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Is gravity 'weak' because it leaks into higher dimensions? 
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#1
Apr912, 11:29 AM

P: 12

I seem to have read something once that suggested that gravity may be leaking into other dimensions/branes, this could explain why this force is so weak.
I've been left thinking that it makes sense as a possibility. If it turns out to be the case, could Dark Energy then be leaked gravitational energy from other dimensions leaking into our Universe? I realise that Dark Energy has the opposite effect to gravity, but somehow I think that makes sense, just like when you move a figure from one side of an = sign to another, the sign flips. Ideas anybody? Thanks 


#2
Apr912, 12:16 PM

P: 526

The idea you're speaking of comes out of nonperturbative string theory, called Mtheory. In MTheory, strings are not the only ingredient. Rather, multidimensional strings, called branes, are fundamental along with ordinary one dimensional strings. MTheory explains string tension, the variable that determines string oscillations and hence particle values, can be explained in terms of how tightly the branes hold the ends of open strings, the strings that make up everything except gravity. Gravity, in string theory, is represented by a closed loop string. Since a brane cannot hold onto a closed loop, since it has no ends, it will occasionally float off of the brane. The problem with supposing dark energy is truly gravity from other branes is it doesn't explain the constancy of the acceleration of the expansion of the universe. But I can't say for sure, considering Mtheory is still in its infancy. I did a search on arxiv for a paper on the topic, but I couldn't find anything. But, some string theorists have proposed that dark matter may be gravity from other branes. The idea may be a source of experimental testing of MTheory in the future. If particle collisions could produce less mass than was originally started with, that would be a sign of strings being 'knocked off' of our brane. 


#3
Apr912, 12:31 PM

P: 12

Hi Mark
Thanks for the welcome and reply. It all makes sense (so much as these things can) re: 'doesn't explain the constancy of the acceleration of the expansion of the universe'... OK, but I thought that the Universe hasn't expanded constantly, I was under the impression that after 'inflation' the expansion slowed considerably and then kinda kept coasting on and that the expansion rate is actually increasing as the binding force of gravity is diluted throughout the increasing volume of the Universe. If the Universe has a finite mass/energy and is a closed system (I know it wouldn't be if gravitational energy was leaking across branes) then the amount of gravitational energy that leaks out would be constant. The energy leaking in to our Universe from other branes would be constant too as the other Universes have a finite/fixed mass, if that is the source of the Dark Energy and associated expansion then it would a constant force. Is that convoluted b.s. or does it make sense? 


#4
Apr912, 12:48 PM

P: 526

Is gravity 'weak' because it leaks into higher dimensions?
The rate of expansion is increasing, yes. But it is most likely doing so at a fixed rate. Once inflation ended, the universe underwent the normal expansion, decelerating over time. The gravity of the totality of everything in the universe out weighed that of dark energy, causing the universe to slow down over time. But as the volume of the universe diluted, dark energy began to have a more prominent role. About 7 billion years ago, the force of dark energy overcame that of gravity, and began to accelerate the expansion of the universe. The important point to remember is that, as far as we know, dark energy's value seems to remain constant, or at least, very close to it. It may lose some density through expansion, but nothing like the amount lost by matter/dark matter. So, in short, yes, the universe is accelerating. But it is doing so according to a fixed rate. If gravitons from other branes were the case, we should see fluctuations in this rate of expansion, not a constant acceleration. 


#5
Apr912, 02:29 PM

P: 12

Super. I like that answer, I need to read up for the first sentence a little; the rest made sense. I get the history of Universal expansion a little more, not that I can picture 7 billion.
I'm guessing 'Inflatons' are hypothetical particles that carry the inflation force? (a kinda antiHiggs?) Antigravity? Maybe one day all this atom smashing will come up with something useful! ;) Branes shouldn't be symmetrical to ours, from what I understand that most of the concept relies on each brane having different qualities. I'm gonna keep thinking... thanks again. 


#6
Apr912, 03:07 PM

P: 526

There isn't an inflation force, but it is repulsive gravity. Here's how it works: In general relativity, mass, energy, and pressure produce the attractive gravity that we are familiar with. This causes a positive curvature in spacetime. Negative pressure, the force that is exerted by a tense rubber band, has the opposite effect  it gives spacetime a hyperbolic shape, resulting in repulsive gravity. Since gravity is just the shape of spacetime, a negative pressure will drive expansion. The inflaton field is a scalar field, meaning it takes a value at every point in space. Essentially, when it was trying to reach it's ground state, it supercooled. This is similar to how purified water can be lowered below 0 degrees Celsius without freezing. The inflaton field would react by filling the universe with large amounts of energy, and as Guth found, a negative pressure. This enormous negative pressure would allow for the inflaton field to reach its true vacuum, and in the process, expand the universe by a huge amount. After reaching this ground state, it would decay into a hot bath of radiation, which would soon give birth to the matter in the universe. 


#7
Apr912, 11:33 PM

Sci Advisor
PF Gold
P: 9,359

Gravity leaking into other dimensions is not a satisfying solution. Were this true, should we not expect gravity from 'other universes' to leak into our universe?



#8
Apr912, 11:43 PM

P: 2,251

gravity is neither weak nor strong in comparison to, say, EM because it acts on mass while EM acts on charge and the two quantities are incommensurable, so they cannot be directly compared. it is true that the attractive gravitational force between two protons is much much less than the electrostatic repulsive force between the same two protons (both fields are inversesquare, so it doesn't matter what the spacing is), and that is because the charge on the protons is in the same ballpark as the Planck charge, but the mass of the protons is far, far less than the Planck mass.
quoting Frank Wilczek: 


#10
Apr1012, 01:02 AM

P: 3,014

This is from Anthony Zee's "QFT in a Nutshell", chapter I.6.
In a [itex]D = (3 + n)[/itex]dimensional world, the potential energy goes as: [tex] V(r) \propto \int{d^{3 + n} k \, \frac{e^{i \mathbf{k} \cdot \mathbf{x}}}{k^2}} \propto \frac{1}{r^{1 + n}} [/tex] If the gravitational force is proportional to the masses of the interacting particles regardless of the number of spatial dimensions, we may write: [tex] V(r) = G_{3 + n + 1} \, \frac{m_1 \, m_2}{r^{1 + n}} [/tex] In natural units ([itex]\hbar = c = 1[/itex]), the dimensions of mass, length, and time satisfy [itex]\mathrm{T} = \mathrm{L} = \mathrm{M}^{1}[/itex], and every physical quantity has only a mass dimension. For example: [tex] \left[ V(r) \right] = [E] = \mathrm{M} \, \mathrm{L}^2 \, \mathrm{T}^{2} \stackrel{\mathrm{n.u.}}{\rightarrow} \mathrm{M} [/tex] Then, the "Universal Gravitational Constant" in [itex]3 + n + 1[/itex] spacetime dimensions has a mass dimension: [tex] \left[ G_{3 + n + 1}\right] = \frac{[E] \, [r]^{1 + n}}{[m]^2} = \mathrm{M}^{2  n} [/tex] Indeed, this constant defines a Planck mass in [itex]3 + n + 1[/itex] spacetime dimensions: [tex] G_{3 + n + 1} = \frac{1}{M_{\mathrm{Pl}, 2 + n}} [/tex] Indeed, in (3 + 1)spacetime, [itex]M_{\mathrm{Pl},4} = 1/\sqrt{G_{4}} \sim 10^{19} \, \mathrm{GeV}[/itex]. This is a HUGE energy scale (energy and mass have the same dimension in natural units), a fact we interpret as the reason why gravity is so weak. But, suppose the extra n spatial dimensions are "curled up" to linear dimensions of the size R. This need not be a microscopic length scale, but is small enough that it had not been detected by gravitational experiments performed on such a small length scale. For distances [itex]r \gg R[/itex], the gravitational flux spreads only in the 3 unfolded linear dimensions, and the gravitational potential energy goes as: [tex] V(r) = \frac{m_1 \, m_2}{M^{2 + n}_{\mathrm{Pl}, 3 + n +1}} \, \frac{1}{R^n} \, \frac{1}{r} [/tex] If we interpret: [tex] \frac{1}{M^2_{\mathrm{Pl},4}} \equiv \frac{1}{M^{2 + n}_{\mathrm{Pl}, 3 + n +1} R^{n}} [/tex] [tex] \frac{M_{\mathrm{Pl},4}}{M_{\mathrm{Pl},3 + n + 1}} = \left( M_{\mathrm{Pl},3 + n + 1} R \right)^\frac{n}{2} [/tex] Now, the Planck mass in [itex]3 + n + 1[/itex] spacetime dimensions may as well be of the order of magnitude of ordinary particle masses (let us choose a range of values 10 eV  100 GeV). This would put the dimensioless ratio of masses: [tex] \frac{M_{\mathrm{Pl}, 4}}{M_{\mathrm{Pl}, 3 + n + 1}} = 10^{17}  10^{27} [/tex] Then, the distance R, should be: [tex] R = \frac{1}{M_{\mathrm{Pl},3 + n + 1}} \, \left( \frac{M_{\mathrm{Pl}, 4}}{M_{\mathrm{Pl}, 3 + n + 1}} \right)^\frac{2}{n} = \frac{1}{M_{\mathrm{Pl}, 4}} \, \left( \frac{M_{\mathrm{Pl}, 4}}{M_{\mathrm{Pl}, 3 + n + 1}} \right)^{1 + \frac{2}{n}} [/tex] This is a monotonically rising function of the dimensionless ratio (the smaller the mass of the Planck particle, the bigger the ratio). For a popular choice [itex]n = 6[/itex], we would have: [tex] R = (10^{19}  10^{31}) \, M^{1}_{\mathrm{Pl},4} = (10^{16}  10^{4}) \, \mathrm{m} [/tex] 


#11
Apr1012, 03:23 AM

P: 10

It goes like this: After a fraction of a second after the big bang (brane collision, whatever) there were only TWO fundamental forces instead of four. They were the Electromagetism force and the Weaknuclear force. The amalgam of these two forces formed the Electroweak force. This is the "inflaton" field you speak about (apparently). Anyway back to the Big Bang timeline, this superforce somehow caused inflation, spreading and recycling (yes recycling not creating as stated by E=mc^2) matter in an incredibly short amount of time. We now call this inflation. But why do we not see the super weak force now? Think of each force as a spinner with two outputs. When you spin the spinner, it gains energy and then soon it looses it and lands on one of the possible outputs. Now apply that to the electroweak force. It had a MASSIVE effect and quickly exhausted it's available energy thus forcing the spinner to land on something. But unlike spinners in real life, forces (when they stop "spinning") land on all the outcomes at the same time. The electroweak superforce has now split into The Weak nuclear force, the strong nuclear force and the Electromagnetic force. But where's gravity? Scientists believe that gravity was the result of the same superforce which spawned dark energy. The balancing scale of gravity vs. dark energy was tipping toward gravity and then for some reason reversed giving dark energy the upper hand. So the answer to the question "Is gravity 'weak' because it leaks into higher dimensions?" is a big no no. Gravity is the same thing as everything else. It just seems to be weaker because there is not much concentration and quantity of the things it has control over (matter). Hope that helped.  Physgeek123 


#12
Apr1012, 12:34 PM

P: 2,251




#13
Apr1012, 02:07 PM

P: 526

As usual with all cosmological models based off of Mtheory, it's interesting, but there is no evidence. http://arxiv.org/abs/astroph/0101507 http://en.wikipedia.org/wiki/Higgs_mechanism Great post DF, that would seem to solve the problem better than gravitons 'leaking' off of our brane. 


#14
Apr1012, 09:06 PM

P: 126




#15
Apr1012, 09:10 PM

P: 15,319




#16
Apr1112, 10:18 PM

P: 110




#17
Apr1212, 04:02 PM

P: 12

Interesting answers all. 


#18
Apr1212, 04:06 PM

P: 12




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