Everything, Everywhere, All At Once

What's going on right now, 70,000 light years away on the nearest galaxy to our own, Canis Major?  Or put it another way, if we're at $x = y = z = t = 0$, what's happening at $y = z = t = 0$ and $x$ = 70,000 light years?

According to Einstein's special theory of relativity it depends who you ask.  The most natural coordinates to use depend on your inertial frame.  Sitting in frame $S$, I may use the coordinates $x,y,z,t$ but someone in frame $S'$ would need to use $x',y',z',t'$ in order to observe the universe operating according to the same physical laws.  To make it more concrete, imagine $S$ and $S'$ share their $y$ and $z$ axes, and $S'$ is moving at velocity $v$ along the $x$ axis of $S$.  In this case the transform for converting between the two reference frames is

$$
\begin{align}
x' &= \frac{x - vt}{\sqrt{1-\frac{v^2}{c^2}}} \\
y' &= y \\
z' &= z \\
t' &= \frac{t - \frac{vx}{c^2}}{\sqrt{1-\frac{v^2}{c^2}}} \\
\end{align}
$$
 

But this means that something happening over there ($x>0$) right now ($t=0$), according to an observer in $S$, happened in the past ($t'<0$) according to an observer in $S'$.  And conversely something happening over there ($x' >0$) right now ($t'=0$), according to an observer in $S'$, is yet to happen ($t>0$) according to an observer in $S$.  Returning to our example of Canis Major: if I am standing still and you are jogging towards Canis Major at 3 m/s then an event occurring in that galaxy which I would describe as happening now, you would say happened 6 hours ago!

Although this is odd it doesn't produce any observable paradoxes because any event on Canis Major from which a signal could have reached us will always be in the past in both reference frames.  And likewise, if any signal from us could possibly reach a particular event on Canis Major, both $S$ and $S'$ will agree it is in the future.  But that leaves a lot of events whose nature (past-like or future-like) are completely relative!  In fact that accounts for 140,000 years of Canis Major history.

Intuitively, it should be possible to say that there is a difference between that which has happened on Canis Major, and that which is yet to happen, even if we're not actually there.  Yet if such a distinction were to have any physical meaning we would need to give up the principle of relativity (the idea that the laws of physics do not depend on your inertial frame) and instead say, there is indeed a special frame of reference.  But there's no evidence for any special frame of reference, and it doesn't simplify any calculation to add one.  Adding a special frame of reference would simply be an act of faith.

 

A Block Universe

So, if we give up on the concept that it is physically meaningful to say there is a distinction between that-which-has-happened and that-which-is-yet-to-occur (on Canis Major, or indeed anywhere other than Right Here) what is the alternative?  The standard answer often used in Special Relativity is the Block Universe.  This describes the universe as something like a 4 dimensional oblong containing point-like "events" (for the purpose of visualization, the block universe is usually drawn with just two spatial dimensions and one time dimension giving the impression that photons emanating from a point trace out a "light-cone").  In the block universe there is no special surface which objectively divides that-which-has-happened from that-which-is-yet-to-occur.  All that we have are past "light-cones", which mark the dividing line between the region from which signals can reach us and the region from which they cannot.  These can be thought of as dividing past from future, but they do so subjectively.

The Block Universe suggests a solution to the problem: that the future is pre-determined.  This nicely does away with any need to distinguish between that-which-has-happened and that-which-is-yet-to-occur, because - in a sense - everything has already occurred and the appearance of a past is a subjective illusion. But is that the only solution?  As it happens there is an alternative which integrates nicely with Quantum Mechanics.  You just have to take the Many Worlds Interpretation literally.

If the Schroedinger Equation applies at the universal scale (and there is no reason to believe it doesn't) then there isn't just one block universe but an infinity of them in a superposition.  Hugh Everett described this as the Universal Wavefunction, which became better known as the Many Worlds Interpretation.  Let's consider a small region of spacetime, for example the interior of an astronomical observatory between 3am and 4am.  Now let's imagine a precise set of events going on in that observatory: let's say there are two astronomers in it and they observe A, B, and C as they look out at the night sky.  But let's go further than that and imagine describing everything that happens in that region of spacetime with infinite precision.  However detailed our description it cannot uniquely specify a block universe.  This is because the superposition contains multiple block universes which exactly match our description of this small region of spacetime but differ outside of  it.  And these differences can be especially large when it comes to the area outside of its past light cone.

 

To take an extreme example: we know that an astronomer in the lab at 4am couldn't know whether or not the Sun blew up between 3:52am and 4am because light takes 8 minutes to travel to Earth from the Sun.  But, if they took the MWI literally they'd not even agree that it either had or had not.  All they'd be able to say is that their current state belongs to some block universes in which it had and to some in which it hadn't.

Returning now to the dwarf galaxy Canis Major. In our reference frame it has 70,000 years of history that is completely unknown to us, and is completely unknowable.  We have to choose how we think about this history, and - unless I've missed something - these are our choices:

• Some amount of this 70,000 years of history has "already happened", with the precise amount determined by a "special" frame of reference that violates the principle of relativity
• There is a single block universe and the future is predetermined, rendering the concept of "already happened" meaningless, as everything has, effectively, already happened.
  
• Each of our experiences define small regions of a block universe.  These regions can be extended to a complete block universe in an infinite number of ways, and are therefore compatible with an inifinite number of alternate histories of Canis Major going back 70,000 years.  None of these extensions on its own represents "the truth" because they all exist in the superposition that is The Universal Wavefunction.