target
Andromeda
There is no clock the universe agrees on. "Now" is a slice you cut through spacetime — and the faster you move, the more it tilts. Drag your speed and watch the same distant events change tense beneath you. The physics is exact. What it means is the oldest argument in physics, and it is below too — fairly, both sides.
Picture two people passing on a pavement, one ambling north, one ambling south. Ask each: what is happening, right now, in the Andromeda galaxy? They will not agree — and not by a little. Their two answers are separated by days of Andromedan history. For one, a fleet has launched; for the other, the decision to launch has not yet been taken. Neither is wrong. They simply sliced the universe at slightly different angles, because they were walking.
This is not a trick of signal-delay or of "what they can see yet." It is about what each is entitled to call simultaneous with this footstep — and in Einstein's universe that is a question with no single answer. The slice of spacetime you call the present depends on how you move through it. Below, you cut the slice yourself.
| event (outside your reach) | separation | its tense, for you |
|---|
● events on your worldline-cone are timelike — every observer agrees on their order. ◆ events outside the cone are spacelike — their tense is yours to tilt. As you move, watch only the diamonds change tense. The solid dots never will: the now-line can lean up to the light cone but never across it, so no motion you choose can put an effect before its cause.
The diagram is Hermann Minkowski's, 1908: time runs up, space runs across, and light travels the 45° diagonals — the light cone. The shallow gold line is the set of events you call this instant, everywhere. At rest it is flat. Move, and it tilts by exactly the angle your worldline tilts the other way — the two scissoring toward the light line as you approach c, like a closing pair of shears.
And here is the whole of it: a far-off event sitting just below your now-line is, for you, already past; lift the line above it and the same event becomes your future. You changed its tense by walking. But the line can never pivot past 45°. So an event inside your light cone — anything you could have caused, or that could have reached you — keeps its order no matter how you move. Simultaneity bends. Cause and effect do not.
The size of the tilt is a single clean formula. A clock at rest distance D that you call "now" sits, for a stationary friend, at time Δt = v·D⁄c² — your speed, times the distance, divided by the speed of light squared. Tiny v, vast D: across a room it is femtoseconds, beneath noticing. Across the gulf to a galaxy, a stroll is worth days.
Two people pass each other at this speed, one walking toward the target and one away: their two "nows" there are — apart — Penrose's pavement. Standing still, β = 0, and your present matches everyone's: the disagreement is exactly proportional to how you move and to how far away you look.
Everything above is the special theory of relativity, exact and a century-tested: the Lorentz transformation, the invariant interval, the relativity of simultaneity. The tool runs that transform directly; the numbers it shows are the numbers the verifier checks. None of it is in dispute among physicists.
What people do with it is. In 1966 C. W. Rietdijk and, in 1967, Hilary Putnam ran the argument forward: if your "now" and a passer-by's "now" each contain a definite, settled state of Andromeda, and the two states differ — one with the fleet launched, one without — then both must already be real. The future event is as fixed and as existent as this sentence. Roger Penrose dramatised the same step in The Emperor's New Mind (1989). Their conclusion is the block universe: past, present and future all equally there, "now" a merely local label, the open future an illusion of being embedded in time.
"Each present moment that comes into existence... must somehow already exist."The Rietdijk–Putnam reading — stated, on this page, as the argument it is, not the verdict it is sometimes sold as.
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