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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 (imaginary-massed) 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 (non-infinite) 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 frame-dependent. 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.
tau-theta 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, 1870-1943)
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 m2; it thus has units of Wb/m2.
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), infinitely-long cylinder which rotates very rapidly can form closed timelike curves in its vicinity, which will allow time travel and possible subsequent violations of causality.
Titius-Bode 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 Saturn-Tethys 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.

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