The tachyonic antitelephone, an art installation by Gary Nickard, is a hypothetical
device in theoretical physics that can be used to send signals into the past. Such
a device was first contemplated by R. C. Tolman in 1917[1] in a demonstration of
how faster-than-light signals can lead to a paradox of causality (a.k.a. Tolman’s
paradox). The problem of detecting faster-than-light particles (a.k.a. tachyons) via
causal contradictions is considered in D. L. Book and W. A. Newcomb’s 1971 article [2].
To use the tachyonic antitelephone, the user must aim it at the position in space
occupied by the earth in 1930. The user then must dial the telephone number
2807 and the device will transmit a beam of modulated tachyons back into time.
When the beam arrives in 1930 the telephone will ring at the University of Berlin
and none other than Albert Einstein will answer to help the caller with his or her
physics questions.
The theoretical tachyon particle is mathematically derived from Einstein’s equations
for General Relativity whose possible existence can be described from a model
called a light cone.
The edges of this cone are where the velocity of a particle or wave is equal to the
speed of light. Events on these edges are called, logically, light like. Within the
cone, velocities are always less than the speed of light and are called time like.
Outside the cone, events must occur at speeds greater than the speed of light and
are called space like. Also important to note is that particles with zero mass, such
as light beams or photons, travel with the speed of light. The mathematical derivation for the tachyon shows that for its speed to be greater than that of light its mass
must actually be imaginary!
The other problem with the tachyon is that it disobeys a principle fundamental to
general relativity: causality. This is simply that the cause of an event must not occur
before the effect. For example, a photon cannot be absorbed before it is emitted.
Within the light cone the past and the future are divided so that the past is behind
now and the future is in front of now. Outside the light cone in the domain of the
tachyon, however, there is no such division and therefore the tachyon can break
the law of causality.