In 1905, our conception of the Universe changed forever when
Einstein put forth his special theory of relativity. Prior to Einstein,
scientists were able to describe every “point” in the Universe with the use of
just four coordinates: three spatial positions for each of the three
dimensions, plus a time to indicate which moment any particular event occurred.
All of this changed when Einstein had the fundamental realization that every
single observer in the Universe, dependent on their motion and location, each
had a unique perspective on where and when every event in the Universe would
have occurred.
Whenever one observer moves through the Universe relative to
another, the observer-in-motion will experience time dilation: where their
clocks run slower relative to the observer-at-rest. Based on this, Einstein
suggested that we could make use of two clocks to put this to the test: one at
the equator, which speeds around the Earth at approximately 1670 km/hr (1038
mph), and one at the Earth’s poles, which is at rest as the Earth rotates about
its axis.
In this regard, however, Einstein was wrong: both clocks run
at exactly the same rate relative to one another. It wasn’t until 1971 that a
proper test could be conducted, and it required a lot more than special
relativity to make it so.
Back when Einstein first put forth his special theory of
relativity, there was a missing element: it didn’t incorporate gravitation into
the mix. He had no idea that proximity to a large gravitational mass could
alter the passage of time as well. Owing to the planet’s rotation and the
attractive gravitational force of every particle that makes up the Earth, our
planet bulges at the equator and gets compressed at the poles. As a result, the
Earth’s gravitational pull at the poles is slightly stronger — by about 0.4% —
than it is at the equator.
As it turns out, the amount of time dilation due to a point
on the equator zipping around the Earth is exactly cancelled by the additional
amount of gravitational time dilation that results from the difference in
gravity at the Earth’s poles versus the equator. Being deeper in a
gravitational field, which the poles are, causes your clock to tick by more
slowly, just as moving faster relative to a stationary observer does.
If you want to account for the rate at which the passage of
time will appear to occur for each and every observer, both the relative motion
effects of special relativity and also the relative effects of gravity — i.e.,
the relative curvature of spacetime between multiple observers — must be taken
into account.
Time dilation was one of the few relativistic phenomena that
was actually predicted even before Einstein put forth the ideas of special and
general relativity, as the consequences of motion close to the speed of light
for distances (length contraction) was worked out in the 19th century by George
FitzGerald and Hendrik Lorentz. If distances changed, then in order to maintain
the proper working of physics that we knew for electrons in atoms (as shown by
Joseph Larmor in 1897) or for clocks in general (as shown by Emil Cohn in
1904), that the same factor — the Lorentz factor (γ) — must factor into time
equations as well.
Although this was very difficult to measure initially, our
growing understanding of the subatomic world soon made it possible. In the
1930s, the muon, a subatomic particle that’s the heavier, unstable cousin of
the electron, was discovered. With a mean lifetime of just 2.2 microseconds,
muons that are produced from cosmic ray collisions in Earth’s upper atmosphere
should all decay within just hundreds of meters. And yet, if you hold out your
hand, about one such muon passes through it with every second, indicating that
they journeyed somewhere around 100 kilometers: a feat that’s physically
impossible without time dilation. As soon as we developed the technology of
cloud chambers, these muons could easily be seen even by the naked eye.
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