# GPB-Einstein - Gravity Probe B

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First found May 22, 2018

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```Gravity Probe B
Testing Einstein’s Universe
STORY 1:
What is Gravity Probe B?
How does it work?
Einstein’s
General Relativity
1916
Leonard Schiff
1960
Forty years later, Leonard Schiff and
colleagues propose the “relativity gyroscope
experiment” to test Einstein’s theory
Yes, spacetime must be curved to produce
orbital paths and equivalent accelerations…
GP-B has four requirements:
1. Build a perfect straight line that can
orbit the Earth stably.
2. Align it with a distant star.
3. Protect it from all forces, except for
“gravity” (or curved spacetime).
4. Watch it very carefully for one year.
1) Build a perfect straight line
that can orbit the Earth stably.
We have four requirements:
1.
Build a perfect straight line that can orbit the Earth stably.
2.
Align it with a distant star.
3.
Protect it from all forces, except for “gravity” (or curved
spacetime).
4.
Watch it very carefully for one year.
A GYROSCOPE
a gyroscope’s spin axis maintains its
orientation as long as it is spinning
What makes a perfect gyroscope?
sphericity
= balance
+
homogeneity
World’s Best Gyroscope
Guinness Book of World Records
Material Fused quartz spheres, coated with niobium metal
Sphericity < 0.3 millionths of an inch (40 atomic layers)
Homogeneity < 2 parts per million
2) Align it with a distant star.
We have four requirements:
1.
Build a perfect straight line that can
orbit the Earth stably.
2.
Align it with a distant star.
3.
Protect it from all forces, except for
“gravity” (or curved spacetime).
4.
Watch it very carefully for one year.
300 light years
IM Pegasi
We have four requirements:
1.
Build a perfect straight line that can orbit the
Earth stably.
2.
Align it with a distant star.
3.
Protect it from all forces, except for “gravity”
(or curved spacetime).
4.
Watch it very carefully for one year.
3) Protect from all forces...
except “gravity”
•Dust
•Atmospheric “wavetops”
•Heat
•Magnetic field
•Weakest thrusters ever!
•Supercooled helium (2K)
4) Watch very carefully for one year
We have four requirements:
1.
Build a perfect straight line that can orbit the
Earth stably.
2.
Align it with a distant star.
3.
Protect it from all forces, except for “gravity”
(or curved spacetime).
4.
Watch it very carefully for one year.
Superconducting metal generates magnetic field when spinning
Must sense any tilt in spin axis > 0.5 milliarcseconds
(~ 2 ten-millionths of a single degree)
EQUIVALENT TO…
…measuring the width of a human hair (~100 microns) from 25 miles!
…measuring Lincoln’s head on a US penny from 3,000 miles
(San Francisco to New York)!
12
9
3
6
…spotting an angle 50 million times smaller than a single minute
on a clock face!
STORY 2:
What is Einstein’s
“curved spacetime”?
Newton’s Inference
In the Principia (1687), Newton states:
“there is a power of gravity pertaining to all bodies,
proportional to the … quantities of matter which they contain.”
Why Wonder @ Gravity?
1) Mercury’s “extra” precession
Newton - 531 arcsecs
Actual - 574 arcsecs
2) No such thing as coincidence
Balance b/w inertia~acceleration
3) How fast is gravity?
“instant propagation” (Newton)
or
“delayed information” (Einstein)
An Alternative Explanation
Say the magic word “gedanken” and
we can make gravity disappear…
Step back and recognize that “gravity” is not a force, but
an experience. It is a collection of observed actions and
behaviors and a set of personal experiences. The role of
science is to provide a theory to coherently explain what
is behind all these observations and experiences.
Newton provided one explanation -- a propagating force
inherent in mass.
Einstein provided an alternative explanation -- mass
follows the curvature of spacetime.
Matter follows the structure of spacetime.
Where spacetime is curved by a mass, other