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| < Science ~ Did the Big Bang really happen? |
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Thu Aug 11, 2005 2:44 pm
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I would like to add additionally that tractor beams and solid state holograms are possible (yet we haven't been able to create them on the macro scale, just the nano scale.) This is possible because of optical tweezing or trapping. Intersting stuff that may help you understand optics a bit better.
http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-26-3562
[/url] |
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| Posted:
Thu Aug 11, 2005 5:21 pm
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Joined: Jun 21, 2005
Posts: 94
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| cwes99_03 wrote: |
| Don't kid yourself into believing that light has any sort of manipulatable mass. Light is not directly affected by gravity. Instead it is described as space time being bent and light traveling the straightest possible path through space time. |
All objects are affected by gravity by the means of curved space-time so I don't understand why you make it look like light is the exception to the rule. Also stars that are brighter but of equivalent rest mass as compared to dimmer stars have stronger gravity fields.
Light exhibits many rest mass like effects such as imparting mometum and having gravity. Some of the effects can be explain with QFT (quantum field theory) |
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| Posted:
Thu Aug 11, 2005 11:25 pm
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Joined: Jun 21, 2005
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| Posted:
Fri Aug 12, 2005 9:21 pm
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Let's consider the source of that information first.
http://van.hep.uiuc.edu/
Then let's read that article closely. The author of that statement is trying to explain relativity to a 16 year old. Second Mike is saying that he likes to think of a box of Photons vs a box of coal. He is going to relate the mc^2 energy of the coal to the (pc)^2 energy of the photons.
They do at least point out that there is not frame in which light can ever be at rest, but then they say the box is a great idea because it creates a frame where the sum of all the momenta of the photons in the box is zero. The only way to get a zero momentum is to treat every photon as a particle with mass and velocity and then cancel out the collective velocities by saying if my frame of reference is the box and all the photons are trapped in the box. Since we already know that photons have no rest mass, this idea is absurd. Secondly, it goes directly against what they said above, that no reference frame can exist where light is at rest. Then he says
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| It's nice that we can think of an atom or a block moving relativistically without having to worry about each subatomic particle inside |
http://van.hep.uiuc.edu/van/qa/section/New_and_Exciting_Physics/Relativity/20020814123426.htm
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In order to answer your first question, we have to introduce the relationship between energy, mass, and momentum in special relativity. Most people are familiar with Einstein's E=mc**2, where c is the speed of light. This equation really only applies to a particle which is standing still, and it is misleading in the case of massless particles. What some people do (not those who routinely use special relativity in their daily work) is to redefine mass to mean E/c**2 to preserve this relationship; under this definition, the mass changes with speed, and we really only then have two names for the same concept. But there still is a valuable idea, called the "rest mass" of an object, which does not depend on the speed with which it travels. People sometimes refer to this mass as m0, and for people who use this stuff every day, that's what "mass" means under all circumstances. For a massless particle, m0 = 0.
The full relativisitic expression for energy in terms of the rest mass of a particle is
E**2 = (m0*c**2)**2 + (pc)**2
where p is the momentum of the particle. This relationship is true for all particles in all frames of reference and is very useful in practice. The momentum of a particle which is traveling at a speed v less than c is given by
p = m0*v*sqrt(1/(1-(v/c)**2))
This equation works for particles traveling at speeds less than c. For v=c, it involves dividing zero by zero. |
I quote this from the URL above it.
Keep trying, maybe you'll prove me wrong. |
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| Posted:
Fri Aug 12, 2005 9:41 pm
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| Posted:
Fri Aug 12, 2005 10:58 pm
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Joined: Jun 21, 2005
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There is a problem with "light in a box." What it is is that light is always freefalling. And this is true inside the box. So light that is always falling cannot contribute to any weight of the box as everything in freefall is weightless.
Since the box will be absorbing light the energy will be in the walls of the box. That is the reason for the increase in the box's mass.
Hope this clears it up! 
Light is always falling! |
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| Posted:
Sat Aug 13, 2005 3:44 am
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It simply states what I have been saying virtual mass has gravity. |
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| Posted:
Sat Aug 13, 2005 4:05 am
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Joined: Jun 21, 2005
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| cwes99_03 wrote: |
| Keep trying, maybe you'll prove me wrong. |
Ok the issue of photons having gravity is such a basic concept that it’s difficult to get any good references for it. But I have found one credible source here:
http://www.newscientist.com/article.ns?id=dn6544
OK for once and for all PHOTONS HAVE GRAVITY!!!!!  |
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| Posted:
Sat Aug 13, 2005 4:41 am
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Joined: Jun 21, 2005
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| Frankinstien wrote: |
| OK for once and for all PHOTONS HAVE GRAVITY!!!!! |
That's GTR. |
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| Posted:
Sun Aug 14, 2005 3:33 pm
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You need to quit posting sites that simply say a single sentence about light and gravity. Until you find a site that can better explain the idea, not simply reference it, then you aren't contributing to the forum.
As for light having mass, it doesn't. Light has momentum, and it is not a simple task of saying momentum is mass multiplied by velocity, therefore removing c we can get a virtual mass.
What I've read saying light has gravity says that if it can be bent by a massive object then it too must have gravity to interact with gravity. I object to this thinking. A better way of looking at it is that everything with mass bends space time. Then objects passing through cruved space time have to travel a straight line in that geometrical coordinate system (i.e. if the space is curved, light will follow a curved path.) I guess it would be analogous to our sailing a "straight" path on the surface of the earth. In fact we are sailing a curved path, unless you are going to tell me that the earth is flat. |
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| Posted:
Sun Aug 14, 2005 3:41 pm
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| Posted:
Sun Aug 14, 2005 5:55 pm
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| cwes99_03 wrote: |
You need to quit posting sites that simply say a single sentence about light and gravity. Until you find a site that can better explain the idea, not simply reference it, then you aren't contributing to the forum.
As for light having mass, it doesn't. Light has momentum, and it is not a simple task of saying momentum is mass multiplied by velocity, therefore removing c we can get a virtual mass. |
You still don't get it! I have never said that light has rest mass! You seem to think that only rest mass has gravity that's where you are WRONG! The reason that the article was posted was to demonstrate that photons do exhibit gravity despite the fact that they are not a form of rest mass.
Below is a quote from http://www2.abc.net.au/science/k2/stn-archive1/posts/topic743.shtm
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The other matter is the possible attraction light has for other objects – does this mean it has mass? The answer is no – and comparison with electrical fields will show how this is possible. Given first that we know energy and momentum couple to gravity, the gravitational analogue to electric charge is not mass, it is what we call the energy momentum 4 vector of a particle (bit of a mouthful, but bear with me…). This is merely the energy-momentum relationship in 4D space-time, where mass is defined invariantly. (Remember above we saw that relativistic mass or mass-energy is frame variant). |
In effect photons will exert a stress tensor that can curve space since photons do have momentum.
Explaination of stress tensor: http://en.wikipedia.org/wiki/Stress-energy_tensor
Get it? Photons are not a form of rest mass but STILL HAVE GRAVITY!
BTW the url you posted doesn't work. To save bandwidth we all understand how photons and all objects are affected by gravity, which is curving space. You need to get over the "only rest mass has gravity" issue. It's really getting tiring... |
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| Posted:
Mon Aug 15, 2005 1:42 pm
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| Posted:
Mon Aug 15, 2005 1:56 pm
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Additionally if you look at that website the very last paragraph is the most interesting part for anyone else who would like to add comments to this forum.
| Quote: |
| The energy and momentum of light also generates curvature of space-time so according to theory it can attract objects gravitationally. This effect is far too weak to have been measured. The gravitational effect of photons does not have any cosmological effects either (except perhaps in the first instant after the big bang). There are far too few with too little energy to make up any noticeable proportion of dark matter. |
So while that author agrees with the idea, or theory that photons should exhibit a gravitational pull on objects, he says there is no possible way to measure the effect because it is so miniscule. Then he goes on to say because it is so miniscule that it can't be an explanation for the additional gravitiational pull currently believed to be caused by dark matter.
I don't know about you but I believe that in 13 billion years, there must be a whole lot of unabsorbed photons filling up "dark" space.
Oh and if photons do have "mass brought about by their energy" would this mean that black holes are getting more massive simply by consuming light. I would suppose this to be true and frankly believable that inside a massive black hole that light could be converted back from energy to mass, but what if instead of being converted to mass, everything inside a black whole was converted into energy. Then we would have the supposed box of those thought experiments mentioned before and it would be possible to measure the mass of the box. The only hard part would be to measure the amount of light entering the box. |
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