Negative Mass [UPSC Science and Technology Notes]

The negative matter is a matter whose mass is of opposite sign to the mass of regular matter. It is an imaginary type of matter which, if it existed will have negative mass and negative energy.

The negative matter is a concept featured in the General Science section of the UPSC Syllabus.

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Overview of Negative Mass

In physics, negative mass is any matter whose mass is of opposite sign to the mass of normal matter. For example, if an object weighs 5 kg, its negative mass will be -5 kg. As a result, such an object with the resulting negative matter would break more than one or more energy condition, while showing some strange properties, stemming from the ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass. It is used in certain speculative hypotheses, such as on the construction of traversable wormholes. Initially, the closest known real representative of such exotic matter is a region of negative pressure density produced by the Casimir effect.

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Concept of Negative Mass

In considering negative mass, it is important to consider which standard concepts of mass are negative. Ever since Isaac Newton first formulated his theory of gravity, there have been at least three conceptually distinct quantities called mass:

  • Inertial mass – the mass m that appears in Newton’s second law of motion, F = m a
  • “Active” gravitational mass – the mass that produces a gravitational field that other masses respond to
  • “Passive” gravitational mass – the mass that responds to an external gravitational field by accelerating.

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The law of conservation of momentum states that active and passive gravitational mass should be identical. Einstein’s equivalence principle speculates that inertial mass must equal passive gravitational mass, and all experimental evidence to date has found these are, indeed, always the same.

Upon further examination of negative mass, it is assumed that the equivalence principle and conservation of momentum continue to apply, and therefore all three forms of mass are still the same, leading to the study of “negative mass”. But the equivalence principle is simply an observational fact and is not necessarily valid. If such a distinction is made, a “negative mass” can be of three kinds: whether the inertial mass is negative, the gravitational mass, or both.

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There have been many case studies regarding the analysis of negative mass, with non-having addressed a fundamental question – what kind of energy and momentum would be necessary to describe non-singular negative mass? Indeed, the Schwarzschild solution for the negative mass parameter has a naked singularity at a fixed spatial position.

The question that immediately comes up is, would it not be possible to smooth out the singularity with some kind of negative mass density. The answer is yes, but not with energy and momentum that satisfies the dominant energy condition. This is because if the energy and momentum satisfy the dominant energy condition within a spacetime that is asymptotically flat, which would be the case of smoothing out the singular negative mass Schwarzschild solution, then it must satisfy the positive energy theorem, i.e. its ADM mass must be positive, which is of course not the case.

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