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Topic: How was the real universe created? (Read 23224 times)
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Death 999
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We did. You did. Yes we can. No.
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Sorry to re-touch on the infinity talk for those bored of it, but this is flawed reasoning. As is the suggestion about an infinite number of stars creating infinite graviational forces in the Universe.
The reason is that distances are also infinite. As the distance tends to infinity the "brightness" from our perspective equals 0.
An infinite number of stars does create an infinite amount of light and an infinite gravitational force.
As for the gravity, well, most of the force cancels, and the rest we can't feel because it affects all parts of our body equally.
As for the light, if the universe were eternal (i.e. having no beginning or ending) and stars as far away as infinity had been pumping out light long enough ago that it got to us by now-- well, then in every direction we would find a star sooner or later, and it would shine on us. If there were interposing matter, it would eventually heat up and emit light on to us. The entire universe would become uniformly as hot as the center of a star.
Just look at the afterglow of the decoupling and that's what I'm talking about - everything really freaking hot, light cannot get far but it is immediately reemitted, and so on.
We require the stretching effect of the expansion of the universe acting on light to keep things cool. It is what decreases the energy density of the universe by increasing the volume while leaving the energy the same.
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The_Ultimate_Evil
Zebranky food
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Devouring Spathi Since 2152
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No, you don't understand.
The force of gravity is governed by the equation:
Fg = G x M1 X M2 / r^2
As radius becomes extremly large Fg = 0.
Gravity may be infinite in the entire Universe (assuming the Universe is infinite and stars are infinite) but at ANY SINGLE POINT the gravitational forces are finite.
You can only be affected by gravitational forces from relatively small finite values of radius, ie the distance between the 2 objects exerting gravitional forces.
The same goes for brightness, the light from stars at ANY SINGLE POINT only exists for relatively small finite values of the distance betwen the objects.
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SubComan
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Hello nurse!
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Totally true, Ultimate_Evil !!!
I couldn't put that into words, but it's evident that in the long run, distance^2 (or radius^2) beats all other factors in both gravity and brightness equations.
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Death 999
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We did. You did. Yes we can. No.
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I do understand gravity rather well, thank you.
Gravity may be infinite in the entire Universe (assuming the Universe is infinite and stars are infinite) but at ANY SINGLE POINT the gravitational forces are finite.
Well, the NET gravitational force is finite, yes...
You can only be affected by gravitational forces from relatively small finite values of radius, ie the distance between the 2 objects exerting gravitional forces.
heh heh... you are missing an important point: as you get further away, the amount of mass pulling also increases.
The inverse square law follows from the following principle: The field you produce is the same for any distance away from you, but is spread out over all the points that are that distance from you. The area of a sphere is proportional to the square of the radius, so the field you produce is the inverse square of the radius.
Now, consider the amount of mass in a thin spherical shell of great radius R around your position. The field that each mass in that shell applies to you has a factor of 1/R squared attached to it. However, the mass in that shell has a factor of R squared, which cancels!*(see below). So once we get into radii large enough that the density of the universe is uniform, the gravitational pull of objects on us, cumulative and not cancelled, is CONSTANT as a function of radius.
Integrate this out to infinity, and we get an infinite total gravitational force.
Inverse square doesn't beat Square.
In any case, good thing for us that gravity is a vector and thus cancels!
However, all of this extended gravity, even though it cancels, does have an effect on the expansion of the universe -- it is the matter pulling itself closer together, resisting the natural expansion of the universe due to the cosmological constant.
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*the volume of a spherical shell of inner radius R and outer radius R+r, with r << R:
the volume of the outer sphere is 4/3 (R+r)^3 = 4/3 (RRR + 2 RRr + 2Rrr + rrr)
the volume of the inner sphere is 4/3 RRR.
Subtract this out, and we get the volume of the shell as
4/3 ( 2RRr + 2Rrr + rrr)
since we are assuming that r << R, we can short-cut this to
8/3 RRr
which has square dependence on the radius of the sphere R.
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« Last Edit: June 04, 2003, 12:50:36 am by Death_999 »
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