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Aki

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Different materials have different properties. Clean glass is an example of a material through which light passes easily - it is transparent. Hold it over a candle and you get soot on it (carbon) - and some parts of the glass are now opaque because carbon blocks out the light.

 

Now, when carbon is under extremely high pressure it turns into diamond, which has very good refractive quality (as in lasers and lenses).

 

So reflection, refraction, scattering, absorption, transparency etc is decided by the material through which light passes. This is how we can study star spectra, for example, by analyzing which parts of the light spectrum reach us and which parts do not.

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I think it was you who misunderstood my answer. :)

 

Materials do not "allow" light to pass through. The property of a material is what makes an object transparent or opaque. Most materials have degrees of both. There are several things which can influence transparency - for example, density, atomic structure, phase (ie, gas, crystal, liquid - observe how light is refracted in water but not in ice).

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OK, now I am confused. I sure don't want to argue with an astrophysicist. But c is only c in a vacuum. The "speed of light"/ thus the speed of a photon (I would assume) is not a constant except in a vacuum. It slows thru mass.

 

FT,

 

Let's see if I can simplify it for you. If a photon of light could have a rest mass, it would be as

though the speed of light had been slowed down to zero. Were you to create some very

dense light passing material, light would not be able to slowed down to rest. Blocking the

light alltogether is not zero either. ==> no rest mass. I would agree with Tormod's

astrophysicist's friend. :)

 

Maddog

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Maybe you misunderstood my question, I know that some materials allow light to pass, but what makes them different from things which don't?
Here's a way of looking at it.

When a photon comes in contact with a molecule, it is absorbed by an electron. This electron then 'moves up' and travels for a distance at the next valence level before falling back to it's original orbit. (Image insert)

In a pack of molecules the photon is continually absorbed and re-emited.

 

That help?

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I find that thinking about any attribute of matter/energy starts to become almost impossible at smaller and smaller scales. For instance, what is charge really? When pondered on a small enough scale, it loses tangible meaning just like mass. I like to think of mass and charge as dimensions just like time and space. Then I just think of the electron as a particle that is not moving in the "charge" dimension, but is sitting still at -1. Mass is more classical, since values are less discrete, and thus you can think of a particle accelerating as one that is losing mass but gaining energy.

 

This brings me to a question of my own: have there been experiments done to determine whether the gravity force travels at the speed of light or whether gravity is "instantaneous" like Newton and GR postulate? Should this experiment deem one of GR and QT right and the other wrong?

 

Gerbus

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I find that thinking about any attribute of matter/energy starts to become almost impossible at smaller and smaller scales. For instance, what is charge really? When pondered on a small enough scale, it loses tangible meaning just like mass. I like to think of mass and charge as dimensions just like time and space. Then I just think of the electron as a particle that is not moving in the "charge" dimension, but is sitting still at -1. Mass is more classical, since values are less discrete, and thus you can think of a particle accelerating as one that is losing mass but gaining energy.

 

This brings me to a question of my own: have there been experiments done to determine whether the gravity force travels at the speed of light or whether gravity is "instantaneous" like Newton and GR postulate? Should this experiment deem one of GR and QT right and the other wrong?

 

Gerbus

 

Gerbus,

 

String Theory has been harping that "charge" is somehow more fundamental whereas

mass is the expression of the particle interacting with the local Higgs field. This is the

theoretical notion which is from QFT.

 

From what I understand, it is thought that gravity moves at speed of light. Somewhat

empirical as I understand it. As far as I am aware no such has experiment has been

accomplished. Of course, how would you go about it. It would be true, if we could detect

gravitational waves from some event in space, that we would be able to determing its

speed. Hope this helps. :hihi:

 

Maddog

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Thanks maddog, that does help. It seems very obvious to me now that such an experiment is a long ways from being accomplished. It's too bad.

 

By the way, how is QFT holding up to scrutiny. QFT is grad school material, and thus I have not broken into it yet, but would like to get an idea of how it is doing so far in describing the world. I understand that the mathematics of it are quite complex, which is unappealing even from a GUT standpoint.

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1) Gravity is not a field force nor a conventional force. It is the bending or Curviture of Space-Time due to energy. Guv = 8(pi)ruv Einstein's field Equation curtousy of NASA.

2) It has no mediating particle. Because Gravity does not interact with Particles.

3) Space-Time Curviture propagates at c.

4) Mass and Energy are one in the same. Something with mass has energy, is energy. something with energy has mass, is mass. A photon, to my knowledge, has no charge. EVERY other particle has charge of sometype. Even the neutron has charge because of it's compisition (up down down) it's net charge is 0 however.

 

I would also venture to say that.

m = r^2

what radius I do not know. I suspect it is a quantazides basic universal distance.

I came to this conclusion when I attempted to convert the equation:

Fg = G m1 m2/ r^2 to Fg = G m/r to E = G I messed up somewhere and came up with the equation E = 299792458 r^4/t^2 I've encountered this same thing in several other equations I would like some help to prove or disprove.

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Hi KAC, welcome!

 

2) It has no mediating particle. Because Gravity does not interact with Particles.

 

The how do you explain gravitational red-shift? Photons escaping from the sun are red-shifted because they do not lose mass but frequency as they speed away from the star.

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How do we know gravitons have a spin of 2 if we've never seen one?

 

This is one of the first fall outs of String Theory by Schwarz and Green back in the late

70's. From what I understand this was speculated from GR as written in "Gravitation" by

Misner, Thorne. This is what lead a lot of people to think String Theory was going some

place. Apparently, it must still be going... and going... ;)

 

Maddog

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E = hf

h = planck's constant

f = frequency

 

A photon's frequency as I know it decreases with distances. when Space-Time is Curved the distance changes. So the photon travels more/less distance? Hmmm anyone know the equation for the Frequency-Distance relationship?

Anyway Gravity doesn't interact with mass or energy. Not in the classical sense. Space-Time Curviture will pass through mass-energy without lossing energy or otherwise being affected... As I understand it. particles are only particles when the interact with another particle other wise they are wave right?

 

well in a particle-particle interaction there is action with equal but oppisite reaction. gravity does not work that way. the two bodies of mass have equal but opposite reaction but Space-Time is not directly affected. You know what I mean?

 

Please some one help out and provide some math here... I won't be getting my Relativity books for some time.

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