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When is mass negetive?
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Apparently, even the scientists are confused................
okk pppl ..according to new researches ..there is matter and antimatter as well ...we know dat evry quantity has its positive as well as negative ... so yesss dere is anti energy and anti mass as well .......now its upto you dat u call its negative mass or anything.....scientists ne negative heliunm and hydrogen bhi bana liya hai though for certain nanoseconds!!!!!!!!!...hope dis satisfies u !!!
Negative mass
Ever since Newton first formulated his theory of gravity, there have been at least three conceptually distinct quantities called mass: inertial mass, "active" gravitational mass (that is, the source of the gravitational field), and "passive" gravitational mass (that is, the amount of force produced in response to gravity). The Einstein equivalence principle postulates that inertial mass must equal passive gravitational mass; while the law of conservation of momentum requires that active and passive gravitational mass must be identical. All experimental evidence to date has found these are indeed always the same. In considering hypothetical particles with negative mass, it is important to consider which of these concepts of mass are negative; however, in most analyses of negative mass, it is assumed that the equivalence principle and conservation of momentum continue to apply.
In 1957, Hermann Bondi suggested in a paper in Reviews of Modern Physics that mass might be negative as well as positive . He pointed out that this does not entail a logical contradiction, as long as all three forms of mass are negative, but that the assumption of negative mass involves some counter-intuitive form of motion.
From Newton's second law:
Thus it can be seen that an object with negative inertial mass would be expected to accelerate in the opposite direction to that in which it was pushed, which is arguably a strange concept.
If one were to treat inertial mass mi, passive gravitational mass mp, and active gravitational mass ma distinctly, then Newton's law of universal gravitation would take the form
Thus objects with negative gravitational mass (both passive and active), but with positive inertial mass, would be expected to be repelled by positive active masses, and attracted to negative active masses.
Negative mass in general relativity
In general relativity, negative mass is generalized to refer to any region of space in which for some observers the mass density is measured to be negative. This can occur due to negative mass, or could be a region of space in which the stress component of the Einstein stress-energy tensor is larger in magnitude than the mass density. All of these are violations of one or another variant of the positive energy condition of Einstein's general theory of relativity; however, the positive energy condition is not a required condition for the mathematical consistency of the theory. (Various versions of the positive energy condition, weak energy condition, dominant energy condition, etc., are discussed in mathematical detail by Visser.)
Morris, Thorne and Yurtsever pointed out that the quantum mechanics of the Casimir effect can be used to produce a locally mass-negative region of space-time. In this article, and subsequent work by others, they showed that negative matter could be used to stabilize a wormhole. Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by negative-mass loops of cosmic string.Stephen Hawking has proved that negative energy is a necessary condition for the creation of a closed timelike curve by manipulation of gravitational fields within a finite region of space;this proves, for example, that a finite Tipler cylinder cannot be used as a time machine.
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