Why is next Tuesday different from Amsterdam's Central Station?
Next Tuesday is off in the future. It hasn't happened yet, and you can't say what it's going to look like. Maybe it will be like today. No big deal. But maybe you'll get hit by a falling meteor on Monday and be in the intensive care ward. Bummer. Amsterdam's Central Station, however, exists now. It's just over the Atlantic Ocean, and even as you read these words people are there, scurrying to get their trains or milling about buying weird Dutch fast food (try the greasy fried rice balls ... yum!).
So Amsterdam Central Station already exists. It's just at a different point in space. But next Tuesday, which is at a different point in time, doesn't exist.
What is up with that?
For physicists like myself, this question of the difference between space and time is rock bottom, fundamental, super important. We can't really do physics without starting with some kind of theory of space and of time and of their relationship.
Newton's giant leap forward, 400 years ago, was to think about space and time as two totally separate domains. Space was the unchanging stage on which the drama of the world played out. Time was a river that flowed at an unchanging rate through every point on that cosmic change.
The power of Newton's conception changed the world, opening the floodgate for the mechanical era and the Industrial Revolution. There was only one small problem with this idea of a separate, absolute space and time.
It was wrong.
Of Albert Einstein's many great achievements, his most profound might be the recognition that space (the location of Amsterdam Central Station) and time (the location of next Tuesday) cannot be so easily separated. And that is why next Tuesday may already exist in the same way as Amsterdam's Central Station does.
Let me tell you about your "world line."
According to Einstein's theory of relativity, the drama of the world is "played" out not on the 3-D stage of space with time acting as an unchanging metronome. Instead, reality is composed of a four-dimensional space-time. There are three dimensions of space: left/right, forward/back, up/down. And there is one dimension of time: past/future. Just as every location on the surface of the Earth already exists in space, every event that has ever happened and ever will happen already exists in space-time.
To see how freaky this idea really gets, let's consider your life for a moment.
Your birth is an event that occurred at some location in space and at some moment in time. Your eventual death is also an event that will occur at some location in space (hopefully a cozy bed) and at some moment in time (hopefully many, many years from now). Those two points define the end points of a line in the four dimensions of space-time. In between, every place you've been or will be and all the moments you occupied those places fill out the shape of this line with its birth-death endpoints.
That is your world line. That is, according to the most powerful theory we have, the true nature of your life. (It's actually a "world volume," but we don't have to obsess on this point).
You are not hurtling into the future. You are not receding away from the past. The flow of time is, according to this interpretation of relativity, nothing more than a kind of optical illusion.
From the joys and sorrows of childhood to the joys and sorrows of old age, all of it has eternally existed — complete and whole — in the 4-D "block universe" of space-time.
So has next Tuesday already happened? Is the flow of time an illusion? World lines and relativistic space-time are certainly part of every physicist's education. But, to be fair, there is a lot of philosophical debate about the meaning and reality of the 4-D block universe of relativity. That debate is going to go on. And, at some point, some theory will replace relativity as we dig even deeper into the nature of the world.
But the new theory might not change all of relativity's perspectives on this most basic question.
And so the question remains.
Has next Tuesday already happened? Does next January already exist? Is the day of your death already fully formed?