INTRODUCTION

From “K.S. Thorne, “Warping Spacetime,” in The Future of Theoretical Physics and Cosmology: Celebrating Stephen Hawking’s 60th Birthday, edited by G.W. Gibbons, S.J. Rankin and E.P.S. Shellard (Cambridge University Press, Cambridge, England, 2003), Chapter 5, pp. 74-104; Chinese translation in Sixty Years in a Nutshell (Hunan Science and Technology Press, Hunan, China, 2005)”:

  • General relativity is Einstein’s law of gravity, his explanation of that fundamental force which holds us to the surface of the Earth. Gravity, Einstein asserted, is caused by a warping of space and time—or, in a language we physicists prefer, by a warping of spacetime.
  • The Earth’s matter produces the warpage, and that warpage in turn is manifest by gravity’s inward tug, toward the Earth’s center.
  • The inward tug is not the only manifestation of spacetime warpage;
  • the warpage is much richer than that.
  • As we shall see, it curves space, it slows the flow of time, and it drags space into tornado-like motions — at least that is what Einstein’s general relativity predicts.

From “Understanding gravity—warps and ripples in space and time”:

  • Is there any physical experiment you could do within the confines of your spaceship to tell whether you really were accelerating through space (assuming there were no windows to look out from), or if, instead, you were inside a spaceship stationary on the surface of Earth? Einstein said no—just as Galileo imagined the indistinguishability between a person inside a smooth-sailing ship (confined without windows) and a person on land, Einstein realised that the effects of acceleration and gravity were indistinguishable too. This is called the equivalence principle.
  • Once Einstein had formulated the equivalence principle, gravity became less mysterious. He could apply his knowledge of acceleration to better understand gravity.
  • The equivalence principle tells us that the effects of gravity and acceleration are indistinguishable. In thinking about the example of the cylindrical ride, we see that accelerated motion can warp space and time.
  • It is here that Einstein connected the dots to suggest that gravity is the warping of space and time. Gravity is the curvature of the universe, caused by massive bodies, which determines the path that objects travel. That curvature is dynamical, moving as those objects move.

GRAVITATIONAL WAVES

From “Understanding gravity—warps and ripples in space and time”:

  • Imagine two very massive objects, such as black holes. If those objects were to collide, they could potentially create an extreme disturbance in the fabric of spacetime, moving outwards like the ripples in a pond. But how far away could such waves be felt? Einstein predicted that gravitational waves existed, but believed they would be too small to detect by the time they reached us here on Earth.
  • So it was with great excitement that on February 11 2016, the scientific community was abuzz with the announcement that a gravitational wave had been detected. We needed instruments capable of detecting a signal one-ten-thousandth the diameter of a proton (10-19 meter). That’s exactly what the Laser Interferometer Gravitational-Wave Observatory (LIGO) equipment, operated by the California Institute of Technology and the Massachusetts Institute of Technology, can do. 
  • In the LIGO experiment, a laser is directed into a large tunnel structure. The laser beam is split so that half of it travels down one of the 4-kilometre-long ‘arms’, and the other half travels down the other 4-kilometre arm at the exact same time. At the end of each arm, a mirror reflects the light from the laser back to where it came from, and the two beams merge back into one. 
  • Normally, the laser beams should recombine at exactly the same time. But if a gravitational wave comes rippling through space while the detectors are switched on, that ripple will stretch one arm of the L-shaped structure before stretching the other. The gravitational wave distorts the passage of the light, resulting in a particular kind of interference light pattern detected at the end.
  • On 11 February 2016, the LIGO teams announced the direct discovery of a gravitational wave matching the signal predicted from the collision of two black holes.

OTHER EXAMPLES OF EXPERIMENTAL EVIDENCE FOR GRAVITY

Few examples of evidence for gravity from “Einstein’s Theory of General Relativity” (Space.com, 2017):

  • Gravitational lensing:
    • Light around a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lie behind it. Astronomers routinely use this method to study stars and galaxies behind massive objects.
    • Einstein’s Cross, a quasar in the Pegasus constellation, is an excellent example of gravitational lensing. The quasar is about 8 billion light-years from Earth, and sits behind a galaxy that is 400 million light-years away. Four images of the quasar appear around the galaxy because the intense gravity of the galaxy bends the light coming from the quasar.
  • Frame-dragging of space-time around rotating bodies: The spin of a heavy object, such as Earth, should twist and distort the space-time around it. In 2004, NASA launched the Gravity Probe B GP-B). The precisely calibrated satellite caused the axes of gyroscopes inside to drift very slightly over time, a result that coincided with Einstein’s theory.
  • Gravitational redshift: The electromagnetic radiation of an object is stretched out slightly inside a gravitational field. Think of the sound waves that emanate from a siren on an emergency vehicle; as the vehicle moves toward an observer, sound waves are compressed, but as it moves away, they are stretched out, or redshifted. Known as the Doppler Effect, the same phenomena occurs with waves of light at all frequencies. In 1959, two physicists, Robert Pound and Glen Rebka, shot gamma-rays of radioactive iron up the side of a tower at Harvard University and found them to be minutely less than their natural frequency due to distortions caused by gravity.

UNANSWERED QUESTIONS

From “Understanding gravity—warps and ripples in space and time”:

  • One unanswered question is whether or not gravity is propagated by the graviton—the proposed (but so-far undetected) particle responsible for gravitational interactions.
  • Even more pressing, we know that general relativity is, in its current form, incompatible with the other pillar of modern physics: quantum mechanics.
  • This is an indication that one or both theories are incomplete, or that we’re missing some other key component.

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