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<blockquote data-quote="freyar" data-source="post: 6690564" data-attributes="member: 40227">There's also a shorter version of some of the same content in post 26.Anyway, I was going to say something about string theory and the equivalence principle, that is, the idea that inertial mass and gravitational mass are the same.&nbsp; So, as has already been discussed, this is a feature of any theory where gravity is really just the geometry of spacetime.&nbsp; That includes general relativity, extensions of general relativity, and also string theory.&nbsp; String theory is interesting since you can start with strings moving on a flat spacetime, but you end up with (an extension of) general relativity (in more dimensions). In that sense, the equivalence principle is &quot;derived.&quot;But it is also possible to find many discussions of violations of the equivalence principle in string theory and elsewhere. The context is that you get a lot more than gravity in string theory and really in most other theories beyond the Standard Model of particle physics.&nbsp; These new types of particles can introduce new forces.&nbsp; If one of these forces interacts differently with, for example, protons than neutrons, then different elements will experience this force differently, so we might mistakenly think that gravity acts differently on different materials. This is sometimes called a &quot;violation of the equivalence principle.&quot;The problem I have with that wording is that no one says that electromagnetic forces violate the equivalence principle. Or the strong or weak nuclear forces.&nbsp; Or even the very short range Higgs force which also must exist.&nbsp; That's because these forces aren't gravity.&nbsp; Well, any other new force wouldn't be gravity either.&nbsp; But I think this is one of those cases where there's a phrase that gets stuck in usage, even though it isn't really appropriate.&nbsp; There are some others in physics I don't like much either.</blockquote>

[QUOTE="freyar, post: 6690564, member: 40227"] There's also a shorter version of some of the same content in post 26. Anyway, I was going to say something about string theory and the equivalence principle, that is, the idea that inertial mass and gravitational mass are the same. So, as has already been discussed, this is a feature of any theory where gravity is really just the geometry of spacetime. That includes general relativity, extensions of general relativity, and also string theory. String theory is interesting since you can start with strings moving on a flat spacetime, but you end up with (an extension of) general relativity (in more dimensions). In that sense, the equivalence principle is "derived." But it is also possible to find many discussions of violations of the equivalence principle in string theory and elsewhere. The context is that you get a lot more than gravity in string theory and really in most other theories beyond the Standard Model of particle physics. These new types of particles can introduce new forces. If one of these forces interacts differently with, for example, protons than neutrons, then different elements will experience this force differently, so we might mistakenly think that gravity acts differently on different materials. This is sometimes called a "violation of the equivalence principle." The problem I have with that wording is that no one says that electromagnetic forces violate the equivalence principle. Or the strong or weak nuclear forces. Or even the very short range Higgs force which also must exist. That's because these forces aren't gravity. Well, any other new force wouldn't be gravity either. But I think this is one of those cases where there's a phrase that gets stuck in usage, even though it isn't really appropriate. There are some others in physics I don't like much either. [/QUOTE]

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