When we started this journey, we first asked if we needed a theory of everything, and if a theory of everything was even possible. But before we jump into the details in our future posts, it would also be appropriate to ask what we should expect from a theory of everything.
Remember that I have defined a theory of everything as an as-yet unknown theory that would allow us to understand our entire universe within one self-consistent framework. When students take my ‘theory of everything’ class, this definition often leads to questions about free will and personality and the origin of preferences. Could such things be a part of a working theory of everything? Would a theory of everything necessarily subsume all other knowledge making it the only thing we would ever need to know and study because we could derive from it everything else, like chemistry and biology, perhaps even biochemistry, and eventually psychology and sociology depending on how far you want to push the idea of everything.
I do not believe so, but I am willing to be agnostic on this point. Personally, I don’t see a reason to use what I envision coming out of a working theory of everything to pursue chemistry, biology, or any other well-established science. We have wonderful methods of understanding and using these fields already, and whereas a theory of everything may give some insights into unsolved or empiric problems in chemistry (for instance), it would most likely prove enormously impractical for day-to-day uses.
Of course, if an eventual working theory of everything is radically different from what we are using today, then all bets are off! When I picture a theory of everything, I am envisioning a sort-of “fixed up” version of our current standard model, one that works better and can handle gravity, perhaps written in a slightly different mathematical language, and that mental picture could be a colossal mistake. I’ll let you know if I’m right when we have a theory of everything. However, one thing we know for sure is that our current standard model must appear as a low-energy limit of a working theory of everything because what we have works pretty well and should be inside a final theory somewhere, so perhaps I am not too wrong with my mental picture.
For these reasons and others some physicists dislike the term ‘theory of everything’ but I believe that a rose by any other name would smell just as sweet. Like very many arguments in science, this is an issue of definitions and context. However, I absolutely believe that words are important, and if you dislike the phrase theory of everything, you can fill in whatever you like.
In his new book Fundamentals, the great Frank Wilzcek writes about these ideas and wishes to make the distinction between a so-called “theory of everything” that could imply an “end of science” much in the way we saw above and what he calls a “final theory.” Wilzcek is saying that a ‘theory of everything’ which literally encompasses everything is not what we are after or even looking for. Instead, we want a theory that would describe all known phenomena, stand up to rigorous testing, and would not be open to change. Wilzcek’s “final theory” would not incorporate many things we might think of when we think of “everything,” and would make no attempt to do so. I like this description, and appreciate the thoughtful distinction. This “final theory” matches my working definition of a “theory of everything,” so I will continue to use the term while acknowledging it can be loaded.
At the other extreme, there are other scientists who think that so much more must be included in a working theory of everything, like the wonderful Max Tegmark. In his book of a similar name, he described what is known as the “mathematical universe hypothesis” which would require us to think about data in the computational sense as part of a so-called theory of everything. A fascinating idea. Again, time will tell.
Sidestepping the very real issue of what a theory of everything will look like, we would like to know what it should tell us about the universe. I am not sure how controversial my list below is amongst physicists, when I ask around I find a similar want-list with some additions and deletions, but I know for sure that not all of the items on my list are independent. This is for you, dear reader, to best emphasize what would come out of a theory of everything even if there is a little overlap.
What do we want? Everything! When do we want it? Now!
Clearly, there are some nuts-and-bolts inside our current theoretical framework that would need to be a part of a theory of everything. For instance, it should include everything we already know and anything that we are currently missing because we do not know about it yet. Are we missing any particles or forces? Are the particles we think of as fundamental actually fundamental? But, this is also not exactly what I would expect from a theory of everything per se, this is what I would call a perfect description of reality but is not what I truly want. What I want is not only the “what,” but the “why.”
To jump to another part of our overall story for a minute, let’s talk about gravity. The relatively simple bread-and-butter Newtonian version of gravity. This is the version you most likely learned in a high school or college physics class. Newton is a very complicated figure in the history of science, but we will sidestep that interesting story and jump to a real concern he personally had with his theory that made him question whether he should publish it at all. Newton felt that his theory was an excellent “description” of gravity, but it didn’t say “why” there was gravity in the first place. We would have to wait for Einstein to understand the “why.” This “why” should be central to a working theory of everything.
So, I do not just want to know the masses of all the particles and how they couple to each other, I want to know why they have the masses and couplings that we observe in the first place. I want to know what the CP phase is and why it appears in the weak sector and not in the strong sector (or if that is even true). This “why” is part of all of the questions to be answered by a theory of everything.
What other questions instantly come to mind?
Well, do the fundamental constants of nature vary with time? Why are there four (large) dimensions of spacetime? Why is there an arrow to time? Is it absolute? Why are there no (local) magnetic monopoles? Why are there (only) three generations of matter? Why are there no fractionally charged/colored objects? What gives rise to the standard model? Why do we have the gauge groups we have? Why is there more matter than antimatter? What is dark energy and dark matter?
We should also caution ourselves with these questions. They may look very silly in the future. If we went back in time to the year 1900 and asked a large group of working physicists what they would ask us one hundred years in the future, those questions would probably make little sense to us. They would be asking about the future of a world that predates quantum mechanics, the mainstream acceptance of atomic theory, and the existence of two of the four fundamental forces. In one hundred years, my questions could be quite banal.
There is one final overarching question that I would also like answered to my satisfaction that is perhaps a little deeper than these other technical questions. Was there any freedom in how our universe came into being? That is to say, is our universe unique or could it have come into being in another way? In many different ways? If so, why did it come into being in this way in particular? The answer to these questions alone, I believe, would give us a great deal of insight into the nature of our reality.
In the end, the universe is out there doing something and it is up to us to figure it out.