# [ExI] BBC Faster Than The Speed Of Light

scerir scerir at alice.it
Thu Oct 27 10:15:49 UTC 2011

```Causality
_Causality_ (http://en.wikipedia.org/wiki/Causality_(physics))  is a
fundamental principle of _theoretical_
(http://en.wikipedia.org/wiki/Theoretical_physics)  _particle physics_
(http://en.wikipedia.org/wiki/Quantum_field_theory)   (and thus a basis of arguments that
various methods of potentially
sending  information faster than c or backwards in time are unworkable). If
tachyons can be used to transmit information faster than light, then
according  to relativity, they can also be used to violate certain simple
understandings of  the causality principle using a scheme sometimes known as the
(http://en.wikipedia.org/wiki/Tachyon#cite_note-Gr.C3.B8n1-15)   This can be understood in
terms of the _relativity of
simultaneity_ (http://en.wikipedia.org/wiki/Relativity_of_simultaneity)  in
special relativity, which says that different _inertial  reference frames_
(http://en.wikipedia.org/wiki/Inertial_frame_of_reference)  will disagree on
whether two events at different locations  happened "at the same time" or
not, and they can also disagree on the order of  the two events (technically,
these disagreements occur when _spacetime  interval_
(http://en.wikipedia.org/wiki/Spacetime#Spacetime_intervals)  between the events is
'space-like',
meaning that neither event lies  in the future _light cone_
(http://en.wikipedia.org/wiki/Light_cone)  of the other)._[17]_
(http://en.wikipedia.org/wiki/Tachyon#cite_note-Jarrell-16)
If one of the two events represents the sending of a signal from one
location  and the second event represents the reception of the same signal at
another  location, then as long as the signal is moving at the speed of light or
slower,  the mathematics of simultaneity ensures that all reference frames
agree that the  transmission-event happened before the
reception-event._[17]_ (http://en.wikipedia.org/wiki/Tachyon#cite_note-Jarrell-16)
However, in
the case of a hypothetical signal moving faster than light, there  would
always be some frames in which the signal was received before it was sent,  so
that the signal could be said to have moved backwards in time. Because one
of  the two fundamental _postulates  of special relativity_
(http://en.wikipedia.org/wiki/Postulates_of_special_relativity)  says that the laws of
physics should work the same way  in every inertial frame, if it is possible for
signals to move backwards in time  in any one frame, it must be possible in
all frames. This means that if observer  A sends a signal to observer B which
moves faster than light in A's frame but  backwards in time in B's frame,
and then B sends a reply which moves faster than  light in B's frame but
backwards in time in A's frame, it could work out that A  receives the reply
before sending the original signal, a seeming challenge to  temporal causality
in every frame. Nonetheless, the event cannot be said  to lack causal
structure as the events are still connected, albeit in this case  in a backwards
relationship. Mathematical details can be found in the _tachyonic
antitelephone_ (http://en.wikipedia.org/wiki/Tachyonic_antitelephone)  article, and an
illustration of such a scenario using _spacetime diagrams_
(http://en.wikipedia.org/wiki/Minkowski_diagram)  can  be found here._[18]_
(http://en.wikipedia.org/wiki/Tachyon#cite_note-17)
It has been argued that we can avoid the notion of tachyons traveling into
the past using the Feinberg reinterpretation principle_[3]_
(http://en.wikipedia.org/wiki/Tachyon#cite_note-feinberg67-2)   which states that a
negative-energy tachyon sent back in time in an  attempt to challenge forward
temporal causality can always be  reinterpreted as a positive-energy tachyon
traveling forward in  time. This is because observers cannot distinguish between
the emission and  absorption of tachyons. For a tachyon, there is no
distinction between the  processes of emission and absorption, because there
always exists a sub-light  speed _reference frame_
(http://en.wikipedia.org/wiki/Frame_of_reference)  shift  that alters the temporal
direction of the
tachyon's world-line, which is not  true for _bradyons_
(http://en.wikipedia.org/wiki/Massless_particle) .
The attempt to  detect a tachyon from the future (and challenge forward
causality)  can actually create the same tachyon and sends it forward in time
(which is itself a causal event).
According to the Feinberg reinterpretation principle every tachyon detector
will register tachyons in every possible detection mode; from the
perspective of  a frame where the registration by the "detector" preceded the
activation of the  "emitter", the "detector" in the past is actually
_spontaneously  emitting_ (http://en.wikipedia.org/wiki/Spontaneous_emission)  tachyons,
only some of which will be intercepted by the  detector in the future.
However, Feinberg's reinterpretation principle, treats  events in a way that the
earlier event is defined as the "emission" and the  later one the
"detection", and one critic claims that it does not, in itself,  solve the causality
problems associated with tachyon information  transmission._[16]_
(http://en.wikipedia.org/wiki/Tachyon#cite_note-Gr.C3.B8n1-15)   This criticism however
is ill-conceived as it completely ignores the causal  structure of reality.
For example, suppose experimenter A could selectively  influence the rate
at which tachyons from her emitter traveled to the detector  of experimenter
B, and B could measure changes in the rate tachyons arrived at  his
detector, so that A could transmit a message to B in binary code. Then in a  frame
where B's detector went off at an earlier time than A influenced her
emitter, relabeling B's device as a "spontaneous emitter" and A's as a  "detector"
would be necessary to make the experiment intelligible as it wouldn't
change the fact that B, by observing the changing rates of tachyons being
spontaneously emitted from his device, could gain information about A's
interactions with her own device in the future. In fact, this experiment would
solidify the principle of causality even more since B can follow the causal link
and make preditions about A's action in the future. Although remote, the
possibility of backward causality is not a real challenge to the principle of
causality, but rather a novel way of understanding an additional aspect of
it.
Some arguments from quantum field theory suggest that even if tachyons
exist,  they cannot be used to transmit information faster than light, either
because  disturbances in the quantum field for a tachyon would not actually
propagate  faster than light, or because the tachyon is impossible to
localize._[19]_ (http://en.wikipedia.org/wiki/Tachyon#cite_note-chase1-18)   Without
the possibility of faster-than-light information transmission, the  problem
of backwards-in-time information transmission can be avoided as  well.
In the theory of _general relativity_
(http://en.wikipedia.org/wiki/General_relativity) ,  it is possible to construct
spacetimes in which particles
travel faster than the  speed of light, relative to a distant observer. One
example is the _Alcubierre metric_
(http://en.wikipedia.org/wiki/Alcubierre_metric) ,  another is of _traversable wormholes_
(http://en.wikipedia.org/wiki/Wormhole) . However,  these are not tachyons in the above
sense, as they do
not exceed the _speed  of light_
(http://en.wikipedia.org/wiki/Speed_of_light)  locally.
_http://en.wikipedia.org/wiki/Tachyon_
(http://en.wikipedia.org/wiki/Tachyon)

```