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Einstein's
Theory of Relativity
8
Ways You Can See Einstein's Theory of Relativity in Real Life
Relativity is one of the
most famous scientific theories of the 20th century, but how well does it
explain the things we see in our daily lives?
Formulated by Albert
Einstein in 1905, the theory of relativity is
the notion that the laws of physics are the same everywhere.
The theory explains the
behavior of objects in space and time, and it can be used to predict everything
from the existence of black holes,
to light bending due to gravity, to the behavior of the planet Mercury in its
orbit.
The theory is deceptively
simple. First, there is no "absolute" frame of reference.
Every time you measure an
object's velocity, or its momentum, or how it experiences time, it's always in
relation to something else.
Second, the speed of light is
the same no matter who measures it or how fast the person measuring it is
going.
Third, nothing can go
faster than light.
The implications of
Einstein's most famous theory are profound.
If the speed of light is
always the same, it means that an astronaut going very fast relative to the
Earth will measure the seconds ticking by slower than an Earthbound observer
will — time essentially slows down for the astronaut, a phenomenon called time
dilation.
Any object in a big
gravity field is accelerating, so it will also experience time dilation.
Meanwhile, the
astronaut's spaceship will experience length contraction, which means that if
you took a picture of the spacecraft as it flew by, it would look as though it
were "squished" in the direction of motion. To the astronaut on
board, however, all would seem normal.
In addition, the mass of
the spaceship would appear to increase from the point of view of people on
Earth.
But you don't necessarily
need a spaceship zooming at near the speed
of light to see relativistic effects.
In fact, there are
several instances of relativity that we can see in our daily lives, and even
technologies we use today that demonstrate that Einstein was right.
Here are some ways we see
relativity in action.
Electromagnets
Magnetism is
a relativistic effect, and if you use electricity you can thank relativity for
the fact that generators work at all.
If you take a loop of
wire and move it through a magnetic field, you generate an electric current.
The charged particles in
the wire are affected by the changing magnetic field,
which forces some of them to move and creates the current.
But now, picture the wire
at rest and imagine the magnet is moving. In this case, the charged particles
in the wire (the electrons and protons) aren't moving anymore, so the magnetic
field shouldn't be affecting them.
But it does, and a
current still flows. This shows that there is no privileged frame of
reference.
Thomas Moore, a professor
of physics at Pomona College in Claremont, California, uses the principle of
relativity to demonstrate why Faraday's Law,
which states that a changing magnetic field creates an electric current, is
true.
"Since
this is the core principle behind transformers and electric generators, anyone
who uses electricity is experiencing the effects of relativity," Moore said.
Electromagnets work via
relativity as well. When a direct current (DC) of electric
charge flows through a wire, electrons are drifting through the material.
Ordinarily the wire would
seem electrically neutral, with no net positive or negative charge. That's a
consequence of having about the same number of protons (positive charges) and
electrons (negative charges).
But, if you put another
wire next to it with a DC current, the wires attract or repel each other,
depending on which direction the current is moving.
Assuming the currents are
moving in the same direction, the electrons in the first wire see the electrons
in the second wire as motionless. (This assumes the currents are about the same
strength).
Meanwhile, from the
electrons' perspective, the protons in both wires look like they are moving.
Because of the
relativistic length contraction, they appear to be more closely spaced, so
there's more positive charge per length of wire than negative charge. Since
like charges repel, the two wires also repel.
Currents in the opposite
directions result in attraction, because from the first wire's point of view,
the electrons in
the other wire are more crowded together, creating a net negative charge.
Meanwhile, the protons in
the first wire are creating a net positive charge, and opposite charges
attract.
Global
Positioning System
In order for your
car's GPS navigation to
function as accurately as it does, satellites have to take relativistic effects
into account.
This is because even though
satellites aren't moving at anything close to the speed of light, they are
still going pretty fast.
The satellites are also
sending signals to ground stations on Earth. These stations (and the GPS unit
in your car) are all experiencing higher accelerations due to gravity than the
satellites in orbit.
To get that pinpoint
accuracy, the satellites use clocks that are accurate to a few billionths of a
second (nanoseconds).
Since each satellite is
12,600 miles (20,300 kilometers) above Earth and moves at about 6,000
miles per hour (10,000 km/h), there's a relativistic time dilation that tacks
on about 4 microseconds each day.
Add in the effects of
gravity and the figure goes up to about 7 microseconds. That's 7,000
nanoseconds.
The difference is very
real: if no relativistic effects were accounted for, a GPS unit that tells you
it's a half mile (0.8 km) to the next gas station would be 5 miles (8 km) off
after only one day.
Gold's
yellow color
Most metals are shiny
because the electrons in the atoms jump from different energy levels, or
"orbitals."
Some photons that hit the
metal get absorbed and re-emitted, though at a longer wavelength. Most visible
light, though, just gets reflected.
Gold is a heavy atom,
so the inner electrons are moving fast enough that the relativistic mass
increase is significant, as well as the length contraction.
As a result, the
electrons are spinning around the nucleus in shorter paths, with more momentum.
Electrons in the inner
orbitals carry energy that is closer to the energy of outer electrons, and the
wavelengths that get absorbed and reflected are longer.
Longer wavelengths of
light mean that some of the visible light that would usually just be reflected
gets absorbed, and that light is in the blue end of the spectrum.
White light is a mix of all the colors of
the rainbow, but in gold's case, when light gets absorbed and
re-emitted the wavelengths are usually longer.
That means the mix of
light waves we see tends to have less blue and violet in it. This makes gold
appear yellowish in color since yellow, orange and red light is a longer
wavelength than blue.
Gold
doesn't corrode easily
The relativistic effect
on gold's electrons is also one reason that the metal doesn't corrode or react
with anything else easily.
Gold has only one
electron in its outer shell, but it still is not as reactive as calcium or
lithium.
Instead, the electrons in
gold, being "heavier" than they should be, are all held closer to the
atomic nucleus.
This means that the
outermost electron isn't likely to be in a place where it can react with
anything at all — it's just as likely to be among its fellow electrons that are
close to the nucleus.
Mercury
is a liquid
Similar to gold, mercury is also a heavy
atom, with electrons held close to the nucleus because of their
speed and consequent mass increase.
With mercury, the bonds
between its atoms are weak, so mercury melts at lower temperatures and is
typically a liquid when we see it.
Your
old TV
Just a few years ago most
televisions and monitors had cathode ray tube screens. A cathode ray tube works
by firing electrons at a phosphor surface with a big magnet.
Each electron makes a
lighted pixel when it hits the back of the screen. The electrons fired out to
make the picture move at up to 30 percent the speed of light.
Relativistic effects are
noticeable, and when manufacturers shaped the magnets, they had to take those
effects into account.
Light
If Isaac Newton had been right in assuming that there
is an absolute rest frame, we would have to come up with a different
explanation for light, because it wouldn't happen at all.
"Not
only would magnetism not exist but light would also not exist, because
relativity requires that changes in an electromagnetic field move at a finite
speed instead of instantaneously," Moore, of Pomona College, said.
"If
relativity did not enforce this requirement … changes in electric fields would
be communicated instantaneously … instead of through electromagnetic waves, and
both magnetism and light would be unnecessary."
Jesse Emspak
Live Science Contributor
Jesse Emspak is a contributing
writer for Live Science, Space.com and Toms Guide. He focuses on physics, human
health and general science. Jesse has a Master of Arts from the University of
California, Berkeley School of Journalism, and a Bachelor of Arts from the
University of Rochester. Jesse spent years covering finance and cut his teeth
at local newspapers, working local politics and police beats. Jesse likes to
stay active and holds a third degree black belt in Karate, which just means he
now knows how much he has to learn.
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