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Horizon

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dr.bob
639441.  Mon Nov 23, 2009 9:02 am Reply with quote

bobwilson wrote:
If you stood on the surface of a really massive body then the horizon would be more than 3 miles (if it was massive enough, you'd be able to see the back of your head).


Although, if the body was that massive, it'd be very hard to see the back of your own head seeing as how it would have been squished into a small puddle of goo.

 
gruff5
639455.  Mon Nov 23, 2009 9:52 am Reply with quote

Davini994 wrote:
I'm confident then that there isn't a horizon on flat ground, as this is just saying that it's hard to see things that are a long way away with great detail.

I guess this is another semantics issue? Looking up "horizon dictionary" on Google, the highest-ranked source gave, among others, these two definitions:

1. the line or circle that forms the apparent boundary between earth and sky.

and

3. the limit or range of perception, knowledge, or the like.


I think (1) would include the infinite flat Earth concept, (3) would not. So, you can pick your preferred definition of the word.

 
bobwilson
639478.  Mon Nov 23, 2009 11:13 am Reply with quote

dr.bob wrote:
bobwilson wrote:
If you stood on the surface of a really massive body then the horizon would be more than 3 miles (if it was massive enough, you'd be able to see the back of your head).


Although, if the body was that massive, it'd be very hard to see the back of your own head seeing as how it would have been squished into a small puddle of goo.


That's just being pedantic ;)

 
Spamperial
641586.  Sat Nov 28, 2009 6:08 pm Reply with quote

Flash wrote:
Would this be an interesting Gen Ig (not in the sense of a trick question, but something that people generally won't know)?

How far away is the horizon?



Would "46 billion light years" be an acceptable response for QI points. 3 miles may be the furthest you one can see along the Earth's surface from standing at sea level, but 46 billion light years is the furthest distance we can see anything!

 
Davini994
641643.  Sun Nov 29, 2009 5:17 am Reply with quote

gruff5 wrote:
I guess this is another semantics issue? Looking up "horizon dictionary" on Google, the highest-ranked source gave, among others, these two definitions:

[i]1. the line or circle that forms the apparent boundary between earth and sky.

Not semantics AFAIC; I'm saying that the horizon on flat ground would be infinitely far away.

 
gruff5
641704.  Sun Nov 29, 2009 8:57 am Reply with quote

Davini994 wrote:
Not semantics AFAIC; I'm saying that the horizon on flat ground would be infinitely far away.

Which I don't dispute & I agree with, but say you can also describe the elevation of the horizon in our field of view. Thus, the elevation of the horizon of Earth is below the "horizontal" and elevation of the horizon on the Moon is lower still and the elevation of the horizon of flat* ground is at 0 degrees.

* idealised flat ground, not taking into account the 46 billion light year horizon of the real universe alluded to by Spamperial.

 
bobwilson
641745.  Sun Nov 29, 2009 11:18 am Reply with quote

Quote:
46 billion light


4.6 surely?

 
Moosh
641748.  Sun Nov 29, 2009 11:26 am Reply with quote

bobwilson wrote:
Quote:
46 billion light


4.6 surely?

Nope, the edge of the observable universe is about 46.5 billion light years away. I'm not totally sure how this works as the universe is only 13.7 billion years old, but I'm told it's something to do with expansion.

 
plinkplonk
951662.  Sun Nov 18, 2012 12:21 pm Reply with quote

Moosh wrote:
bobwilson wrote:
Quote:
46 billion light


4.6 surely?

Nope, the edge of the observable universe is about 46.5 billion light years away. I'm not totally sure how this works as the universe is only 13.7 billion years old, but I'm told it's something to do with expansion.


Would the figure that it is 46.5 billion light years away be wrong then? Given that the light didn't take 46.5 billion years to travel from there to here? Perhaps we need a new yardstick?

 
suze
951680.  Sun Nov 18, 2012 1:53 pm Reply with quote

It's because space itself has gotten bigger over time, as Moosh noted there.

If we were to limit ourself to the "Minkowski space"* of special relativity, then the distance from the middle to the edge could not possibly be greater than the speed of light times the age of the universe.

But if we understand general relativity (which I am certainly not claiming to do) we need not limit ourselves to that. The Universe started as a point, and has steadily blown up like a balloon - the edges get further from the middle all the time. There are hard equations which you can consult if you want to know how much further how quickly.

Now, at this point, it becomes very clear that I know one seventh of fuck all about Physics. I start asking questions like "But if the Universe is getting bigger all the time, what is outside it that is being pushed out the way". Apparently you're not allowed to ask that.

Accordingly, I'll get out the way and leave the floor free for someone who actually knows what they're talking about.


* Minkowski space is a theoretical four dimensional concept of space. Named after a Polish guy who was briefly Einstein's Physics teacher.

 
gruff5
951728.  Mon Nov 19, 2012 1:17 am Reply with quote

yes, quite right.

The furthest galaxies that Hubble has imaged "look" as though they are about 13.7 billion l.y. away (ie size and brightness-wise), but the objects from which that light has come are now 46 billion l.y. away.

 
dr.bob
951741.  Mon Nov 19, 2012 5:08 am Reply with quote

suze wrote:
I start asking questions like "But if the Universe is getting bigger all the time, what is outside it that is being pushed out the way". Apparently you're not allowed to ask that.


This is science, not religion: you're allowed to ask anything.

The only downside is that some questions have answers that are really quite complicated to explain to someone without a deep understanding of maths, physics, topology, and the concept of "infinity". Some questions also have the answer of "we don't know" (though this being science, I prefer to phrase that answer as "we don't know yet").

Anyway, for a much better attempt at an explanation for what the universe is expanding into, I recommend you cast an eye over this webpage:
http://curious.astro.cornell.edu/question.php?number=274

 
PDR
951754.  Mon Nov 19, 2012 7:28 am Reply with quote

And indeed this one

PDR

 
'yorz
951756.  Mon Nov 19, 2012 7:32 am Reply with quote

You were one step ahead of me, PDR :-)

 
PDR
951758.  Mon Nov 19, 2012 7:41 am Reply with quote

Spooky!

PDR

 

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