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Center of the observable universe

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JamesCC
1097559.  Sun Oct 12, 2014 3:07 am Reply with quote

QI L Series Episode 2

Stephen Fry: "what would you find at the exact center of the observable universe?"
[...]
Jason Manford: "Isn't the center of the universe the person who is looking out?"
Stephen Fry: "You are so absolutely right it hurts."
[...]
Stephen Fry: "At the exact center of the observable universe you will find the unbearable lightness of Johnny Vegas, or whoever happens to be observing it."


It seems to me that the mistake here is the use of the word 'observable', which means 'able to be observed', or 'Able to be noticed or perceived; discernible:'.

'The observable universe' is all of the universe that can be observed, which broadly speaking, means all of it. (There's a TL;DR later if the following is boring you)


You could argue that it might rule out a few bits, such as the singularities of black holes, the center of planets and suns, and so on, but I would suggest that they can be observed, even if not from close up. Additionally, if a being managed to get to such a place then they absolutely could observe it, albeit possibly very briefly. So that is definitely 'observable'.

I also think that dark matter (assuming the theories are correct) is observable - it's just a matter of your method of observation.
Likewise very small objects such as sub-atomic particles.


When you bring in dimensions it gets a bit more complicated. For example, the dimension of time: the ability to observe the past universe is dependent on your location (or a time machine or faster than light travel). We can observe the past universe in other places, the further away the further back we can see, due to the speed of light. Then we just have to wait long enough for us to be able to observe the present (or future) universe in those locations, though by that point it would have become the past universe of course. Observing the past universe near to us would require time travel, or travelling far away faster than light (assuming we are observing using something, such as light or radio waves, that travels at the speed of light or slower). Observing the future universe just requires the application of patience (and survival).

So on that basis, since it would be theoretically possible to observe all the way back to the Big Bang (assuming that theory is indeed correct) by choosing or being in the right location in the universe, the universe is 'observable' right back to the Big Bang, and by waiting, right up to whatever happens at the end (if there is such a thing), and perhaps beyond.

Similar to the issues with time, if we accept that there are more than 4 dimensions (which seems to be the current theory) then parts of the universe that are separated from us 'along' one or more of those other dimensions could be tough to observe, but I could be wrong - perhaps any physics experts could comment on that. And saying that it is not observable assumes that no being will ever, now or in the future, be able to observe them, and that's not a certainty. Also, as with the solution of choosing the right position to see back to the big bang, it would be theoretically possible for a being to be in the right place 'along' one or more of those dimensions, and thus be able to observe it, and so that is 'observable' too.


Even if you consider any parts of universes in parallel universes (if they exist), it is still theoretically possible that something could observe them (though we would probably never know it unless we find a way to travel or communicate between parallel universes).


I would also add that an important component of this is who can do the observing. I think that we have to allow for the possibility of alien observers, and observers using more advanced technology, and observers in different locations/dimensions/parallel universes as described above. Also does the observer have to be sentient? I would suggest not always, they only have to 'perceive', not record, analize, study, etc, though in some cases where you would require technology to observe (such as sub-atomic particles) sentience would be required, unless the observers are really really small, or given the right technology (think of a cow strapped to portable atom-smasher! :) ).

Of course if one allows the observer to be an omniscient, omnipotent and omnipresent God then that definitely rules everything at every time 'observable' without the complications of having to theoretically posit individual observers in the hard-to-reach places. Similarly a sufficiently advanced theoretical being outside the limits of the universe could probably achieve the same effect.


TL;DR:
So, coming back to the point, the 'observable universe' is the same as 'the universe'. The center is therefore NOT 'whoever happens to be observing it', or the Sun, or Johnny Vegas. It is a point that could be calculated (and that attempt has been made several times that we know of), possibly allowing for other dimensions or parallel universes as desired. It also might move over time.


To get the answer that Stephen said was correct you would need to use the word 'observed' in some specific way.

If you just said 'the observed universe' then we could argue for a while about the possibility of life (whether sentient or not) elsewhere, observing things - see above.

If you were more specific then you could narrow it down. The Earth would arguably be 'the center of the human-observed universe'; even then, you could also say the Sun, since the Earth orbits around it so the Sun would be the center of the average position of observation... except the Sun moves too, as does the galaxy, and so on!

And let's face it, asking where the center of the human-observed universe is would give away the point QI was trying to make, though it would open up a discussion of the movement of the Earth through that universe.

 
Posital
1097598.  Sun Oct 12, 2014 8:22 am Reply with quote

So are you saying the centre is a bit more to the left than we previously expected?

 
JamesCC
1097605.  Sun Oct 12, 2014 8:28 am Reply with quote

No, apparently it's a bit to the right:
http://www.politicalcompass.org/images/internationalchart.png

(from http://www.politicalcompass.org/)

 
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.

 

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