Callender Saves and Kills Time

By Phyllis Jeffrey – Can the emerging field of quantum gravity "save" the concept of time from Einstein's theory of general relativity? Seeking answers on October 7, 2016 was philosopher of science Craig Callender, in a talk hosted by the Philosophy and Physics of Space-Time cluster, entitled “A Tale of Two Times.”

Our intuitive certainty about the way time works—an accumulation of past events ceding ground to a continually renewing present—was deeply shaken by Einstein’s theory. Arguing that time and space are part of a single phenomenon called space-time, with gravity an effect of the warping of space-time by large masses (mountains, for example), Einstein unsettled the established understanding of time as the flowing, constant background on which events play out.

Quantum mechanics developed as a way to study the behavior of atoms and subatomic particles once it began to be observed that elements’ behavior at this scale appeared to violate the laws of physics. For instance, quantum mechanics has demonstrated that light can act as a wave or as a stream of particles, depending on how and when it is measured. As studies in this field have repeatedly shown, measurement can even affect the past behavior of an atom or subatomic particle. A 2015 study at the Australian National University using helium atoms, for example, confirmed that the path taken by an atom was affected by future measurement.

So is time real or not? With the conceptual impasse of general relativity and quantum mechanics, this question has been the subject of renewed interdisciplinary debate and discussion among theoretical physicists and philosophers. 

One thing after another

Callender proposed that to arrive at some resolution concerning the nature (or very existence!) of time, we would do well to look to quantum gravity. Specifically, he turned to two distinct research programs within the field: Causal Set Theory and canonical Quantum Gravity, delving into the very different ramifications each holds for the status of time.

Causal Set Theory developed, Callender explained, from a curiosity as to whether the principles of general relativity could be arrived at probabilistically—that is, by positing space-time as made up of discrete sets of events. In this approach, the first event represents the “big bang,” but this big bang births other events which exist in a “space-like” relation to each other. Since probabilities can be assigned to the likelihood of one event birthing another—and thus producing time as we experience it—Causal Set Theory appears to have the potential to “save” the original concept of time.

However, in seeking to “faithfully approximate” space-time, does this approach succeed in fully bridging the quantum to the universal scale of general relativity? Does it solve the dilemma of bridging relativistic time with the manifest time we all seem to live in? Callender suggested there is a tension between how much a theory is faithful to relativistic time, and the extent to which is does justice to an account of manifest time. Ultimately, Callender found that Causal Set Theory does not completely rescue time.

Killing time for good

Next, he took up canonical Quantum Gravity, an approach that began simply as an attempt to see how gravity functions at the small level—and that “kills” time for good. It renders the whole gravitational field at a quantum scale—thus, in place of a particle you have a “spatial scale factor” and in place of momentum, an “extrinsic curvature.” With the entire gravitational field sliced up into discrete units, time might be seen as an effect of scale. In the “heavy,” big universe, time does not exist. But in its sub-systems, it might—as a structuring principle.

We could, therefore, seem to experience time even without its really existing “out there.” But trying to explain time without time presents difficulties of language and cognition. For instance, Callender noted, the heavy universe is said to move more “slowly” than the lighter sub-systems—revealing the difficulty of talking about the absence of time without invoking time.

“Killing time,” Callender concluded, “is as hard as saving it.” 

Craig Callender is a professor of philosophy at UC San Diego. Learn more at his faculty webpage.