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No dark matter?
#61
RE: No dark matter?
(May 8, 2011 at 5:16 pm)little_monkey Wrote: Sorry for being late on this thread. However, that is totally wrong. The speed of light near a black hole, or anywhere else, is the same. Deny that, and you're denying the foundation of Special and General Relativity.

Inside the black hole, the light doesn't escape because it is bent by gravity into a circle, and so doesn't escape, but its speed remains the same. Outside the BH, your clock will slow down and your ruler will shrink, nevertheless, you will end up measuring the speed of light to be c.


Actually, this is not quite true. The problem with general relativity is that things can be coordinate dependant. Take for example the Schwarzschild geometry: from the time Schwarzschild came up with it during the first world war (whilst fighting in the trenches, no less) up until the 1960's, everyone thought that the singularity at the schwarzschild radius was a real physical effect, but it turns out to be just an artifact of the coordinates.
The coordinate speed of light can be non-constant (or =/= c), but the proper speed is always c. An observer that doesn't know about relativity that wants to measure the speed of light in a non-inertial frame will measure (distance travelled by light measured by my ruler) / (time elapsed on my clock) which is the average coordinate speed which could be something other than c.

For the example of a black hole you gave, the coordinate speed of light at the surface of a black hole is zero whereas it's proper speed is c.


Regardless, the quran says nothing about any of this!
Galileo was a man of science oppressed by the irrational and superstitious. Today, he is used by the irrational and superstitious who claim they are being oppressed by science - Mark Crislip
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#62
RE: No dark matter?
(May 8, 2011 at 5:16 pm)little_monkey Wrote: The speed of light near a black hole, or anywhere else, is the same.
The speed of light depends on the material (or lack of material) it is passing through. Light travels fastest in a vacuum, but is slower when traveling through, say, air or water.
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#63
RE: No dark matter?
(May 8, 2011 at 5:16 pm)little_monkey Wrote:
(May 5, 2011 at 4:31 am)Wormhole199 Wrote:
(May 5, 2011 at 4:01 am)lilphil1989 Wrote: A non-local frame means a global frame...

No it doesn't mean a global frame. It means that the frame is not at the same location as the observed event. Like you measuring the speed of light near a black hole; in your clock and ruler it is a few meters/sec.

Sorry for being late on this thread. However, that is totally wrong. The speed of light near a black hole, or anywhere else, is the same. Deny that, and you're denying the foundation of Special and General Relativity.

Inside the black hole, the light doesn't escape because it is bent by gravity into a circle, and so doesn't escape, but its speed remains the same. Outside the BH, your clock will slow down and your ruler will shrink, nevertheless, you will end up measuring the speed of light to be c.

Suppose that you have a clock and a ruler (which is not rotating with respect to stars) and that you are not accelerating (inertial). Locally (where you are) you will always measure the speed of light at 299792.458 km/sec. However in the presence of gravity if I am at a different location than yours then I could measure the speed of light at your location to be any value smaller than or greater than 299792.458 km/sec. It depends on where I am and where you are (it depends on locations). So in the presence of gravity the speed of light becomes relative (variable depending on the reference frame of the observer). This does not mean that photons accelerate or decelerate. This is just gravity causing clocks to run slower and rulers to shrink.

To measure the speed of light at a different location than yours first you have to define a local inertial frame. (If your frame is non-inertial, rotating for example, then the following is not for you). With the same clock and ruler the velocity of light becomes a variable depending on the reference frame of the observer. You can find the full derivation by Einstein himself (1955) from the full theory of general relativity in the weak field approximation:

'The Meaning of Relativity', A. Einstein, Princeton University Press (1955).

See pages 92-93, eqn (107); the variable velocity of light expressed in coordinates is:

[Image: speed_of_light_23.gif]

If your frame of reference is near a black hole and you are measuring the speed of light outside gravitational fields then the result is a zillion km/sec.

If your frame of reference is outside gravitational fields and you are measuring the speed of light near a black hole then the result is a few meters/sec. This scenario by convention is called the coordinate speed of light.

If your frame of reference is at the same location where light is propagating (both near a black hole or both outside gravitational fields) then the result is always 299792.458 km/sec. But remember that you defined a local inertial frame to begin with.

So it is absolutely meaningless to talk about the speed of light without defining your frame of reference.

My favorite site: Speed of Light.
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#64
RE: No dark matter?
(May 8, 2011 at 11:48 pm)Wormhole199 Wrote:
(May 8, 2011 at 5:16 pm)little_monkey Wrote:
(May 5, 2011 at 4:31 am)Wormhole199 Wrote:
(May 5, 2011 at 4:01 am)lilphil1989 Wrote: A non-local frame means a global frame...

No it doesn't mean a global frame. It means that the frame is not at the same location as the observed event. Like you measuring the speed of light near a black hole; in your clock and ruler it is a few meters/sec.

Sorry for being late on this thread. However, that is totally wrong. The speed of light near a black hole, or anywhere else, is the same. Deny that, and you're denying the foundation of Special and General Relativity.

Inside the black hole, the light doesn't escape because it is bent by gravity into a circle, and so doesn't escape, but its speed remains the same. Outside the BH, your clock will slow down and your ruler will shrink, nevertheless, you will end up measuring the speed of light to be c.

Suppose that you are outside gravitational fields, and that you have a clock and a ruler (which is not rotating with respect to stars), and that you are not accelerating. Based on your clock and ruler what is the measured speed of light near a black hole?


c
(May 9, 2011 at 3:00 am)lilphil1989 Wrote:
(May 8, 2011 at 5:16 pm)little_monkey Wrote: Sorry for being late on this thread. However, that is totally wrong. The speed of light near a black hole, or anywhere else, is the same. Deny that, and you're denying the foundation of Special and General Relativity.

Inside the black hole, the light doesn't escape because it is bent by gravity into a circle, and so doesn't escape, but its speed remains the same. Outside the BH, your clock will slow down and your ruler will shrink, nevertheless, you will end up measuring the speed of light to be c.


Actually, this is not quite true.

Which part is not true?

Quote:The problem with general relativity is that things can be coordinate dependant. Take for example the Schwarzschild geometry: from the time Schwarzschild came up with it during the first world war (whilst fighting in the trenches, no less) up until the 1960's, everyone thought that the singularity at the schwarzschild radius was a real physical effect, but it turns out to be just an artifact of the coordinates.

Yes that's true. For an observer falling into a black hole, he would not observe anything significant. He would not realize that any light he would be sending out would never go beyond the event horizon.

Quote:The coordinate speed of light can be non-constant (or =/= c), but the proper speed is always c.
The speed of light is always the proper speed, unless specified differently. This means observers can detect when their motion is accelerated by studying light signals.

Quote:An observer that doesn't know about relativity that wants to measure the speed of light in a non-inertial frame will measure (distance travelled by light measured by my ruler) / (time elapsed on my clock) which is the average coordinate speed which could be something other than c.

The speed of light does not vary because some observer happens to be ignorant of the laws of physics. His corrections can amply be supplied by one who does know relativity.

Quote:For the example of a black hole you gave, the coordinate speed of light at the surface of a black hole is zero whereas it's proper speed is c.

The light I referred to in my post was emanating from inside the black hole. If there was anything wrong on that, please enlighten me.




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#65
RE: No dark matter?
(May 9, 2011 at 8:08 am)Wormhole199 Wrote: So it is absolutely meaningless to talk about the speed of light without defining your frame of reference.
Coordinate systems are just a convenient way to label points. They confer no intrinsic properties to space-time.

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#66
RE: No dark matter?
Quote:The speed of light is always the proper speed, unless specified differently.

Of course the proper speed of light is always c, (I'd even say it was tautologically true, since it's pretty much just the statement that a null path is a null path) and I'm not disagreeing with that. In fact, a few posts ago I said exactly that.
My point was that two accelerated observers can measure different coordinate lightspeeds.

Quote:The speed of light does not vary because some observer happens to be ignorant of the laws of physics. His corrections can amply be supplied by one who does know relativity.

I didn't say that. I meant that if he doesn't know to correct his coordinate time interval such that light is travelling on a ds=0 path, then the coordinate speed he measures can differ from c. Does this mean that the "true" speed of light is not c? Well, that depends entirely on what you mean by the speed of light in a non-inertial frame. Perhaps the best way to deal with the problem is just to construct a locally inertial frame, in which it is obvious and the coordinate speed and the proper speed have the same value.



Galileo was a man of science oppressed by the irrational and superstitious. Today, he is used by the irrational and superstitious who claim they are being oppressed by science - Mark Crislip
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#67
RE: No dark matter?
(May 9, 2011 at 6:17 pm)lilphil1989 Wrote:
Quote:The speed of light does not vary because some observer happens to be ignorant of the laws of physics. His corrections can amply be supplied by one who does know relativity.

I didn't say that. I meant that if he doesn't know to correct his coordinate time interval such that light is travelling on a ds=0 path, then the coordinate speed he measures can differ from c. Does this mean that the "true" speed of light is not c? Well, that depends entirely on what you mean by the speed of light in a non-inertial frame. Perhaps the best way to deal with the problem is just to construct a locally inertial frame, in which it is obvious and the coordinate speed and the proper speed have the same value.
The way to do physics properly is to start with Lorentz invariant equations and then figure out what it means in a specific coordinate system. From there you can figure out what is velocity or force or any other quantity. Needless to say that these equations will be written in tensorial form. To make matter easy, you define the proper time (τ) of the worldline of a particle as the parameter that parametrize the worldline of that particle. You then define the velocity u = dx/dτ, which has components ( dt/dτ, dx/dτ, dy/dτ, dz/dτ), etc. In that development of relativity, the speed of light is ALWAYS constant. If you throw that out, then you might as well throw out relativity altogether.

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#68
RE: No dark matter?
I would also have to argue that there is evidence for dark matter to exist that we can observe also. Lawrence Krause did his work on this area and stated that it gives off a weak gravitational force, also light likes to go around high concentrations of dark matter.

The figures of dark matter and dark energy also tie in with the universe being flat. We are also aware that our universe is flat due to a certain type of way to measure the universe to see if the light that comes back is stretched, shrunk or the same size as it was before indicating what kind of universe we live in (that being flat).

My explanation is pretty piss poor so I will link you directly to the video. It is about an hour long and although I grasp all the concepts, he didn't go into detail on the measurements of the universe for me to fully understand HOW he did it.

http://www.youtube.com/watch?v=7ImvlS8PLIo
(April 20, 2011 at 7:43 am)alemcodon Wrote:
Quote:sorry to bursted your bubbles but dark matter and the bogus theory above - are exactly that bogus theories.
I think it's pretty ballsy to come out and just say that when it is the model current physicists are using today. If it was bogus they would not be using it.

Quote:I think its probably going to be a phenomenon that we (or budding zweisteins) would never be able to explain.
I was about to label this a hypothesis before I read the assumed conclusion at the end.

Quote:If dark matter does exist then is it at a perfect balance in EVERY galaxy, from trillions of billions of galaxies, where it keeps order and prevents mass mayhem. Why is it not in some galaxies the stars on the outer edge could be going too fast, or too slow? (thus causing mayhem and 'randomness' - in which our universe is built???)
Dark matter attracts dark matter to form bigger balls/spheres/clumps of dark matter. From what I know I do think there is reason to believe there is a weak attraction between matter and dark matter but someone may need to back me up on this as I am not 100% sure on this

Quote:Ich bin muslim (yeah yeah save your curses for an anti islamic forum)....the quraan tells us angels are keeping order in the stars - so my personal belief would therefore be this order is being kept by angels, and not dark energy/mass. Although it would be feasible to decsribe the angels as dark energy since we cannot see them, and they have a impact/force which cannot be seen or measured.
You can call dark matter what ever you like. Call it angels if you really want. However the difference between changing the word in for another is not the same as dark matter having magical properties that has a conscious mind that goes 'I will do this because this galaxy needs to hold together', there is no reason to suggest there is conscious thought behind it.

Quote:Angels are described as being made of 'light', which is kinda like a metaphore as humans are made from 'earth', although our mass is physically from the earth, it is not just mud we're made from, so angels are made from light, or various components of light.
That depends who you ask and what kind of angel.

The rest of your post was just babble about how the Ko'ran is right because so I don't see any further need to quote you other than your lack of understanding of the dark matter theory. There is nothing wrong with that as I don't understand it fully myself but at least I do not claim it is bogus on a whim.

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#69
RE: No dark matter?
Interesting read: CRESST uncovers hint of dark matter
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