- 3986
- EINSTEIN'S THEORIES
- they still hold up? The theory of gravity, called general
relativity (GR), was first cooked up by Albert Einstein in a magnificent feat
that took seven years to complete and provided amazing insights into how the
world works.
-
------------ 3986 - EINSTEIN'S THEORIES - they still hold up?
- 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.
-
- Matter and
energy tell space-time how to bend, and the bending of space-time tells matter
how to move. The actual mechanics of
this stastement take 10 equations to describe, with each one is 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.
-
- 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.
-
- By the
early 1900s, it was a well-known problem in solar system dynamics, but not one
that caused much controversy. Einstein
thought Mercury was giving him a clue about gravity. 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 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. 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. 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 relies on the good old-fashioned “Doppler effect”. If something is moving away from you, the
sound it produces will get stretched out, shifting down to lower
frequencies. 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. The next time you're pulled over, you can use the opportunity to
reflect on the nature of gravity.
-
- 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.
-
- 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.
-
May 5, 2023 EINSTEIN'S THEORIES
- they still hold up? 3980
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--------------------- --- Saturday, May 6, 2023
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