Hubble Telescope

[warpedskydiver 0]

We are actually pretty close to the downtown area.

[billvon 3,006]

> Doesn't this mean that the light source there is never seen because light
>is being expanded away more quickly than it can move toward us? Kind of
>like an airplane traveling at 100 mph airspeed into a 120 mph headwind
>- it will never be able to move forward relative to a static ground observer.

Not really; the speed of light is largely invariant. And that leads to behavior that is very non-obvious.

Let's take an example. (We'll use the train example because everyone uses the train example.) Train A leaves the platform going east at 60% of the speed of light; train B leaves the platform going west at 60% of the speed of light. That would lead one to believe that they would never see each other once they left the station, since the light from one train couldn't possibly reach the other (their relative velocity is greater than lightspeed.)

However, they do in fact see each other. To show why this is so, imagine a man with a laser standing by the track. Just as train A passes, he fires his laser at train B. Someone on train A fires a laser at train B at exactly the same time.

Now, we can probably easily agree that the person on train B would see the stationary laser; after all, the train is only going .6C and the laser light is going 1C. However, that person also sees the laser from the train at exactly the same moment, because the speed of light is a constant. There is no way for train A to fire a "slow" laser at train B, at the instant the beam emerges from both lasers they are both going at exactly the same speed.

There will be one important difference, though. If both lasers were green lasers, they will not be seen as green when they arrive. They will both be red-shifted. Indeed, even if the guy on train B did not know who fired which laser, he could discover that simply by measuring the wavelength of the light. Whichever beam was more red-shifted came from the train, since its relative velocity is higher.

[warpedskydiver 0]

Which train would be hit by the GBU27 first?

[Andrewwhyte 1]

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Whichever beam was more red-shifted came from the train, since its relative velocity is higher.

The relative velocity of the less red shifted one is 1C; how much higher is the other.

[lawrocket 3]

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At that distance, though, the universe is expanding at greater than the speed of light.

That's probably an incorrect assumption.

Again, looking at Hubble's constant. The speed of light does not appear to be an asymptote. Rather, the potential speed of expansion appears to be infinite.

The speed of expansion is relative to distance. The further away something is the more quickly is expands. Think of an uninflated balloon - the bigger it gets the more quickly it expands and the distances grow and grow the further away from a certain point you get.

I would think that there is visible light we can see from distant galaxies or quasars that will slowly fade NOT because they burned out but because they are moving away from us more quickly than the speed of light. The further away, the more quick the relative expansion.

This expansion is theorized to continue into the future. 100 billion years into the future, we likely will be unable to see any stars in the sky because they will too far away. No stars, galaxies, etc. They've all gotten too far away to see - and the continued expansion will make the universe a very cold place, anyway.

My wife is hotter than your wife.

[elias123 0]

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If you really need more information I can refer you to some excellent texts and resources.

Please do

"In a mad world, only the mad are sane"

[billvon 3,006]

>The relative velocity of the less red shifted one is 1C; how much higher is the other.

The velocity of the beams is exactly the same. However, the velocities of the sources are different. This has two effects:

1) The Doppler shift of the light results in the more rapidly receding source to be more red-shifted.

2) Time dilation (due to the significant relative motion of the trains) also causes a redshift, greater on the source that is moving away the most rapidly.

[warpedskydiver 0]

http://www.amazon.com/gp/product/images/0691130272/ref=dp_image_0?ie=UTF8&n=283155&s=books

I found that it was a well written and useful book not only for my class but for future reference.


http://www.amazon.com/gp/product/images/0030062284/ref=dp_image_0?ie=UTF8&n=283155&s=books
This one was a good introduction text.

http://www.netlibrary.com.proxy.devry.edu/

And of course the LoC

http://www.loc.gov/index.html

LoC is my favorite research tool, we should all make the best use of the most thorough collection of publications and arts that is known to man.

[cloudclimr 0]

you said what?

[Andy9o8 2]

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100 billion years into the future, we likely will be unable to see any stars in the sky because they will too far away.

So what? By that time I'll be retired.

[champu 1]

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>This article got me thinking. If Hubble is looking "back in time" 13
>billion years, with a little more advanced technology, what happens if it
>looks back 13.8 billion years? Would it be possible to see the big bang?

We can see it now; we call it "cosmic background radiation." We're inside the sphere of expanding timespace that is the universe, so we can't see it from outside (of course.) But from inside we can see the afterglow of the edge, and it's pretty uniform in all directions.

wouldnt that be an interpretation of what we think we see? although i like the thought of seeing the end of the world, it makes me smile :)

A couple interesting things to read about on this topic are the COBE and Wilkinson missions.

[aresye 0]

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This expansion is theorized to continue into the future. 100 billion years into the future, we likely will be unable to see any stars in the sky because they will too far away. No stars, galaxies, etc. They've all gotten too far away to see

Well, that sucks for dudes with NVG's.

Skydiving: You either learn from other's mistakes, or they'll learn from yours.

[popsjumper 2]

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Train A leaves the platform going east at 60% of the speed of light; train B leaves the platform going west at 60% of the speed of light.

Tsk, tsk, tsk...the two trains ran into each other at the station. Dang big crash before they got off to a good start.

Bill, turn them around so they both are leaving town.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[kallend 2,027]

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At that distance, though, the universe is expanding at greater than the speed of light.

That's probably an incorrect assumption.

Again, looking at Hubble's constant. The speed of light does not appear to be an asymptote. Rather, the potential speed of expansion appears to be infinite.

The speed of expansion is relative to distance. The further away something is the more quickly is expands. Think of an uninflated balloon - the bigger it gets the more quickly it expands and the distances grow and grow the further away from a certain point you get.

I would think that there is visible light we can see from distant galaxies or quasars that will slowly fade NOT because they burned out but because they are moving away from us more quickly than the speed of light. The further away, the more quick the relative expansion.

This expansion is theorized to continue into the future. 100 billion years into the future, we likely will be unable to see any stars in the sky because they will too far away. No stars, galaxies, etc. They've all gotten too far away to see - and the continued expansion will make the universe a very cold place, anyway.

General relativity allows distant galaxies to be receding from each other faster than c on account of the expansion of spacetime. That means light emitted in one galaxy will never reach the other.

The accelerating expansion rate due, it is suggested, to dark energy means that distant galaxies currently visible to us will at some time in the future go outside our visible universe.

The c limit relates to local velocities as described by special relativity.

...

The only sure way to survive a canopy collision is not to have one.

[kallend 2,027]

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> Doesn't this mean that the light source there is never seen because light
>is being expanded away more quickly than it can move toward us? Kind of
>like an airplane traveling at 100 mph airspeed into a 120 mph headwind
>- it will never be able to move forward relative to a static ground observer.

Not really; the speed of light is largely invariant. And that leads to behavior that is very non-obvious.

Let's take an example. (We'll use the train example because everyone uses the train example.) Train A leaves the platform going east at 60% of the speed of light; train B leaves the platform going west at 60% of the speed of light. That would lead one to believe that they would never see each other once they left the station, since the light from one train couldn't possibly reach the other (their relative velocity is greater than lightspeed.)

However, they do in fact see each other. To show why this is so, imagine a man with a laser standing by the track. Just as train A passes, he fires his laser at train B. Someone on train A fires a laser at train B at exactly the same time.

Now, we can probably easily agree that the person on train B would see the stationary laser; after all, the train is only going .6C and the laser light is going 1C. However, that person also sees the laser from the train at exactly the same moment, because the speed of light is a constant. There is no way for train A to fire a "slow" laser at train B, at the instant the beam emerges from both lasers they are both going at exactly the same speed.

There will be one important difference, though. If both lasers were green lasers, they will not be seen as green when they arrive. They will both be red-shifted. Indeed, even if the guy on train B did not know who fired which laser, he could discover that simply by measuring the wavelength of the light. Whichever beam was more red-shifted came from the train, since its relative velocity is higher.

That is a special relativity explanation. The expansion of the universe requires a general relativity explanation.

...

The only sure way to survive a canopy collision is not to have one.

[lawrocket 3]

Thanks, prof!

My wife is hotter than your wife.

[elias123 0]

Muchos gracias amigo

"In a mad world, only the mad are sane"

[kallend 2,027]

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Thanks, prof!

This wiki article explains it fairly well, and describes the differences between special and general relativity in an expanding universe:

en.wikipedia.org/wiki/Expansion_of_the_universe

While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, there is no such theoretical constraint when space itself is expanding. It is thus possible for two very distant objects to be moving away from each other at a speed greater than the speed of light (meaning that one cannot be observed from the other). The size of the observable universe could thus be smaller than the entire universe.

...

The only sure way to survive a canopy collision is not to have one.

[lawrocket 3]

Thanks again, prof. Exactly what I was thinking.

Next time I'm in Chi-town I'll owe you a beer. Good beer.

My wife is hotter than your wife.