- 4208 - SPACETIME - which is it space or time? The idea that time and space are best described as spacetime, and that spacetime itself is a dynamical thing that tells us about gravity, has really held up over the past many years of data-taking.
--------------------- 4208 - SPACETIME - which is it space or time?
- The start of the
20th century, physicists had a problem: The speed of light was always the speed
of light. How can that be true?
-
- If you threw a
baseball out of a train going 20 mph, it would travel the speed at which you
threw it plus 20 mph, just as Isaac Newton’s laws predicted. However, if you
aimed a flashlight out of a train going 20 mph, the light would travel the
speed of light—no more, no less—no matter your perspective. And according to
Newton’s picture of the universe, that didn’t make any sense. To me either?
-
- We didn't have a
theory that would explain why light was special. It turned out to be something Albert Einstein
would soon propose: the idea of “spacetime”.
Spacetime is a necessary consequence of the fact that all observers
measure the same value for the speed of light, no matter how fast they are
going.
-
- For common-day
experience, as well as most experiments, space and time being separate is
totally fine. But if you want to make a
general statement about how the universe works, then you really need to view
them as one object, spacetime.
-
- In 1905, building
on existing experimental and theoretical work, Einstein published the theory of
special relativity. Among other things, the theory combined space and time into
a single entity that he called spacetime.
Einstein took the question ‘What if the speed of light is just the same
to everyone?’ seriously. And spacetime grew out of that thought experiment.
-
- It all starts
with the concept of different frames of reference. How a person experiences the
world depends on their individual frame of reference. Two people standing
together on a moving train will perceive one another as stationary. But an
observer standing outside the train will perceive both of those people as in
motion, chugging along at the speed of the train.
-
- Zoom out even
farther, and another observer floating in space will perceive the person
standing outside the train as in motion as well, spinning along with the Earth
while in orbit around the sun, which in turn is flying through the galaxy.
-
- What Einstein
realized is that something similar happens with time: Different people will
experience the passage of time differently, depending on their frame of
reference. The key to understanding how this works is the universal speed of
light.
-
- Imagine a single
quantum of light, a photon, bouncing up and down between two mirrors that are
facing each other. Traveling at the speed of light, the photon should bounce at
regular intervals, like a steadily ticking clock.
-
- A person standing
on a moving train with this photon clock will see the photon moving up and down
in a line. To a person standing outside the moving train, on the platform,
however, the photon will seem to move in a different way. Not only will the
photon bounce up and down, it will also move forward with the train, making a
triangle image.
-
- From one frame of
reference—on the train—the photon follows the shortest possible path, a
straight line. From another—on the platform—it follows a stretched-out zig-zag
path instead. The puzzle Einstein faced
becomes apparent if you imagine two photon clocks, one sitting stationary on
the platform, and the other whizzing by in the train.
-
- If the speed of
light is constant regardless of the frame of reference, then to the person on
the train, the photon clock next to them will tick more quickly, while to the
person on the platform, that same clock on the train will tick more slowly.
This effect is called “time dilation”. A similar thought experiment, with the
photon clock tipped on its side, shows that objects are more compact along the
direction of the train's motion, an effect called “length contraction”.
-
- This works out
mathematically. It’s only when you combine the different measurements from the
different frames of reference of space and time that all the observers will
agree on the result, suggesting that space and time are inextricably linked.
-
- If you allow both
space and time to change in a connected way, then everyone agrees that light
moves at the speed of light. Once you
combine them, everything kind of follows naturally. The equations are very
beautiful and elegant.
-
- Spacetime is
four-dimensional. We can move through the three dimensions of space at will,
passing up and down, side to side, or forward and backward. But we are also
always moving through a fourth dimension: time.
The only difference is that we as humans cannot decide to move forwards
and backwards in time.
-
- Because we don't
have control over it, it’s hard for our human brains to conceptualize the
fourth dimension of spacetime. One way
to think about spacetime is it’s really just space and time. It’s what you’re familiar with. The only
difference is that your space and time are not absolute; someone else may
measure a different space and a different time. The physical object that is absolute
is the combination: spacetime.
-
- A decade after
Einstein published the theory of special relativity, he published a new work
that made spacetime a bit easier to conceptualize. The 1905 theory of special relativity wasn’t
complete: It couldn’t account for gravity. This was fine for experiments on
Earth, where gravity is weak, but the theory failed to describe situations in
which gravity played a significant role.
-
- In 1915, Einstein
published the theory of general relativity. Unlike the flat spacetime described
by special relativity, the spacetime described by general relativity could be
curved. Einstein explained that the dynamical curvatures of spacetime were
caused by massive objects, such as the sun.
-
- Imagine spacetime
as a trampoline. Massive objects warp spacetime, similar to the way a bowling
ball sitting on a trampoline causes it to sag.
An object traveling through curved spacetime will follow a different
path than an object traveling through flat spacetime.
-
- In some cases,
curves around a massive object will transform the path of another object into
an orbit, like the elliptical path the Earth follows around and around the sun.
In some sense, the universe itself is spacetime. You can't even start to think about it unless
you are thinking of it in terms of this combined spacetime. It's like this
crucial stage on which everything we understand plays out.
-
- Spacetime is not a
new idea, but it’s still important in scientific study today. In the last few
years, spacetime has been front and center in many exciting discoveries. The Event Horizon Telescope, in 2019
published the first ever image of a black hole.
-
- A black hole is a
curvature of spacetime with gravity so strong nothing can escape it, not even
light. And that’s just what scientists found when they combined data from
multiple observatories into a single picture.
It is amazing that somebody wrote down a series of equations, and 100
years later, that set of equations predicted the existence of this object and
perfectly described what it would look like.
-
- The Laser
Interferometer Gravitational-Wave Observatory announced the discovery of
“gravitational waves” in 2016.
Gravitational waves are ripples in spacetime caused by the acceleration
of massive and dense objects like neutron stars and black holes.
-
- If we move faster
maybe time will slow down and we will live longer?
-
-
November 2, 2023
4300
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