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Jeremiah Walgren
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Joined: 24 May 2003
Posts: 1937
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 Posted: 21 Feb 2006 02:02:52 pm Post subject: |
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| Watch it buddy, or I'll start throwing some neutrinos at you. |
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Radical Pi
Advanced Member
Joined: 23 Nov 2005
Posts: 455
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| Posted: 21 Feb 2006 03:08:14 pm Post subject: |
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What's your favorite flavor of quark? 
strange |
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Tiberious726
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Joined: 07 Oct 2005
Posts: 284
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| Posted: 21 Feb 2006 05:12:00 pm Post subject: |
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JoeImp wrote: Light does not have mass.
And yes, the speed of light varies in different substances, and yes, we can/have manipulated it.
[post="65570"]<{POST_SNAPBACK}>[/post]
light has mass; all energy has mass; my example for this is hawkings radiation which has less mass so it can escape from black holes (we all know that it can move faster than light under our current model so it must have less mass)
EDIT: my favorite quark would be charm
Last edited by Guest on 21 Feb 2006 05:12:44 pm; edited 1 time in total |
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Jeremiah Walgren
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Joined: 24 May 2003
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| Posted: 21 Feb 2006 07:14:33 pm Post subject: |
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Quote: my example for this is hawkings radiation which has less mass so it can escape from black holes (we all know that it can move faster than light under our current model so it must have less mass)
As I remember, Hawking radiation is nothing more than particles or antiparticles being blasted away from black holes.
It's been a while, but this is what I remember. In the quantum foam of the Universe, particles and antiparticles are being formed from the energy contained in a photon (or something), living for a very short amount of time and then combining to be annihilated into photons/energy again.
At the horizon of a black hole, one of these particle-antiparticle pairs is caught in the gravitational field and sucked in while the other member of the pair is blasted out into space (I forget how it receives the energ to do so). These particles constitue Hawking radiation.
Explain what you mean by all energy having mass. |
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Fr0stbyte124
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Joined: 26 Jan 2006
Posts: 98
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| Posted: 21 Feb 2006 11:08:28 pm Post subject: |
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Once again, mass is not needed to be affected by gravity, only to produce it. All objects fall at the same speed, even light. (Technically, that's only if the larger object is chained-down, but with the Earth, that's usually negligible)
Gravity is the effect of distortion in 4D space-time caused by the presence of mass that makes things appear to orbid around massive objects, when in fact, they move in straight lines. |
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Jeremiah Walgren
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Joined: 24 May 2003
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| Posted: 22 Feb 2006 12:45:53 am Post subject: |
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I will relate an analogy that I think will convey how light can be affected by gravity. I heard it eight years ago, though, so if it's outdated or entirely wrong, feel free to point that out. Just beware I might defend it if you don't provide a sufficient argument...
I believe I related this earlier, but I lack the energy to copy and paste. Besides, it'll save time putting it here again anyway.
Think of space as a giant bed sheet, as it were. Spread out nice and flat. You can even have a bunch of your friends do this if you want to see it for yourself.
Now, take a ball of some sort and place it in the middle. Note how the sheet sags to the point of where the ball is. This is the curvature mass causes on space-time.
To simulate how gravity affects light, then, take a marble or small sphere and shoot it by the larger sphere in the middle. Note how the smaller sphere bends towards the larger sphere in the middle and follows a curved path past it (and might even strike it). This is how gravity affects light.
In the case of a black hole, the larger sphere is so dense that it essentially creates a very deep indentation in the sheet that the smaller spheres (light) just fall into.
...
So, how'd I do? |
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Fr0stbyte124
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Joined: 26 Jan 2006
Posts: 98
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| Posted: 22 Feb 2006 06:46:41 pm Post subject: |
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| And before anyone points out something is illogical in the analogy, remember that the sheet exists in 4D, and it can do a whole lot of stuff that makes no sense. |
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Arcane Wizard
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Joined: 02 Jun 2003
Posts: 8993
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| Posted: 22 Feb 2006 07:25:52 pm Post subject: |
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Jeremiah Walgren wrote: In the case of a black hole, the larger sphere is so dense that it essentially creates a very deep indentation in the sheet that the smaller spheres (light) just fall into.
...
So, how'd I do?
[post="70496"]<{POST_SNAPBACK}>[/post] Sounds about right.
For a graphic impression think of this Simpsons episode:
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Super Speler
Super Awesome Dude
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Joined: 28 Nov 2005
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| Posted: 22 Feb 2006 09:52:07 pm Post subject: |
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| Wait... how can light speed up to normal after it leaves the medium? |
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Jeremiah Walgren
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| Posted: 23 Feb 2006 04:34:58 pm Post subject: |
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It doesn't. It is simply allowed to travel without being hindered by atoms in the way.
If you'll remember, it was the process of the photons being absorbed and then emitted that caused the delay. Since there are no more things to absorb and emit said photons then it can proceed at it's regular pace, if you will.
Or, at least, not as many things to absorb and emit... |
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Arcane Wizard
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| Posted: 23 Feb 2006 05:36:51 pm Post subject: |
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Shouldn't light have different speeds in all sorts of bodies then?
(effective speed, from x to y in z time, not the speed at which the particles actually move.. when they move)
Last edited by Guest on 23 Feb 2006 05:37:56 pm; edited 1 time in total |
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Jeremiah Walgren
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| Posted: 23 Feb 2006 07:37:26 pm Post subject: |
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| I would guess so. I mean, I've taken powerful lights and put my hand over them to witness my skin glowing, as it were. I imagine there's really not much of a difference between my hand and glass at the atomic level, besides density or something like that. |
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Arcane Wizard
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| Posted: 23 Feb 2006 08:06:48 pm Post subject: |
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What then of all those experiements that kept/keep getting a constant value for c?
I was taught that this absorbing of photons by nuclei was only possible in rare special cases. Like how they managed to freeze light in that experiment I posted about. |
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Super Speler
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| Posted: 23 Feb 2006 09:37:53 pm Post subject: |
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C is constant always. C is the speed of light in a vacuum not in every medium. Light moves at different speeds in different mediums.
Last edited by Guest on 23 Feb 2006 09:38:23 pm; edited 1 time in total |
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Jeremiah Walgren
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Joined: 24 May 2003
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| Posted: 24 Feb 2006 12:20:22 am Post subject: |
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Quote: What then of all those experiements that kept/keep getting a constant value for c?
Which... ones?
Quote: I was taught that this absorbing of photons by nuclei was only possible in rare special cases. Like how they managed to freeze light in that experiment I posted about.
I was taught that as well. However, electrons have no such restriction from what I have learned. Since there are literally billions of them in a small amount of substance, their effect can add up...
Quote: C is constant always. C is the speed of light in a vacuum not in every medium. Light moves at different speeds in different mediums.
You mean it takes light different amounts of time to pass through different mediums. As you said, "C is constant always." C is, after all, the speed of light. |
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Arcane Wizard
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| Posted: 24 Feb 2006 08:20:08 am Post subject: |
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And mediums excist of vacuum for the most part. : ) (from a light particle's point of view)
Last edited by Guest on 24 Feb 2006 08:20:29 am; edited 1 time in total |
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Super Speler
Super Awesome Dude
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| Posted: 24 Feb 2006 03:32:19 pm Post subject: |
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| I still don't understand how the momentum returns. |
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Jeremiah Walgren
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| Posted: 24 Feb 2006 07:35:19 pm Post subject: |
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Let's assume, then, that a photon has managed to pass through nearly all of a glass windows without coming into contact with any of the electrons or nuclei residing therein. Just as it's about to leave the glass, however, it hits the last electron. We shall analyze what happens here.
Now then, E = mc^2. Since c is a velocity, this can be rewritten (momentarily) as E = mv^2 or E = mv*v. Referring back to momentum, then, we have p=mv. Substituting this back into E = mv*v, we now have E=pv. Replacing v with c (the velocity of the photon in question), we have E = pc. Solving for p will give us the momentum of the photon, then. So, p=E/c.
A quick step-by-step overview of the math:
Code:
E=mc^2
c=v
E=mv^2
E=mv*v
p=mv
E=pv
v=c
E=pc
p=E/c
Continuing on, then.
Looking at our new equation (p=E/c) that gives the momentum of the photon, we can see that it is entirely dependent upon the energy of said photon. With that said, we can now look at what happens when the photon hits the electron.
There is a certain property of electrons that comes into play here. Electrons can only exist at certain energy levels for extended periods of time. (From what I understand, this is because of the wave-particle duality of nature which I won't elaborate on in this post.) So when this photon hits the electron, the energy of the photon is transferred to the electron and causes it to gain energy. Sadly, the photon is annihilated in this process.
However, generally this won't be enough energy to raise it to the next level. The photon's energy, then, will cause the electron to enter a "virtual" energy level, as it were, and the electron will exist there for a very, very short amount of time. Once it can no longer exist at this virtual level, it will release all of the energy it gained in the form of a photon and return to its previous energy level.
Kind of how it is with some math thrown in:
pi/pf (initial momentum, final momentum)
Ei/Ef (final energy, final momentum)
c (speed of light - constant)
The initial momentum.
pi = Ei /c
Ei = pi*c
The final momentum.
pf = Ef/c
Ef = pf*c
The final energy is equal to the intial energy.
Ef = Ei
Substituting.
pf*c = pi*c
Simplified.
pf = pi
Probably a lot of jarbled thoughts, but I think I covered it as best as I can... Still lost on how the momentum is conserved? |
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Fr0stbyte124
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Joined: 26 Jan 2006
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| Posted: 24 Feb 2006 09:27:00 pm Post subject: |
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Quote: Ei/Ef (final energy, final momentum)
(inital energy, final energy)?
That momentum isn't really a property of the light, since it has no mass. That's the momentum passed onto the atom. Since it isn't getting that momentum from kinetic energy, it absorbs the actual energy that the photon is made of. It loses that momentum when it spits the light back out.
Have you ever seen those glass bulbs with the rotating fins on the inside? (one side is reflective, the other is black) They work on the same property.
Also, lasers work in a similar manner, by passing an EMF through the medium to artificially raise the energy levels. Then, lo and behold, real light starts pouring out, with the side affect of each photon being the exact same wavelength traveling with the exact same vector.
I don't suppose anyone happens to have a textbook on quantum mechanics? Meh, google's good too, I guess. |
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Jeremiah Walgren
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Joined: 24 May 2003
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| Posted: 24 Feb 2006 10:12:39 pm Post subject: |
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| Yeah, thanks Fr0stbyte124. I was rushing a bit when I typed that... |
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