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Center of the observable universe

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RLDavies
1097621.  Sun Oct 12, 2014 9:38 am Reply with quote

The observable universe isn't the same as the observed universe or the entire universe.

As long as we stick with the physical law that no information can travel faster than c (speed of light in a vacuum), then there are definite limits to how much universe we can theoretically observe, even assuming perfectly powerful instruments.

The Big Bang was roughly 13.5 billion years ago. The farther away we look, the further back in time we see as well. This means no-one can see farther than 13.5 billion light years, because that would take the observation back to the moment of the universe's creation. If the entire physical universe is larger than that, everything farther away than that limit is absolutely unobservable.

There are also more immediate limits. If the Sun exploded right now, we wouldn't know about it for eight minutes. If Sirius exploded, we wouldn't know for eight and a half years; and for the Andromeda galaxy, it would be two and a half million years before the news reached us. There's no way to get the information any sooner, since it's already travelling at the speed of light.

The overall result is what's called a light cone, centred on the observer. Or rather two light cones, one extending into the future and one into the past. Everything within the cone is -- at least theoretically -- observable and can be interacted with by that observer. Everything outside the cone is in the "absolute elsewhere", and can neither influence nor be influenced by the observer in any way.

https://en.wikipedia.org/wiki/Light_cone

 
JamesCC
1097624.  Sun Oct 12, 2014 9:54 am Reply with quote

RLDavies wrote:
The observable universe isn't the same as the observed universe or the entire universe.

As long as we stick with the physical law that no information can travel faster than c (speed of light in a vacuum), then there are definite limits to how much universe we can theoretically observe, even assuming perfectly powerful instruments.


But again, the definition of 'observable' (admittedly from the online Oxford dictionary, if anyone has the physical one then by all means post what it says there) is 'Able to be noticed or perceived; discernible'.

It does not in any way specify who is doing the noticing, perceiving or discerning (or when, or where the observer is), only that it is possible for something to be noticed, perceived or discerned.

So while there are definite limits to how much universe WE can theoretically observe, I would argue that there are no limits to how much universe there is that can be observed by some theoretical observer, whether human, alien, or other, in the future, present or past.


Of course, that begs the question 'What is not observable?' (as opposed to 'what is not observable by us at present?')

It's a tough question, but I'd definitely put forward the existence, or lack of one, of a supreme being that does not wish to be observed. Possibly both the position and state of a quantum entity at a certain time? Parallel universes? True altruism by a politician?

I'm struggling to think of anything else. (For example, thoughts will almost certainly be observable in the future; various sub-atomic particles and their behaviour will be or already have been observed) Can anyone think of other things that are not observable?

 
dr.bob
1097733.  Mon Oct 13, 2014 5:33 am Reply with quote

JamesCC wrote:
We can observe the past universe in other places, the further away the further back we can see, due to the speed of light.
<snip>
So on that basis, since it would be theoretically possible to observe all the way back to the Big Bang


That's not quite correct.

Being able to observe things that happened back in time relies on an uninterrupted path of light travelling through the universe. Unfortunately, in the very early universe, things were so hot that electrons and protons flew around independently of each other in a plasma. This caused photons of light to be absorbed and reflected by the free electrons (and partly by the free protons). For this reason, the early universe was opaque.

It was only around 378,000 years after the big bang that the universe cooled sufficiently to allow electrons and protons to bind together and form atoms. Once this occurred (the so-called era of "recombination") then photons were capable of moving through the universe unimpeded.

So, at best, we're only able to detect photons from around 378,000 years after the big bang.

 
dr.bob
1097734.  Mon Oct 13, 2014 5:37 am Reply with quote

RLDavies wrote:
The Big Bang was roughly 13.5 billion years ago. The farther away we look, the further back in time we see as well. This means no-one can see farther than 13.5 billion light years


That's not quite correct.

Current estimates for the edge of the observable universe put it at about 46 billion light years away. This is due to the curved nature of spacetime combined with the fact that, not only are objects moving away from us, but the space itself is expanding with time.

This is why general relativity makes my head hurt.

 
djgordy
1097736.  Mon Oct 13, 2014 5:49 am Reply with quote

dr.bob wrote:
but the space itself is expanding with time.

This is why general relativity makes my head hurt.


Are you sure it isn't that time is expanding with space?

 
dr.bob
1097739.  Mon Oct 13, 2014 6:13 am Reply with quote

Or both.

 
CharliesDragon
1097740.  Mon Oct 13, 2014 6:24 am Reply with quote

The point Stephen was making perfectly as far as I'm concerned, is that the center of the universe that I can observe is me. Everything else is outwards and in the past.

To be honest I don't understand your problem with that position. It relies on solid, well-proven science. We might detect more stuff in the future with better technology, but there is a limit to how far back/out we can see, and whatever we see out in the universe will be in the past.

Now someone accuse me of being of the selfish generation, I did just say I was the center of the universe. :P

 
JamesCC
1097742.  Mon Oct 13, 2014 6:29 am Reply with quote

dr.bob wrote:
Being able to observe things that happened back in time relies on an uninterrupted path of light travelling through the universe.
[...]
So, at best, we're only able to detect photons from around 378,000 years after the big bang.


Thanks, Dr. Bob, that's my 'interesting fact for the day'. Much appreciated.

I suppose theoretically if one could accurately measure and model every single thing in the universe then one might be able to model back past that point... :) All you'd need is a spare universe or two to contain the necessary computer! :(

All things considered, I'm happy to let everything before 378,000 Post Big Bang be considered as 'unobservable'!

 
JamesCC
1097744.  Mon Oct 13, 2014 6:48 am Reply with quote

CharliesDragon wrote:
The point Stephen was making perfectly as far as I'm concerned, is that the center of the universe that I can observe is me. Everything else is outwards and in the past.

To be honest I don't understand your problem with that position.


I may be flagilating a deceased equine, but my point (as opposed to my problem!) is that Stephen did not specify 'observable by you', or 'observable by humans'. He simply said 'observable'.

My point is that, with that word used, the answer is wrong. Had the wording been tweaked a little it would have been fine. Had he said 'human-observed universe' then the answer would have been spot on, albeit easier to get correct.

Look, it's not a great example (it's kind of late here) but: 'predictable' (in the sense of 'able to be predicted'. Things that are (accurately) described as predictable are not limited to things that can be predicted by you, or even by the human race, past, present and future; they are simply things that could be predicted if someone (or some being) had sufficient knowledge, understanding and possibly computing power.

Likewise things that are 'observable' are things that can be observed, and that's not limited to things that can be observed by the human race.

 
Troux
1097747.  Mon Oct 13, 2014 7:28 am Reply with quote

Stephen's definition of "observable" assumes the clause of 'without exerting any effort'. JamesCC's definition assumes 'with exerting all conceivable efforts'. Similar to arguing whether man is a 'flightless' animal.

 
RLDavies
1097758.  Mon Oct 13, 2014 8:51 am Reply with quote

Except the phrase "observable universe" is a standard term in theoretical physics, with an agreed meaning. It's not just some words the QI scriptwriters threw together. The (an) observable universe is, by definition, centred on the observer, whoever and wherever that observer might be. It doesn't mean "what humans can see with current equipment".

 
CharliesDragon
1097768.  Mon Oct 13, 2014 9:21 am Reply with quote

That's what I'm working out from, too, the theoretical physics definition.

Whoever and whatever the observer is, the center will be on them. There doesn't seem much getting around that. If an alien being can observe a greater deal and more dimensions of the universe than we can, either naturally or due to technology, that being is still the center of what it is observing.

 
Troux
1097770.  Mon Oct 13, 2014 9:26 am Reply with quote

You're right, it's not Stephen's definition at all. Similarly, flightless is a zoological term that can take on a different meaning in layman's terms. The show has done this a few times; hardwood, softwood, berry, nut, bean, etc.

 
dr.bob
1097897.  Tue Oct 14, 2014 4:43 am Reply with quote

JamesCC wrote:
I suppose theoretically if one could accurately measure and model every single thing in the universe then one might be able to model back past that point... :) All you'd need is a spare universe or two to contain the necessary computer! :(


Not even then, I'm afraid.

The key phrase in that post is "accurately measure". In order to model the universe with any degree of accuracy, you would need incredibly accurate measurements of the velocity and position of all those sub-atomic particles. Unfortunately, that bastard Heisenberg put the kibosh on that idea back in 1927 so, even with your two-universe-sized computer, you still wouldn't be able to do it :(

 
dr.bob
1097898.  Tue Oct 14, 2014 4:47 am Reply with quote

JamesCC wrote:
I may be flagilating a deceased equine, but my point (as opposed to my problem!) is that Stephen did not specify 'observable by you', or 'observable by humans'. He simply said 'observable'.

My point is that, with that word used, the answer is wrong. Had the wording been tweaked a little it would have been fine. Had he said 'human-observed universe' then the answer would have been spot on, albeit easier to get correct.


So here's a quibble that Johnny Vegas might be interested in:

If we take the term "observable universe" to mean the universe observable by human beings, then we have to consider the Earth's orbit around the Sun.

The patch of sky observable at any point is defined by the Earth's position around the Sun. Even radio telescopes, which can happily observe during the day, are unable to see through the Sun, so the part of the sky behind the Sun is always unobservable at that time.

Of course, we can observe it at a different time of year, but that implies that, to build up a picture of the "observable universe", one would have to average over all of the sky observable over the course of a solar year.

In which case, the centre of the "observable universe" would indeed be the Sun :)

 

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