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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 x of an inertial frame with x0 in metres and c>0 (Inertial frames and inertial charts of the affine Minkowski model). Fix w>c and consider the family Eξ:=x−1(cwξ, ξ, 0, 0),ξ∈R, a prearranged array of devices at x1=ξ triggered at coordinate time t=ξ/w, so the pattern is a sweep of coordinate speed w>c along the row.

(a) For ξ<η the displacement is Eη−Eξ=(cw(η−ξ), η−ξ, 0, 0), whose interval is (η−ξ)2(1−c2/w2)>0, so successive events are spacelike separated; in particular no event Eη lies in J+(Eξ) or J−(Eξ) for η≠ξ, by the coordinate criterion (The closed future cone in inertial coordinates: description, finite sums, and the coordinate criterion for causal reachability).

(b) Let b:=c/w∈(0,1). The passive boost with parameter be1 (The passive Lorentz boost with parameter beta) maps the displacement to time component γb(cw(η−ξ)−b(η−ξ))=γb(η−ξ)(cw−cw)=0, so in the boosted chart xb the whole family consists of events simultaneous at one coordinate time; taking b>c/w produces the opposite sign γb(η−ξ)(cw−b), 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 x and origin event o=x−1(0,0,0,0), a constant c>0, a real w>c, the family Eξ=x−1((c/w)ξ,ξ,0,0) of events, and for ∣b∣<1 the boost B(be1) with γb=(1−b2)−1/2.

[L1]

For the spacelike pair (E0,E1) with displacement D=(c/w,1,0,0) and spatial unit direction e1, and for every ∣b∣<1, the chart xb of the inertial frame with the same origin o and basis ea′:=B(be1)−1ea satisfies xb=B(be1)∘x and xb0(E1)−xb0(E0)=γb(D0−b∣Ds∣); in general ∣D0∣<∣Ds∣ 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).

[L2]

The passive boost acts in the (0,1)-plane by x′0=γ(x0−bx1), x′1=γ(x1−bx0) and is linear in the coordinate column (The passive Lorentz boost with parameter beta).

[L3]

Diagonal form and classification: η(D,D)=−(D0)2+∣Ds∣2, and D is spacelike exactly when η(D,D)>0, timelike exactly when η(D,D)<0 (Timelike, null, spacelike and causal vectors of a Lorentz vector space).

[L4]

Coordinate criterion for reachability: for events r,s with D=x(s)−x(r), s∈J+(r) iff D0≥∣Ds∣, and s∈J−(r) iff −D0≥∣Ds∣ (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).

[L5]

Chart linearity: x(s)−x(r)=E−1(s−r) for events r,s (Inertial frames and inertial charts of the affine Minkowski model).

Verification

technique · direct
1.1givenL3L4L5algebra

For ξ<η the displacement of the family is Eη−Eξ=((c/w)(η−ξ),η−ξ,0,0) by [L5], so η(Eη−Eξ,Eη−Eξ)=−(c/w)2(η−ξ)2+(η−ξ)2=(η−ξ)2(1−c2/w2)>0 because w>c and η−ξ>0; hence the displacement is spacelike by [L3]. For η>ξ the first criterion of [L4] would require (c/w)(η−ξ)≥η−ξ, which fails since c/w<1, and the second would require −(c/w)(η−ξ)≥η−ξ, which also fails; the case ξ>η 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).

2.1step 1.1L1L2L5algebra

Set b:=c/w∈(0,1). The vector E1−E0 is spacelike by step 1.1, so [L1] applies with the origin event o and gives the chart xb=B(be1)∘x of an inertial frame; by linearity [L2] and [L5], xb(Eη)−xb(Eξ)=B(be1)(Eη−Eξ), whose time component is γb((c/w)(η−ξ)−b(η−ξ))=γb(η−ξ)(c/w−c/w)=0 for all ξ,η; hence all events Eξ share the coordinate time xb0. For a parameter b′>c/w with b′<1, which exists because c/w<1, the same computation gives xb′0(Eη)−xb′0(Eξ)=γb′(η−ξ)(c/w−b′), of sign opposite to η−ξ; so the temporal order of the sweep runs forward in x, is frozen in xb and runs backward in xb′; this is (b).

3.1step 1.1step 2.1given∎

Steps 1.1 and 2.1 compute only the locations and coordinate differences of the family Eξ 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 Eξ 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

Dependency tree · 2 levels

Sources