When physicists speak of a Theory of Everything, what they are hoping to find
is a single underlying theoretical framework that applies to all physical phenomena in the universe.
This is hugely ambitious and would by definition and, in a very real sense, be all encompassing,
but that does not necessarily mean it would “explain everything.”
When we hear “Theory of Everything”, at face value we might expect it to be a scientific theory that
literally explains everything that has, does, or will happen — some system for perfect knowledge
and prediction of all physical phenomena. The idea of such perfect physical explanation
brings to mind the famous quote of 19th-century physicist and mathematician Pierre-Simon Laplace:
“An intellect which at a certain moment would know all forces that set nature in motion,
and all positions of all items of which nature is composed, if this intellect were also vast
enough to submit these data to analysis, it would embrace in a single formula the movements of the
greatest bodies of the universe and those of the tiniest atom; for such an intellect
nothing would be uncertain and the future just like the past would be present before its eyes.”
One might think that, for contemporary physicists, the Theory of Everything would be that
single formula by which perfect prediction of the future would, in principle, be possible.
But a lot has changed since the 1800s.Today, we have a profoundly deeper understanding of
the underlying forces of nature than Laplace and his contemporaries did. Most physicists would now
agree that a Theory of Everything would not result in perfect prediction of every physical event.
The first blow to perfect physical prediction is the indeterminacy of quantum mechanics.
Our current best theory of the structure of matter is commonly understood to be fundamentally
probabilistic, not giving exact predictions. Further, the Heisenberg uncertainty principle
forces us to give up on any hope of perfectly knowing everything about the state of any physical
object at any time, even in principle. While quantum mechanics is not the Theory of Everything,
any ultimate fundamental theory would have to incorporate what we have learned from it,
and it is not at all clear how such a theory could avoid the
indeterminism and uncertainty we have become accustomed to.
The second blow to perfect physical prediction is purely mathematical,
based on Godel’s Incompleteness Theorem. In 1931 Kurt Godel proved, in brief, that a general class
of consistent mathematical theories cannot be complete, that is, there are true facts
that cannot be proved from within the theory. It seems fairly clear that any mathematical Theory
of Everything should be such a consistent mathematical theory, strongly suggesting
that it would necessarily be incomplete. Some have interpreted this to mean that it would be
impossible to mathematically state a Theory of Everything in a finite number of steps,
while others have argued that it would simply mean that there are certain questions the theory could
not answer. Either way, independent of any appeal to quantum mechanics,
Godel’s Theorem cuts against the possibility of perfect physical prediction.
A final blow comes from the difficulty, if not impossibility, of reducing all
physical phenomena to the interactions described in a fundamental theory of physics.
We have touched on this in other episodes, but many scientists and philosophers think
that the complex behavior of condensed matter physics, chemistry, biochemistry, and biology
cannot simply be “read off” of a perfect physical Theory of Everything, even in principle.
While the classical notion of perfect predictability has been largely abandoned,
that does not mean that finding the Theory of Everything, properly understood, is an unworthy
goal or that finding such a theory would not be a remarkable and foundational human achievement.
That said, there have been arguments back and forth among physicists and
other scientists in the past decade over the feasibility of achieving such a goal and,
especially, whether the current strategies for finding it are working or not.
Predicting the future of scientific progress is always a risky business. That said, as a particle
physicist, I still hold out hope that we will find ways of unifying our best theories of physics,
namely quantum mechanics and general relativity. Whether that Theory of Everything will look
something like the string theory or loop quantum gravity which physicists are already working on,
I don’t know. I suspect, though, that there will need to be new observational and
theoretical developments, perhaps even another “revolution” in how we think about physics.
This doesn’t mean we’ll have to discard General Relativity and Quantum Mechanics, but
we may need to look at them in a deeper light.
If we had such a theory, I would expect it would be an amazing window into the underlying order
and symmetry in the universe that helps provide the foundation for the beauty and complexity that
we already observe in every physical system. That said, we know that such a theory will not
actually explain everything, even if we restrict ourselves to just scientific questions. The
Theory of Everything would ground our knowledge of Nature, but we will still need to work on higher
level physics, chemistry, and biology to fill out our understanding of the physical world, and
the work of philosophy to place it in the context of human knowledge and understanding as a whole,
let alone to answer questions about how we live our lives as individuals and a society.
Ultimately, the greatest accomplishments of human reason and science
only make sense in light of the God who created the physical order and gave us the
gifts we need to explore and investigate it and to understand, at least in some sense, everything.
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