Barak Shoshany

Barak Shoshany's picture

Areas of Research:

"There is a theory which states that if ever anyone discovers exactly how to quantize gravity, the universe will instantly disappear and be replaced by a universe in which quantizing gravity is even harder. There is another theory which states that this has already happened." (Paraphrased from "The Restaurant at the End of the Universe" by Douglas Adams.)

I am a PhD student in the quantum gravity group at Perimeter Institute, under the supervision of faculty member Laurent Freidel. I graduated from the Perimeter Scholars International program with an MSc in theoretical physics. Before coming to Perimeter Institute I obtained a BSc in mathematics & physics from Tel Aviv University (Israel) and did summer internships at Weizmann Institute of Science (Israel) and CERN (Switzerland).


Quantum Gravity

My main field of research is called quantum gravity. There are two extremely precise and well-tested theories used by physicists to describe the universe: quantum mechanics and general relativity. These two theories are very different from each other in almost every possible way: mathematically, physically, conceptually and even philosophically. Usually that's not a problem, since quantum mechanics applies to very small things like elementary particles and atoms, while general relativity applies to very big things like stars and galaxies.

However, there are certain extreme situations, such as inside black holes or shortly after the big bang, which are within the realms of validity of both theories at once. In these scenarios, we simply do not know what happens. "Quantum gravity" is what we call the currently unknown theory which might someday be able to tell us what's going on. The most we have at the moment are some speculative ideas, but no complete theory, and in particular, nothing close to a true scientific theory that can be tested experimentally.

The focus of my PhD thesis is a particular speculative theory called loop quantum gravity. I decided to work on this theory because I'm interested in understanding the concept of quantum spacetime. Generally, when we apply quantum mechanics to theories, we find that what we thought was continuous is actually fundamentally discrete. For example, when we combine quantum mechanics with electromagnetism, we find that the electromagnetic field is actually made of discrete photons. General relativity tells us that gravity is a consequence of the curvature of spacetime. Thus, in a theory of quantum gravity, one might expect spacetime itself to be made of discrete fundamental elements.

One of loop quantum gravity's most celebrated results is its prediction that spacetime is indeed fundamentally discrete at the quantum level. Space appears smooth and continuous to us only because this discreteness is at extremely small distance scales, many orders of magnitude smaller than even atoms. This situation is analogous to digital photos: though they may look continuous, zoom in enough and you will start seeing that they are actually made of discrete pixels.

Loop quantum gravity predicts that, if you zoom in close enough on spacetime, you will see that it is also made of discrete entities. For my PhD thesis, I'm trying to understand the nature of this quantum spacetime and, in particular, how it relates to the usual classical spacetime described by general relativity. In simple terms, I am trying to understand what the fabric of the universe is “made of”, and what it looks like when you "zoom in all the way", as predicted by loop quantum gravity.

This was the subject of my first two peer-reviewed publications, Phys. Rev. D 99, 046003 (2019) which I wrote with Laurent Freidel (my supervisor) and Florian Girelli, and Phys. Rev. D 100, 026003 (2019) which I wrote on my own. These two papers deal with the easier case of 3 spacetime dimensions: two of space and one of time. I am currently working on a new paper which generalizes our results to 4 spacetime dimensions, which is the actual number of dimensions our universe seems to have.

It is important to note that we do not currently know if loop quantum gravity is the correct theory of quantum gravity. Therefore, I cannot claim with any kind of certainty that the research I'm doing has any relevance to the real universe. However, this is the essence of research in theoretical and mathematical physics. We work on speculative theories, developing them at the purely mathematical level, until eventually we arrive at a sufficiently complete and consistent theory which can be tested by experimentalists. Loop quantum gravity is still very far from that stage, but I'm confident that it will get there eventually. My research is just a small part of the overall mathematical analysis and development of the theory.

Faster-than-Light Travel and Time Travel

Another field of research that I am currently working on is the possibility of traveling faster than the speed of light, or back in time. It is a well-known fact that the theory of relativity forbids faster-than-light travel. Unfortunately, this makes space travel very limited, since other solar systems are located many light-years away. However, physicists have discovered theoretical methods of faster-than-light travel which are fully compatible with relativity.

One such possibility is a wormhole, which is simply a shortcut from one point in space to another. While a spaceship will travel slower than light inside the wormhole, when it emerges on the other side it will be in a completely different solar system or even galaxy. Another possibility is a warp drive, which consists of a warp “bubble” which moves faster than light. This does not violate relativity, since the bubble is not a massive object, it is part of space itself, and thus its speed is not restricted. A spaceship can travel slower than light inside the bubble – it can even be at rest – and yet move faster than light along with the bubble.

It turns out that whenever faster-than-light travel is possible, time travel to the past is also possible. However, this violates causality and creates consistency paradoxes. The most famous such paradox is the “grandfather paradox”: if a time traveler goes back in time and prevents their grandparents from meeting, then the time traveler will never be born – but then they will not be able to go back in time and prevent their grandparents from meeting in the first place.

I am currently working with my two summer students, Mir Jalal and Jacob Hauser, on two different projects in this field of research.

The first project, with Mir Jalal, is to find out if warp drives can be more than just mathematical equations, and potentially be constructed in reality. This has to do with the fact that the construction of warp drives seems to require “exotic matter”, which is matter with negative energy. Such matter does not seem to exist in our universe, but we do know that quantum effects may allow us to create negative energy in very small quantities. This may in principle make warp drives possible to construct, but at the moment this seems very unrealistic.

The second project, with Jacob Hauser, is about solving time travel paradoxes. We are trying to construct a mathematical model where, when the time traveler emerges from their time machine in the past, they actually find themselves in a parallel universe, which is a copy of their original universe up to that point in time. Therefore, any change they make – such as preventing their grandparents from meeting – only happens in this parallel universe, and does not affect the original universe from which they came. They can prevent their birth in the parallel universe, but not in the original one. This avoids a paradox, and everything is perfectly consistent. However, while this is fairly easy to describe in words, it is not easy to do mathematically, and this is what our project is about.

In addition, I recently taught a course on faster-than-light travel and time travel, for which the lecture notes are available at arXiv:1907.04178.

Teaching and Outreach

In addition to research, I am very enthusiastic about teaching, science communication and outreach. I have extensive experience in teaching both mathematics and physics to students from diverse backgrounds, and I regularly participate in outreach events at Perimeter Institute and elsewhere.

Curriculum Vitae

My CV is available here.

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