- 4582 - RELATIVITY - The Evidence for Einstein's Theory? - 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. The theory of gravity is general relativity (GR), first cooked up by Albert Einstein himself in a magnificent feat that took seven years to complete and provided amazing insights into how the world works.
--------------------------- 4582
- RELATIVITY - The
Evidence for Einstein's Theory?
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- It's easy enough to state the bare essence
of the theory, "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 a whopping 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.
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- If 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.
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- 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 GR, 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, 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 comes 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. That's 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.)
-
- Cops can take advantage of this shift by
bouncing a light off your car to catch you speeding. 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 GR,
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.
-
- In all regards, GR 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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October 21, 2024 RELATIVITY - The
Evidence for Einstein's Theory 4582
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