Gravitational Waves Detected!

This is the weekly Q & A blog post by our Research Professor in Philosophy, Dr. William Lane Craig.

Question

A 17 year old Indian from the Middle East who's a big fan of your work for Christ.

My question deals with recent discoveries in physics. How would the new discovery of gravitational waves affect Lorentzian relativity, the Kalaam argument and the A-theory of time?

Xavi

India

Dr. William Lane Craig’s Response

The announcement that experimentalists have discovered gravitational waves is one more triumph of Einstein’s gravitational theory, called the General Theory of Relativity, which ultimately led to the prediction of the beginning of the universe at some time in the finite past.

“How would the new discovery of gravitational waves affect Lorentzian relativity?” Einstein’s contemporary H. A. Lorentz, whose equations constitute the core of the Special Theory of Relativity, disagreed with Einstein about the physical interpretation of the equations. Because Lorentz was not, like Einstein, a verificationist, he thought it perfectly legitimate to hold that absolute quantities like duration and length exist even if they are not empirically detectable by us. On Lorentz’s view the perceived relativity of time and space is due to the distortion of our physical instruments when in absolute motion. In the famous Michelson-Morley experiment in 1887 their interferometer failed to detect the Earth’s absolute motion because the arms of the instrument along which light rays travel shrink in the direction of motion.

In the Ligo experiment laser beams are similarly measured along the two arms of a conceptually similar interferometer device. The interference patterns produced by the beams indicate that the two arms change length due to the gravitational waves. This is precisely what Lorentz would have expected. Our physical measuring devices are distorted by causal forces acting upon them.

In the application of the General Theory of Relativity to cosmology, there emerges a cosmic time which is a parameter that is independent of spatial coordinates and so is the same for all observers in the universe, regardless of their motion. When cosmologists say that the universe is 13.8 billion years old, they do not mean merely in Earth’s time; rather they are speaking of this cosmic time which is the same for all observers and measures the proper time of the universe since its inception. The existence of such cosmic time is right in line with Lorentz’s view and represents in a real sense the resurrection of absolute time.

“How would the new discovery of gravitational waves affect the Kalaam argument?” By corroborating yet another experimental prediction of the General Theory, the detection of gravitational waves confirms that theory and thus the cosmological model based upon it. The standard Big Bang model predicts the beginning of the universe and thus provides empirical evidence in support of the argument’s premiss that “The universe began to exist.”

More than that, however. Some physicists have been pessimistic about our ability to probe more deeply into the universe’s past because the early universe was so dense that it was opaque. Since our instruments have heretofore relied on electro-magnetic radiation (such as visible light, radio waves, X-rays, and so on) to look out into space and thus back into the past, astronomers encounter a horizon beyond which they cannot see. But here’s the thing that has scientists so excited: gravitational waves enable us to see beyond that barrier and further back into the past.

Neil Turok, director the Perimeter Institute for Theoretical Physicsat the University of Waterloo in Canada, is quoted as saying,

If astronomers can use gravitational waves to do what Turok imagines, it can remove the last bastion of scepticism of those wanting to deny that the universe began to exist.

Finally, “How would the new discovery of gravitational waves affect the A-theory of time?” The discovery is neutral with respect to whether one thinks that time is tensed (A-Theory) or tenseless (B-Theory). On the tenseless interpretation gravitational waves are taken to be ripples in the geometry of spacetime. They would be like wrinkles on a sheet. On a tensed theory they would be radiation propagating through space at a finite velocity. It’s interesting how in popular accounts of the discovery these two interpretations get conflated. The gravitational waves are described as ripples in spacetime passing by us. This is incoherent. Since time is already built into spacetime, spacetime does not and cannot change, nor are waves propagated through it. On the tenseless, spacetime interpretation the waves are geometrical distortions, whereas on the tensed interpretation the gravitational radiation is travelling through space.

The tenseless, geometrical interpretation has been criticized by physicists like Steven Weinberg as a hindrance to integrating gravitation with the other fundamental forces of nature, since it is treated not as a force but geometrically. Perhaps this new discovery will abet thinking of gravity as a force propagated through space, thereby undermining spacetime realism and the B-Theory of time.

This post and other resources are available on Dr. William Lane Craig's website: www.reasonablefaith.org

Learn more about Dr. Craig’s book, A Reasonable Response.

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[1] Cited in “Gravitational waves: breakthrough discovery after two centuries of expectation,” by Tim Radford, The Guardian, February 11, 2016 https://www.theguardian.com/science/2016/feb/11/gravitational-waves-discovery-hailed-as-breakthrough-of-the-century