◬ Light and 'now'

Everything you see is the past

Light takes time to arrive: the Moon is 1.3 seconds ago, the Sun 8.3 minutes, Mars 3 to 22 minutes, the Andromeda galaxy 2.5 million years. You never see what a distant thing is doing 'now' — only what it was doing when the light left. Established

Correct for the delay — and it still doesn't work

You might hope to reconstruct a shared 'now' by allowing for light-travel time. Relativity says you can't do it uniquely. Observers moving relative to each other, each correcting perfectly for delay, disagree about which distant events were simultaneous (Einstein, 1905). Established

For events far enough apart that no signal could pass between them — outside each other's light cones — observers can even disagree about their order. For events that could affect each other, everyone agrees on the order, so cause never comes after effect.

So what is 'now'?

If no way of slicing spacetime into moments is special, many physicists and philosophers conclude that all times are equally real — the block universe (eternalism). Others hold that only the present exists (presentism) and relativity leaves room for it. Contested This is hole H4.

With two time dimensions

A moment would no longer be a slice but a line through a time plane — and even perfect knowledge of it wouldn't fix what comes next (see Two Films). The mathematics is Established; whether it tells us anything about our universe is Speculative.

At cosmic scale

Our view of the universe is one enormous past light cone. The oldest light we see left 13.8 billion years ago; the matter that sent it is now about 46 billion light-years away. And because expansion is accelerating, there are galaxies we can see today that a message sent now will never reach. Established within the standard model of cosmology.

How we know

• Light's speed is measured to be the same in every direction and for every source (Michelson–Morley, 1887, and its modern versions to about one part in 10¹⁸).
• Relativity of simultaneity isn't measured directly — it follows from the same symmetry (Lorentz invariance) that time dilation does, tested in particle accelerators, atomic clocks and GPS.