Tell me more ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.
up vote 5 down vote favorite
2

I was reading this article this morning:

http://www.bbc.co.uk/news/science-environment-13539914

Scientists have discovered a gamma-ray burst whose light has taken 13.14 billion years to reach Earth. This much is cool and interesting. However, the assumption is then stated that this is "the most distant single object yet spied by a telescope".

But hold on a minute. It is also known that galaxies are moving away from each other at incredible speeds, most faster than the speed of light, because the space/time between them is expanding (or something like that... I'm no scientist!)

So these so-called amazingly distant objects, well for starters, they don't exist any more... They are things that happened billions of years ago. But not only that, they are probably objects that were only a few million light years away from us when they actually took place. So surely then, the objects themselves aren't the most distant ones, but the light from them has been distorted such that the light has taken that long to reach us?

Furthermore, if the galaxies are spreading out faster than the speed of light, who is to say this explosion actually happened 13.1 billion years ago? Isn't it possible that the light was created say 5 billion years ago, but has taken much longer to reach us because of the expanding space between the galaxies?

I'm sure this stuff has already been considered by scientists, but I find it weird the way news articles always assume that just because light travels at a specific speed, that it's always going to take the same amount of time to reach us.

Or am I getting it wrong? I'd love to know!

:-Joe :)

share|improve this question
1  
agreed, this is a pure physics question – Anonymous May 26 '11 at 11:48
1  
and as far as we have measured, no galaxy is moving faster than the speed of light, relative to any other galaxy. Where did you got this strange idea? – Jader Dias May 26 '11 at 12:31
I've never heard the idea that 'space/time is expanding' myself – Phoshi May 26 '11 at 13:43
2  
@Jader Dias--this isn't true--it's possible to view objects on different sides of the sky that would be outside of each others' light cones. – Jerry Schirmer May 26 '11 at 16:17
3  
"It is also known that galaxies are moving away from each other at incredible speeds, most faster than the speed of light" Moving apart, yes. Faster than $c$, no. Though in the standard model of cosmology there was a time (inflation) when the expansion outpaced light, and there will come such a time again as dark energy dominates the cosmology. – dmckee♦ May 26 '11 at 16:38
show 6 more comments

migrated from skeptics.stackexchange.com May 26 '11 at 16:11

3 Answers

up vote 5 down vote

Of course the expansion of space is being considered by astronomers. In fact, it's pretty much the only thing they are considering. The redshift due to expansion of space is the way that astronomers know that it came from 13.14 billion years ago. What you do is look at the lightwaves very carefully. They will be stretched out (which looks like redshifting) due to the expansion of the universe. The longer they have been flying along, the more stretched out/redshifted they will be. The group cited in the article measured a redshift (a measure of this stretching) of 9.4, which is the largest we've ever observed; we conclude that this light has been traveling longer than any other light we've observed from a single source (the cosmic background radiation is way more redshifted).

A number of very clever methods allow us to identify just how long the light must have been traveling for it to received a particular amounts of redshift. This is an application of the relation known as Hubble's Law. If you use it, you find that light with a redshift of 9.4 has been traveling for about 13.14 billion years.

This method is used so routinely that you'll hear cosmologists talk about time in terms of redshift, like "Ionization occurred at redshift 17" rather than "Ionization occured 13.5 billion years ago" (those numbers are made up).

share|improve this answer
Also it may be worth noting that the object from which the light originated is currently much more distant than 13.14B ly away, if it still exists. – JYelton May 27 '11 at 15:42
@JYelton not sure I agree entirely - simultaneity is determined by the speed of light. There isn't really a good notion of "currently" that should go faster than the speed of light. – kharybdis Jun 2 '11 at 17:36
I think my brain just imploded. :) – Joe Jun 7 '11 at 9:54
Actually, saying that any galaxy is X billion lightyears away is a misnomer. Because, due to the expansion of the universe, you can't translate its age to its actual distance (as in metres). Heck, it doesn't even make sense to speak of an actual distance because, as pointed by Spencer Nelson, there is no more simultaneity at these scales. You can't figure out where the object "currently" is. The only thing you can say is that it is so far that its light has taken X billion years to arrive. – KPM Jan 16 '12 at 23:58
up vote 2 down vote

One of the reasons why scientists like to took at distant objects is because they know that, by doing so, they're looking back in time: a distant object is an object from the past. A very distant object (like the one you cited) is an object from near the beginning of the universe (which is interesting to people who want to know about the beginning of the universe).

share|improve this answer
So is it just the media who are speculating "furthest distance" when in fact that is in impossible concept given the warping of spacetime? – Anonymous May 26 '11 at 11:47
1  
@Joe - As far as I know, your sentence "galaxies are spreading out faster than the speed of light" is an 'impossible concept': the theory of 'special relativity' implies that the relative speed of two masses can't reach the speed of light. – Anonymous May 26 '11 at 11:53
3  
@Anonymous: That's not quite right. It's not that two masses can't move away from each other faster than c, but rather that a massive particle can't move relative to space faster than c. That means that you cannot accelerate a massive particle to the speed of light, but, for example, you can accelerate two particles to 99% of c, and shoot them in opposite directions. At that point, they are moving away from each other at a rate faster than c, but they themselves can never reach c. – voithos May 26 '11 at 17:10
1  
@Colin: I may be misunderstanding something, but I don't think my original point disagrees with yours: that galaxies, given enough space between them, can in fact appear to move faster than the speed of light. – voithos May 27 '11 at 17:58
1  
My understanding was that the bubbles of spacetime between the galaxies are expanding faster and faster and eventually will be faster than the speed of light: universetoday.com/13808/… – Joe Jun 7 '11 at 10:04
show 7 more comments
up vote 0 down vote

Looking at a distant object, billions of light years distant, has many connotations.

By being so incredibly far away, you're seeing past events. But this does not necessarily mean the object no longer exists. It may still be around, but certainly could be very different.

I recommend checking out some of the starter Astronomy videos like this one at Khan Academy, it helps me get a handle on the scale of things and the physics behind it.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.