How statement and proof provenance work
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A prearranged sweep with superluminal coordinate speed: successive events are spacelike separated and can be made simultaneous
Example
Worked computation in the chart of an inertial frame with in metres and (Inertial frames and inertial charts of the affine Minkowski model). Fix and consider the family a prearranged array of devices at triggered at coordinate time , so the pattern is a sweep of coordinate speed along the row.
(a) For the displacement is whose interval is , so successive events are spacelike separated; in particular no event lies in or for , by the coordinate criterion (The closed future cone in inertial coordinates: description, finite sums, and the coordinate criterion for causal reachability).
(b) Let . The passive boost with parameter (The passive Lorentz boost with parameter beta) maps the displacement to time component , so in the boosted chart the whole family consists of events simultaneous at one coordinate time; taking produces the opposite sign , so the apparent temporal order of the sweep runs forward, frozen or backward in different inertial frames even though no event of the family is causally related to another (Spacelike-separated events: the spacelike coordinate inequality in every inertial chart, and an explicit family of inertial frames realizing every time-order sign).
(c) No signal statement is made or needed: the computation is a property of the spacetime locations of independently triggered events, and it exhibits the class of spacelike-separated patterns that a kinematic superluminal speed bound cannot exclude. It is a statement about the locations of the events, it asserts nothing about any of them being the reception of a signal, and it does not model the fixed-speed relay kinematics of the hypothetical superluminal model treated on the companion page.
Facts & Assumptions
Given: An inertial frame with chart and origin event , a constant , a real , the family of events, and for the boost with .
For the spacelike pair with displacement and spatial unit direction , and for every , the chart of the inertial frame with the same origin and basis satisfies and ; in general in every inertial chart (Spacelike-separated events: the spacelike coordinate inequality in every inertial chart, and an explicit family of inertial frames realizing every time-order sign).
The passive boost acts in the -plane by , and is linear in the coordinate column (The passive Lorentz boost with parameter beta).
Diagonal form and classification: , and is spacelike exactly when , timelike exactly when (Timelike, null, spacelike and causal vectors of a Lorentz vector space).
Coordinate criterion for reachability: for events with , iff , and iff (The closed future cone in inertial coordinates: description, finite sums, and the coordinate criterion for causal reachability, Causal reachability, causal futures and pasts, causal diamonds, and reachability by future causal paths in affine Minkowski space).
Chart linearity: for events (Inertial frames and inertial charts of the affine Minkowski model).
Verification
For the displacement of the family is by [L5], so because and ; hence the displacement is spacelike by [L3]. For the first criterion of [L4] would require , which fails since , and the second would require , which also fails; the case is the same computation with the roles exchanged, so no event of the family lies in the causal future or past of another; this is (a).
Set . The vector is spacelike by step 1.1, so [L1] applies with the origin event and gives the chart of an inertial frame; by linearity [L2] and [L5], , whose time component is for all ; hence all events share the coordinate time . For a parameter with , which exists because , the same computation gives , of sign opposite to ; so the temporal order of the sweep runs forward in , is frozen in and runs backward in ; this is (b).
Steps 1.1 and 2.1 compute only the locations and coordinate differences of the family and the chart changes of those coordinate differences; the family is prearranged and independently triggered, no displacement of the family is causal in either direction, and nothing in the computation identifies an event with the reception of a signal or with the relay model of the companion page; the sweep is a kinematic pattern whose temporal order is frame-dependent without any causal relation among its events, and no choice principle is used.
Depends on
Spacelike-separated events: the spacelike coordinate inequality in every inertial chart, and an explicit family of inertial frames realizing every time-order sign
The closed future cone in inertial coordinates: description, finite sums, and the coordinate criterion for causal reachability
Causal reachability, causal futures and pasts, causal diamonds, and reachability by future causal paths in affine Minkowski space
The passive Lorentz boost with parameter beta
Timelike, null, spacelike and causal vectors of a Lorentz vector space
Inertial frames and inertial charts of the affine Minkowski model
Dependency tree · 2 levels
Sources
- R. C. Tolman, The Theory of the Relativity of Motion (University of California Press, Berkeley, 1917): Chapter V, Sections 45-52 pp. 49-55 (kinematical rate of a clock, the idea of simultaneity, velocities greater than that of light with the order-of-cause-and-effect reversal formula (28)-(29), and the blocks caveat that kinematic patterns without causal connection are not restricted); Internet Archive full text of the 1917 printing
- B. Crowell, Special Relativity, revision 2020-12-10 (author-published undergraduate textbook, 246 PDF pages): Section 1.3.2 pp. 22-23 (light-rectangles as causal diamonds with invariant area), Section 2.1 pp. 43-45 (the modest causality postulate P1 and the limits of a general causality principle), Section 4.7 pp. 110-113 (superluminal transmission of information, the tachyonic antitelephone attributed to Tolman, reported tachyon searches); signature (+---) converted to (-+++) and c = 1 formulas converted to x^0 = c t