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Dr. Mabuse
Septic Fiend
Sweden
9687 Posts |
 Posted - 07/15/2005 : 13:53:41 [Permalink]
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quote:
Originally posted by dv82mattquote:
Originally posted by markie
Yeah photons are supposed to have zero "rest" mass, but since they travel at light speed they effectively do have mass; at least we say they have momentum.
It appears that you are correct here. In fact, I believe that this has been experimentally verified by detecting radiation pressure on thin reflective foils among other obsevations.
I've been told you can do the experiemt yourself.
Either hang a few feet sheet of aluminium foil in a string, keep your ear close to it and have someone use a photo-flash on it: you should be able to hear the foil rustle if your surrounding is silent.
Or use the photo-flash on a cymbal on a drummer's set.
I'll try to remember this experiment tomorrow, when I have access to a photo-flash.
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dv82matt
SFN Regular
760 Posts |
Posted - 07/16/2005 : 12:26:34 [Permalink]
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I've been thinking about how my conjectures in the OP might fit in with general relativity.
Consider the common rubber sheet analogy to Einstien's spacetime.
For those who might not be familiar with it, imagine a frictionless rubber sheet. When a large marble is placed on it it makes an indentation on the sheet. It is easy to see how a smaller marble might roll around larger one in an orbit.
Now lets incorporate anti-matter and anti-gravity into the analogy. We can see that gravity is represented by a downward indentation (or dimple) in the fabric of spacetime, so let's imagine that antigravity can be represented by an upward indentation (or pimple) in the fabric of space time.
Supposing, for the sake of illustration, that the earth were made of anti-matter, then we might imagine the marble representing earth forming a peak in the rubber sheet rather than a valley. Clearly then a marble representing the moon could not maintain an orbit. Also since the earth is gravitationally bound it would immediately fall apart with the dertius quickly sliding in every direction down the slope of spacetime.
One could imagine a 'topological' map of the universe with areas of zero net gravity representing 'sea level', areas of positive gravity being below sea level and areas of negative gravity being above sea level.
Most of the volume of the universe then, would be fairly uniform and slightly above 'sea level', but in areas where matter is concentrated, such as galaxies, there would be a dramatic dive to far below sea level. The topology would become highly irregular as gas clouds, planets, stars, black holes and whatnot all contribute their mass to the warping of spacetime. |
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Dr. Mabuse
Septic Fiend
Sweden
9687 Posts |
Posted - 07/16/2005 : 14:40:17 [Permalink]
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Matt, in the rubber-sheet analogy the indentation of the sheet is the result of positive mass. Would you then suggest that anti-matter would have negative mass, or that only the force created by the mass is negative?
F=(G*m1*m2)/r²
If the mass is negative, then all antimatter will be spheres under the rubber sheet trying to rise up through it, causing the upward indentation of the rubber sheet, yet concentrations of antimatter will stay intact and not break up. |
Dr. Mabuse - "When the going gets tough, the tough get Duct-tape..."
Dr. Mabuse whisper.mp3
"Equivocation is not just a job, for a creationist it's a way of life..." Dr. Mabuse
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dv82matt
SFN Regular
760 Posts |
Posted - 07/16/2005 : 15:29:35 [Permalink]
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quote:
Originally posted by Dr. Mabuse
Matt, in the rubber-sheet analogy the indentation of the sheet is the result of positive mass. Would you then suggest that anti-matter would have negative mass, or that only the force created by the mass is negative?
Only the force created by the mass is negative.
quote:
F=(G*m1*m2)/r²
If the mass is negative, then all antimatter will be spheres under the rubber sheet trying to rise up through it, causing the upward indentation of the rubber sheet, yet concentrations of antimatter will stay intact and not break up.
Yeah, that would be the case if the mass were negative. Observations of cosmic rays have all but ruled out the existence of galaxies made of anti-matter, so for the moment it's safe to say that this conjecture (that the mass and the gravity are negative) has been falsified.
I did a bit more thinking on this topic. It seems to me that there would be three basic types of mass. All of these masses would be positive, meaning that they all respond to the curvature of spacetime in the same way.
First there would be the regular matter that we are all familiar with. This type of mass would generate a positive (or attractive) gravitational field, expressed in our rubber sheet analogy as a dimple.
Next is anti-matter. This type of mass would generate a negative (or repulsive) gravitational field, expressed in our rubber sheet analogy as a pimple.
Last is electromagnetic radiation. For the purposes of this thread I've included it as a mass since it has momentum and is responsive to gravitational fields. Also in relativity energy is equivalent to mass through the equation E=mc2. This type of mass/energy would generate no gravitational field at all.
All of these masses would accelerate towards a positive gravitational field (or dimple) and away from a negative one (or pimple). |
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Dave W.
Info Junkie
USA
26020 Posts |
Posted - 07/16/2005 : 20:01:26 [Permalink]
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It might be important to note that the force of gravity, F, in that famous equation Mab posted will only be negative if one mass is positive and the other negative. Two negative masses (or masses which generate a negative gravitational field) will still attract each other with a positive value for F, since -1 × -1 = 1.
Take two positive masses P, with a negative mass N equidistant between them. If you work through the equations, the force on the P masses is proportional to P(4N+P) - of course the net force on the N mass is always 0. FP becomes zero when P = 4N (since N is negative). In other words, if the P masses are less than four times the N mass, they get repelled and fly off into space. If the P masses are more than four times the N mass, the P masses attract each other until... KABLOOIE! 2N of the total mass is converted to gamma rays.
If the P masses are precisely four times the N mass, however, a steady state will exist in which the repulsive force from the N mass exactly balances the attractive force from the other P mass. F equals 0.
These are all point masses, of course.
Now consider this: one of the P masses (when precisely four times the N mass) is split into two X masses, each precisely twice the N mass, and separated from each other by one-hundredth the distance to the N mass, perpendicular to the imaginary line all the masses sat on in the earlier examples. Calculating the three force vectors for each of these four masses results in the P mass accelerating away (slowly, since the X masses are no longer directly opposed from it), and the X masses accelerating away (also slowly). But after a while, due to their proximity, the X masses (which are made of glue) will come together, recreating a P mass equal to four times the N mass, so the accelerations of the P and X masses would drop over time, and once the two X masses stuck together, would be zero. They (and the P mass) would just drift away from the N mass with whatever momentum they'd gained to that point in time.
Now reset that whole experiment, only this time make the P mass 4.0000001 times the N mass, and the X masses 2.00000005 times the N mass. If the distance between the P and N masses (and thus the distance between the N and center-of-gravity of the X masses) is large enough, they again start out accelerating away from the N mass, but as the X masses come together, the acceleration slows, stops, and then becomes an attractive force between the P and X masses until, once again... KABLOOIE!
How odd is that? Well, pretty odd since the only point masses we know of are black holes, and they're very f-ing strange, anyway. How this all would work with "real" masses - perhaps modeled as big clouds of point masses, I can only guess at. |
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dv82matt
SFN Regular
760 Posts |
Posted - 07/16/2005 : 21:58:14 [Permalink]
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quote:
Originally posted by Dave W.
It might be important to note that the force of gravity, F, in that famous equation Mab posted will only be negative if one mass is positive and the other negative. Two negative masses (or masses which generate a negative gravitational field) will still attract each other with a positive value for F, since -1 × -1 = 1.
Yeah, the equation would have to be modified. Perhaps to F=q(G*m1*m2)/r² where q=1 for matter, q=-1 for anti-matter and q=0 for EM radiation.
I had to read through your cosmic trick shot a few times before I got it. Very weird.
Negative mass has all sorts of counter-intuitive properties as evidenced by the wikipedia article origionally posted by Plyss.quote:
For a negative value of mp with positive value of ma, F is negative (repulsive). At first glance it would appear that a negative mass would accelerate away from a positive mass, but because such an object would also possess negative inertial mass it would accelerate in the opposite direction from F. Furthermore, it can be shown that if both masses are of equal but opposite mass, then the combined system of positive and negative particles will accelerate indefinitely without any additional input into the system.
By the way, Dr. Mabuse have you had any luck detecting the pressure of light? |
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Dr. Mabuse
Septic Fiend
Sweden
9687 Posts |
Posted - 07/17/2005 : 04:36:13 [Permalink]
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| I forgot it the day after my post about it, but I've sent my girlfriend a PM about it. Question is, did she remember to do it? I'll find out in a short while. |
Dr. Mabuse - "When the going gets tough, the tough get Duct-tape..."
Dr. Mabuse whisper.mp3
"Equivocation is not just a job, for a creationist it's a way of life..." Dr. Mabuse
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Send them unarmed civilians for target practice..
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furshur
SFN Regular
USA
1536 Posts |
Posted - 07/22/2005 : 08:21:42 [Permalink]
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If anti-matter creates anti-gravity that would mean that anti-matter has anti-mass, since gravity is the measure of bending of space about a mass. So anti-gravity would mean that space was bending the 'other way' (whaterver that means in 4 dimensions) due to an anti-mass. The idea of an anti-mass creates all kinds of problems such as:
-F = -m X A
or
-E = -mC^2 A bomb made of anti-hydorgen would cause building to assemble themselves and 1,000s of people and animals to suddenly come to life. Well not really, but you see the difficulty with the idea of a negative mass. The changes in momentum for matter and anti-matter collisions would certainly have observed if there was really anything to the negative mass hypothesis......
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If I knew then what I know now then I would know more now than I know. |
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dv82matt
SFN Regular
760 Posts |
Posted - 07/22/2005 : 13:08:59 [Permalink]
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quote:
Originally posted by furshur
If anti-matter creates anti-gravity that would mean that anti-matter has anti-mass, since gravity is the measure of bending of space about a mass.
There are three conceptually distinct quantities called mass. They are inertial mass, passive gravitational mass and active gravitational mass. I am only proposing that the active gravitational mass of antimatter is reversed.quote:
So anti-gravity would mean that space was bending the 'other way' (whaterver that means in 4 dimensions) due to an anti-mass.
That's right, although it would be more accurate to say that it's due an anti-active gravitational mass, but that's a bit of a mouthful. 'Negative gravitational field' is clearer.quote:
The idea of an anti-mass creates all kinds of problems such as:
-F = -m X A
or
-E = -mC^2 A bomb made of anti-hydorgen would cause building to assemble themselves and 1,000s of people and animals to suddenly come to life. Well not really, but you see the difficulty with the idea of a negative mass. The changes in momentum for matter and anti-matter collisions would certainly have observed if there was really anything to the negative mass hypothesis......
In these equations 'm' refers specifically to inertial mass (and perhaps also to passive gravitational mass) but not to active gravitational mass.
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