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When’s Happening?

August 23, 2011

The other night I watched a documentary about time and how it only goes one way and how that’s remarkable. Coupled with a throwaway line in the book I’m reading it got me thinking about events. Mind you, none of these so-called insights are revolutionary in any other place than in my own brain.

In that book we have people who have developed faster-than-light travel, so one evening at a particular place they look up in sadness at the light of a battle they as a society fought 800 years ago. The throwaway line was something to the effect of that event happening at that observed instant, relativistically speaking.

It got me thinking and I realised in a way it was right.

What is an event? How do we know something has happened? The obvious answer is that we know it when we see it. The question is then: Has the event happened before we observed it?

A no-nonsense approach would be to say that even though we hadn’t seen it yet, it still happened. It seems fairly obvious; just imagine an observer closer to the event, so that they knew about it before us. Or, you know, the epicentre of the event itself.

Far as I know, information cannot travel faster than the speed of light in vacuum, which makes for interesting scenarios.

The Sun is about eight light-minutes from Earth (or vice versa if you want to be heliocentric about it). All sunlight we see is eight minutes old. Anything that happens on the Sun won’t affect us until at least eight minutes after it happened there.

Now imagine that the Sun somehow disappeared. Just vanished. We would not see this for another eight minutes, that seems intuitive enough, once we’ve accepted that light is not instantaneous.

What’s rather peculiar, though, is that the Earth would still follow its curved path around the Sun for another eight minutes. Even though the Sun would no longer exist, we would still be pulled towards it for another eight minutes.

If it was not so, then Earth would abandon its curved path for a linear one the very instant the Sun vanished. And if that was so, then information would have reached us before light did. We would observe the sky change is unexpected ways before we would see and feel the light and the heat go out.

But nothing can travel faster than light, not even information like that.

This means that there is no way that something can affect us before light could have reached us. There is no way something can causally influence anything else before that imagined sphere of light has reached the “observer.”

Does this mean that the timing of events is purely subjective?


Think of our vanishing Sun again. Up until we see the Sun go out, all our readings and all our observations would say that the Sun was still there. We would enjoy its heat, its light, its gravity.

Things get really interesting once we imagine someone on Venus, ~6 light-minutes from the Sun. They would experience this event before we did. However, the information of the Sun’s disappearance would reach us at the same time as the information about Venus’ receiving that same information. So all planets closer to the Sun would appear to us as going linear at the exact same time.

(This assumes that all planets are lined up; they seldom are.)

Mars would be even more interesting. Mars is ~13 light-minutes from the Sun, so information about the disappearance would not affect it until about five minutes after we would us. And then it would take about five more minutes for that information to reach us.  We would observe Mars following an orbit around a star we would already have seen disappeared.

That’s fucking mind-blowing.

It also makes me think that events don’t really happen at a particular instant but rather at a certain point in time and forever onwards.

Which every physicist in the world probably already knows and which probably is the whole fucking point of those neat theories of Einstein’s.


From → General

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