Planck units

In physics, Planck units are physical units of measurement originally proposed by Max Planck. They form a system of natural units because they are defined exclusively in terms of the following fundamental physical constants — the units are natural because the numerical values of these five universal constants become 1 when expressed in units of this system.

Related Topics:
Physics - Physical - Units of measurement - Max Planck - Physical constant - Unit - Natural

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An increasingly common convention in the literature of particle physics and cosmology is to use reduced Planck units in which 8pi G=1 (so called because the Planck mass is reduced by sqrt{8pi} in these units). These units have the advantage of removing a factor of 8pi from the Einstein equation, Einstein-Hilbert action, Friedmann equation, and the Poisson equation for gravitation, at the expense of introducing one into Newton's law of universal gravitation. Another convention which is occasionally seen is to set 16pi G=1, which sets the coefficient of R in the Einstein-Hilbert action to unity.

Related Topics:
Particle physics - Cosmology - Planck mass - Einstein equation - Einstein-Hilbert action - Friedmann equation - Poisson equation - Law of universal gravitation

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The Planck units are often semi-humorously referred to by physicists as "God's units". They eliminate anthropocentric arbitrariness from the system of units: some physicists believe that an extra-terrestrial intelligence might be expected to use the same system.

Related Topics:
God - Anthropocentric

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Natural units can help physicists reframe questions. Perhaps Frank Wilczek said it best (June 2001 Physics Today):

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...We see that the question is not, "Why is gravity so feeble?" but rather, "Why is the proton's mass so small?" For in Natural (Planck) Units, the strength of gravity simply is what it is, a primary quantity, while the proton's mass is the tiny number ...

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The strength of gravity is simply what it is and the strength of the electromagnetic force simply is what it is. The electromagnetic force operates on a different physical quantity (electric charge) than gravity (mass) so it cannot be compared directly to gravity. To note that gravity is an extremely weak force is, from the point-of-view of natural units, like comparing apples to oranges. It is true that the electrostatic repulsive force between two protons (alone in free space) greatly exceeds the gravitational attractive force between the same two protons, and that is because the charge on the protons are approximately a natural unit of charge but the mass of the protons are far, far less than the natural unit of mass.

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Natural units have the advantage of simplifying many equations in physics by removing conversion factors. For this reason, they are popular in quantum gravity research.

Related Topics:
Physics - Quantum gravity

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Newton's Law of universal gravitation

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: F = G rac{m_1 m_2}{r^2}

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::becomes

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:: F = rac{m_1 m_2}{r^2} using Planck units.

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Schrödinger's equation

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:

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