The laws list
h to Huygen's construction.

Jump links.
See Planck constant.
Hall effect
When charged particles flow through a tube which has both an electric field and a magnetic field (perpendicular to the electric field) present in it, only certain velocities of the charged particles are preferred, and will make it undeviated through the tube; the rest will be deflected into the sides. This effect is exploited in such devices as the mass spectrometer and in the Thompson experiment. This is called the Hall effect.
Hawking radiation (S.W. Hawking; 1973)
The theory that black holes emit radiation like any other hot body. Virtual particle-antiparticle pairs are constantly being created in supposedly empty space. Occasionally, a pair will be created just outside the event horizon of a black hole. There are three possibilities:
  1. both particles are captured by the hole;
  2. both particles escape the hole;
  3. one particle escapes while the other is captured.
The first two cases are straightforward; the virtual particle-antiparticle pair recombine and return their energy back to the void via the uncertainty principle.

It is the third case that interests us. In this case, one of the particles has escaped (and is speeding away to infinity), while the other has been captured by the hole. The escapee becomes real and can now be detected by distant observers. But the captured particle is still virtual; because of this, it has to restore conservation of energy by assigning itself a negative mass-energy. Since the hole has absorbed it, the hole loses mass and thus appears to shrink. From a distance, it appears as if the hole has emitted a particle and reduced in mass.

The rate of power emission is proportional to the inverse square of the hole's mass; thus, the smaller a hole gets, the faster and faster it emits Hawking radiation. This leads to a runaway process; what happens when the hole gets very small is unclear; quantum theory seems to indicate that some kind of "remnant" might be left behind after the hole has emitted away all its mass-energy.

Hawking temperature
The temperature of a black hole caused by the emission of Hawking radiation. For a black hole with mass m, it is
T = (hbar c3)/(8 pi G k m).
Since blackbody power emission is proportional to the area of the hole and the fourth power of its thermodynamic temperature, the emitted power scales as m-2 -- that is, as the inverse square of the mass.
See Dirac constant.
Heisenberg uncertainty principle
See uncertainty principle.
henry; H (after W. Henry, 1775-1836)
The derived SI unit of inductance, defined as the inductance of a closed circuit in which an electromotive force of 1 V is produced when the electric current varies uniformly at a rate of 1 A/s; it thus has units of V s/A.
hertz; Hz (after H. Hertz, 1857-1894)
The derived SI unit of frequency, defined as a frequency of 1 cycle per s; it thus has units of s-1.
Hooke's law (R. Hooke)
The stress applied to any solid is proportional to the strain it produces within the elastic limit for that solid. The constant of that proportionality is the Young modulus of elasticity for that substance.
hoop conjecture (K.S. Thorne, 1972)
The conjecture (as yet unproven, though there is substantial evidence to support it) that a nonspherical object, nonspherically compressed, will only form a black hole when all parts of the object lie within its event horizon; that is, when a "hoop" of the event horizon circumference can be rotated in all directions and will completely enclose the object in question.
Hubble constant; H0 (E.P. Hubble; 1925)
The constant which determines the relationship between the distance to a galaxy and its velocity of recession due to the expansion of the Universe. Since the Universe is self-gravitating, it is not truly constant. In cosmology, it is defined as
H = (da/dt)/a,
where a is the 4-radius of the Universe. When evaluated for the present, it is written
H0 == H(t = now).

The Hubble constant is not known to great accuracy (only within about a factor of 2), but is believed to lie somewhere between 50 and 100 km/s/Mpc.

Hubble's law (E.P. Hubble; 1925)
A relationship discovered between distance and radial velocity. The further away a galaxy is away from is, the faster it is receding away from us. The constant of proportionality is the Hubble constant, H0. The cause is interpreted as the expansion of spacetime itself.
Huygens' construction; Huygens' principle (C. Huygens)
The mechanical propagation of a wave (specifically, of light) is equivalent to assuming that every point on the wavefront acts as point source of wave emission.
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