I forget the ratio of the two, but the gravitational attraction is MUCH weaker than the electromagnetic.

I thought someone said there was going to be free beer!

Gravity gets no stronger (in proportion), as far as we can tell, down at the subatomic scale.

The model for gravity has a strength like 1/r^2, just as the electromagnetic force. As far as we can tell right now, it's a wimp at all scales.

Interestingly, the strong nuclear force is "short-range" in that it drops to nothing after a certain distance. However, at subatomic distances, it outstrips even the power of the EM force. This is why stable nuclei don't fly apart.

I think it is in comparison to the other forces (Strong, Weak, and electromagnetic) that gravity is weak.

You're right. The difference is a factor of about 10^-15, IIRC. This is just in terms of the "coupling constants", and doesn't depend on the charge of the body. (The coupling constants are related to Coulomb's "k" and Newton's "G". Also, "charge" usually means electric charge, but it could be any charge. A gravitational charge is mass.)

So far I've never need a decent case presented that says gravity pulls and isn't a push effect. Of course that would be as hard to prove as the differnce between gravity and acceleration.

Eh? Gravity is an attractive force because the "gravitational charge" - mass - is always positive. Or are you trolling? I think you are - your post contains enough complete errors that it almost got me...

Doesn't quite cut it - if that were the point, then you could always say that humans simply haven't discovered the "negative charge" yet. But all particles known to man have the same kind of "charge", and the force pulls them together whereas with all the 3 other forces, objects with the same charge are pulled apart!

And that is the fundamental difference between gravity and the other forces, besides its weakness.

Interesting idea: if gravity didn't have the "same chrage attracts" property and there were different charges, it would be so weak as to be immeasurable. Perhaps there are dozens of yet undiscovered forces that are as weak as gravity but follow the "different charges attract" principle?

Michael "Brazil" Borgwardt --- Member of #WASHU# and Her would-be guinea-pig.

The "we just haven't found it" argument has many flaws, not the least of which is that you can look as long as you like on Earth, you're not going to stumble across a unicorn.

Also, let's talk about what negative mass entails: If you go for negative mass, I'll use the kiddie E=mc^2 and point out that negative mass would lead to having negative energy (don't confuse this with potential energy, or a negative differential).

Now, if negative energy was an "allowable" number, we would see all kinds of very odd things in quantum mechanics that we do not see, at all, and we'd expect to see them quite easily. They haven't been observed yet. Basically, think about the creation of two particles, one with positive energy, one with negative energy. This wouldn't violate any conservation laws (assuming you made one a baryon and another an anti-baryon, charges opposite, blah blah). We would then see particles simply appearing out of nowhere and staying there, all the time. The vacuum would blaze up and be, well, solid particles. We don't see this, ergo, no negative energy, ergo, no negative mass.

Now, you could "decouple" the two numbers and say, "perhaps the 'mass' in gravitational attraction doesn't have to do anything with the 'mass' in E=mc^2." Unfortunately, that's also a problem if you look at anything in General Relativity.

As for the "other forces," we keep looking. Every so often they'll revive a fifth and sixth force routine, I think the last time I remember that happening was circa 1990, but it has yet to pan out.

Also, only in EM fields do the same charges repel. In strong, they hook to each other, there's no real analogue of "charge" there, unless you count baryon number. Weak I'm not so sure about, you have to start talking about neutral currents and stuff my profs never got to when I got my degree.

Where do anti-protons and anti-electrons fit in? (BKA antimatter)

Does antimatter have negative mass?

What happens to all the energy that is released in a matter-antimatter collision? Since mass cannot just appear out of nowhere, (1st Law of Thermodynamics?) and can only be converted from energy or energy to matter, wouldn't it stand to reason that since the matter got converted to energy, that the anti-matter got converted to anti-energy?

The weird thing is is that if there is no such thing as anti-energy, what happened to the antimatter? Did it stay as antimatter? This would lead to a catastrophic problem. If a matter -antimatter collision resulted in a conversion of matter to energy and preservation of antimatter, then eventually all matter in the universe would collide with anti-matter and leave the universe as a huge field of energy. However, if the collision DOES convert the antimatter to anti-energy, wouldn't these (energy and anti-energy) cancel each other out (and thus we wouldn't see a large amount of energy released)?

Matter consists of leptons, antileptons, quarks, and antiquarks. Baryons (including but CERTAINLY not limited to protons and neutrons) are built out of quarks and antiquarks, three of them, to be exact. Mesons (a class of particles you have neglected) are made of two quarks. No, quarks don't ever appear singly. Leptons aren't quarks. In the lepton family, we have electrons, muons, tauons, antielectrons (positrons), antimuons, and antitauons.

"Electrons have a negative charge but no mass." Wrong!

Electrons have mass. The mass is smaller than that of a proton, and it would take roughly 1,300 of them, if I recall, to equal that mass, but, yes, they definitely have mass.

Same thing goes for positrons.

Also wrong "a proton is essentially a neutron combined with a positron." First, neutrons weigh more than protons. Second, you're forgetting about conservation of baryon number.

When a neutron beta-decays, it typically falls apart into a proton, an electron, and an electron anti-neutrino.

Never ascribe to maliciousness that which can be adequately explained by incompetence.

A proton decays into a neutron by emitting a positron and an electron-neutrino. Welcome to conservation of lepton number, as well as possibly spin.

Don't get your physics off of kiddie webpages.

In this scenario, we have to conserve a few quantities.

• Charge: this is why we have to have a positron, a proton can't just drop to a neutron, we have to conserve charge by kicking off a positron.
• Baryon number: this is why the proton turns into a neutron (although, to do this, you have to add energy, protons mass slightly less than neutrons)
• Lepton number: Leptons in equals leptons out. A positron has lepton number of -1, which is balanced by the +1 of the electron-neutrino.
• Spin: Spin conservation is what led to first the hypothesis for, and then the eventual detection of, neutrinos.

Maybe we need a Particle Physics for Slashdotters webpage. Any suggestions?

Check up on the Casimir effect. Negative energy has been created in the lab. Not much mind you, but enough to know that it exists.

Is it enough to know.......?
Je'ne ces't pas!

The Casimir effect is NOT negative energy.

Nor is it a bottomless source of energy, as people seem to assume.

Just because two plates fall towards each other, that doesn't mean that it is negative energy. Potential energy is not negative energy. Otherwise, you could say that potential energy from gravitation is negative energy (hey, stuff falls down, right?)

Also, amusingly, the two plate thing creates an "attraction," but, depending on the geometry of the two surfaces, you can also get a repulsion, where the two surfaces would push apart. The math is really, really ugly, though.

So, again, Casimir effect is NOT negative energy. Also, all of my previous stuff stands. Negative net energies would cause all kinds of things to happen that we just do not see.

Casimir effect is a purely QM phenomenon. In a very weird way, it's like the Van Der Waals forces. I'm not sure if you've ever done the basic blackbody stuff, but, imagine a box, say the size of the universe, with two metal plates floating in the midst of it. You can have all kinds of allowable, full wavelenghts of photons (virtual, hey, this is QM) on either side of the plate. Big wavelengths, light-years long. Little ones, nanometers long. Anyway, the possible push that QM allows on the plates is just a little bit bigger than the push generated by the possible push between the plates, thus generating what appears to be an attraction (really just unbalanced pushing from both sides).

I'm not sure what you mean by a "push" effect. There are two possible interpretations I can think of. First (and least likely), you mean gravity can be repulsive. Repulsive gravity has never been observed. The second (and more likely) interpretation is that the interaction of gravity involves mediating particles that push instead of pull. ie. When body A is gravitationally attracted to body B, A is in between B and the mediator, rather than have the mediator in the middle.

This is a little tricky. You probably learned that energy and momentum is always conserved. Well, that's not quite true. (I don't want to get too wordy here; when I say "true", I mean "consistent with the theories currently accepted by the people doing this research.") In fact, Heisenberg's Uncertainty Principle allows the Universe to cheat. Particles produce fields around them, full of whatever particles mediate the forces that particle can feel. These mediators are created from nothing; energy is borrowed from the Universe. They can do this, as long as they can't get caught. Heisenberg's principle gives conditions under which this energy violation can never be measured. As long as it can't get caught, it can break any rule it wants.

This lies at the heart of the current explanations of forces. Mediating particles appear and disappear, and transfer energy and momentum from one particle to another. These can act to repel or attract two objects, according to their charges. The electromagnetic force has infinite range because its mediator is a photon; light has no mass. Massless mediators can have arbitrarily low energy, and can therefore live an infinite amount of time. The weak nuclear force, on the other hand, can choose any of three mediating particles, but they all have mass. Since mass and energy are equivalent (as shown by Einstein), there is a minimum amount of energy needed to conjure up that much mass. That's why the weak force has an effective range on the order of a few Angstroms.

The current proposal for gravity involves a massless mediator that has never been observed. (The mediators for all the other forces have been produced in particle accelerators.) Having the mediators go around the object to come in from behind means it has to live longer, which reduces the amount of energy it can have. Nothing prohibits it, but the most efficient mediators will take the shortest path between bodies A and B.

Different size animals have their world dominated by different effects. To paraphrase something i read somewhere: if you drop each of these animals from a height of 5m:

• an insect flutters to the ground very slowly
• a mouse is unharmed
• a cat is unharmed - if it lands correctly
• a man breaks his legs
• a horse splatters

In large part, the difference in the examples you give is not so much gravity, but other issues

I don't think your argument:

In all cases, the amount of damage sustained is controlled primarily by the 'square-cube' law - the strength of the body's structural components goes up as the square of the size, but the mass goes up as the cube of the size.

See here for more.

The formula for calculating the gravity between two objects from Newton is:
Fg = G * m1 * m2 / d^2

If Newton's equation is taken at face value, without factoring in theories of how gravity may be quantitized, then the sense of scope is amazing:

A single particle, no matter how small, can move an inch and change its influence on the Universe by some infinitesimal amount.

Moreover, if the force of gravity increases dramatically at short distances, it may be possible for the next generation of accelerators -- such as Europe's Large Hadron Collider scheduled to begin operation in 2005 -- to create black holes, regions smaller than the radius of the extra dimensions where gravity is so strong that nothing can escape.

Jesus. I need to sleep or get laid as soon as possible. This computer shit is bad for your head.

I need to sleep or get laid as soon as possible. This computer shit is bad for your head.

Doing both nearly every night makes life a lot nicer. I know from personal (continuing) experience. *grin* And for those few remaining nights, there's Gnutella / plaympeg. ;^)

Liar, huh? Scroll down and see that, yes, I have a significant other. And a beautiful and lovable one at that.

Yeah, whatever.

It was a pretty good original post. But the downside is that the record companies and artists wouldn't make as much money as they normally do now (unless of course someone wants to buy the song at a few million).

I also must agree about the moderation. There's no way to stop it. Most people moderate only new articles, and then only moderate other posts that have been moderated up (me-too syndrome).

"Evil will always triumph over good, because good is dumb." - Dark Helmet (Spaceballs)

LOL! I always figured humanity's last word would be something like, "Oops."

"There is no shot you can take that I cannot simply deny." - Ertai, wizard goalie

LOL! I always figured humanity's last word would be something like, "Oops."

How about, "What do you mean, it doesn't have an 'off' switch?!?"

-- I looked into the abyss, and the abyss looked into me--and we both winked.

The four forces, in order from weakest to strongest, are

• gravity
• sex
• death
• guilt

Sex. death and guilt were unified early in the 1800's. Gravity remains ununified.

----
Bitch-slapped by karma.

It's worth pointing out that when we fall down it's the eletromagnetic force holding the earth we fall on to together that causes the damage, not gravity. I've yet to see someone's leg broken by gravitational forces!

Never ascribe to maliciousness that which can be adequately explained by incompetence.

Also, I wonder what sort of experiment could possibly test whether these extra dimensions are really needed for grand unification - or whether they're an unnecessary complication.

As for experiments... well, the article discusses experiments to test for the existence of these "large" extra dimensions, but "small" (Planck-scale) effects are much harder to see.

Couldn't you have used "Overrated?"
---
If you're not browsing at -1,Nested you're not getting the whole story.

That's right. They're comparing the values of the coupling constants, which are dimensionless quantities (ie. no units), so they can look at things on the same scale. The coupling constant determines the strength of the interaction. (It also has a really bad name; it turns out that it's not constant.)

Dimensions do not have a size. Objects have sizes in a set of dimensions.

Of course dimension have a size, due to the fact that they are either limited in some extent or infinite. For instance time has a definite starting point and may be infinite in size (an open universe) or finite in size (a closed universe with a Big Crunch). Since the Universe isn't infinitely large the three spatial dimensions we're familiar with also have a size.

In superstring theory the compactified dimensions have a size corresponding to the Planck length in usual models - approx 10^-33 metres. There are models in which the size of these dimensions could be much larger, as in the article, and in those the idea in the article could be made to mesh with superstring theory.

Saying a dimension has a "size" is an unfortunate wording born in an attempt to reach a broader audience. Dimensions have scales. I'm a little fuzzy on this part, since I'm in particle physics and not cosmology, but the idea is that the other dimensions could be "curved in" on themselves in such a way that their existance would be masked from our perceptions. I have yet to find a decent layperson's explanation for this, but I'll keep looking. The mathematical gymnastics in the theory do lead to characteristic length scales.

Not quite: you can compare the gravitational force of an electron or proton to the electric force effecting it. If you compare the repulsive force between two electrons to the gravitational force between them, you'll get around 10^40 ratio.

The notion sounds deceptively simple: besides the familiar three dimensions of space there may be other dimensions, too small to see yet perhaps as large as a millimeter.

Dimensions do not have a size. Objects have sizes in a set of dimensions.

It is true by your perception of a dimention it doesn't have size, since you're used to think about infinite dimentions. However, imagine a dimention is wrapped around another dimention, and thus creates a loop - this loop have a limited size - after one milimeter of movement along this dimention you return to your initial point. So this is how dimentions may have size.

Sela

Well, so the question changes from "why is gravity so much weaker" to "why are the electromagnetic, strong and weak atomic charges (if present at all) so much greater than the gravitational ones in all particles known to human science?

Same difference.

Michael "Brazil" Borgwardt --- Member of #WASHU# and Her would-be guinea-pig.

Michael "Brazil" Borgwardt --- Member of #WASHU# and Her would-be guinea-pig.

Michael "Brazil" Borgwardt --- Member of #WASHU# and Her would-be guinea-pig.

Definitely not right.

Never confuse equality of quantity with equality of property. Example: A dollar buys me two dollars. However, apples are not the same thing as dollars. Apples are tender, dollars are legal tender. Apples are round, dollars are rectangular and flat.

Conversion may occur under some limited conditions, as well.

For example, I may not transform an electron directly into energy. You gasp! No, I'm not kidding. I have to have an electron and a positron to do that. Nor can I convert a photon into just an electron.

Saying that "mass=energy" and "charge=energy" ignores all kinds of basics, like conservation of baryon number, conservation of lepton number, conservation of charge, etc.

Matter and energy are not the same two things. You may exchange one for another, under limitations, but don't think that they are in any way identical.

The notion sounds deceptively simple: besides the familiar three dimensions of space there may be other dimensions, too small to see yet perhaps as large as a millimeter.

Dimensions do not have a size. Objects have sizes in a set of dimensions.

In this description of the universe, some dimensions do have a size. You are perhaps familiar with the idea that the universe is closed -- ie, travelling along a geodesic will take you to where you started. You can imagine a 2d closed universe as a donut with n holes (n=0 gives a sphere) .

But you don't need to have all dimensions closed. You could have some dimensions curl up on themselves (topological compactification) and others that stretch out into infinity. The 2d example of this is a very long paper towel tube.

And finally, you can have some dimensions curl up much more tightly than others. 2d example again: the hula-hoop. A 2d scientist living on a hula hoop might not realize he is in a 1-holed donut shape, since part of the curvature tensor is so tight. Instead, he may think he is a 1d scientist living in a circle.

That is what this idea is all about. Using scary mathematics you can imagine any number of additional tightly curled dimensions to our apparently 3d universe. The question of what effect these extra dimensions would have, and how we can test for them, is what people are working on.

So far it is an exciting idea, but there have been no huge breakthroughs lately. I am much more interested in lattice QCD computation, but that is more personal prejudice than anything else.

...Leptons and quarks are fundamental particles...

Refresh my chemistry and physics, what are Leptons and quarks?

Sorry, its been a few years