Causality and prohibition of motion faster than light
See also: Causality Causality is the process of making something happen. Often it denotes a necessary relationship between one event and another event (called effect) which is the direct consequence of the first. This two event type of causality is known as accidental causality. Another variety, essential causality, has one event seen in two ways. Aristotle's example and Tachyonic antitelephone The tachyonic antitelephone is a hypothetical device in theoretical physics that can be used to send signals into one's own past. Such a device was first contemplated by R. C. Tolman in 1917 in a demonstration of how faster-than-light signals can lead to a paradox of causality . The problem of detecting faster-than-light particles (a.k.a. tachyons) Diagram 2. Light coneIn diagram 2 the interval AB is 'time-like'; i.e., there is a frame of reference in which event A and event B occur at the same location in space, separated only by occurring at different times. If A precedes B in that frame, then A precedes B in all frames. It is hypothetically possible for matter (or information) to travel from A to B, so there can be a causal relationship (with A the cause and B the effect).
The interval AC in the diagram is 'space-like'; i.e., there is a frame of reference in which event A and event C occur simultaneously, separated only in space. However there are also frames in which A precedes C (as shown) and frames in which C precedes A. If it were possible for a cause-and-effect relationship to exist between events A and C, then paradoxes of causality would result. For example, if A was the cause, and C the effect, then there would be frames of reference in which the effect preceded the cause. Although this in itself won't give rise to a paradox, one can show[24][25] that faster than light signals can be sent back into one's own past. A causal paradox can then be constructed by sending the signal if and only if no signal was received previously.
Therefore, one of the consequences of special relativity is that (assuming causality Causality is the process of making something happen. Often it denotes a necessary relationship between one event and another event (called effect) which is the direct consequence of the first. This two event type of causality is known as accidental causality. Another variety, essential causality, has one event seen in two ways. Aristotle's example is to be preserved), no information or material object can travel faster than light Faster-than-light communications and travel refer to the propagation of information or matter faster than the speed of light. Under the special theory of relativity, a particle (that has mass) with subluminal velocity needs infinite energy to accelerate to the speed of light, although special relativity does not forbid the existence of particles. On the other hand, the logical situation is not as clear in the case of general relativity, so it is an open question whether there is some fundamental principle The chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of closed timelike curves that preserves causality (and therefore prevents motion faster than light) in general relativity.
Even without considerations of causality, there are other strong reasons why faster-than-light travel is forbidden by special relativity. For example, if a constant force is applied to an object for a limitless amount of time, then integrating F = dp/dt gives a momentum that grows without bound, but this is simply because approaches infinity Infinity refers to several distinct concepts – usually linked to the idea of "without end" – which arise in philosophy, mathematics, and theology. The word comes from the Latin infinitas or "unboundedness." as v approaches c. To an observer who is not accelerating, it appears as though the object's inertia is increasing, so as to produce a smaller acceleration in response to the same force. This behavior is in fact observed in particle accelerators A particle accelerator is a device that uses electric fields to propel electrically-charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: linear accelerators and circular accelerators.
<<Table of Contents Special relativity (also known as the special theory of relativity or STR) is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein (after the considerable and independent contributions of Hendrik Lorentz, Henri Poincaré and others) in the paper "On the Electrodynamics of Moving Bodies" | Next>> | Show All>>
