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Given the fact that gravity is the weakest of all forces, how does a gas cloud manage to collapse on itself under gravity and start a fusion reaction, outweighing the electromagnetic and nuclear forces between atoms? Even if the mass is greater, it would mean that the electromagnetic and nuclear forces between atoms would also be greater. How does gravity overcome these forces?

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The unique thing about gravity is that it is always attractive. Your intuition would be correct if the gas cloud was made purely of electrons, for example, as then the electromagnetic repulsion would be much greater than the gravitational attraction. But because there are both positive and negative charges in the gas cloud, the electromagnetic forces cancel out over long distances. Eventually the gas cloud can accumulate so much mass that it can overcome the other forces, despite gravity's relative weakness at the inter-particle level.

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  • $\begingroup$ "the electromagnetic forces cancel out over long distances" -- what role is played by the fact that gravity acts over longer distances (i.e. weakens less quickly) than the stronger forces? Am I even right in thinking that's the case? $\endgroup$ – Peter Duniho Jun 26 '20 at 0:18
  • $\begingroup$ @PeterDuniho: Both electromagnetic forces and gravitational forces are effectively infinite range. More precisely, they're both inversely proportional to the square of the distance between the objects. In contrast, the strong & weak nuclear forces are (roughly speaking) proportional to $e^{-r/\lambda}/r^2$, where $\lambda$ is a length on the scale of nuclei; this exponential factor means that the forces fall off much more quickly than for the "long-range" forces of electromagnetism & gravity. $\endgroup$ – Michael Seifert Jun 26 '20 at 0:40
  • $\begingroup$ I have to say - gravity is not always attractive. The acceleration of the universe shows that. i.e. in units of the critical density, present density of matter + dark energy - pressure dark energy = -1 (more or less). Since there is a minus sign in front of the whole thing, the acceleration of the universe is positive. This is all gravity, it is just that some terms take the opposite side. $\endgroup$ – Mr Anderson Jun 26 '20 at 6:33

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