QUESTION:  Can the HST actually see so far that it can look to the past?
 
ANSWER from Joe Pesce on April 10, 1996:
The answer to this question relies on the fact that the speed
of light is finite; approximately 186,000 miles per second
(or 300,000,000 meters per second).  Now while this is VERY fast there
is a limit and  therefore it takes time for light to travel some distance
(time = distance/velocity).  Even though the speed of light is
so very large, distances in space are enormous.  The distance to our
Sun (the nearest star) is about 150,000,000,000 meters, and so how
long does it take light to travel from the Sun to the Earth?
Right, it takes light a little over 8 minutes to travel from the
Sun to the Earth.  Or, put another way, we are seeing the Sun
as it was 8 minutes ago, that is, 8 minutes in the past.  Something
that happens on the Sun right at this moment, wont be seen by us
for 8 minutes.

Okay, for the Sun we can say it is 150,000,000,000 meters
away or we can say it is 8 LIGHT MINUTES away (it takes light 8
minutes to get here).  Other astronomical objects are much more
distant.  For example the next nearest star is 4.3 LIGHT YEARS away.
What does that mean?  Well, it takes light, zipping through space,
4.3 YEARS to travel the distance (which is?  let's see, there are
31,600,000 seconds in a year, or 135,880,000 seconds in 4.3 years,
and light travels at 300,000,000 meters per second, so this object
is 40,000,000,000,000,000 meters away! THIS IS WHY ASTRONOMERS
REFER TO LIGHT YEARS RATHER THAN METERS!). And therefore, the light
we see today from this object left the object 4.3 years ago.
Now you see the connection between the speed of light and the distance
to the object, and a fundamental Astronomical measurement, the light
year (which measures both time and distance).

Now let's jump WAY out there - so far that it is almost meaningless
to talk about meters, inches, or whatever.  With big enough
telescopes, we can observe objects BILLIONS of light years away.
This means that it has taken light billions of years to travel
the immense distances between the object and us.  So in this case,
as indeed in every measurement we make, we are looking at the object
as it appeared in the past; we see the Sun as it was 8 minutes ago,
we see this very distant object as it was billions of years ago.
So, our telescope (or even eyes for nearby objects) can be looked
at as a time machine - we aren't seeing our past necessarily, but
the past of some other object.  This, I think, is actually one
of the neater things about astronomy.