# The Block Universe

April 8, 2022

If we take Spacial Relativity into account, we find something extraordinary regarding our existence.

There is no universal time that everyone agrees on. Time depends on the motion of an object. The faster an object is, the slower time runs for it.

Two observers traveling at different speed experiences time differently and as a result does not agree on simultaneity of events.

Above is a spacetime diagram that shows the relativity of simultaneity. The horizontal axis denotes the space and the vertical axis the time. As motion is relative any observer can claim to be stationary and have own reference frame. You can use Lorentz transformation to switch the reference frames when multiple observers are present in a diagram. In this case the green reference frame is considered to be of a stationary observer. The blue and the red reference frame are for two observers moving in opposite directions from the stationary one.

The vertical grid lines are for stationary paths through the spacetime, that is a stationary object in a particular reference frame would follow the vertical lines. On the other hand the horizontal lines are the lines of simultaneity, that is two points on the same horizontal line are simultaneous in respect to that particular reference frame.

Now the reference frames can differ between two observers traveling at different speed. We can show this through Lorentz transformation of the spacetime grids as shown in the above picture. We can see that the grid lines no longer align between the reference frames. That means two events simultaneous in one reference frame can occur at different times in respect to another reference frame.

For example, the events A and B are simultaneous in the green reference frame. But A happens before B in the red reference frame. On the other hand, A happens after B in the blue reference frame.

So, your future can be someone's past and your past can be someone else's future.

Thus, your past and future must be exist. It is intuitive to think that the past is already fixed since we are moving forward in time. But the future must also be predetermined. Because if your future is not already fixed then it could not have been the past for someone else.

Now, since the past and the future both are fixed, it can be concluded that the entire universe is predetermined. When taken the four dimensions of spacetime into account, the universe would look like a infinite block that is eternally unchanging. What we experience as present, is but a thin slice of "now" moving through the block universe like a needle of a gramophone playing an unchanging vinyl record.

This is the Eternal Block Universe.

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