Exploring the Weakness of Gravity: Unifying the Four Fundamental Forces

[Techno-Raver]

If all four fundamental forces were once unified and equal, why is gravity so much weaker than the other three forces?

 

[nrqed]

If all four fundamental forces were once unified and equal, why is gravity so much weaker than the other three forces?

well, if I knew why I would have a good shot at a Nobel prize!

Nobody really knows why. One idea is inspired by string theory/ brane models scenarios. In string theory, gravity is mediated by closed strings whereas the other forces are mediated by open strings. And it turns out that open strings have their ends attached to submanifolds (the so-called branes) whereas closed strings may propagte freely in all dimensions. If our universe is one of those branes, this would explain why gravity appears weaker: the force is "spreading" out in all dimensions and appears to us weaker. The other forces mediators are confined within our brane and do not "leak" out in all the dimensions.

 

[Farsight]

This is by a science fiction writer so shouldn't be considered serious physics, but it's "open your mind" interesting.

http://www.npl.washington.edu/AV/altvw98.html

Why is gravity so weak? Why are the color forces between quarks so strong? In the standard model of particle physics, why are there so many different energies at which distinct fundamental forces are supposed to "unify", and what determines these widely separated energies? The answers to these questions may be provided by extra dimensions curled into loops a millimeter around. In other words, our universe may be only a millimeter across, in directions we are not yet able to perceive. In this column we'll consider millimeter-size extra-dimensional loops and their implications...

 

[Mindscrape]

This may or may not be on the topic, but does anybody know that since Fermi Labs found that neutrinos have mass how much more of the universe's mass is now accounted for? Is the search for gravitons still taken as seriously?

 

[ZapperZ]

This may or may not be on the topic, but does anybody know that since Fermi Labs found that neutrinos have mass how much more of the universe's mass is now accounted for? Is the search for gravitons still taken as seriously?

And this is certainly off-topic, but give credit where credit is due. The first laboratory/facility that verified the existence of neutrino mass is Super Kamiokande in Japan, not Fermilab. Fermilab only recently got into the neutrino business with MINOS, that just announced their first set of results a couple of months ago.

And no one is looking for gravitons right now. Gravitons and "gravity waves" as the ones being looked for with LIGO are not the same thing.

Zz.

 

[Farsight]

Sure is some interesting stuff out there.

http://www.psc.edu/science/Winicour/winicour.html

"To clinch the case, scientists at Caltech and MIT, with funding from the National Science Foundation, are building LIGO, the Laser Interferometer Gravitational-Wave Observatory...

 

[ZapperZ]

This is by a science fiction writer so shouldn't be considered serious physics, but it's "open your mind" interesting.

http://www.npl.washington.edu/AV/altvw98.html

Why is gravity so weak? Why are the color forces between quarks so strong? In the standard model of particle physics, why are there so many different energies at which distinct fundamental forces are supposed to "unify", and what determines these widely separated energies? The answers to these questions may be provided by extra dimensions curled into loops a millimeter around. In other words, our universe may be only a millimeter across, in directions we are not yet able to perceive. In this column we'll consider millimeter-size extra-dimensional loops and their implications...

You'll notice that this is OLD. The Adelberg's group at U. of Washington has already verified gravity up to sub-micron scales with NO deviation in G. This implies that there are no "extra-dimensional loops" at the millimeter scale.

Zz.

 

[George Jones]

You'll notice that this is OLD. The Adelberg's group at U. of Washington has already verified gravity up to sub-micron scales with NO deviation in G. This implies that there are no "extra-dimensional loops" at the millimeter scale.

Zz.

I'm not sure what happened to this

http://cosmicvariance.com/2005/08/12/rumors-of-new-forces

I'll try and take a closer look this afternoon.

 

[ZapperZ]

I'm not sure what happened to this

http://cosmicvariance.com/2005/08/12/rumors-of-new-forces

I'll try and take a closer look this afternoon.

They themselves are not sure of it, and we only have the report of their presentations at the APS April Meetings, which is where this was first revealed. So I would suggest we wait till such a thing is published.

In any case, it is still NOT at the mm scale that all of these "extra" dimensions for gravity have been predicting.

Zz.

 

[Farsight]

OK thanks Zapper.

Techno-Raver: I'm not sure about this, but I picked up somewhere that Gravity is so much weaker than eg Electromagnetism because of a dimensional difference. Both can be considered as curvature or distortion of spacetime, but gravity is a gentle distortion of the spacetime in our familiar dimensions, while the Coulomb forces is a severe distortion in unfamiliar, smaller dimensions. These other dimensions are like 10^40 smaller than the ones we're used to, and the amount of distortion is the perceived "force" strength.

http://www.wordwizz.com/pwrsof10.htm

"This is a visual journey consisting of 42 images -- 42 powers of ten. At one end of the journey is the immensity of the known universe, 13.7 billion years old with a radius of at least 12 billion light years (and probably much larger). At the other end of the journey is a depiction of the three quarks within a proton.

 

[George Jones]

In any case, it is still NOT at the mm scale that all of these "extra" dimensions for gravity have been predicting.

It's my impression that theory/experiment has ruled out extra dimensions at the anything above the mm scale. All scales below this are still fair game.

 

[scott1]

Nobody really knows why. One idea is inspired by string theory/ brane models scenarios. In string theory, gravity is mediated by closed strings whereas the other forces are mediated by open strings. And it turns out that open strings have their ends attached to submanifolds (the so-called branes) whereas closed strings may propagte freely in all dimensions. If our universe is one of those branes, this would explain why gravity appears weaker: the force is "spreading" out in all dimensions and appears to us weaker. The other forces mediators are confined within our brane and do not "leak" out in all the dimensions.

Yes string theory does have a good idea about why gravity is so weak you might want to watch thesehttps://www.physicsforums.com/showthread.php?t=8241" for more about string theory.In cosmology was gravity soppsed to become sepreate form the other field froces first. Could this have somthing to do with the reason why gravity is so weak.
EDIT: Also I just remberd somthing. That there's a theory called mond theory that says that gravity strength changes. I think it might of also been callled varible gravity.

 

[DM]

And no one is looking for gravitons right now. Gravitons and "gravity waves" as the ones being looked for with LIGO are not the same thing.

Can anybody ever look for gravitons knowing they're virtual particles?

Can anybody look for any virtual particles for that matter?

 

[rbj]

If all four fundamental forces were once unified and equal, why is gravity so much weaker than the other three forces?

well, if I knew why I would have a good shot at a Nobel prize!

Nobody really knows why.

personally, i would like to know in what sense do you mean that gravity is weak?

since gravity (in the classical sense) acts on mass and, say, E&M acts on this property called electric charge, you cannot compare the two. it depends on how much mass you have on one hand and how much charge you have on the other. i don't think that gravity is any "weaker" than, say, E&M. the force of gravity between two Planck masses is equal to the force of E&M between two Planck charges.

if, when it boils down to it, that you say that the attractive gravitational force between two protons (or pick your fundamental particle) alone in free space is far, far weaker than the repulsive electrostatic force between the same two protons, you're right, it is. and that is because the charge of the two protons is very nearly the natural unit of charge, but the masses of the two protons is far, far less than the natural unit of mass and that is why the gravitational force between them is neglegible.

the real question to ask is: why are the masses of the fundamental particles so, so small?

 

[ZapperZ]

Can anybody ever look for gravitons knowing they're virtual particles?

Can anybody look for any virtual particles for that matter?

How did you think we confirmed the existence of the W and Z particles that mediate the weak interactions, the gluons that mediate the strong interactions, etc... etc? Virtual photons are not verified? Let's throw out QED.

Zz.

 

[DM]

How did you think we confirmed the existence of the W and Z particles that mediate the weak interactions, the gluons that mediate the strong interactions, etc... etc? Virtual photons are not verified? Let's throw out QED.

Actually, I don't know.

I've simply read that virtual particles cannot be directly detected, which makes perfect sense to me. Whether there are practical ways to detect them or not is of great interest since theoretically it sounds very difficult.

 

[ZapperZ]

Actually, I don't know.

I've simply read that virtual particles cannot be directly detected, which makes perfect sense to me. Whether there are practical ways to detect them or not is of great interest since theoretically it sounds very difficult.

They can't be directly dectected, but the effects of their existence have ramifications that are measureable. You do not get "free quarks" either, yet we have many predictions that have been verified experimentally based on the quark model.

You should know by now that in physics, unless things are experimentally verified, we normally do not award Nobel Prizes for it, especially if it is theoretical work.

Zz.

 

[DM]

You should know by now that in physics, unless things are experimentally verified, we normally do not award Nobel Prizes for it, especially if it is theoretical work.

In which of course, I do.

Your previous post perplexed me. It gave me the interpretation that someone actually directly detected virtual particles, or if not, that there were ways of detecting them and therefore enabling physicists to apply the same experimental procedure to detect gravitons.

 

[ZapperZ]

Your previous post perplexed me. It gave me the interpretation that someone actually directly detected virtual particles, or if not, that there were ways of detecting them and therefore enabling physicists to apply the same experimental procedure to detect gravitons.

What's perplexing about that?

Zz.

 

[DM]

What's perplexing about that?

You agreed that virtual particles cannot be directly detected.

Are you now implying they can?

Again, I'm left somewhat perplexed.

 

[ZapperZ]

You agreed that virtual particles cannot be directly detected.

Are you now implying they can?

Again, I'm left somewhat perplexed.

You don't put a detector, and detect virtual particles. You can see what the theory says, especially on higher order interactions, and THEN, measure THAT. Why do you think QED can calculate the electron gyromagnetic ratio to such high degree of accuracy that agrees with experimental observation?

When the theoretical predictions using such model works, then one tends to put a lot of weight on the validity of it. If you look at how physics is done, you'll notice that a lot of what you accept to be true works this way, where a series of consistent experimental observation agrees with what is predicted by the theory. Even the so-called "real" particles are verified this way - how do you think we detect neutrinos?

Zz.

 

[Farsight]

Anyhow, gravity.

If the "action at a distance" forces like gravity and electromagnetism can be considered as some kind of spatial distortion, think about a horizontal pole, in a gym. You're clinging on to it with you arms and legs wrapped around it. And it's a thousand feet long.

Now I come along and bend the pole into a U. But the pole is so long you hardly notice that it's bent. The distortion is slight.

Now we repeat with a pole that's only ten feet long. This time you really notice it. The distortion is huge.

You can perhaps apply this analagy to gravity. The same distortion is over two different scales, and the distortion on the large scale feels locally weak.

 

[scott1]

A little offtopic question:
Are the other 3 field froces the same sterngth and gravity is the only field force that isn't the same stergth. Or are they differn't(which is probally the correct answear since there's a weak and a strong nuclear force)

If they all have differn't stergths they why are physicst only seem to wonder why gravity is so weak?

 

[Techno-Raver]

As I've research M Theory, I've read that in the 11th dimensional framework, gravity is leaking into the 11th dimension, losing its strength. The 11th dimension is said to be where parallel universes coexist, so that would imply that gravity is leaking out and being shared between universes.

Theoretically...:yuck:

 

[Mindscrape]

I dunno. You should always becareful what you read on String Theory because it has ventured into the realm of Popular Science, and a lot of the times Popular Science will give a really wishy-washy view that can be misleading. What you read I remember reading about in Brian Greene's book, so it's probably right, but be careful about accepting views on String Theory, or just String Theory in general. (Remember, it's still just a theory!)

 

[Farsight]

Perhaps we could talk about what a force like gravity or electromagnetism actually is?

I don't think it's some spooky action-at-distance thing caused by messenger particles, or some ZZZZZZZZ magical attraction like a cartoon magnet. IMHO it has to be the distortion of some fundamental property of spacetime. We can't see this distortion. But we can feel it.

 

[rbj]

i'm re-asserting my question: in what sense (hopefully comparing apples to apples) do you (anyone here) mean when you say that the gravitational force is weak?

 

[Farsight]

For the record, I don't think it's weak.

If I might reiterate without boring you guys: Sure it's weak on the local scale, but that's because it's on a bigger scale. Like the two scaffold poles, both bent into a U. One's only ten foot long, so if you're clinging to it, you really notice the distortion. The other one's a million miles long. It's just as distorted, but you don't notice it so much because it's all so much bigger.

 

[Locrian]

So clowns can do handstands.

 

[rbj]

For the record, I don't think it's weak.

If I might reiterate without boring you guys: Sure it's weak on the local scale, but that's because it's on a bigger scale. Like the two scaffold poles, both bent into a U. One's only ten foot long, so if you're clinging to it, you really notice the distortion. The other one's a million miles long. It's just as distorted, but you don't notice it so much because it's all so much bigger.

but that still implies in an apple-to-apple comparison that the gravitational pole is so much bigger than the E&M pole. for a Planck mass clinging to the gravitational pole and a Planck charge clinging to the EM pole, the curvature is the same.

 

[George Jones]

personally, i would like to know in what sense do you mean that gravity is weak?

Often, in this sense.

why are the masses of the fundamental particles so, so small?

In other words, why does Nature sprinkle an electron so liberally with electric charge and so conservatively with gravitational charge.

The answer could be "That's just the way it is." or it could be due to some profound new physics.

Or it could be that the gravitational charge of an electron is not (relatively) small, and that, as nrqed oulines, gravity leaks out into other dimensions while other forces don't.

 

[rbj]

In other words, why does Nature sprinkle an electron so liberally with electric charge and so conservatively with gravitational charge?

we agree that this is the question to ask (and anwer). so does Frank Wilzcek.

The answer could be "That's just the way it is." or it could be due to some profound new physics.

i agree that there is potentially some profound physics in answering this. i think it answers the same question of "why is the size of atoms so much bigger than the natural unit of length?" since

$$a_0 = \frac{m_P}{m_e \alpha} l_P$$

Or it could be that the gravitational charge of an electron is not (relatively) small,

not from a POV of natural units. the mass of even a proton or neutron is exceedingly small (in terms of the natural unit of mass).

and that, as nrqed oulines, gravity leaks out into other dimensions while other forces don't.

i remember seeing that hypothesis from the Brian Greene NOVA special ("The Elegant Universe"). for some reason, this seems speculative while the salience of Planck Units (or something close to them - i think that $4 \pi G$ and $\epsilon_0$ should be normalized for the most natural units) seems to be right there. we know that these dimensionful scaling factors go away if we measure and describe physical quantities in terms of natural units. then we have a basis for saying something is really big or really small.

 

[George Jones]

not from a POV of natural units. the mass of even a proton or neutron is exceedingly small (in terms of the natural unit of mass).

But Planck units change as the number of spacetime dimensions changes.

The gravitational constant in $D$ dimensions is given by

$$G^{\left( D \right)} = G \left( l_{C} \right)^{D-4},$$

where $G$ is the usual gravitational constant and $l_C$ is the "cirumference of compactification" of the extra spatial dimensions.

The Planck mass, for example, is the product of appropriate powers of $G^{\left( D \right)}$, $c$, and $\hbar$. When I work this out for, say, $D = 6$ and an $l_C$ of 10 microns, I get much less discrepancy between the Planck mass and the mass of fundamental particles, and between the Planck mass and the Planck charge (if gravity and not electromagnetism leaks into the extra dimensions).

This why the work at the University of Washington is so important.

Chapter 3 of Zwiebach's book A First Course in String Theory gives a readable presentation of these ideas. Problems 3.9 and 3.10 are quite interesting.

Zwiebach points out that this just trades the mass hierarchy problem for a length hierarchy problem. Why is the Planck length so much smaller than than the compactification scale?

 

[Farsight]

Did anybody say what one of these action-at-a-distance forces actually is?

I'd like to establish what an action-at-a-distance force is before talking about one being weaker than the other. Is there a consensus or official line here? In layman's terms?

I don't like to rely on Wikipedia:

http://en.wikipedia.org/wiki/Action_at_a_distance_(physics )