Size of the universe
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09-07-2013, 08:15 AM
RE: Size of the universe
(09-07-2013 08:10 AM)TwoCultSurvivor Wrote:  Boom. Well, there goes my head.

Look on the bright side, you'd never learn this on a fundy forum Smile

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09-07-2013, 08:26 AM
RE: Size of the universe
That is true.

One of the reasons I raised this topic, though, is that it's a common criticism of theism that if God created stars on day 4, how could anyone see them, considering that some are millions and billions of light years away?

It seems to me that any answer to the original question I posed can be appropriated (misappropriated) by theists, who can offer some claptrap about that whole space-time, universe-expansion stuff being how God answers the day 4 question. I don't know if I'm being clear, but that's what I was trying to work through.
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09-07-2013, 08:32 AM
RE: Size of the universe
(01-07-2013 02:05 PM)ridethespiral Wrote:  
(01-07-2013 01:54 PM)TwoCultSurvivor Wrote:  I get that: That's why I picked 28 billion miles as a marker.

See, my brain can't wrap itself around this:

If I go in one direction at the speed of light for 14 billion years, and you go in the opposite direction for 14 billion years, how can we be MORE than 28 billion light years apart?

Also, how does space expand? What does that even mean? I can't wrap my head around it.

Some Galaxies are actually moving away from us faster than the speed of light, that does not mean that they are not moving faster than the speed of light, but that we are also moving in the opposite direction at insane speeds....Also expansion is accelerating (which is super weird, 'dark energy' is the best explanation we have for this right now).

Think of space as a giant expanding balloon, and all the galaxies are on the skin of the balloon moving outward from the center point as it inflates.

No actually. No galaxy, even from the perspective of one moving away from another is moving faster than the speed of light. If you drive a car down the road at night with the headlights on at 60 mpg your headlight isn't traveling at the speed of light plus 60 mpg. In the early moments of the universe matter did travel faster than the speed of light but at the time quantum mechanics and what we now know to be physics hadn't been established but it didn't take long (relatively) for things to slow down. The universe is expanding and it is speeding up but we don't know how long it will occur. Eventually all objects might approach the speed of light and the universe will go dark.

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09-07-2013, 08:45 AM
RE: Size of the universe
Ok, let me try to make my comment a little more clear.

I am standing at a fixed point. I have a flashlight in my right hand and another in my left. I extend my arms and turn each flashlight on. In one year, the light from the flashlight in my right hand will travel one light year to my right. In that same time, the light from the flashlight in my left hand will travel one light year to my left. Someone one light year to my left will be able to see that light. someone one light year to my right will be able to see THAT light.

How far are these two people from each other?
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09-07-2013, 08:55 AM
RE: Size of the universe
(09-07-2013 08:45 AM)TwoCultSurvivor Wrote:  Ok, let me try to make my comment a little more clear.

I am standing at a fixed point. I have a flashlight in my right hand and another in my left. I extend my arms and turn each flashlight on. In one year, the light from the flashlight in my right hand will travel one light year to my right. In that same time, the light from the flashlight in my left hand will travel one light year to my left. Someone one light year to my left will be able to see that light. someone one light year to my right will be able to see THAT light.

How far are these two people from each other?

Depends - how are they moving? How are you moving? (you're never moving relative to yourself, but the three observers can have relative motion).

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09-07-2013, 09:01 AM
RE: Size of the universe
Assume all other things being equal. I started by saying I am at a fixed point.
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09-07-2013, 09:05 AM
RE: Size of the universe
Let's remove the impossible hypothetical and replace it with a more possible (but still bloody unlikely) hypothetical.

A star explodes. Four light years away from that star is a planet. Four years after the star explodes, the people on that planet observe it. Four light years away in the exact opposite direction there's another planet. Four years after the star explodes, the people on THAT planet observe the explosion. The planets and the exploding star are not moving relative to each other. How far apart are the two planets?
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09-07-2013, 09:08 AM
RE: Size of the universe
(09-07-2013 08:45 AM)TwoCultSurvivor Wrote:  Ok, let me try to make my comment a little more clear.

I am standing at a fixed point. I have a flashlight in my right hand and another in my left. I extend my arms and turn each flashlight on. In one year, the light from the flashlight in my right hand will travel one light year to my right. In that same time, the light from the flashlight in my left hand will travel one light year to my left. Someone one light year to my left will be able to see that light. someone one light year to my right will be able to see THAT light.

How far are these two people from each other?


The easy answer is 2 light years apart. What is more complicated is explaining how they can be 2 light years apart but from the perspective of the light traveling the other light traveling in the other direction still was not traveling at faster than the speed of light. I'd defer you to Albert Einstein as he can explain it better than I can.

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09-07-2013, 09:12 AM
RE: Size of the universe
(09-07-2013 09:05 AM)TwoCultSurvivor Wrote:  Let's remove the impossible hypothetical and replace it with a more possible (but still bloody unlikely) hypothetical.

A star explodes. Four light years away from that star is a planet. Four years after the star explodes, the people on that planet observe it. Four light years away in the exact opposite direction there's another planet. Four years after the star explodes, the people on THAT planet observe the explosion. The planets and the exploding star are not moving relative to each other. How far apart are the two planets?

Same answer. 8 light years apart. But if you were on a surfboard riding the wave of photons from the supernova and looked back behind you the light traveling in the opposite direction would still appear (not just appear but actually be) to be traveling at the speed of light. Not twice the speed of light.

“Whenever you find yourself on the side of the majority, it's time to pause and reflect.”

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09-07-2013, 09:17 AM
RE: Size of the universe
(09-07-2013 09:05 AM)TwoCultSurvivor Wrote:  Let's remove the impossible hypothetical and replace it with a more possible (but still bloody unlikely) hypothetical.

A star explodes. Four light years away from that star is a planet. Four years after the star explodes, the people on that planet observe it. Four light years away in the exact opposite direction there's another planet. Four years after the star explodes, the people on THAT planet observe the explosion. The planets and the exploding star are not moving relative to each other. How far apart are the two planets?

8 light years, but that's because they are all stationary relative to each other. They each measure lightspeed the same (by definition), but they'll also measure the time it takes and the distance it crosses as being the same.

Incidentally, at 4 light years away from a nova, the planets' inhabitants are probably quite fried.

(09-07-2013 09:05 AM)TwoCultSurvivor Wrote:  Four light years away ... Four light years away in the exact opposite direction ... not moving relative to each other. How far apart are the two planets?

Consider a slightly modified situation. The star (A) and first planet B are not moving relative to each other. Second planet C is moving towards 'both' (they share a reference frame, so its motion is the same relative to either) at, say, half lightspeed (that is to say, planet C sees the other two moving towards it at half lightspeed).

How it works is, observer C sees light travelling from A, and sees it taking less than four light years to reach them. Since lightspeed is constant, less time means the distance must be less as well.

If you'd like, I can bust out the old classic Special Relativity primer: trains!

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