Expert: Jayendra Upadhye - 10/18/2004

Question
-------------------------
Followup To
Question -
-------------------------
Followup To
Question -
Hi!
How many lightyears away is the most remote object we can see today?

Let's say that it is X lightyears away.

Does this mean that light that is emitted from that object "now" will reach us x years from now?

But the universe is expanding, after x years the distance between us and the object is more than x lightyears. So will it not in fact take X + Y years for the light to reach us?

And what about the light that we see  in our telescope from this object? Has it been on the way x years? But it left the object at a time when it was closer to us. That seems to mean that it has been on the way less than x years. But of course: we have moved apart since then so maybe the light has in fact been on the way x years or even more than x years?
Can you sort it out?
Regards
Claes
Answer -
Hi,
You are correct!
And I cannot sort it out if you mean getting absolute actual "current" distance to a celestial object.
Let me explain.
I some how sense you are a bit upset (dont ask me how ;) i could be wrong you know..but just hear what i have to say and take what you find agreeable.
What we mean when we say "this star is x light years away!"
actually means "it was x light years away so many years ago!".
Obviously we have no means of knowing whether in the intervening period it went supernova or went bust just like that.. or whatever!
Knowing its red shift (cosmological + actual), we CAN go on to "extrapolate" where it "should be" currently.

But you will agree such a prediction involves un underlying premise that "all has been well with it" till such time as we "get the latest"!

Most astronomers are NOT concerned with what they CANT possibly know (as no observation is possible till "current" light beam reaches them from the star which may be 10s of years hence!), but in what they CAN see.

Ofcourse if the star happens to be 1 billion light years away and is supermassive, they can say "hey this guy will fade away in a few million years!"..but that would still be a conjecture.[pardon me for implying such stars are visible to astronomers as individual stars..any thing at that distance has to be of galactic proportions or incredibly bright ..quasarlike..to be visibly resolved into a point].

Thus most distances given to us are on the basis of "distance when observed".
Hence the distance unit "light-year" conceptually helps, though its purely a unit of physical distance and has nothing to do with time in its units.

regarding your last para all i can say is that yes, light has travelled x years after exiting the star, and before it
was recieved.
Absloute distances obtained by observing cepheid variables, type 1-a supernovas etc also are NOT exempt from this uncertainity regarding actual "current" position.That always involves extrapolation.
use this link:-
http://cas.sdss.org/dr3/en/proj/advanced/hubble/distances.asp
It also deals in relative distances only.
Jayen

Hi Jayen!

Thanks for answer. How did you see that I was (a little) upset? I was in fact. Because I have tried to get answer from the other expert, but we don't understand each other. He gave up and probably found me stupid. While I found his answers off target, like if he didn't understand what I asked.

So with you I decided not to blurt out the full question at once but to take it step by step. In the first step we have now cleared out that "x lightyears away" means that the star was x lightyears away when the light was emitted and the light we see has been emitted x years ago and that nobody knows for sure how far away that star/galaxy really is right now. That tallies with what I have read: that while looking at far away objects we are looking back in time.

Now step 2. If X is VERY BIG, does the reasoning still hold? Let's say X is 13 billion. It should mean that we are looking 13 billion years back in time, right?

But at that time the universe was little. We (or our future place in space) were then much closer to the object in question. So it shouldn't have to take 13 billion years for the light to reach us.

So why did it in fact  take that long time? What has the light been doing during all this time? Rushing between two objects which are in fact disappearing from each other with a speed a substantial fraction of the speed of light?

Or maybe the rule (that looking x lightyears away = looking x years back in time) holds only for comparatively small values on X? So we can never in that way "see" the early universe?

Or maybe the most remote object we can see is only 50 million lightyears away so the problem I have in mind will never materialize?

Regards

Claes
Answer -
Hi claes,
thank you for confirming my judgement about the "upset" part. Guess it was a hunch. (hunches it seems improve with age!).
Now on to your argument.
>Now step 2. If X is VERY BIG, does the reasoning still >hold? Let's say X is 13 billion. It should mean that we >are looking 13 billion years back in time, right?

>But at that time the universe was little. We (or our >future place in space) were then much closer to the object >in question. So it shouldn't have to take 13 billion years >for the light to reach us.

But WE (the observers) weren't around 13 billion years ago!
so any light that came "our" way missed us by a margin in time of 13 billion years!
By the time we "looked", the object itself was displaced over such a huge distance in space that we managed to only see "how it was" 13 billion years ago! Thanks to the limitations of space-time!

Claes! One last thing! If you are trying to pick holes in the current theory regarding the expanding universe, do so.

But! (and there are many if's and buts included in those two "!"s), DO compartmentalise the "doubting tom" from the "learning tom"! Else the battle will be lost.

Afterall what is a battle if amongst unequals?
compartmentalise your doubts, (they are to be cherished as it is the doubting thomases that make progress possible!), but do not let that distract you from "the given gospels" of science. (after all they too are the results of "deep doubting and searching" by other toms before you!). Arm yourself with these, before your own "search" commences!

For if the doubting toms lead us forward, their achievements in the end become part of the "given gospels",
and starting points for new doubters!
wouldnt you agree?
Jayen

Hohoho! "Doubting Tom"! No, I am not doubting established and proven science. This is a language problem, not primarily a scientific one. Specialists and laymen use different languages and I think even different ways of thinking. Which means that we laymen kan be confused without necessarily believing that we have thought out something new. The problem is to follow how the specialists think when you can't see how what they say fit together and make sense. It is not a "battle" (your word) only a striving for understanding.

"But WE (the observers) weren't around 13 billion years ago!"
I am well aware of that. That was why I wrote "We (or our future place in space) were then ....."

"...so any light that came "our" way missed us by a margin in time of 13 billion years!
By the time we "looked", the object itself was displaced over such a huge distance in space that we managed to only see "how it was" 13 billion years ago! Thanks to the limitations of space-time!"

Sorry, but here is a language problem again. The little word "so" in the beginning of the citation seems to indicate that you mean that since we didn't exist 13 billion years ago we missed the light that was emitted 13 billion years ago. Like if you mean that "yes, the light reached us after only three weeks, because the universe was so little then, but nobody was there to see it".  But the rest of the citation seems to confirm that it really took 13 billion years for that light to reach us. So it didn't reach us after only three weeks.
It doesn't fit (and I am not battling, only trying to understand).

Well, the other expert said that the light emitted 13 billion years ago in fact reached "us" after short time, since the universe was little then. But we were not there then to see it. He didn't elaborate on the real problem, that if we REALLY see 13 billion years back in time we should have seen a little universe where it shouldn't have taken 13 billion years for the light to reach us.

I get the impression that the experts say that "yes it took only three weeks for that light to reach "us" and it took 13 billion years for that light to reach us."

In my follow up question to you I at the end imagined three solutions to the problem. Non of them had anything to do with doubting science, only three possible answers which I found meaningful. You didn't comment on any of them.

Well, I am 60 years old and probably a little stubborn. Thanks for your time anyway, I enjoyed the discussion.

Regards

Claes  

Answer
Hi Claes,
Sorry!
Hope you will bear me out a bit more.
I have been far from succinct as you have demonstrated so accurately.
The CRUX lies in the fact that though the universe was "much" smaller then, it was in the least as wide as 13 billion light years!
so i will try it another way.
let me put it in tabular steps.

(1)
time:       13 billion years ago
mutual distance:   13 billion Light years
ray status: the ray starts out towards us
our status: dont exist (note-1)

(2)
time:       between 13 billion years and now
mutual distance:   13 billion Light years and increasing
ray status: the ray continues towards us
our status: continue to not exist.

(3)
time:       now
mutual distance:   13 billion + distance as extrapolated using hubble's criteria.
ray status: recieved          
our status: we exist and do see the object. (note-2)

note-1:
The universe was smaller, but certainly at least as wide as 13 billion light years! (meaning now it is much more wide).
So both of us are true, the universe was smaller then!
But we did not exist to see ANY previous rays sent out by the object before 13 billion years that might have reached "our location" 13 billion years ago.
note-2:
We exist and see the object as it was where it was 13 billion years ago. However, the "real" object has in the meantime receded further.each second we see the object, we see it from its new location that many seconds after zero time of 13 billion years!
Now is there any contradiction?

Actually current estimates say the universe is 156billion years wide and was about 15 to 16 billion light years wide 13 billion years ago.
So no doubt we are seeing the edge of it as it was 13 billion years ago, when its physical boundary was 15 to 16 billion light years end to end!

There is also the fact that the universe actually expanded faster than the speed of light.
If you are wondering how a universe 1 or 2 billion years old (13 billion years ago) could be wider than 2 billion light years.
Ref the site:-http://www.space.com/scienceastronomy/mystery_monday_040524.html
hope you find this explanation satisfactory.
Jayen