- 4481
- EINSTEIN'S -
theory of relativity? - Albert Einstein's famous theory of
relativity has been borne out in the real world, measured in eclipses,
distorted galaxies and even the universe's structure. General Relativity (GR), is the “theory of
gravity” first cooked up by Albert Einstein in a feat that took him seven years
to complete. It provided amazing
insights into how the world works.
-
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- EINSTEIN'S -
theory of relativity?
-
- It's easy enough to state the essence of the
theory in a simple statements: "Matter and energy tell space-time how to
bend, and the bending of space-time tells matter how to move." But the
actual mechanics take 10 equations to describe, with each one very difficult
and highly interconnected with the others.
-
- Out of all the features of his new theory,
Einstein was proudest of its ability to explain the details of the orbit of
Mercury. That innermost planet has a slightly elliptical orbit, and that
ellipse ever-so-slowly rotates around the sun. In other words, the place where
Mercury is farthest from the sun slowly changes with time.
-
- mIf you apply simple Newtonian gravity to
the sun-Mercury system, this change over time, called “precession”, doesn't
show up. Isaac Newton's view is
incomplete. Once you add in the gentle gravitational nudging and tweaking due
to the other planets, almost all of the precession can be explained, but not
all.
-
- By the early 1900s, it was a well-known
problem in solar system dynamics, but not one that caused much controversy.
Most folks just added it to the ever-growing list of "slightly weird
things we can't explain about the universe" and assumed that we would find
a mundane solution some day.
-
- But Einstein thought Mercury was giving him
a clue. When, after years of attempts, he was able to flex his general
relativistic muscles and explain precisely the orbital oddities of Mercury, he
knew he had finally cracked the gravitational code.
-
- Before Einstein put the finishing touches on
the big General Relativity, he came to some startling realizations about the
nature of gravity. If you're isolated on a rocket ship that accelerates at a
smooth and constant 1g — providing the same acceleration as Earth's gravity
does — everything in your laboratory will behave exactly as it would on the
planet's surface, Einstein reasoned. Objects will fall to the ground at the
same speed as on Earth; your feet will stay firmly planted on the floor, etc.
-
- This equivalence between gravity (as
experienced on Earth) and acceleration (as experienced in the rocket) propelled
Einstein forward to develop his theory. But hidden in that scenario is a
surprising insight. Imagine a beam of light entering a window on the left side
of the spaceship. By the time the light crosses the spaceship to exit, where
will it be?
-
- From the perspective of an outside
observer, the answer is obvious. The light travels in a perfectly straight
line, perpendicular to the path of the rocket. During the time the light was
passing through, the rocket pushed itself forward. The light will then enter
the rocket at one window, say, near the tip, and exit near the bottom, close to
the engines.
-
- From the inside the spacecraft, though,
things seem strange. In order for the light to enter a window near the tip and
exit near the engines, the beam's path has to be curved. Indeed, that's exactly
what you see. And, since gravity is
exactly the same as acceleration, light must follow curved paths around massive
objects.
-
- It's difficult to observe this one
experimentally, because you need a lot of mass and some light that passes close
to the surface to get a detectable effect. But the 1919 solar eclipse proved
just the right opportunity, and an expedition led by Sir Arthur Eddington found
the exact shifting of distant starlight that Einstein's nascent theory had
predicted.
-
- Another interesting result pops out of
creative thought experiments surrounding general relativity. This conclusion
relies on the good old-fashioned Doppler effect, but it's applied to an
unfamiliar scenario.
-
- If something is moving away from you, the
sound it produces will get stretched out, shifting down to lower frequencies,
the “Doppler effect”. The same is true of light: A car moving away from you
appears ever-so-slightly redder than it would be if the vehicle were
stationary. (The redder light, the lower the frequency.)
-
- So, if movement shifts light's wavelength,
then acceleration can too: A bit of light traveling from the bottom to the top
of an accelerating rocket will experience a redshift. And under General
Relativity, what goes for acceleration goes for gravity. Light emitted from the surface of the Earth
will shift down into redder frequencies the farther upward it travels.
-
- It took a few decades to conclusively
demonstrate this prediction, because the effect is so tiny. But in 1959, Robert
Pound and Glen Rebka proposed, designed, built and executed an experiment that
enabled them to measure the redshift of light as it traveled a few stories up
the Jefferson Laboratory at Harvard University.
-
- Even with all that evidence, we continue
to put general relativity to the test. Any sign of a crack in Einstein's
magnificent work would spark the development of a new theory of gravity,
perhaps paving the way to uncovering the full quantum nature of that force.
That's something we currently don't understand at all.
-
- But in all regards, General Relativity
passes with flying colors; from sensitive satellites to gravitational lensing,
from the orbits of stars around giant black holes to ripples of gravitational
waves and the evolution of the universe itself, Einstein's legacy is likely to
persist for quite some time.
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-
May 25, 2024 EINSTEIN'S
- theory of relativity? 4481
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