This video is brought to you by NordVPN. Click the link in the description for
a special offer for Arvin Ash viewers. Many physicists have devoted their entire lives to
answer one question, “how does the universe work?” The seeds of some theories often started with
a guess, which initially sounded crazy. For example, it was once considered crazy to think
that light comes in packets of energy, now called photons, when Albert Einstein first proposed it
in 1905. But it better explained observable phenomena such as the photoelectric effect,
where electrons are emitted from a metal plate only by light beyond a certain frequency not
intensity, and it helped advance the idea that quantum mechanics is the most accurate
way to describe the true nature of reality. Fast forward some 120 years later to today.
And advancements in quantum mechanics, has led to a mathematical framework that
brings us tantalizingly close to a theory of everything. This framework is called
quantum field theory. Its various aspects describing nearly all particles and forces
in the universe is consolidated into the most accurate and proven theory of reality, called
the standard model. Is this the final theory? No, we know this is not the end. This theory
doesn’t explain gravity. And so we are still in search of that crazy new idea
that leads us to an even more accurate and complete description of the universe. Did
I say crazy? Enter string theory, originally thought to explain hadrons, that is, particles
like protons and neutrons, in the late 1960s. This theory may seem insane as it proposes
crazy ideas such as a universe made of vibrating strings and with at least 10 dimensions! Mind you,
we only know of 4, the three spatial dimensions and time. String theory claims that somehow at
least 6 other dimensions have managed to stay hidden from even the best experiments
we’ve made. But it appears to have an answer explaining gravity. And many physicists have
spent nearly their entire careers studying it. The big question is how do the crazy ideas of
string theory fit with the now well established quantum field theory
and the standard model. Are they at all compatible? How are they different? Can
they both be correct? That’s coming up right now…
String theory and quantum field theory are both
ways to model the universe in a…wait, ah is this Germany?
Let's try that again...
String theory and quantum field theory are both…in London?
Oh, I get it now,
it’s because I’m using NordVPN, today’s sponsor. They can establish a secure encrypted connection
between your device and a remote private server in one of 50 plus countries, like Germany or the
U.K. Your virtual location can be in any of these countries. So for example, if you’re currently
geo-restricted from certain online content, it’s not a problem with NordVPN. In addition, it
encrypts all your data, so third parties like your snoopy service provider can’t know your online
activities. And your data is more secure from hackers. They have a special offer for Arvin Ash viewers right now.
You can get 4 months extra on a 2 year plan if you click the link in the description,
NordVPN.com/Arvin. And it’s completely risk free with a 30 day money-back guarantee. Give
it a try and stay safe online. Now, where were we…String theory
and quantum field theory are both ways to model the universe in a useful mathematical framework,
but they approach the fundamental nature of the universe from different perspectives.
String theory is a theoretical framework that posits that the smallest building blocks of the
universe are not zero-dimensional point-like particles, but tiny,
one-dimensional "strings" that vibrate at various frequencies and in different dimensions,
and that different combinations of frequency and dimensions can describe all the different types
of matter, forces and energy in the universe. On the other hand, quantum field theory postulates
that all particles are excitations in fields that span the entire universe, and that forces are due
to the interaction of these particles mediated by other particles called bosons. It is the
mathematical framework describing how particles and forces interact at the subatomic level. And
our best and most accurate understanding of the universe, based on this framework,
is consolidated in what is known as the standard model of particle physics. This standard model,
is represented by all the equations you see here. If you want to know more about what these equations mean,
I have a dedicated video here on that. Both String theory and quantum field
theory are based on the principles of quantum mechanics and special relativity. And just
like quantum field theory is a mathematical framework, so is string theory. There are
several string theories, all related to a deeper overarching theory called M-theory.
Both theories in principle can mathematically describe all particles and forces.
But we know quantum field theory, or QFT, works. In fact, it’s the most accurate theory we have
ever had. Its predictions concur with experimental results in some cases to 10 parts in a billion.
So why do we even bother with the crazy ideas of string theory? The biggest reason is that it so
happens that a certain combination of a closed string vibrating in 10 dimensions can model a
particle that looks identical to the graviton. The graviton is the theorized missing boson
particle of the standard model that would carry the gravitational force,
similar to the way photons carry the electromagnetic force, W and Z bosons carry
the weak force, and gluons carry the strong force. Keep in mind that gravitons are theoretical only.
They have never been detected. They are predicted by String theory. And that's why the theory was so
popular some decades ago. Physicists thought that it
could provide the mathematical leap we needed to integrate gravity into quantum field theory. And
if that could be done, it would solve the holy grail of physics – the quantization of gravity.
Interestingly, String Theory was first proposed in the late 1960s and early 70s,
to describe the forces that keep hadrons like protons and neutrons glued inside the nucleus
of atoms. But it turned that it was not needed because. a quantum field theory called Quantum
Chromodynamics (QCD) worked quite well, and is today the established theory that quite
accurately describes the strong nuclear force. However, this exercise with string theory,
was not a complete waste because it led to Scientists finding that you could mathematically
model a particle with zero mass and spin of 2. These are the properties of a graviton.
Why is zero mass and spin of 2 required to model a graviton? Well, we know that gravity propagates at
the speed of light and has an infinite range. Only a massless boson particle could have such
properties. It’s akin to Electromagnetism which also obviously propagates at the speed
of light and has infinite range. And its carrier particle is the massless photon.
The spin 2 requirement is a bit complicated to understand. There are two main reasons. First,
spin 2 is required to model an exclusively attractive force which is what gravity is. Spin 1,
for example like in photons, the carriers of electromagnetism, can model both a repulsive
and attractive force. Second, a graviton must have a spin of 2 because that’s the
way spacetime would have to be quantized in order to agree with General Relativity,
which we know works, and is the current best model we have for gravity. So whatever theory
we get has to agree with General Relativity. But simply having a description of gravity
in string theory is not something to get too excited about, because Quantum
Field Theory can also describe gravitons – as excitations in a theoretical graviton field.
But the problem with trying to quantize gravity in quantum field theory is something called
renormalization. This is the procedure of making a theory valid to infinitesimally small lengths.
When we try to do this, that is when we try to mathematically quantize spacetime itself,
instead of keeping it as a background on which fields exist, we get infinities in
the equations. So quantizing gravity using QFT, doesn’t really work. The analogy can
be made something like this - spacetime is the background on which quantum fields exist like paint exists
on a canvas. So when you try to make the canvas itself into a paint, the painting falls apart.
If you want to know the details what exactly falls apart, I made a video about it right up here.
Unlike QFT, String theory solves the issue of quantizing gravity. But there is a big price
we pay for being able to so And that is that we have to contend with 6 to 7 extra dimensions,
10 or 11 in total. This is a problem because these dimensions have never been detected.
And this is not for lack of trying. Scientists have conducted test after test to try to detect
these extra dimensions, but they have all failed. Now you might say, well we need to throw it out
then, and come up with some other crazy idea. But not being able to find these extra dimension does
not necessarily make String theory incorrect. Scientists have come up with explanations about
where the missing dimensions might be. One is compactification. It can be thought of like this:
Imagine holding a piece of thread or floss. From far away, it looks like a
thin one dimensional line. But as you look at it more closely, realize it’s not just a line,
it has a two dimensional surface. And if you look at it under a microscope, you would see
that it has an intricate 3D structure. The idea is the same for string theory. We just
see objects from a point of view where things look three dimensional, but if we could zoom in
incredibly close, closer than we are technically capable of doing today, we would discover compact
dimensions that are hiding from us. Another possible solution to the
missing dimensions is that there could be super dimensional branes or membranes. The
idea is that here is that the universe may have more dimensions than we have access to,
that there could be a 4th spatial dimension, are isolated from other dimensions. We would
have to exist outside of our local brane to see these branes or extra dimensions.
The problem is that detecting dimensions on such a tiny scale requires such large energy levels that,
in practice, that we can’t verify it with our current technology and experimental methods. But
this doesn’t mean that the mathematics of higher dimensional spaces is invalid.
After all, it took 100 years to detect gravitational waves which were predicted
by Einstein way back in 1916 and not discovered until 2015. Maybe
we can still work with string theory. If we could detect these extra dimensions,
the reward would be staggering. It would be significant evidence indicating that string
theory is likely the superior model of the universe, and that QFT is simply an
approximation. This would be similar to the way that we found how General Relativity is
a superior model of gravity to Newton’s, and that Newtons equations are just good approximations.
Could the two theories, quantum field theory and string theory be combined to make a kind
of composite theory? It doesn’t seem so. The main problem is the extra
dimensions. But are there any similarities? In QFT we define particles as point-like fuzzy
vibrations having zero dimensions. They have no defined size. They are excitations in a field
that exists in 4 dimensions, 3 spatial and 1 time. As I showed in a prior video, they can
be mathematically modeled like oscillating springs in a mattress. The mattress can be
thought of as the field. So the universe could be looked as a bunch of mattresses with springs
oscillating. The different energy levels of the oscillations would correspond to the number of
particles. In other words, the amplitude of the excitation represents the number of particles.
In string theory, a particle is a one dimensional object vibrating in at least 10 dimensions. It has
a length, but no width or height. It can be an open or closed string, similar to a rubber band.
This 1D string can have a size. It is near the Planck Scale, that is about 10^-33 cm.
You can think of this as analogous to the strings in a guitar. It can vibrate to
make sounds. And just like the frequency of the vibration determines the pitch of the sound, the
frequency of strings in string theory determines the type of particle it is. So one string can act
like different particles depending on how it vibrates, and in which dimensions it vibrates.
But similar to how the amplitude in QFT corresponds to the number of particles,
the amplitude of the vibrations in string theory also corresponds to the number of particles.
This is about as far as the similarities go. The multiple undetectable dimensions makes
string theory seem strange. The main idea that still drives interest in it is that it could
result in a unified quantum theory by combining the standard model, which explains the strong,
weak, and electromagnetic force, with general relativity, which explains gravity. But General
Relativity, is not a quantum theory, and breaks down at the quantum level. It gives us infinities.
For example, it gives us a singularity, which is an object of infinite mass density inside a
black hole and at the big bang. This does not seem physical, so is probably a glitch in the theory.
Many physicists are convinced that gravity must also be quantum,
because nature would not be quantum for everything but somehow make an exception
gravity. So we need new physics to quantize it. String theory, on the other hand can be used to
make a meaningful theory of gravity. In this case, the excitation or particle of gravity, called the
graviton, is modeled as a closed vibrating string. You might be asking, why does the theory need
all these dimensions. The simple answer is that its mathematics does not work in our
4 dimensions. It only works if we allow the strings to vibrate in at 6 extra dimensions or at least 10 dimensions.
Now, I should mention that extra dimensions could also be theorized in Quantum Field Theory.
But so far, we have not needed them to make the theory work. The math works fine in our observable
four dimensions, as long we ignore gravity. Now, if you think that all this
multiple-dimensional talk is insanity, I would not blame you. We are just trying
to solve a really hard problem, probably the hardest in all of physics. We can’t afford to
throw out any crazy-sounding theory, just based intuition or incredulity. We should
evaluate all crazy ideas based on their merit. Unfortunately, string theory doesn’t appear to
be a great alternative because its predictions are either incorrect or untestable. This is why
it’s getting out of favor by many physicists. On paper, it’s nicely allows us to model gravity,
but in practice, it doesn’t bring us any new answers. it’s a good mathematical exercise,
which has limited physical significance. But keep in mind that science evolves,
and even though quantum field theory seems to be the better framework today, it doesn’t mean that
it will remain this way in the future. We know that there is something beyond the standard model;
we just don’t know what that is. Could that be string theory? It doesn’t appear so,
but its mathematics could provide the insight we need to come up with the next crazy idea. If you
enjoyed this video, I think you might enjoy my video on quantum field theory and string
theory. That's coming up in about 5 seconds. I’ll see you in the next video my friend.