Hossein Alizadeh: A Universe where time runs backward? What sets apart one plausible mathematical solution from another is…Reality
Writes Professor Hossein Alizadeh:
In just published research, theoretical physicists are asking Is there a universe where time runs backward?
Any serious theory in theoretical physics is based on a rigorous mathematical framework. At the core of the mathematical language of these physical theories lie what are called differential equations. For instance, Einstein’s General Relativity, with all its elegance and profoundness, is a set of differential equations that could have numerous possible solutions. Any acceptable mathematical solution to these equations could be the representative of a physical reality in outside world. For instance, black holes, with all their bizarreness and unimaginable characteristics, were first conceived not as an amusement for an avid sci-fi reader, but as a mathematical solution to the differential equations of General Relativity. Naturally, one may wonder what criterion sets apart one plausible mathematical solution from another equally plausible solution. The answer is the “Reality”. When an experiment is conducted or observation is done, the nature tells us which model is rooted in the outside reality, and which is merely a mathematical solution without a physical representation.
Often, the articulated physical implications of such mathematics sound quite farfetched and incredible. One of the prime examples of such propositions is the idea of extra dimensions conceived in string theory, and in multiple theories of quantum gravity. For instance, some variations of string theory hypothesize up to twenty-six extra dimensions. For a theoretical physicist, these extra dimensions, first and foremost, are a tool to make the theory’s math work. Then if the math checks out, it is time to think what implications these mathematical tools would have in outside world. Yet, the ultimate judge in physics is the observation. If these extra dimensions could be observed in a certain experiment, and they are not, then no matter how elegant or profound a theory is, it has to be replaced by a better one.
Another example of this nature, which has garnered a lot of attention recently, is the hypothesis of Multiverse, that hypothesizes about the existence of a vast number of universes other than ours. This hypothesis primarily tries to solve one of the biggest unsolved questions of modern physics, that is, the cosmological constant problem. One may wonder about the testability of this hypothesis. After all, how can we have access to other universes, while there are parts of our own universe, which are outside our causal cone and will never be accessible to us? Doesn’t this violate one of the major tenets of modern philosophy of science, which considers falsifiability as a must for any serious scientific theory? Before one gets caught up in this question, we should point out that this hypothesis does indeed produce testable predictions, such as the existence of certain elementary particles with precisely predicted masses, spins, etc. Of course, no such particles have been found yet, but they have not been ruled out either.
In the newly published work, Boyle, Finn, and Turok present a theory, whose main purpose is to solve the mystery of dark matter, which is another major open question in the physics of 21st century. In this work the authors extend CPT symmetry to a universe other than the current universe. CPT symmetry (Charge, Parity, Time) is a fundamental symmetry of physical laws that implies if in a system contained in our universe, all the electric charges change their sign, i.e. matter becomes anti-matter, and if the said system is replaced with its mirror image, and if time reversal occurs, i.e. time becomes negative, then all the physical laws would remain intact under such transformations. CPT symmetry is profoundly fundamental, and it has never been observed to be violated in any experiment.
Boyle, et al. extend CPT symmetry to a universe other than our universe. More specifically, the authors propose another universe which is the CPT symmetric version of ours. This hypothesized universe would be filled with anti-matter, the arrow of time would be backwards there, i.e. in the direction of decreasing entropy, and it would be connected to our universe via Big Bang. Anti-matter has all the usual properties of normal matter, including mass, charge, and quantum spin, with the difference that it has the opposite charge of normal matter. For instance, positron, the anti-matter of electron, shares all its properties, with the only difference that it has positive electric charge.
One may find such proposal of a mirror universe quite implausible, but one should remember that the ultimate judge in physics is observation. This theory predicts several observable characteristics of early universe perturbations. If these predictions are observed in relevant experiments, then the idea of a mirror universe would be a surprise turn of events.