead to the idea that traveling
faster than the speed of light == traveling backwards in time"

It's just a point of language, but that's silly. Seeing a video of France is
very different from traveling to France, so one would imagine that seeing the
past would not be described as "traveling" backwards in time.

edit: although, you may think about going faster than the speed of light as
traveling backwards in time because of the "effects" of time dilation when
your speed is > c

~~~
wtallis
Faster than light travel isn't classical time travel, until you turn around.
If you only travel away from Earth FTL, then that doesn't automatically
produce nasty paradoxes. However, if instead of sticking around to see all the
light pass you by, you just turn around and head back to Earth, again FTL,
then you can arrive before you left, and cause all sorts of trouble.

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ulvund
Strictly speaking, everything we see happened in the past :)

~~~
sambeau
err.. the first sentence of the link:

    
    
      From my understanding of light, you are always 
      looking into the past based on how much time 
      it takes the light to reach you from what you 
      are observing.

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ck2
We cannot even resolve the lunar lander site on the moon with Hubble.

How the heck exactly are you going to take advantage of that distance or
mirrors?

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socratees
Or, you can videotape yourself and then watch the past whenever you want.

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zohebv
The question has been protected on stackoverflow, so answering here. Yes a
mirror 13.5 light years away can do the trick. But there is some fundamental
lack of understanding regarding relativity that has lead to such a question.
In fact, the top answer is incorrect on several levels.

1\. Technically the only way a person can travel 27 light years in 27 years is
by travelling through out at the speed of light. Ignoring the infinite amount
of energy required for such travel, there is another aspect of such travel
that is not being considered. Time Dilation. So assuming you were born and
take off at the speed of light. Yes you will be able to observe your birth on
earth 27 years later, but you would still be a baby and not 27 years old. Time
would have been effectively frozen for you as you traveled.

2\. The top answer says that you are always viewing and hearing the past. You
are hearing the past, but you are actually viewing the present. The light
radiating out of an event you are observing are also causality horizons. In
your reference frame you are observing past events depending on the distance.
However thanks to relativity and Lorentz contraction, there are reference
frames where the distance between you and the event can be arbitrarily close
to zero, effectively making the events simultaneous i.e. when you see an event
you are effectively watching it as it happens simultaneously, you are not
watching the past. This is also an alternative explanation for 1. i.e. the
baby cannot grow older if you put it 27 light years away in 27 years time. It
will be able to watch its birth, because its only just been born.

<http://en.wikipedia.org/wiki/Minkowski_diagram>

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maeon3
If you could figure out a way to invert all the constants and principles in
the universe, then we could relive our past, in reverse. tea cup fragments on
the floor would assemble into a teacup then accelerate up into the air and
then sit on the table. The universe would play out, right back to it's
formative moments. Then you could watch yourself being born, though you would
have to wait years for it to render.

~~~
llambda
I don't understand the physics but I do believe Stephen Hawking disproved
this[1]; he said that after entertaining the idea that as the universe
contracts time might reverse he concluded later this was wrong.

[1] A Brief History of Time (the documentary film)

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ignifero
I wonder if there is research into gravitational lenses or other possible
sources of naturally occuring space mirrors or lenses that we could use to
collect light rays from the earth's past, maybe to detect its spectral
content.

~~~
hugh3
There isn't any research, but only cuz it's damn impossible. The number of
photons ever emitted by Earth in the distant past which are gonna ever come
back to Earth is... very very small.

~~~
ignifero
Yet, since there are not many similar atmospheres in the region, if there were
any backscattered radiation it wouldn't be impossible to detect.

