 tachyon

A purely speculative particle, which is presumed to travel faster
than light. According to Einstein's equations of special
relativity, a particle with an imaginary rest mass and a velocity
greater than c would have a real momentum and energy. Ironically,
the greater the kinetic energy of a tachyon, the slower it
travels, approaching c asymptotically (from above) as its energy
approaches infinity. Alternatively, a tachyon losing kinetic
energy travels faster and faster, until as the kinetic energy
approaches zero, the speed of the tachyon approaches infinity;
such a tachyon with zero energy and infinite speed is called
transcendent.
Special relativity does not seem to specifically exclude
tachyons, so long as they do not cross the lightspeed barrier and
do not interact with other particles to cause causality
violations. Quantum mechanical analyses of tachyons indicate that
even though they travel faster than light they would not be able
to carry information faster than light, thus failing to violate
causality. But in this case, if tachyons are by their very nature
indetectable, it brings into question how real they might be.
See Occam's razor; compare tardon, luxon.
 tachyon paradox

The argument demonstrating that tachyons (should they
exist, of course) cannot carry an electric charge. For a
(imaginarymassed) particle travelling faster than c, the less
energy the tachyon has, the faster it travels, until at zero
energy the tachyon is travelling with infinite velocity, or is
transcendent. Now a charged tachyon at a given (noninfinite)
speed will be travelling faster than light in its own medium, and
should emit Cherenkov radiation. The loss of this energy will
naturally reduce the energy of the tachyon, which will make it go
faster, resulting in a runaway reaction where any charged tachyon
will promptly race off to transcendence.
Although the above argument results in a curious conclusion,
the meat of the tachyon paradox is this: In relativity, the
transcendence of a tachyon is framedependent. That is, while a
tachyon might appear to be transcendent in one frame, it would
appear to others to still have a nonzero energy. But in this case
we have a situation where in one frame it would have come to zero
energy and would stop emitting Cherenov radiation, but in another
frame it would still have energy left and should be emitting
Cherenkov radiation on its way to transcendence. Since they
cannot both be true, by relativistic arguments, tachyons cannot be
charged.
This argument naturally does not make any account of quantum
mechanical treatments of tachyons, which complicate the situation
a great deal.
 tardon

A particle which has a positive real mass and travels at a speed
less than c in all inertial frames.
Compare tachyon, luxon.
 tardyon

See tardon.
 tautheta paradox (1950s)

When two different types of kaons, tau and theta (today tau refers
to a completely different particle) decay, tau decays into three
particles, while the theta decays into two. The tau and theta
differ only in parity; and at the time, it was thought that parity
was strictly conserved, and that particles differing only in
parity should behave exactly the same. Since the two decay
differently, a paradox ensued. The paradox was resolved when
experiments carried out according to F. Yang and T.D. Lee's
theoretical calculations indeed indicate that parity is not
conserved in weak interactions.
 tesla; T (after N. Tesla, 18701943)

The derived SI unit of magnetic flux density, defined the magnetic
flux density of a magnetic flux of 1 Wb through an area of 1 m^{2};
it thus has units of Wb/m^{2}.
 thermodynamic laws

 First law of thermodynamics

The change in internal energy of a
system is the sum of the heat transferred to or from the
system and the work done on or by the system.
 Second law of thermodynamics

The entropy  a measure of the
unavailability of a system's energy to do useful work  of a
closed system tends to increase with time.
 Third law of thermodynamics

For changes involving only perfect
crystalline solids at absolute zero, the change of the total
entropy is zero.
 Zeroth law of thermodynamics

If two bodies are each in thermal
equilibrium with a third body, then all three bodies are in
thermal equilibrium with each other.
 Thomson experiment; Kelvin effect (Sir W. Thomson [later Lord Kelvin])

When an electric current flows through a conductor whose ends are
maintained at different temperatures, heat is released at a rate
approximately proportional to the product of the current and the
temperature gradient.
 Tipler machine

A solution to Einstein's equations of general relativity that
allows time travel. An extremely dense (on the order of the
density of neutron star matter), infinitelylong cylinder which
rotates very rapidly can form closed timelike curves in its
vicinity, which will allow time travel and possible subsequent
violations of causality.
 TitiusBode law

See Bode's law.
 transition temperature

The temperature (dependant on the substance involved) below which a
superconducting substance
conducts electricity with zero resistance; consequently, the
temperature above which a superconductor loses its superconductive
properties.
 Trojan points

L4 and L5, the two dynamically stable Lagrange points
(under certain conditions).
 Trojan satellites

Satellites which orbit a body at one or the other Trojan points
relative to a secondary body. There are several examples of this
in our own solar system: a group of asteroids which orbit in the
the Trojan points of Jupiter; daughter satellites which orbit in
the Trojan points of the SaturnTethys system, and an additional
satellite (Helene) which orbits in the forward Trojan point
of Saturn and Dione.
 twin paradox

One of the most famous "paradoxes" in history, predicted by A.
Einstein's special theory of relativity. Take two twins, born on
the same date on Earth. One, Albert, leaves home for a trip
around the Universe at very high speeds (very close to that of
light), while the other, Henrik, stays at home at rests. Special
relativity predicts that when Albert returns, he will find himself
much younger than Henrik.
That is actually not the paradox. The paradox stems from
attempting to naively analyze the situation to figure out why.
From Henrik's point of view (and from everyone else on Earth),
Albert seems to speed off for a long time, linger around, and then
return. Thus he should be the younger one, which is what we see.
But from Albert's point of view, it's Henrik (and the whole of the
Earth) that are travelling, not he. According to special
relativity, if Henrik is moving relative to Albert, then Albert
should measure his clock as ticking slower  and thus Henrik is
the one who should be younger. But this is not what happens.
So what's wrong with our analysis? The key point here is that
the symmetry was broken. Albert did something that Henrik did
not  Albert accelerated in turning around. Henrik did no
accelerating, as he and all the other people on the Earth can
attest to (neglecting gravity). So Albert broke the symmetry, and
when he returns, he is the younger one.
