- 4239 - EINSTEIN'S SPACETIME - In Einstein’s famous theory of relativity the concepts of immutable space and time aren’t just put aside, they’re rejected. Space and time are now woven into a coexisting fabric. We live in a four-dimensional universe. Space and time alone cease to exist; only the union of those dimensions remains, “spacetime”.
------------------------------ 4239 - EINSTEIN'S SPACETIME
- This is what
physicists give as a single word: “spacetime”. Take four pens from your desk at
work. Start with two and make a cross on your desk, so that they are sitting
perpendicular two each other. Now add a third pen and position it so that it’s
perpendicular to both of the first two. To do this you can no longer let the
pencils sit on the desk, which is a two-dimensional surface. You have to hold
our contraption in the air, which is a part of the three-dimensional world that
we’re used to.
-
- But we don’t live
in a merely three-dimensional world. Take your fourth pen and pierce it through
the other three pens so that it’s perfectly perpendicular to all of them. And
no angles less than 90 will do.
-
- You can’t. There
are only three spatial dimensions. And beyond that, you can’t even think of a
fourth spatial dimension, because our brains evolved in a three-dimensional
world.
-
- But while you can’t
hold in your mind the concept of a fourth dimension, you can experience it. The
flow of time from past to future sits as its own dimension. You, holding your
odd arrangement of pens, spans the dimension of time from the moment you first
put it together until it all falls apart. For that duration, you created a
four-dimensional piece of sculpture.
-
- Physicists bind the
three spatial dimensions and singular temporal dimension into the unified
framework of “spacetime”. In the physics of Newton, that spacetime is the
stage, and we the objects and contents of the universe are the actors. We hit
our marks and say our lines, and the stage provides the background on which we
perform our interactions. We know where the other actors are based on that same
stage. They are on their marks saying
their lines at the appointed cues.
-
- But in general
relativity, the stage itself becomes the starring actor. To make a theory of
gravity compatible with the relativity of space and time (now spacetime),
Einstein realized that gravity is not a force at all, at least not of the kind
envisioned by Newton.
-
- Spacetime is not a
fixed stage, but a flexible membrane that suffuses the entirety of existence.
It is a thing, an object and entity in its own right, a dynamical actor in the
cast of characters in the universe. The universe consists of the usual
assortment of particles and radiation and all their wonderful interactions, and
those interactions don’t happen on top of spacetime but along with spacetime.
-
- Mass and energy
(which do not need to be strictly separated, as they are now regarded as
equivalent thanks to Einstein’s special theory) change the shape of spacetime
in their vicinity. You, reading this very text, are an entity in the universe,
a thriving bundle of matter. You have mass and you have energy. The spacetime
within you, throughout you, and near you is now distorted thanks to that mass
and energy. Space and time are changed thanks to your existence.
-
- And the rest of the
universe responds. If we imagine an infinitely tiny particle traveling on a
trajectory near you, we can ask how that particle responds to your presence. In
the language of Newton we would say that your body exerts a gravitational
field, an invisible influence, and that the particle responds to that invisible
influence by way of the sensation of a gravitational force, which changes its
direction.
-
- But in the
language of Einstein there are no forces, no invisible strings. There is only
spacetime and spacetime alone. In Einstein’s radical reevaluation of gravity,
all objects travel in straight lines, always and forever. But the spacetime
that those objects must traverse bends beneath them.
-
- A hiker making
their way from one waypoint to another may travel in a straight line according
to a map and according to their feet, which are always placed directly in front
of one another with every step, but must follow the bends and curves (not to
mention the gnarled tree roots) along the trail, lest they get lost in the
wilderness.
-
- And so objects,
test masses, planets, beams of light, always travel in straight paths, but
“straight” only in the sense that every step forward is always placed directly
in front of the previous. The spacetime underneath and around those objects
bends, and so too does the path the object takes.
-
- This is gravity,
according to Einstein. Mass and energy bend the shape of spacetime, and the
bending of spacetime becomes an actor, affecting the paths and motions of
everything else. With this awakening of gravitational insight, Einstein was
able to successfully marry his concept of special relativity, that there are no
fixed, universal standards of reference, with the gravitational force.
-
- Not even
spacetime itself is immune from the relativistic effects of differing
viewpoints, and that mutation of spacetime gives rise to our experience of
gravity.
-
-
November 25, 2023
EINSTEIN'S SPACETIME 4239
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