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10 Things Einstein Got Right

10 Things Einstein Got Right

One
hundred years ago, on May 29, 1919, astronomers observed a total solar eclipse in
an ambitious  effort to test Albert
Einstein’s general theory of relativity by seeing it in action. Essentially, Einstein
thought space and time were intertwined in an infinite “fabric,” like an
outstretched blanket. A massive object such as the Sun bends the spacetime blanket
with its gravity, such that light no longer travels in a straight line as it passes
by the Sun.

This
means the apparent positions of background stars seen close to the Sun in the
sky – including during a solar eclipse – should seem slightly shifted in the
absence of the Sun, because the Sun’s gravity bends light. But until the
eclipse experiment, no one was able to test Einstein’s theory of general
relativity, as no one could see stars near the Sun in the daytime otherwise.

The
world celebrated the results of this eclipse experiment— a victory for
Einstein, and the dawning of a new era of our understanding of the universe.

General
relativity has many important consequences for what we see in the cosmos and
how we make discoveries in deep space today. The same is true for Einstein’s slightly
older theory, special relativity, with its widely celebrated equation E=mc². Here
are 10 things that result from Einstein’s theories of relativity:

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1. Universal Speed Limit

Einstein’s
famous equation
E=mc²

contains “c,” the speed of light in a vacuum. Although
light comes in many flavors – from the rainbow of colors humans can see to the
radio waves that transmit spacecraft data – Einstein said all light must obey
the speed limit of 186,000 miles (300,000 kilometers) per second. So, even if
two particles of light carry very different amounts of energy, they will travel
at the same speed.

This
has been shown experimentally in space. In 2009, our Fermi Gamma-ray Space Telescope detected two photons at virtually the same moment, with one carrying a million
times more energy than the other. They both came from a high-energy region near
the collision of two neutron stars about 7 billion years ago. A neutron star is
the highly dense remnant of a star that has exploded. While other theories
posited that space-time itself has a “foamy” texture that might slow down more energetic
particles, Fermi’s observations found in favor of Einstein.

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2. Strong Lensing

Just
like the Sun bends the light from distant stars that pass close to it, a
massive object like a galaxy distorts the light from another object that is
much farther away. In some cases, this phenomenon can actually help us unveil
new galaxies. We say that the closer object acts like a “lens,” acting like a
telescope that reveals the more distant object. Entire clusters of galaxies can
be lensed and act as lenses, too.

When
the lensing object appears close enough to the more distant object in the sky,
we actually see multiple images of that faraway object. In 1979, scientists
first observed a double image of a quasar, a very bright object at the center
of a galaxy that involves a supermassive black hole feeding off a disk of
inflowing gas. These apparent copies of the distant object change in brightness
if the original object is changing, but not all at once, because of how space
itself is bent by the foreground object’s gravity.

Sometimes,
when a distant celestial object is precisely aligned with another object, we
see light bent into an “Einstein ring” or arc. In this image from our Hubble Space
Telescope,
the sweeping arc of light represents a distant galaxy that has been lensed,
forming a “smiley face” with other galaxies.

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3. Weak Lensing

When
a massive object acts as a lens for a farther object, but the objects are not specially
aligned with respect to our view, only one image of the distant object is
projected. This happens much more often. The closer object’s gravity makes the
background object look larger and more stretched than it really is. This is
called “weak lensing.”

Weak
lensing is very important for studying some of the biggest mysteries of the
universe: dark matter and dark energy. Dark matter is an invisible material
that only interacts with regular matter through gravity, and holds together
entire galaxies and groups of galaxies like a cosmic glue. Dark energy behaves like
the opposite of gravity, making objects recede from each other. Three upcoming
observatories – Our Wide Field Infrared
Survey Telescope,
WFIRST, mission, the European-led Euclid space mission with NASA participation,
and the ground-based Large Synoptic Survey Telescope
— will be key players in this effort. By surveying distortions of weakly lensed
galaxies across the universe, scientists can characterize the effects of these persistently
puzzling phenomena.

Gravitational
lensing in general will also enable NASA’s James Webb Space telescope to look
for some of the very first stars and galaxies of the universe.

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4. Microlensing

So
far, we’ve been talking about giant objects acting like magnifying lenses for
other giant objects. But stars can also “lens” other stars, including stars
that have planets around them. When light from a background star gets “lensed”
by a closer star in the foreground, there is an increase in the background
star’s brightness. If that foreground star also has a planet orbiting it, then
telescopes can detect an extra bump in the background star’s light, caused by
the orbiting planet. This technique for finding exoplanets, which are planets
around stars other than our own, is called “microlensing.”

Our Spitzer Space Telescope, in collaboration with ground-based
observatories, found an “iceball” planet through microlensing. While
microlensing has so far found less than 100 confirmed planets,  WFIRST could find more than 1,000 new
exoplanets using this technique.

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5. Black Holes

The
very existence of black holes, extremely dense objects from which no light can escape, is a prediction
of general relativity. They represent the most extreme distortions of the
fabric of space-time, and are especially famous for how their immense gravity affects
light in weird ways that only Einstein’s theory could explain.

In
2019 the Event Horizon Telescope international collaboration, supported by the
National Science Foundation and other partners, unveiled the first image of a black hole’s event
horizon,
the border that defines a black hole’s “point of no return” for nearby
material. NASA’s Chandra
X-ray Observatory, Nuclear
Spectroscopic Telescope Array (NuSTAR),
Neil Gehrels Swift Observatory, and Fermi Gamma-ray Space Telescope all looked
at the same black hole in a coordinated effort, and researchers are still
analyzing the results.

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6. Relativistic Jets

This
Spitzer image shows the galaxy Messier 87 (M87) in infrared light, which has a
supermassive black hole at its center. Around the black hole is a disk of
extremely hot gas, as well as two jets of material shooting out in opposite
directions. One of the jets, visible on the right of the image, is pointing
almost exactly toward Earth. Its enhanced brightness is due to the emission of light
from particles traveling toward the observer at near the speed of light, an
effect called “relativistic beaming.” By contrast, the other jet is invisible
at all wavelengths because it is traveling away from the observer near the
speed of light. The details of how such jets work are still mysterious, and
scientists will continue studying black holes for more clues. 

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7. A Gravitational Vortex

Speaking
of black holes, their gravity is so intense that they make infalling material
“wobble” around them. Like a spoon stirring honey, where honey is the space
around a black hole, the black hole’s distortion of space has a wobbling effect
on material orbiting the black hole. Until recently, this was only theoretical.
But in 2016, an international team of scientists using European Space Agency’s XMM-Newton and our Nuclear Spectroscopic Telescope Array (NUSTAR) announced they had observed the signature of wobbling
matter for the first time. Scientists will continue studying these odd effects
of black holes to further probe Einstein’s ideas firsthand.

Incidentally,
this wobbling of material around a black hole is similar to how Einstein
explained Mercury’s odd orbit. As the closest planet to the Sun, Mercury feels
the most gravitational tug from the Sun, and so its orbit’s orientation is
slowly rotating around the Sun, creating a wobble.

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 8. Gravitational Waves

Ripples
through space-time called gravitational waves were hypothesized by Einstein
about 100 years ago, but not actually observed until recently. In 2016, an
international collaboration of astronomers working with the Laser Interferometer
Gravitational-Wave Observatory (LIGO)
detectors announced a landmark discovery: This enormous experiment detected the
subtle signal of gravitational waves that had been traveling for 1.3 billion
years after two black holes merged in a cataclysmic event. This opened a brand
new door in an area of science called multi-messenger astronomy, in which both
gravitational waves and light can be studied.

For example,
our telescopes collaborated to measure light from two neutron stars
merging after LIGO detected gravitational wave signals from the event, as
announced in 2017. Given that gravitational waves from this event were detected
mere 1.7 seconds before gamma rays from the merger, after both traveled 140
million light-years, scientists concluded Einstein was right about something
else: gravitational waves and light waves travel at the same speed.

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9. The Sun Delaying Radio Signals

Planetary
exploration spacecraft have also shown Einstein to be right about general
relativity. Because spacecraft communicate with Earth using light, in the form
of radio waves, they present great opportunities to see whether the gravity of
a massive object like the Sun changes light’s path.  

In
1970, our Jet Propulsion Laboratory announced that Mariner VI and VII,
which completed flybys of Mars in 1969, had conducted experiments using radio
signals — and also agreed with Einstein. Using NASA’s
Deep Space Network (DSN), the two Mariners took several hundred radio measurements for
this purpose. Researchers measured the time it took for radio signals to travel
from the DSN dish in Goldstone, California, to the spacecraft and back. As
Einstein would have predicted, there was a delay in the total roundtrip time
because of the Sun’s gravity. For Mariner VI, the maximum delay was 204
microseconds, which, while far less than a single second, aligned almost
exactly with what Einstein’s theory would anticipate.

In
1979, the Viking landers performed an even more accurate experiment along these
lines. Then, in 2003 a group of scientists used NASA’s Cassini Spacecraft to repeat these kinds of
radio science experiments with 50 times greater precision than Viking. It’s
clear that Einstein’s theory has held up! 

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10. Proof from Orbiting
Earth

In
2004, we launched a spacecraft called Gravity
Probe B
specifically designed to watch Einstein’s theory play out in the orbit of
Earth. The theory goes that Earth, a rotating body, should be pulling the
fabric of space-time around it as it spins, in addition to distorting light with
its gravity.

The
spacecraft had four gyroscopes and pointed at the star IM Pegasi while orbiting
Earth over the poles. In this experiment, if Einstein had been wrong, these
gyroscopes would have always pointed in the same direction. But in 2011,
scientists announced they had observed tiny changes in the gyroscopes’
directions as a consequence of Earth, because of its gravity, dragging space-time
around it.

10 Things Einstein Got Right

BONUS: Your GPS! Speaking of time delays, the
GPS (global positioning system) on your phone or in your car relies on Einstein’s
theories for accuracy. In order to know where you are, you need a receiver –
like your phone, a ground station and a network of satellites orbiting Earth to
send and receive signals. But according to general relativity, because of
Earth’s gravity curving spacetime, satellites experience time moving slightly
faster than on Earth. At the same time, special relativity would say time moves
slower for objects that move much faster than others.

When
scientists worked out the net effect of these forces, they found that the
satellites’ clocks would always be a tiny bit ahead of clocks on Earth. While
the difference per day is a matter of millionths of a second, that change
really adds up. If GPS didn’t have relativity built into its technology, your
phone would guide you miles out of your way!

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