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Gravitational lensing

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Boris
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Posted on Monday, 15 January, 2007 - 12:10 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

When light is bent by gravity, are all frequencies bent the same amount or do the frequencies separate like with a glass lens or prism?
I love deadlines. I like the whooshing sound they make as they fly by.
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Eagre
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Posted on Monday, 15 January, 2007 - 01:42 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Boris -

Light is not "bent" by gravity in the sense that it is by entering a prism of different refractive index. In the presence of mass it simply follows the curvature of space caused by the mass according to Einstein's equations (of general relativity). Thus no diffraction.

Ed Grens
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Boris
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Posted on Monday, 15 January, 2007 - 07:51 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks.

Next question:

Is the speed of light (C) the same now as it has always been?

Could a younger/smaller/denser the universe have a slower C?
I love deadlines. I like the whooshing sound they make as they fly by.
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Terry
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Posted on Monday, 15 January, 2007 - 09:28 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Are you in the right forum ?

Terry
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Grab
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Posted on Monday, 15 January, 2007 - 10:20 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

When you say "speed of light", you need to qualify it as "speed of light in vacuum". If it's in anything else, it's slower.

The best answer you're going to get at the moment is "probably". Cosmology relies on the principle that space is homogenous (ie. there's no areas where our known laws of physics don't apply) and that the various constants *are* (ie. God didn't just flick a switch a couple of million years after startup to make it behave differently).

There are a few hypotheses floating around that suggest constants have changed over time, but they're nothing more than suggestions as far as I know. They could be right, as could the "cold dark matter" hypothesis, or the "modified Newtonian dynamics" hypothesis, or the "string theory" hypothesis. There's a bunch of these suggestions hanging around, none of which have much in the way of evidence so far.

One thing to note though is that in a younger (and hence smaller) universe, distances were shorter. So whilst light wouldn't travel faster, it'd travel between two points in less time because the two points were closer together.

Graham.
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Scott2734
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Posted on Monday, 15 January, 2007 - 04:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Grab,

If you look around on the net you will find that light is the same speed no matter where it is or what is going through. When scientits were doing test to see if they could slow it down, they thought they had done it. They found out that light didnt slow down when it went through something, it just bounced around a lot inside the material making it come out a differant way, so it seemed that it had slowed down.But, it didnt, it had just took a longer route out of the medium.

There are 3 scientits now that said they have done it by doing a pulse frequency wave with lasers. There still waiting on the results.
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Epithumia
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Posted on Monday, 15 January, 2007 - 06:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Do you have a reference for that Scott? What you've said is the opposite of what I've always been led to believe.

For example, I thought the refractive index of a material was the ratio between the speed of light in the medium to the speed of light in a vacuum.

If this has something to do with the phase velocity vs the group velocity of the light then I'm going to need to read up a bit. I don't recall being taught about that!

Rob

PS. Wikipedia:
Note that the speed of light referred to is the observed or measured speed in some medium and not the true speed of light (as observed in vacuum). It may be noted, that once the light has emerged from the medium it changes back to its original speed and this is without gaining any energy. This can mean only one thing—that the light's speed itself was never altered in the first place.

Learning all the time! Add this to the list of lies they taught us at school!

(Apologies for multiple edits. Bad typing.

)

(Message edited by epithumia on 15 January, 2007)

(Message edited by epithumia on 15 January, 2007)

(Message edited by epithumia on 15 January, 2007)

(Message edited by epithumia on 15 January, 2007)
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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Obiwan
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Posted on Monday, 15 January, 2007 - 07:00 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Testing it would sure be a bugger.

Want to read something interesting?? Look up how long it takes light generated at the interior of the sun to reach the out side where it's emitted.
Do Not Hit The Fly That Lands On The Tigers Head.
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Obiwan
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Posted on Monday, 15 January, 2007 - 08:29 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Oh yeah, what I mentioned, falls in line with what Scott said about the speed being the same. It talks about the time it takes for the light generated at the core of the sun to reach the outter limits where it's emitted.

But because of the density of the sun, it takes a VERY long time for the light to actually work it's way out (I'll leave the time up to you).

So the light is traveling at a constant speed, the same speed, it's just taking longer because it has to bounce off of so much material.

I just realized that what I had mentioned was the same as what he was talking about.

(but the time it takes, is really impressive)
Do Not Hit The Fly That Lands On The Tigers Head.
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Boris
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Posted on Tuesday, 16 January, 2007 - 12:33 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks peeps.

Graham.

“One thing to note though is that in a younger (and hence smaller) universe, distances were shorter. So whilst light wouldn't travel faster, it'd travel between two points in less time because the two points were closer together.”

This is the point I’m trying to make. I think you’re wrong.

My ‘hypothesis’ if it can be called that, is that there was just as much SPACE in the younger/smaller universe as there is today. It was just denser. This is how I think of it: (Caution 4-D rubber sheet analogy alert)

Space is expanding. The distance between each body in the universe is getting larger. But they are not really moving in the normal speed/time sense. Rather, like dots on a rubber sheet, as the sheet is stretched, the dots get further apart, but none of them have ‘moved’ across the surface of the rubber sheet. If you imagine the universe as a 4-D sphere (rubber balloon) and the galaxies are drawn on the balloon in felt pen, then inflating the balloon shows in 3-D what the 4-D universe is doing when it expands. So all the objects get further away from each other but none have ‘moved’. Now, here’s the tricky bit:

Imagine a line of ‘marker’ objects placed in a straight line exactly 186000 miles apart. (One light-second apart). A beam of light travelling along the markers would pass one every second. So the speed of the light could be measured directly at 186000 miles/second.

Now leave the markers in place for say a billion years. They are further apart because space has expanded, but at the same time they have all remained perfectly still. You are saying that the light now takes longer than one second to get between markers. I am suggesting that the light will always take one second to get between markers. In the distant future where the universe has expanded and in the distant past where it was contracted. The speed of light is a function of the density of space.

Well that’s what I think.

I’m hoping to get some information from the learned members of this chat zone as to whether this is a new theory or a stupid theory or an old theory or whatever.
I love deadlines. I like the whooshing sound they make as they fly by.
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Obiwan
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Posted on Tuesday, 16 January, 2007 - 02:04 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think you'r "light second" example is flawed. Yes the distance is increased, so the time will take longer, not stay the same.

If I'm not mistaken Einestine has already proved that the speed of light doesn't change. (at least in relation to space, and not it's medium) That's why it's a "constant" in the first place, sort of like Keck's (I think that's the name for the expansion constant)

So if what you were saying is true, the speed of light wouldn't be lambda, it would be keck's constant times lambda.

But of course I could be wrong, and that ballon could be getting ready to 'pop' tonight!
Do Not Hit The Fly That Lands On The Tigers Head.
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Scott2734
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Posted on Tuesday, 16 January, 2007 - 02:54 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

This could make some realy nice EPE projects ! Hint Hint. Maybe a light speed tester, to test the speed.A frequency reader would be nice too.Break the light up into a spectrum and be able to read the frequency of each color. A light pulser/pusher iv always wanted to test.They have a simple setup on the net to try this one.The theory is if you can pulse the light waves at just the right time behind each other, they can actualy speed up light.I dont know how far they got with this, i havent looked at the site for a while.There are a bunch of other projects too dealing with light.
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John_becker
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Posted on Tuesday, 16 January, 2007 - 10:46 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Gulp, Scott - pass :-)

On the main matter, I used to know John Mason, then President of the Royal Astronomical Society. He & I often had chats in the car about such things as I drove him to the station from where we worked, he as editor of Astronomy Now mag.

He said the quoted speed of light is in relation to that through a vacuum, where it is constant. And weren't Einstein's theories based on that fact? - though I dont know that he ever had the ability to test or measure the speed.

And the question is unanswered - what caused light's energetic movement in the first place (religious considerations aside) ?

J
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Grab
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Posted on Tuesday, 16 January, 2007 - 11:04 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Sure, Scott - light doesn't actually slow down or speed up in different materials, it just travels a longer route. Same effect from the outside though. :-)

Boris, I think I'm right, although I'm only basing it on my NewScientist reading so I could well be wrong. :-)

Your rubber sheet example works perfectly. The two points are in the same relative positions, but the distance between them increases. 1 metre remains 1 metre though, and 1 second remains 1 second. So you're suggesting that light was originally a lot slower, and is becoming progressively faster and faster as space expands. I don't believe that's the case, although as I say I could well be wrong.

Regarding the variable speed of light, you might be interested in http://en.wikipedia.org/wiki/Variable_speed_of_light. I wasn't aware of that hypothesis before, but that's actually proposing that light was originally *faster* than it is now. Your suggestion is that light was originally *slower* and is now *faster* to cover the greater distance, which doesn't fit that.

Graham.
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Boris
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Posted on Wednesday, 17 January, 2007 - 02:40 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Ok.

If the marker objects were exactly one light second apart, and each object was a sphere of one light second in radius, then they would all be in contact with each other like a string of pearls. Then the universe expands. Does a gap appear between the marker spheres?

No, of course not, the spheres expand also, the gaps between atoms expand and so do the atoms themselves. This would have a drastic effect on all the ‘special’ cosmological constants that allow suns to work, chemistry and life and so on unless all the constants change too. The nuclear forces holding atoms together must change, and logically gravity and light must change also.

I think the expansion of space is actually more like an expansion of causality and effects everything. Including time. So measuring the speed of light locally will always give the same distance/time result, regardless of the size of the universe, but the ‘time’ in the equation passes at a different rate in different epochs of the universe, so the light is travelling effectively at different speeds.
I love deadlines. I like the whooshing sound they make as they fly by.
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Boris
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Posted on Thursday, 18 January, 2007 - 07:13 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Ok.

Imagine a piece of string extended between two points in space. Then the universe expands a bit.

Does the string break?
I love deadlines. I like the whooshing sound they make as they fly by.
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Grab
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Posted on Thursday, 18 January, 2007 - 09:39 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

No, because the string expands too. It doesn't change the fact that the string is now longer, so walking along it at a constant speed will take longer.

The nuclear forces holding atoms together must change, and logically gravity and light must change also.

That logic doesn't follow.

Since none of us are astrophysicists, you're unlikely to get a definitive confirmation/denial from any of us though! :-) Try writing into NewScientist for their Last Word page, maybe...

Graham.
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Obiwan
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Posted on Thursday, 18 January, 2007 - 09:40 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)


Since none of us are astrophysicists, you're unlikely to get a definitive confirmation/denial from any of us though! :-)


Not that some may not try!

That's why the doppler effect works. The speed of light is the same, it's just shifted into the red zone (from white). If it changed, we'd have no doppler effect. Everything would look like "white" light.
Do Not Hit The Fly That Lands On The Tigers Head.

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