Size of the universe
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09-07-2013, 10:03 AM
RE: Size of the universe
Ok, but if we keep the simplicity of it, then you see the basis of my question.

You said eight light years, right? So how do I not conclude that the light from the supernova traveling to planet B is not moving at a relative velocity of 2c (relative to the light traveling to planet C)?

In other words, if two objects are traveling away from each other, each at the speed of light, isn't their relative speed 2c?
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09-07-2013, 10:06 AM
RE: Size of the universe
(09-07-2013 10:03 AM)TwoCultSurvivor Wrote:  In other words, if two objects are traveling away from each other, each at the speed of light, isn't their relative speed 2c?

Not when they are traveling at the speed of light. I'd have go back and freshen up on special relativity to better explain it. I know it doesn't seem to make sense but when you start diving into particle physics and quantum mechanics space and physics get pretty weird.

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09-07-2013, 10:14 AM
RE: Size of the universe
(09-07-2013 10:03 AM)TwoCultSurvivor Wrote:  Ok, but if we keep the simplicity of it, then you see the basis of my question.

You said eight light years, right? So how do I not conclude that the light from the supernova traveling to planet B is not moving at a relative velocity of 2c (relative to the light traveling to planet C)?

In other words, if two objects are traveling away from each other, each at the speed of light, isn't their relative speed 2c?

No.

Just no! The fundamental precept of the theory of relativity is that all observers measure the speed of light to be constant (er, disregarding media of differing optical densities for now).

Sounds batshit, yeah, but it's actually provable.

The first indication was the Michelson-Morley (and subsequent derived) experiment(s). Which measured lightspeed at various points in the Earth's orbit - if there were some 'fixed' cosmic background ('aether'), then the earth would have different relative motion to it at different points, which, if it affected the speed of light, would be measureable. Turns out that isn't so.

Subsequent experiments confirmed the existence of time dilation and/or length contraction (which are sort of two sides of the same coin) - that is, for moving observers, if lightspeed is the same and distance is different (because one of the observers is moving to create a longer path), then the perceived time must change (since velocity is constant and distance changes). Or, if lightspeed is constant and time is measured the same, then, relative motion causes the distance to be measured differently.

That's still only special relativity, mind; general relativity is actually easier to show (via observation of gravitational lensing). It's also necessary to make satellites work...

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09-07-2013, 10:20 AM
RE: Size of the universe
That, and that plugging superluminal velocities into the relativistic equations gives you all kind of paradoxical and impossible results.

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09-07-2013, 10:27 AM
RE: Size of the universe
So relative to each other, they're each traveling at the speed of light, no faster, but four years later they are eight light years apart, just as they would be if their relative velocity was 2c?

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09-07-2013, 04:11 PM
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?

Prolly 2ly times whatever the following simplifies to...

The first measurement of the expansion of space occurred with the creation of the Hubble diagram. Using standard candles with known intrinsic brightness, the expansion of the universe has been measured using redshift to derive Hubble's Constant: H0 = 67.15 ± 1.2 (km/s)/Mpc. For every million parsecs of distance from the observer, the rate of expansion increases by about 67 kilometers per second.[6][7][8] Since distant objects are observed further back in time, there is a one-to-one correspondence between the distance to a distant galaxy and the amount of time that has passed since the light being observed was emitted from that galaxy. Thus, Hubble's Constant can be thought of as an acceleration which in the local universe is equivalent to approximately meters per second per second, though this value is on large-scales dependent on how one defines the distance between two points and how one measures the elapsed time. Cosmologists often adopt comoving coordinates which remove the expansion altogether.

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09-07-2013, 04:37 PM
RE: Size of the universe
Well why didn't you just say so?
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09-07-2013, 04:55 PM
RE: Size of the universe
(09-07-2013 04:37 PM)TwoCultSurvivor Wrote:  Well why didn't you just say so?

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09-07-2013, 08:32 PM
RE: Size of the universe
Okay, I'm gonna throw in my own 2 cents here. Since galaxies are moving and space-time is expanding between them, the galaxies, which would normally be 28 billion light-years away if we assume that both galaxies are moving at the speed of light for 14 billion years, can actually be much further away than that due to the expansion of space-time during that period. I've never so much as taken a physics class, so I could be waaaay off. Is that the general concept of what we're talking about?

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09-07-2013, 08:41 PM
RE: Size of the universe
(09-07-2013 08:32 PM)FrostyMan Wrote:  Okay, I'm gonna throw in my own 2 cents here. Since galaxies are moving and space-time is expanding between them, the galaxies, which would normally be 28 billion light-years away if we assume that both galaxies are moving at the speed of light for 14 billion years, can actually be much further away than that due to the expansion of space-time during that period. I've never so much as taken a physics class, so I could be waaaay off. Is that the general concept of what we're talking about?

No, that's the general idea.

'Course, that specific question is an odd one; as far as the universe's expansion goes, in most senses that is what causes apparent relative motion in distant cosmic objects. Now, for something only ("only") dozens of light years apart, it's essentially negligible.

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