This videos is sponsored by Wondrium. Sign up for free, and check out their great physics
content by clicking the link in the description. Look around the room you’re in, what do you see?
You see stuff. That stuff is somewhere in space and you can tell whether it’s moving
or not. If it’s moving, it has a direction and speed. If this applied to particles,
they would have a position and velocity. And if you could measure that, you could predict
exactly where the particle would be in the future, and where it had been in the past.
It would be a mechanical, clockwork universe where everything was predictable and deterministic.
In principle, if we had all the positions and velocities for all particles in the universe,
we could predict the entire future of the universe, as well as precisely know the past.
This was the way the universe was thought to be about a hundred years
ago. It was a classical picture of the universe defined by classical physics.
Then quantum mechanics came along and poured cold water over this idea.
This changed our view of everything. At the most fundamental level and the tiniest scales,
reality turns out to be probabilistic. The precise location of a particular particle cannot be
predicted in advance, even in principle. Yet this idea as defined by the rules of quantum mechanics,
is the best and most accurate theory we have about the way the universe really works. It can account
for all the forces of nature…except gravity. It does not fit in the quantum mechanical model.
Why is gravity so different? Are we so sure that this is what reality really is. Why can’t gravity
be modeled by quantum mehanics? Let’s examine this and find some answers…coming up right now…
The best theory we have of gravity is the general theory of relativity by Einstein,
and it has been proven to be correct in test after test over the past 100 years. But it
is still a classical theory, not quantum. So why can’t we leave well enough alone? Why
must we quantize gravity? Let me give you a very simple answer. Because quantum mechanics works.
It makes predictions and the predictions fit with observable data in some cases
to a one part in a billion. It is the most accurate theory of reality we have ever had.
Now you may say, well, but you just said that General Relativity has been
proven correct in test after test for a hundred years. Yes, that’s true…mostly.
You see, General relativity falls apart at quantum scales.
For example, it cannot describe the singularity inside a black hole where gravity theoretically
becomes infinitely large at an infinitesimally small scale, or the presumed singularity at
the Big Bang. These singularities are where quantum mechanics, the theory of the very small,
meets gravity, the theory of the very large. The equations of general relativity break
down here because it cannot describe how gravity works on quantum scales.
Another example of this incompatibility is in the double slit experiment. This is a test in quantum
mechanics when single photons or electrons are fired one at a time through a double
slit. They show up as individual photons on a detection screen behind the slits.
This looks fine for a while, but when thousands of these individual particles are fired one-at-a-time over time,
an interference pattern develops, as if the particles are waves. This is the classic test
demonstrating the wave-particle duality of nature. The equations of quantum mechanics predict
this behavior because they show that these particles are not like little cannon balls
but are like a wave described by a wave function. And the wave function tells us the probability of
finding these particles on any particular part of the screen. They tell us that particles are waves
until some kind of interaction occurs, at which point the wave becomes localized like a particle.
In the case of the double slit experiment, the interaction of the photon or electron
wave occurs when it hits the screen, and what you see on the screen is a point-like wave,
which we perceive as an individual particle. But a photon or electron, just like any quantum
particle must also have a gravitational effect. General relativity tells us that
anything with energy or mass affects the curvature of spacetime. This is what gravity
is – the curvature of the background space and time in which all matter resides and moves.
But General Relativity does not tell how this gravity behaves with a quantum wave function.
If a photon or electron or any other particles is a wave prior to an interaction, that is,
it could be anywhere until the moment we measure it, where is its gravitational effect located?
General Relativity does not tell us how gravity can behave like a wave function.
There is no wave function for gravity. The wave-like description of the particle
prior to it hitting the screen does not have a corresponding
gravitational description in General relativity. Yet, since the particle has mass and/or energy,
it must affect spacetime and thus influence gravity. But we don’t know how this works because
we don’t have a quantum description of gravity. So the bottom line is that we know quantum
mechanics works, and it works very well at the smallest scales. And it can be shown that
quantum mechanics at its limits results in classical mechanics on large scales.
On the other hand, we know that General Relativity works very well at large scales.
But the problem is general relativity does not work at all at the smallest scales.
This simply cannot be because gravity must work at the smallest scales, otherwise its cumulative
effects would not work at large scales. This is why most physicists think General
Relativity must be brought into the fold of quantum mechanics. It is currently incomplete.
It’s highly accurate at macro scales, but must somehow be described by, as
yet, undiscovered laws that quantum scales. The problem is this. In quantum mechanics,
we have matter particles called fermions and force particles called bosons.
All interactions between matter particles are mediated by the force particles. In other words,
these bosons of the standard model are responsible for the three forces – electromagnetism,
the strong force and the weak force. And all these interactions
happen with space and time as the background. Gravity doesn’t fit this picture because as
described by general relativity, gravity is not a force. Gravity in general relativity
is due to a disturbance, or a warping, or curving of the background spacetime itself.
There is no force-carrying particle in general relativity
which mediates gravity between matter particles. Gravity is embedded within the fabric or canvas of
the background in which matter and energy resides. This is much different than the picture we have of
the quantum mechanical world, which can be thought of as the layers of paint on top of
the background canvas. The waves of matter particles would be like the different paint
colors. And the interactions of these paint colors would create a beautiful painting,
a painting that represents our beautiful universe. But gravity is the canvas itself. And it is not
an unmoving background monolith like it is in a typical painting. Is it undulating,
warping and curving such that it affects the overall look of the painting, which is
analogous to how gravity affects our universe. Now I want to point out that just because
General Relativity, as we understand it today, does not fit quantum mechanics,
it does not mean that it is incorrect. It is not incorrect. It is just incomplete.
This is similar to the way that Isaac Newton’s laws of gravity are not incorrect simply because
Albert Einstein found a more fundamental theory. In fact, Newton’s laws were used to send people
to the moon. General relativity shows that Newton’s laws are correct within its limits.
Only when Newton’s laws don’t work at extremely high levels of gravity such as very close to the
sun or near black holes, does General Relativity provide a more complete picture. For example,
General relativity correctly described the precession of Mercury, but Newton’s laws did not.
Similarly any quantum theory of gravity must show that General Relativity is correct at its
classical limits. It would just describe a more complete picture of gravity at quantum scales.
So we don’t have a quantum theory of gravity, even though this has been one of the most
active areas of research for decades. Why is quantizing gravity so difficult?
I’m going to explain this in the simplest way I can think of, but this is still a very difficult
area to understand, so don’t feel bad if you don’t get it with just one viewing.
Some of the smartest physicists in the world couldn’t figure this out, including
Einstein himself. Here’s the basic problem. You’ve probably heard that infinities arise
when physicists try to quantize gravity. Where are these infinities coming from?
To explain this, let’s use a Feynman diagram. This diagram represents an electron and a positron
annihilating to create an energetic photon, which then converts back to an electron and a positron.
But quantum uncertainty is such that the photon in the middle on its way to turning
into an electron and positron, can convert to any one of a different number of particles,
for example it can turn into a top quark and anti top quark which annihilates,
or it can turn into an electron and positron and back into a photon.
Or it can turn into muon particle, bottom quark, even a W particle on
its way to becoming an electron and positron. It can do this because the uncertainty principle
allows creation and annihilation of virtual particles. This means that particles can be
created by borrowing energy from the vacuum of space for a very short period of time,
as long as that energy is put right back to the vacuum almost immediately.
Since matter and energy are interchangeable, this energy can be in the form of
just about any fundamental particle. And it can do this 10 times, 100 times,
1000 times, actually an infinite number of times before turning into an electron and positron
again. In other words, it can turn into a myriad of particles and it can do this a myriad of times.
When describing this mathematically, we have to take all the momentums of all the particles
and all potential interactions between the various particles into account. There turns
out to be an infinite number of combinations of interactions . This is where the infinities
crop up in the equations of quantum mechanics. But a few brilliant scientists, most notably,
Richard Feynman, found a way to get rid of these infinities using clever mathematical tools,
for example by showing that the infinite quantities themselves don’t matter, but only
how those quantities change. So even though the equations contain meaningless infinities, useful
information can still be extracted from them. This process is called renormalization. And viola,
we have a quantum theory that works and the standard model is saved.
But this renormalization process does not work when taking gravity into account. This
is because gravity is not a force according to general relativity.
It is all about the curvature of space-time. It is not about particles as is the case in quantum
field theory, it is about the background on which these particles or fields reside.
So you might understand the problem here. If we use the same Feynman diagram as above,
but now instead of just the math representing the myriad of particle interactions,
we also have to take into account every possible configuration of spacetime underneath every one
of those possible interactions, this makes it such that we now can’t get rid of the infinities.
The math can’t be solved. So we are stuck. Now, just because we can’t solve it does not
mean that solutions don’t exist. They probably do, but a completely new approach is needed.
Some valiant efforts have been made. Two of the most well known attempts are loop
quantum gravity and string theory. I have a video on this if you want to learn more.
Basically, loop quantum gravity attempts to quantize the background spacetime itself,
describing reality as a pixilated landscape somewhat like the pixels on your TV screen.
So instead of finding a particle that mediates a gravitational force, it quantizes the entire
spacetime background itself
And String theory is an attempt to describe reality as a completely new paradigm where
everything is a string vibrating in multiple unseen small dimensions.
It treats gravity as a force which is mediated by a new particle called a graviton.
But the problems with both these theories so far is that they largely don’t explain
observed phenomena, and they don’t make any NEW testable predictions. So we are still stuck.
Even Einstein until his last day on earth had been attempting to resolve
this issue of reconciling gravity with quantum mechanics, but was unsuccessful.
But in all likelihood there is a solution out there waiting to be
discovered. It’s probably going to take the right
individual with a bold and perhaps radical new approach to viewing the real reality.
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