Is time quantized? In other words, is there a fundamental unit of time that could not be divided into a briefer unit?

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John Baez is a member of the mathematics faculty at the University of California at Riverside and one of the moderators of the on-line sci.physics.research newsgroup. He responds:

“The brief answer to this question is, ‘Nobody knows.’ Certainly there is no experimental evidence in favor of such a minimal unit. On the other hand, there is no evidence against it, except that we have not yet found it. There are no well-worked-out physics theories incorporating a fundamental unit of time, and there are substantial obstacles to doing so in a way that is compatible with the principles of General Relativity. Recent work on a theory of quantum gravity in which gravity is represented using loops in space suggests that there might be a way to do something roughly along these lines–not involving a minimum unit of time but rather a minimum amount of area for any two-dimensional surface, a minimum volume for any three-dimensional region in space and perhaps also a minimum ‘hypervolume’ for any four-dimensional region of space-time.”


Time: Is it real or an illusion of the mind?

Time: Is it real or an illusion of the mind?

William G. Unruh is a professor in the department of physics and astronomy at the University of British Columbia. He offers this reply:

“There is certainly no experimental evidence that time–or space for that matter–is quantized, so the question becomes one of whether there exists a theory in which time is quantized. Although researchers have considered some theories in which there is a strict quantization of time (meaning that all times are an integer multiple of some smallest unit), none that I know of has ever been seriously regarded as a viable theory of reality–at least, not by more people that the original proponent of the theory.

“One could, however, ask the question in a slightly different way. By putting together G (Newton’s constant of gravity), h (Planck’s constant) and c (the velocity of light), one can derive a minimum meaningful amount of time, about 10-44 second. At this temporal scale, one would expect quantum effects to dominate gravity and hence, because Einstein’s theory links gravity and time, to dominate the ordinary notion of time. In other words, for time intervals smaller than this one, the whole notion of ‘time’ would be expected to lose its meaning.

“The biggest obstacle to answering the question definitively is that there exists no really believable theory to describe this regime where quantum mechanics and gravity come together. Over the past 10 years, a branch of theoretical physics called string theory has held forth the greatest hope, but it is as yet far from a state where one could use it to describe the nature of time in such a brief interval.”

Another, somewhat iconoclastic perspective on this question comes from William G. Tifft, a professor of astronomy at the University of Arizona:

“There are several ways to answer this question. 1) There is no conclusive evidence that time is quantized, but 2) certain theoretical studies suggest that in order to unify general relativity (gravitation) with the theories of quantum physics that describe fundamental particles and forces, it may be necessary to quantize space and perhaps time as well. Time is always a 1-dimensional quantity in this case. 3) My own work, which combines new theoretical ideas with observations of the properties of galaxies, fundamental particles and forces, does suggest that in a certain sense time may indeed be quantized. To see this we need some background information; in this scenario, time is no longer 1-dimensional!

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