Jimmy Higgins

Ignorant question here. Gravity of what? While energy and mass are equivalent, doesn't energy have no mass? No mass, then no gravity right or atleaat in the form we recognize today. Or can you have gravity through highly compacted energy? Like Einstein's equation, E= mc^2. That isn't the full equation. Its E^2 = p^2c^2 + (mc^2)^2, where it lets particles with no mass have energy. Can this be applied within the field of gravitation?

eh

Do you photons with a rest mass fo zero, have mass in motion, since they carry energy?

<a href="http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html" target="_blank">http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html</a>

strubenuff

Gravity of the universe...everything is attracted to everything else. It slowed down before it inflated.

DNAunion

DNAunion: No, inflation is an extremely brief episode of exponential expansion that occurred somewhere around 10^-35 seconds after the "Big Bang" and lasted for far less than a second. In that very short amount of time, the volume of the Universe increased to be "billions and billions" of times its pre-inflation value, with distant areas receding from each other at speeds greater than the speed of light. After that, normal expansion resumed. Inflation solves several paradoxes that the standard Big Bang model cannot.

Devilnaut

DNAunion:

Expansions also spew stuff out into a preexisting space. Explosion or expansion is just shorthand to describe the big bang. Boro's point was that an explosion is basically equivalent to an expansion. They both bring the same amount of meaning to the table.

Friar Bellows

Yes, I think so. According to the general theory of relativity, the source of gravitation (or more correctly, the source of the curvature of spacetime) is a tensor called the stress-energy tensor, which is defined the following way:

"given any two tangent vectors u and v at a point p in spacetime, the stress-energy tensor represents how much energy-momentum-in-the-v-direction is flowing through the point p in the u direction."

But this effect for photons is so tiny it has never been measured, even on a cosmological scale where you're dealing with zillions of photons. I'm not sure whether this effect was tiny in the very early universe, though.

That depends on how flexible you are with the notion of "mass". There seems to be so many different kinds of mass: gravitational mass, inertial mass, relativistic mass, longitudinal mass, transversal mass, rest mass.

By the way, that John Baez site is a good site to learn the basics of general relativity.

[ December 08, 2002: Message edited by: Friar Bellows ]</p>

DNAunion

DNAunion: No, there's a difference. Here, let's all pretend.

Take your hands and ball them up into two fists, and place them knucle to knuckle. Now imitate an explosion. Come on everyone, don't be shy. Now, see your fingers jutting out into space - what's in between them? If the Big Bang happened like that explosion, what would be between the "fingers" that shot out? Does the Universe consist of a multitude of finger-like spacetime projections with vast gaps of complete nothingness between them? Note that the same problem arises if you look at a firework and imagine it to be a Universe-creating explosion: all of those "tendrils" jutting out with complete nothingness between them.

Now let's pretend again. This time, take your two hands a cup them together to form a hollow sphere and pretend there is a deflated balloon inside. Now, imitate expansion - imagine the balloon filling up with air and expanding indefinitely without bursting. See how there are now no "fingers" of spacetime jutting out into nothingness, generating enormous gaps within the Universe?

Friar Bellows

I did a bit of web searching, a bit of rummaging, and found this excellent introductory paper on inflation:

<a href="http://arxiv.org/abs/astro-ph/0005003" target="_blank">An Exposition on Inflationary Cosmology</a>

(click on the PDF link; I suspect most of you have PDF reading software)

It's 90-odd pages long and written at an undergraduate level, so don't expect to gain anything more than a partial understanding of this complicated subject. Still, it's better than reading a popular-level book, I think.

Devilnaut

I do see what you're suggesting DNAunion. I don't think it is particularly incorrect to refer to the big bang as an explosion (as I'm not aware of anything in the definition of "explosion" that excludes this type of "expansion"), it is probably more accurate to think of it simply as expansion.

Anyway no more thread derailment w/ semantics

Boro Nut

It depends what you mean by semantics.