## The Day the Universe Got Bent

(General Relativity)

Student: “Dr. Einstein, Aren’t these the same questions

as last year’s final exam?”

Einstein: “Yes; But this year the answers are different.”

### Einstein Took a Big Step Back From Classical Physics

and Saw Inertia, Spacetime, and Gravity in a Whole New Way

It took Einstein ten years of incremental insights to get from his special theory of relativity to his stunningly accurate description of gravity: general relativity.

By the way, what’s so special about “special” relativity? The simple answer is that special relativity deals with an observer and a subject that are moving at constant, unchanging speeds. They are in “inertial frames of reference,” not experiencing external forces. Einstein wanted a more *general* description of how spacetime works.

Einstein also knew that Newtonian gravity did not fit into his relativistic view of the universe. Just two years into his quest for a new theory of gravity, Einstein had what he called “the happiest thought of my life!” He was thinking about someone falling off the roof of a house. Newton would think of this from the perspective of someone watching the person fall — and see it as the external force of gravity pulling the unfortunate person down. Einstein thought of it from the perspective of the person who was falling. The person in free fall does not feel any external force. Einstein decided that gravity was not an external force. In the spirit of the elastic spacetime of special relativity, Einstein realized that mass warps spacetime. The unfortunate person falling off a roof is simply being caught up in warped spacetime — spacetime that has been bent by the mass of the Earth.

That all sounds a little goofy, but general relativity makes a number of unique predictions about the real world. In this chapter, I line out several of those predictions and the experimental results that have supported them. From the Eddington experiment a century ago to the **L**aser **I**nterferometry **G**ravity-wave **O**bservatory’s confirmation of gravity waves a decade ago, everything we can test supports the general theory of relativity.

Once again, I could not figure out any you-can-do-this-at-home activity for this chapter. Most of us do not have a monster telescope sitting around the house. Or access to a couple of mile-long vacuum tubes to turn into a LIGO gravitational wave detector.

But you might be amused to peruse the links below:

### Here are the links from the text in Chapter 10:

71. Deeper Dive “General relativity priority dispute – Wikipedia.”

https://en.wikipedia.org/wiki/General_relativity_priority_dispute

72. Deeper Dive How Time Dilation Causes Gravity, and How Inertia Works YouTube. 13 Jun. 2021,

https://www.youtube.com/watch?v=QNOFxmcECPQ*Warning: Starts with ads.*

73. Great Video “Brian Greene Explores General Relativity in His Living Room.” 25 Nov. 2015,

https://www.youtube.com/watch?v=uRijc-AN-F0

74. Deeper Dive “Gravitational Lensing.” 30 May. 2019,

https://hubblesite.org/contents/articles/gravitational-lensing

75. Deeper Dive “The ecliptic is the path of the sun – EarthSky.” 27 Jan. 2017,

https://earthsky.org/space/what-is-the-ecliptic/

76. Deeper Dive “Eddington experiment – Wikipedia.”

https://en.wikipedia.org/wiki/Eddington_experiment

77. Great Video “Einstein and Eddington (TV Movie 2008) – IMDb.”

https://www.imdb.com/title/tt0995036/

78. Deeper Dive “What is gravitational lensing? – EarthSky.” 8 Sep. 2021,

https://earthsky.org/space/what-is-gravitational-lensing-einstein-ring/

79. Deeper Dive “Pluto discovered – HISTORY.”

https://www.history.com/this-day-in-history/pluto-discovered

80. Deeper Dive “Precession of the perihelion of Mercury.”

https://aether.lbl.gov/www/classes/p10/gr/PrecessionperihelionMercury.htm

81. Biography “Karl Schwarzschild – Important Scientists”

https://www.physicsoftheuniverse.com/scientists_schwarzschild.html

82. Deeper Dive “Black hole – Wikipedia.”

https://en.wikipedia.org/wiki/Black_hole

83. Deeper Dive “Schwarzschild radius – Wikipedia.”

https://en.wikipedia.org/wiki/Schwarzschild_radius

84. Deeper Dive “Hawking radiation – Wikipedia.”

https://en.wikipedia.org/wiki/Hawking_radiation

85. Deeper Dive “Information May Leak from Black Holes at Dial-Up Speeds ….” 14 Mar. 2008,

https://www.scientificamerican.com/article/information-may-leak-from

86. Deeper Dive “Wobbling Shadow of the M87* Black Hole | Event Horizon ….” 23 Sep. 2020,

https://eventhorizontelescope.org/blog/wobbling-shadow-m87-black-hole

87. Deeper Dive “Albert Einstein was right (again)” 9 Jul. 2021,

https://www.abc.net.au/news/science/2021-07-29/albert-einstein-astronomers-detect-light-behind-black-hole/100333436

88. Great Video “PBS Space Time | LIGO’s First Detection of Gravitational Waves!.” 24 Jul. 2016,

https://www.pbs.org/video/pbs-space-time-ligo/*Warning: May start with ads.*

89. Deeper Dive “Gravitational Waves Detected 100 Years After … – LIGO Caltech.” 11 Feb. 2016,

https://www.ligo.caltech.edu/news/ligo20160211

90. Deeper Dive “NIST Pair of Aluminum Atomic Clocks Reveal Einstein’s ….” 23 Sep. 2010,

https://www.nist.gov/news-events/news/2010/09/nist-pair-aluminum-atomic-clocks-reveal-einsteins-relativity-personal-scale

91. Deeper Dive “Tests of general relativity – Wikipedia.”

https://en.wikipedia.org/wiki/Tests_of_general_relativity