# The Weakness of Gravity

1. Mar 28, 2007

### Kt_Atis

Today after watching a show called "Parallel Universe" and they talked about Parallel Universes of course. But the interesting thing is that they said that gravity is weak. For example. Like how the whole earth pulls on you and yet you are able to pick things up in a heartbeat with no problem at all. So they later come to the conclusion that gravity is leaking from our universe into an 11th deminsion. This is called M-Theory. Later they say that what if gravity wasn't leaking from our universe but to it. What if it came from that other universe inwhich the gravity would be as strong as the other forces but by the time it reached us it would only be a faint signal.

So my question is how likely is this scenerio.

2. Mar 28, 2007

### rbj

the thing that bothers me about the concept of the weakness of gravity is that when you get to specifics, it isn't only a comparison of the force of gravity to the forces of other fundamental interactions, say electromagnetic, but there are other quantities involved like the amounts of mass and charge. and these quantities are not commensurable ("comparing apples to oranges"), so since the force of gravity depends on how much mass you have and the force of E&M depends on how much charge you have, who says that gravity as a class of interaction is weaker than EM as an interaction? we can come up with a mass-to-charge ratio where the forces would be equal.

$$\sqrt{ \frac{1}{G 4 \pi \epsilon_0} } = \frac{m_P }{q_P}$$

with a mass/charge ratio of that, the strength of gravity is the same as the strength of EM. but elementary particles have a much less mass/charge ratio than that. so that is why, for charged elementary particles, the gravitational interaction seems to be insignificant - because the charge on the protons is approximately the Planck unit of charge but the mass of the protons is far, far less than the Planck mass.

i like how Frank Wilczek puts it:
http://www.physicstoday.org/pt/vol-54/iss-6/p12.html [Broken] June 2001 Physics Today

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