An interview with Martin Bojowald

This post is also available in: Spanish

What is the take of Quantum Loop Theory on space and how is it different from other quantum theories of gravity?

In loop quantum gravity, space is discrete in much the same way as energies are discrete in atoms. Space can thus be viewed as being made out of elementary constituents just like the atoms of matter. In this way, loop quantum gravity provides a more fundamental, structured picture of space than is realized in other quantum theories of gravity.

What is the treatment of time in Quantum Loop Gravity? Does it have the same status as space, as in General Relativity, or does it play a different role?

In the initial mathematical formulation of loop quantum gravity space and time play different roles, which has sometimes led to the charge that the theory breaks the covariance of general relativity. There are atoms of space but no atoms of time. Time or change arises when atoms of space and matter interact with each other, forming new atoms by subdivision or exciting existing atoms. An excited spatial atom occupies a larger amount of volume, and so by creating new spatial atoms and exciting existing ones a growing space, just like the expanding universe, arises. Even though space and time seem to play different roles in this active interacting picture, there are consistency conditions which ensure that the dynamical processes described in this way respect covariance properties as they are known from general relativity. Showing that these consistency conditions are realized in all situations remains one of the major questions in the development of the theory.

Recently Stephen Hawking said that, with M theory, God is not necessary anymore since Space-time creates itself with the aid of gravity. Is this also the case in Quantum Loop Theory?

No, even though loop quantum gravity provides a more fundamental description of space-time than M theory, it does not allow for the creation of space-time from nothing. Some atoms of space must already be present before new ones can be created by subdivision. Irrespective of which theory one uses, the answers to such questions are mainly up to the interpretation and perhaps the desires of the beholder.

Lately there has been a heated discussion amongst physicists because of the lack of experimental predictions in some new theories, which nonetheless continue to be developed extensively. How does Quantum Loop Theory fare in that respect? Do you agree with the opinion that theories that do not make experimental claims -yet- are not scientific?

Theories are often developed based on principles (such as general covariance) which have been extracted and condensed in abstract form from previous observations, but contact with experiments is often lost in such a long process. This is the case also for quantum gravity, which aims to combine the principles underlying general relativity and quantum physics. Requiring that any such theoretical development must make experimental claims from the outset would be too strict and counterproductive. But there must always be a perspective for experimental tests; theoretical developments in physics cannot go on for decades without producing at least the possibility of falsifiability by experiments, the crucial property of scientific research. Theorists can often avoid observational pressure by introducing sufficiently many parameters in their constructions and tuning them so as to remain consistent with observations. But if such contraptions become too intricate, one normally loses trust in the theory even if it formally remains uncontradicted. As for loop quantum gravity, it currently does not appear to make unique predictions; the fundamental interplay of its spatial atoms is too complicated to be controllable in all details. Nevertheless, its characteristic features of discreteness imply certain effects in the formation of structure in an expanding universe which have already made the theory falsifiable. In fact, several constructions of cosmological models have already been ruled out, and detailed computations and evaluations of its implications for the cosmic microwave spectrum are in progress.

Physics makes more and more reference to objects which are very far from our perceptive range: from electrons to parallel universes or extra dimensions. This makes the notion of “real” a bit more fuzzy than it used to be. What do you think is “real” in the Universe? I. e. is it the particles, the fields, the patterns? Are we just information?

“Information” is itself a fuzzy enough notion to encompass everything, so I do not consider it a useful characterization of what is real. “Reality” is an intuitive notion, and when used for physical objects it often seems to designate things that can be imagined with the classical outlook on the world which we grow up with before having to learn about quantum physics. Indeed, in science (as opposed to philosophy) the notion of reality was questioned seriously only after the advent of quantum physics. Thus, an answer to the question of what is real in the Universe, which clearly has quantum aspects, must first provide an extension of the notion of reality to quantum objects. I think that the most fundamental aspect of the universe which can be assigned reality is change. Irrespective of how we notice the change (for instance by a difference in the information we have about an object, or by the process of gathering information) the change itself is real. (As a corollary, time is not real because it is a prerequisite for describing change.)

Is there an external reality? Is there any way of checking?

The principle of humility implies that, yes, there is an external reality but, no, there is no way of checking.

Finally and on a much lighter note: what is the meaning of life?

To have fun.

See this author’s biography.


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