Gravity?

[Juggernaut]

Originally posted by Sagacious Dolphin
A greater rate in change of potential (with respect to distance) would.

*cough* Yes, that's what I meant.

[KrazyHorse]

Originally posted by Lancer
What is a 'graviton'?

A figment of the imagination of those who wrongly believe that gravity is a force like any other.

[KrazyHorse]

Originally posted by our_man
Gravity...

I've always wondered - does Archimedes Principle still hold true in space? If so, couldn't the bouyant force it generates offset the effects which the absence of gravity causes in astronauts?

Which Archimedes principle? A floating object displacing its own mas obviously has no meaning, as there is no such thing as floating on the non-existent surface of the liquid. A submerged mass will still displace its own volume, however.

[KrazyHorse]

Note: there is no buoyant force in an inertial frame, as it is caused by a downward pull on the liquid medium.

[TCO]

Just read an old sci fi story about some rocket pilots that had to make a aseveral day run at 3 G's and were wasted by it. Anybody know what the effects of long time exposure to serveral G's is?

[KrazyHorse]

Nope. I doubt they've ever done that to a person. They killed enough people running them up to 20g's and holding them there for 15 seconds. I think there were limits, even for the USAF of the 50s.

[Urban Ranger]

What about magnetic monopoles?

[TCO]

Originally posted by KrazyHorse
Nope. I doubt they've ever done that to a person. They killed enough people running them up to 20g's and holding them there for 15 seconds. I think there were limits, even for the USAF of the 50s.

Yeah, I've heard about the high G work for short periods of time. (for fighter pilots doing a dive pullout.)

Interesting thing about this story, written in 1953 was that it seemed to know about all the pullout research. But the situation poisited was a high speed run (to deliver medicine to Pluto!) at 3.5 G's. (takes about 16 days.) 3 G's for a few minutes is no big deal. Just wonder what the effect would be for days.

[Lancer]

"A figment of the imagination of those who wrongly believe that gravity is a force like any other"

KH, that's interesting. Though I'm sure you can make such a statement based on science, I had the same inclination as a gut feeling. How can a bit of something go out to another bit of matter and pull on it? First it has to go outward, and then when it gets there it has to pull towards itself.

Lots of things push, yes? Light, sound waves, wind... When wind hits you it continues in the same direction as it went in when
it travels towards you. It doesn't blow to you and then, when it gets to you, push you in the direction it came from. That's what gravity does. It seems to obey a different set of laws entirely. I call these laws, the 'WTF' laws.

Also, consider a wind that blows in all directions from every bit of matter in existance...man, it makes my brain hurt.

[Adalbertus]

Just read an old sci fi story about some rocket pilots that had to make a aseveral day run at 3 G's and were wasted by it. Anybody know what the effects of long time exposure to serveral G's is?

About the feeling a woman might have after several consecutive days ****ing with someone of twice her weight

A floating object displacing its own mas obviously has no meaning, as there is no such thing as floating on the non-existent surface of the liquid

partly wrong. Floating requires gravity, sure. But there can be liquids without gravity. If you don't do anything to it the liquid will assume the shape of a sphere, due to the surface tension. Depending of the interactions between the liquid and the object you want to "float" the object will be either dragged into the liquid or be repelled. The same effect as wetting.

[Adalbertus]

Lots of things push, yes? Light, sound waves, wind... When wind

The fundamental forces (gravity, electromagnetism, weak and strong interaction) don't work like a wind. It's, for the last three, an exchange of "virtual" particles which are constantly emitted and absorbed by a charged body. "Virtual", because they locally violate conservation of energy, but the time these particles "exist" is so short that the violation is permitted by the uncertainty relation. Unlike a wind, this can also result in an attraction.
I'm not working in this areas, so I cannot give an explanation on how everything "really" works, even if an explanation exists in a form other than three books of maths.
The best working theory for gravity, General Relativity, explains the gravitational attraction by a curvature of spacetime. So the moon moves along a straight line but the straight line is curved in spacetime (which is one problem many people have with Relativity: It requires a geometry different from what we are used to, with our three space dimensions and one time dimension which are entirely separated. Relativity mixes this up a bit).

What about magnetic monopoles?

Some physicists firmly believe in them, some don't. I don't. We'll have to see.

[Rogan Josh]

KH: Why don't you like the graviton? It seems a nice, perfectly natural extension of the ideas which Adalbertus has already explained (about virtual force carriers) to gravity.

Let me add one point. All of these force carrying particles come about from the requirement that symmetries be local. For example, it was observed that quantum mechanics has a U(1) symmetry: basically, physical observables only depend on the modulus squared of the wavefunction, so you can multiply all the wavefunctions everywhere by a constant phase exp(i*phi) and get exactly the same results. However, since special relativity tells us that we cannot send information faster than the speed of light, we cannot change the wavefunction everywhere at the same time (indeed, what does 'the same time' even mean?). Logically then, the symmetry should be 'local': we should be able to change the phase of the wavefunction locally (at a point) without affecting the results. When you look through the equations, you find that in order to do this you need to have an extra particle, which, in this case is the photon.

Similarly, the insistence of SU(2) symmetry and SU(3) symmetry leads to the requirement of the W and Z bosons and the gluin respectively, all now seen in experiment. Amazing!!!

Now, one of the exciting things in particle physics these days is the search for Supersymmetry: that there is an extra fundamental symmetry of the universe, between bosons and fermions (particles with different spins). Global supersymmtry answers a lot of problems we have with the current models and is present in all string theories. However, the really exciting thing, to my mind, is that in order to make supersymmetry local, we need a new particle (just like before) which is the graviton. We can explain gravity as local supersymmerty!!! Even more amazing!!!

The really intreguing question is: why is gravity so weak?

[Dauphin]

I don't like gravitons because of the equivalence principle.

If gravitons are emitted by mass-energy bearing particles then you can tell the difference between a non-inertial frame of reference and gravitional attraction.

[Dauphin]

we need a new particle (just like before) which is the graviton

If the photon is propogated by Electric and Magnetic fields, the graviton will be propogated by .......???

Gravity is akin to the electric charge (sort of), so is there as yet unmeasured gravity equivalent to the magnetic field? Would a moving mass current generate an ever so weak magneto-gravitional field which repels other mass-currents?

[Solly]

It seems that physicists don't have a clue what makes gravity and make up rules and theories...

How can a force be exerted on me without any other properties at all...

[Ari Rahikkala]

Forgive me for understanding ~nothing about virtual particles, but here's one question that bugs me:

AFAIK electrical force works over infinite lengths. Photons move, of course, at lightspeed (or is this different for virtual particles? If so, how is it possible?). Since a photon has to be exchanged for electrical force to be carried (right?), the uncertainty principle wouldn't care of a photon that goes from one end of the currently existing universe to another (i.e. something like 14 million years, right?)... Which would IMHO sound a bit, err... weird. So, what's the contradiction in my contradiction?

[Lancer]

" Relativity mixes this up a bit"-Adalbertus

Mixes me up a bit too...for instance how can curves in spacetime exist between every tiny bit of sand, or bit of anything, in the universe? Spacetime must have more curves than Maralyn Monroe... If a "virtual" particle can only exist because it does so for such short a time that it doesn't count, how the heck does it manage to pull on every other bit of mass in the galaxy? That is one busy bee. Oh yeah, curves in spacetime. *bangs head on wall for a while*

[Ramo]

the uncertainty principle wouldn't care of a photon that goes from one end of the currently existing universe to another

The Heisenburg Uncertainty Principle states that one cannot know the precise position and momentum of a particle at the same time (as well as other coupled quantites such as energy and time, which, BTW, is essential for the operation of gauge bosons). I don't know what you're referring to...

[Lancer]

LightEning, 14 Billion years, with a B. With the new components the Hubble telescope will be able to look back 13 billion years, "possibly to the first light."

[Sarxis]

Gravity does use up a bit of fuel in the form of heat energy caused by pressure. Thus eventually (I mean, over the course of a very long time), even a planet would simply evaporate from heat lost out into space.

As you increase pressure, the particles in a system become more active and energetic and will emit radiations. In the case of stars, the energy state of their atoms becomes so high that plasma and fusion occur, making a lot of radiation.

In the case of black holes, the surface gravity becomes so extreme that the actual fabric of space-time around it becomes excited and displaces the so-called 'virtual particle pairs' (read 'A Brief History of Time' by Stephen Hawking) that radiation is produced.

It hasn't yet been determined if there is a naturally occuring anti-gravity force, though such a force would explain why the galaxies in our visible part of the universe are accelerating away from each other (just another possible theory).


Gravity Stuff

[KrazyHorse]

partly wrong. Floating requires gravity, sure. But there can be liquids without gravity

Yes, but those liquids, while having a surface, do not have an equivalent of "water-level". Should have used a different choice of words, I suppose.

[Lung]

Originally posted by Boddington's
It seems that physicists don't have a clue what makes gravity and make up rules and theories...

How can a force be exerted on me without any other properties at all...

Because there is no force. "Gravitational force" is a misnomer, as the term "force" is used to describe the effects of gravity, albeit incorrectly. It's not easy to understand, but the secret lies in the curvature of spacetime. From our 3-dimensional perspective, celestial bodies orbit in ellipses around larger celestial objects, but in 4-dimensional spacetime, they are effectively moving in a straight line! Put another way - what is the shortest route between New York and London? Is it straight across the water, or is it through the earth? On a flat world map, airline routes appear to curve, but on a globe, they go straight (at least on the surface).

Dimensionally, we are to space what a map is to the globe. These may not be the best analogies, but they might give you a better perspective.

*awaits correction from Ramo*

[KrazyHorse]

Not a bad explanation.

[Lung]

Another way to look at it is this:

If you walk along the equator and see a longitudinal line perpendicular to the equator. Just to be sure, you measure the angle, and sure enough, it's 90 degrees. You walk another couple of hundred miles, and you see another. Again, you find it's 90 degrees. You turn and follow the longitudinal all the way to the north pole, only to find the other longitudinal line you passed earlier! However, if they were the same angle, they must be parrallel, right? Wrong. Why? Because you've applied 2 dimnesional geometry to a 3-dimensional world. Applying 3-dimensional geometry to 4-dimensional spacetime will also fail. in other words, your straight line is another dimension's curve.

At least that's my understanding of the geometry of multidimensional spacetime

[KrazyHorse]

BTW, those paths-of-least distance are called geodesics, and geodesics on a sphere are called great circles.

[FrostyBoy]

this is interesting

http://www.shortnews.com/start.cfm?i...&rubrikid1=936

[ravagon]

More correctly, "gravitational force" as it is currently understood, results in the curvature of space-time. Mathematically one of the fundamental constants (the permeability I think?) is thought to effectively becomes a tensor quantity with an anisotropic "shear" in 3d space (Absolutely terrible choice of words though I'm afraid ).
At the moment, without a working model of quantum gravity, the graviton as the postulated charge-carrier for gravity in a grand unified theory is still speculation...

The possibility of anti/negative gravitation has been suggested as one of the possibilities of the "missing mass" phenomenon but is not generally well-regarded. Quite why I don't know but I'd guess the scientific community find the dark-matter theories easier to swallow.

[Rogan Josh]

There seems to be some confusion here which has been caused by Einstein (he was always a problem). Einstein's theory of gravity is actually not valid at the quantum level. It is not a quantised theory and makes no attempt to be so you should think of it as relating to gravity as Maxwell's equations relate to electromagnetism (Maxwell's equations incidentaly are relativistically invariant despite being published before Einstein's theory of special relativity!).

Furthermore, the idea of virtual particles is a little bit misleading if you delve too far into it. It is just a nice physical way of picturing things thought up by Feynman, but strictly speaking it is just an interpretation of the field theory. Strictly speaking all particles are virtual, or at least one can never see a 'real' one because real particles, by definition, have infinite life-times - or rather live infinitely long without interacting with anything. The very act of seeing a particle renders it virtual (if you see what I mean).

The physical picture of virtual particle exchange mediating forces is often explained by analogy. Imagine 2 skaters on the ice, facing each other but some distance apart. One has a heavy ball, which he throws to the other. The act of throwing the ball will push him backwards (because he pushes the ball forwards) and the other guy will also be pushed backwards because when he catches the ball. Therefore it looks like they have a repulsive force between them. Indeed, if they continuously throw balls at each other they will be continually repulsed. (This analogy is of course going to brea down for attractive forces.)

Now, it is actually a bit more complicted than that, because a source is throwing out these 'balls' in all directions. Therefore the intensity of these balls passing a point will go down like the square of the distance, and this is exactly why the electromagnetic (and gravitational) force goes like 1/r^2.

I am of course simplifying things, but that is the basic idea.

If you want to have a geometrical and quantum description of gravity you have to learn some string theory....

[Urban Ranger]

Originally posted by Rogan Josh
However, since special relativity tells us that we cannot send information faster than the speed of light, we cannot change the wavefunction everywhere at the same time (indeed, what does 'the same time' even mean?).

Is it because of the Lorentz-Fitzgerald transformations? I can't remember anything in SR to prevent FTL travel otherwise.

[DaShi]

It comes from Einstein's correction of Newton's 2nd Law:

F=mv/t

where
m=m0/(1-(v^2)/(c^2))^1/2


Which is derived from the Lorentz transformation