Changes

===The [[Cockcroft and Walton Experiment]]===

===The [[Cockcroft and Walton Experiment]]===

This one is quite famous, because it was one of the earliest observations of phenomena involving mass change.  But, as noted above, it can't really be taken as a "proof" of realitivity.

This one is quite famous, because it was one of the earliest observations of phenomena involving mass change.  But, as noted above, it can't really be taken as a "proof" of relativity.

===Bending of Light in a Gravitational Field===

===Bending of Light in a Gravitational Field===

Predictions of general relativity turn out to be obscure and difficult to test.  The two most famous predictions were the bending of light in a gravitational field and the precession of the perihelia of orbiting planets.

Unlike special relativity, predictions of general relativity turn out to be obscure and difficult to test.  The two most famous predictions in the early days were the bending of light in a gravitational field and the precession of the perihelia of orbiting planets.  Several other phenomena have arisen more recently.

The first of these was famously tested during a total eclipse in 1919.  That test was somewhat muddled by an incorrect initial calculation, by several people including Einstein himself, of what the effect would be, and some "cherry picking" of the data to be used.<ref>''Einstein's Luck'', John Waller, Oxford University Press, ISBN 0-19-860719-9</ref>  The data selection could be considered "manipulation" or "fudging", by a person (Arthur Eddington) who had a personal stake in the outcome.  His analysis techniques would not pass muster today.  The announcement of this test, flawed though it was, made Einstein world-famous.<ref>[http://einstein.stanford.edu/Library/images/lightsAllAskewBig.jpg The New York Times, Nov. 10, 1919]</ref><ref>[https://www.worldbooklearning.com/how-did-an-eclipse-help-make-albert-einstein-famous/ How did an eclipse help make Albert Einstein famous]</ref><ref>[https://blog.supplysideliberal.com/post/44934715350/how-albert-einstein-became-a-celebrity How Albert Einstein became a celebrity]</ref><ref>[https://www.sciencealert.com/general-relativity-experiment-confirms-accuracy-for-galaxies "The public's mind was blown by the results, effectively turning Albert Einstein into a household name and cementing the concept of general relativity into physics books."]</ref>

The first of these was famously tested during a total eclipse in 1919.  That test was somewhat muddled by an incorrect initial calculation, by several people including Einstein himself, of what the effect would be, and some "cherry picking" of the data to be used.<ref>''Einstein's Luck'', John Waller, Oxford University Press, ISBN 0-19-860719-9</ref>  The data selection could be considered "manipulation" or "fudging", by a person (Arthur Eddington) who had a personal stake in the outcome.  His analysis techniques would not pass muster today.  The announcement of this test, flawed though it was, made Einstein world-famous.<ref>[http://einstein.stanford.edu/Library/images/lightsAllAskewBig.jpg The New York Times, Nov. 10, 1919]</ref><ref>[https://www.worldbooklearning.com/how-did-an-eclipse-help-make-albert-einstein-famous/ How did an eclipse help make Albert Einstein famous]</ref><ref>[https://blog.supplysideliberal.com/post/44934715350/how-albert-einstein-became-a-celebrity How Albert Einstein became a celebrity]</ref><ref>[https://www.sciencealert.com/general-relativity-experiment-confirms-accuracy-for-galaxies "The public's mind was blown by the results, effectively turning Albert Einstein into a household name and cementing the concept of general relativity into physics books."]</ref>

:No [[Nobel Prize]] was was awarded for this.  A few years later Einstein won the Nobel Prize for the (unrelated) [[Photoelectric effect]].

:No [[Nobel Prize]] was was awarded for this.  A few years later Einstein won the Nobel Prize for the (unrelated) [[Photoelectric effect]].

The second "classical" test of general relativity was the advance of the perihelion of the orbit of Mercury.  There are many complex effects contributing to this, including gravitational perturbations from other planets and the effect of the oblateness of the Sun.  These are hard to calculate accurately, but, by 1900 it was known quite accurately that there was an "anomalous" precession, that is, a precession beyond all other known effects, of 43 arc seconds per century.  This is a very tiny effect, but astronomical measurements were sufficiently accurate by that time to show it clearly.

The second "classical" test of general relativity was the advance of the perihelion of the orbit of Mercury.  There are many complex effects contributing to this, including gravitational perturbations from other planets and the effect of the oblateness of the Sun.  These are hard to calculate accurately, but, by 1900 it was known quite accurately that there was an "anomalous" precession, that is, a precession beyond all other known effects, of 43 arc seconds per century.  This is a very tiny effect, but astronomical measurements were sufficiently accurate by that time to show it clearly.

:This created quite a problem&mdash;physicists by then were accustomed to having their theories check out very accurately.  One proposal that was made, by Simon Newcomb and Asaph Hall, was that the exponent of the radius in the gravitational formula wasn't exactly 2.  He showed that, by choosing an exponent of <math>2+\delta</math>, the precession, as a fraction of a full orbit per planet's year, is <math>\delta/2</math>.  By setting <math>\delta</math> to .000000157, that is, an exponent of 2.000000157, Newcomb was able to get a precession of .000000078 revolutions per Mercury year, or 43 arcseconds per Earth year.  Whatever value is chosen for <math>\delta\,</math>, it gives the same precession, per revolution, for all orbiting bodies, but gravitational effects from other planets diminish that effect the further the planet is from the sun.

:This created quite a problem&mdash;physicists by then were accustomed to having their theories check out very accurately.  One proposal that was made, by Simon Newcomb and Asaph Hall, was that the exponent of the radius in the gravitational formula wasn't exactly 2.  He showed that, by choosing an exponent of <math>2+\delta</math>, the precession, as a fraction of a full orbit per planet's year, is <math>\delta/2</math>.  By setting <math>\delta</math> to .000000157, that is, an exponent of 2.000000157, Newcomb was able to get a precession of .000000078 revolutions per Mercury year, or 43 arcseconds per Earth year.  Whatever value is chosen for <math>\delta\,</math>, it gives the same precession, per revolution, for all orbiting bodies, but gravitational effects from other planets diminish that effect the further the planet is from the sun.

:These error bars, and that of the general relativity prediction, all overlap.

:These error bars, and that of the general relativity prediction, all overlap.

[[Image:Cassini-science-289.jpg|right|thumb|The Shapiro effect: A spacecraft signal dipping into a gravity well around the [[Sun]] is delayed slightly.]]

[[Image:Cassini-science-289.jpg|right|thumb|The Shapiro effect: A spacecraft signal dipping into a gravity well around the [[Sun]] is delayed slightly.]]

*Another is the ''Shapiro effect'', involving time delay in radio signals passing through the gravity well of the Sun or a planet.  Various spacecraft have confirmed this.

Another is the ''Shapiro effect'', involving time delay in radio signals passing through the gravity well of the Sun or a planet.  Various spacecraft (such as Cassini) have confirmed this.<ref>http://www.nature.com/nature/journal/v425/n6956/full/nature01997.html</ref>

 

*Another is ''gravitational time dilation''. This is an effect separate from the time dilation of special relativity.  It was tested by the Pound-Rebka experiment in 1959. No Nobel Prize was awarded for this.  Professor Pound had previously shared in the 1952 Nobel for his contributions to Nuclear Magnetic Resonance imaging.

*Later in the 20th century, even more subtle phenomena were tested.  One was the phenomenon of ''gravitational radiation'', or "gravitational waves".  These waves are incredibly difficult to observe, and had never been observed until 2015.  But extremely dense binary pulsars radiate gravitational waves with sufficient energy loss that, even though we can't detect the waves from Earth, we can see the effect of the energy loss from the radiation.  The extreme precision of the timing of pulses from pulsars makes it possible to observe their energy loss with great accuracy.  Observations by Hulse and Taylor of the pulsar pair known as B1913+16, found the energy loss to be consistent with the predicted radiation.  This required choosing orbital parameters to be consistent with observation, in the same way that Kepler chose orbital parameters to fit observed planetary motion to his theory.  The rotating pulsars have since moved such that Earth is now out of the beams.  The assumptions required for this, in addition to those listed above, were that pulsars behave consistently.  The 1993 Nobel Prize in physics was awarded for this.

Gravitational time dilation is an effect separate from the time dilation of special relativity.  It was tested by the Pound-Rebka experiment in 1959.  No Nobel Prize was awarded for this.  Professor Pound had previously shared in the 1952 Nobel for his contributions to Nuclear Magnetic Resonance imaging.

*In late 2015 (and announced in 2016), the LIGO instruments directly detected gravitational waves allegedly from colliding [[black hole]]s.  See [[Gravitational waves]].  Unlike the waves from orbiting pulsars, that could only be detected by the loss of rotation energy, the waves from colliding black holes were strong enough to be detected directly.  The assumptions required for this, in addition to those listed above, were that the objects emitting the radiation (generally believed to be black holes, of course) satisfy the Schwartzschild solution to relativistic mechanics.  The 2017 Nobel Prize in physics was awarded for this.

Later in the 20th century, even more subtle phenomena were tested.  One was the phenomenon of ''gravitational radiation'', or "gravitational waves".  These waves are incredibly difficult to observe, and had never been observed until 2015.  But extremely dense binary pulsars radiate gravitational waves with sufficient energy loss that, even though we can't detect the waves from Earth, we can see the effect of the energy loss from the radiation.  The extreme precision of the timing of pulses from pulsars makes it possible to observe their energy loss with great accuracy.  Observations by Hulse and Taylor of the pulsar pair known as B1913+16, found the energy loss to be consistent with the predicted radiation.  This required choosing orbital parameters to be consistent with observation, in the same way that Kepler chose orbital parameters to fit observed planetary motion to his theory.  The rotating pulsars have since moved such that Earth is now out of the beams.  The assumptions required for this, in addition to those listed above, were that pulsars behave consistently.  The 1993 Nobel Prize in physics was awarded for this.

*In 2018 an observation of the 3-body system PSR J0337+1715, consisting of 2 white dwarfs and a pulsar, was consistent with (and hence tended to confirm) general relativity under conditions of extremely high gravity, to enormous precision.  The assumptions made were the same as those listed above.  One of the researchers stated "If there is a difference [between observation and the prediction of the Equivalence Principle], it is no more than three parts in a million."<ref>https://www.nbcnews.com/mach/science/einstein-s-theory-relativity-passes-its-toughest-test-yet-ncna889021</ref>

In late 2015 (and announced in 2016), the LIGO instruments directly detected gravitational waves allegedly from colliding [[black hole]]s.  See [[Gravitational waves]].  Unlike the waves from orbiting pulsars, that could only be detected by the loss of rotation energy, the waves from colliding black holes were strong enough to be detected directly.  The assumptions required for this, in addition to those listed above, were that the objects emitting the radiation (generally believed to be black holes, of course) satisfy the Schwartzschild solution to relativistic mechanics.  The 2017 Nobel Prize in physics was awarded for this.

*In 2018, observations of a star orbiting the supermassive black hole "Sagittarius A*" at the center of our galaxy, showed that the light was stretched from the black hole's gravity just as relativity predicted.  The star orbits Sagittarius A* at speeds of up to 16 million miles per hour.  This was the first observation of such intense gravity.<ref>https://www.nbcnews.com/mach/science/einstein-s-theory-relativity-aces-its-first-black-hole-test-ncna896641</ref>

In 2018 an observation of the 3-body system PSR J0337+1715, consisting of 2 white dwarfs and a pulsar, was consistent with (and hence tended to confirm) general relativity under conditions of extremely high gravity, to enormous precision.  The assumptions made were the same as those listed above.  One of the researchers stated "If there is a difference [between observation and the prediction of the Equivalence Principle], it is no more than three parts in a million."<ref>https://www.nbcnews.com/mach/science/einstein-s-theory-relativity-passes-its-toughest-test-yet-ncna889021</ref>

*An additional test of general relativity was performed with radio signals to the Cassini spacecraft.<ref>http://www.nature.com/nature/journal/v425/n6956/full/nature01997.html</ref>

In 2018, observations of a star orbiting the supermassive black hole "Sagittarius A*" at the center of our galaxy, showed that the light was stretched from the black hole's gravity just as relativity predicted.  The star orbits Sagittarius A* at speeds of up to 16 million miles per hour.  This was the first observation of such intense gravity.<ref>https://www.nbcnews.com/mach/science/einstein-s-theory-relativity-aces-its-first-black-hole-test-ncna896641</ref>

*Two other effects, ''geodetic precession'' (also known as "de Sitter precession"), and ''frame dragging'' (also known as the "Lense-Thirring effect") were tested by the "Gravity Probe B" satellite early in the 21st century.<ref>http://prl.aps.org/accepted/L/ea070Y8dQ491d22a28828c95f660a57ac82e7d8c0</ref><ref>http://www.digitaljournal.com/article/306430</ref><ref>http://www.nap.edu/html/gpb/summary.html</ref><ref>http://www.sciencenews.org/view/generic/id/73870/title/Gravity_Probe_B_finally_pays_off_</ref><ref>https://www.nasa.gov/mission_pages/gpb/</ref><ref>http://einstein.stanford.edu/</ref><ref>http://spectrum.ieee.org/aerospace/space-flight/the-gravity-probe-b-bailout</ref><ref>http://www.engadget.com/2011/05/06/nasa-concludes-gravity-probe-b-space-time-experiment-proves-e/</ref>  The precision required to observe this was phenomenal.  The results were announced on May 4, 2011.

''Geodetic precession'' (also known as "de Sitter precession"), and ''frame dragging'' (also known as the "Lense-Thirring effect") were tested by the "Gravity Probe B" satellite early in the 21st century.<ref>http://prl.aps.org/accepted/L/ea070Y8dQ491d22a28828c95f660a57ac82e7d8c0</ref><ref>http://www.digitaljournal.com/article/306430</ref><ref>http://www.nap.edu/html/gpb/summary.html</ref><ref>http://www.sciencenews.org/view/generic/id/73870/title/Gravity_Probe_B_finally_pays_off_</ref><ref>https://www.nasa.gov/mission_pages/gpb/</ref><ref>http://einstein.stanford.edu/</ref><ref>http://spectrum.ieee.org/aerospace/space-flight/the-gravity-probe-b-bailout</ref><ref>http://www.engadget.com/2011/05/06/nasa-concludes-gravity-probe-b-space-time-experiment-proves-e/</ref>  The precision required to observe this was phenomenal.  The results were announced on May 4, 2011.

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