Hi there, physics fans.
In our last episode, I talked about how the force of gravity is quite different from the
other forces.
The subatomic forces are caused by force carrying particles jumping back and forth, while gravity
is caused the bending of space and time.
It seems like there should be some version of gravity that works in the world of the
small – something called quantum gravity.
That sounds like a great subject for today’s episode of subatomic stories.
Ever since the early days of quantum mechanics and general relativity, scientists have tried
to merge the two, without success.
Even today, we don’t know the correct way to combine them.
Scientists apply the same techniques to quantum mechanics and gravity that they used to merge
electromagnetism, quantum mechanics, and special relativity.
It worked for electromagnetism, but for gravity an infinite number of infinities popped up.
It just didn’t work.
While that technique didn’t work, we know that a quantum theory of gravity is necessary.
Obviously, gravity must also exist in the microcosm.
So, well, we obviously need a theory that describes it.
While we don’t know what a theory of quantum gravity entails, we do know a few things.
For instance, we know some of the characteristics of the quantum carrier of gravity.
In electromagnetism, it’s the photon – a massless particle with no electrical or strong
charge and a spin of one.
In the strong force, it’s the gluon, which is a massless particle with no electrical
charge and a spin of one.
The gluon does have the strong force charge.
For the weak force, there are two particles of force, the W and Z bosons.
They have mass, and one of them has electrical charge.
Both have a spin of one.
In gravity, the force carrying particle must be the graviton.
It has no strong or electrical charge, it is massless, and it has a spin of two.
In fact, in 1965, Nobel Prize winning physicist Steven Weinberg proved that any massless boson
of spin two leads to Einstein’s theory of general relativity.
So, if we find any massless spin two boson, it must be the graviton.
There are some who wonder if Einstein’s theory of gravity as the bending space and
time can coexist with a graviton, but it can.
You just have to realize that general relativity is simply an approximate theory.
Now gravity is an extremely weak force.
It is – ballpark – ten to the forty power times weaker than the strong force and ten
to the thirty five power times weaker than the weak force.
There is no realistic chance we will ever directly measure the classical graviton.
There is a potential loophole.
We don’t know why gravity is so much weaker than the other forces, but there is a theory
that says that there might exist additional dimensions of space that gravity can enter,
but not the other forces.
If that’s the case, then gravity has simply more dimensions to spread out in and that’s
why it appears weaker.
I made a long form video on these possible extra dimensions.
They’re complete theoretical and probably don’t exist.
But they are definitely interesting.
There are two theories that have been proposed that might be correct theories of quantum
gravity.
Note that neither of them has been proven to be true.
So, don’t believe either of them.
The first is called superstrings.
This theory imagines that the smallest form of matter isn’t particles, but rather small
and vibrating strings, shaped like little sticks of spaghetti or hula hoops.
These strings vibrate and different vibration patterns explain the different known particles.
I made a long form video that tells you more about it and the URL is in the description.
Scientists have been looking at superstrings for decades and there are no predictions of
the theory that can be tested.
I like the idea a lot, but there’s no reason to believe it’s true.
One of the really nice features is that superstring theory is a candidate theory that explains
everything.
It’s nice work if you can get it.
There is another theory of quantum gravity, called loop quantum gravity.
Loop quantum gravity doesn’t aspire to be a theory of everything.
It is just a theory of quantum gravity.
Basically, it assumes that space and time are quantized.
In the theory, there is a smallest bit of space and a smallest bit of time.
It’s kind of like how a smooth sand dune can be made of individual grains of sand.
One consequence of the quantized theory of space is that the speed of light should be
different for different wavelengths.
However, there is no data that supports that conjecture.
Indeed, there is data that refutes it.
I made a long form video about loop quantum gravity as well.
Still, there is no question that physics really needs to understand gravity at the quantum
level.
It’s an open question, just begging for a young scientist to come up with a good idea.
So if you’re a student, looking for a huge project to take on, this is a good one.
Be warned though, hundreds of scientists have worked on it for a century, without success.
There is definitely a Noble Prize waiting for the individual who cracks the problem.OK,
that seems like a daunting and optimistic place to stop.
Let’s see what this week’s questions are.
Hi there, physics friends.
It’s question and answer time.
And this week, the questions were amazing – just amazing.
I wish I could have gotten to all of them.
Sadly, I had to pick just a few, but I hope you’ll enjoy my selection.
I’d like to start with a retraction.
Pavel Czerski properly pointed out that I said some unnecessarily disparaging things
about fans of pineapple pizza.
We scientists should be open-minded to opinions that don’t break the laws of physics.
So, in the spirit of acceptance and inclusion, I’ll have some pineapple pizza in the near
future, and I invite you all to join me.
But can we at least agree that people who put ketchup on hotdogs are heretics?
Moving on, I have an IOU to pay off.
DasItMane asked if anyone thinks spacetime is quantized or if we’re assuming it’s
smooth.
The first part of this video addresses the possibility that spacetime is quantized.
That’s one of the tenets of loop quantum gravity.
We don’t know it’s correct – indeed the one prediction made by at least one version
of quantum gravity was invalidated.
That prediction is that the speed of light depends on wavelength.
Scientists studied light from gamma ray bursts emitted over ten billion years ago.
In spite of travelling so far, the wavelengths all arrive at nearly the same time.
But it’s a tricky business and work continues.
There is another complicated facet and it ties intimately with the mathematics.
At a deep and fundamental level, the mathematics of general relativity and the standard model
assume that spacetime is what mathematicians call smooth, continuous, and differentiable,
which just means no matter how short a distance you can imagine, there is always a shorter
distance.
To describe quantum gravity, we need different mathematics.
Now it is possible to use the continuous mathematics of calculus and differential equations to
describe quantized systems.
We do that in quantum mechanics, but the whole question highlights the intimate interconnection
between the science and the mathematics in this case.
Good question with deep implications.
Dr. Deuteron admonishes me that a density of 1 teragram per cubic meter is correct for
white dwarfs and that neutron stars are denser and that’s true.
However, neutron stars are thought to be surrounded by a crust of lower density – with specifically
the density of white dwarf material.
Neutrinos first encounter that outer environment and do not penetrate deeply into a neutron
star.
But he or she is right, and I should have been clearer and not glossed over that complication.
It’s an occupational hazard of trying to answer complex questions.
What do you do?
By the way, I’d like to thank Dr. Deuteron for reading through the comments and helping
answer them.
Their answers are generally well informed and high value.
Pranav Shroti asks how I can say that the equations of relativity fail at the speed
of light, but we use them.
Hi Pranav.
This is again a math thing.
Relativity simply take what one observer measures and uses those values to predict what another
observer will see.
Essentially all relativity equations have this basic structure, which is that the prediction
equals some ordinary stuff divided by this square root containing both the second person’s
velocity and the speed of light.
If the second person is moving at the speed of light – which physicists write as c – we
can put this in the equation and we see how that makes us divide by zero.
And, like my shirt from the last episode pointed out, things fall apart when you divide by
zero.
The takeaway message is that some of the equations of relativity apply for speeds lower than
c, but they simply don’t apply for velocities equal to c.
Chandan Bhuyan and others asked a question about time dilation and clocks.
This is a super common question or claim and that is while clocks slow down when they are
moving at high speeds, but time doesn’t actually change.
There were even some questions as to what effect this has on biology.
The answer is simple.
The time dilation effect has nothing to do with clocks, whether they be mechanical, electrical,
or the beating of a heart.
It has to do with time.
Let me use the time transformation equation to drive home the point.
It says that that the time experienced by a moving observer can be predicted by this
equation.
Let me draw your attention to the important variables.
The time experienced by the moving observer depends on only the location and time of the
stationary observer and the velocity between them.
And the speed of light, of course.
Notice that there are no clocks or anything mechanical, electrical, or biological.
It is literally just about time.
Time just passes differently for different observers.
Steven Karmazenuk agrees that general relativity is real, but asks if it is a complete theory.
Hi Steven – the answer to your question is quite clearly no.
For instance, in this episode and the previous one, I have pointed out that while relativity
works spectacularly well in the cosmic sphere, it fails miserably in the quantum one.
In addition, even in the cosmic sphere there are questions.
For instance, galaxies rotate faster than we can explain using the observed amounts
of matter and theory of general relativity.
Physicists believe that the explanation is a substance called dark matter, but we haven’t
directly observed dark matter.
One possible explanation is that general relativity needs to be modified or replaced.
That’s a possibility, and I’ll talk about the whole dark matter thing in an upcoming
episode.
If you want a sneak peak, I made some a long form video on the topic.
Then there is the fact that the equation I showed you for general relativity was the
1915 version.
It neglected a form of gravity that is causing the universe to expand faster and faster.
We call that form of gravity dark energy and it will be the topic of an upcoming future
video as well.
And then there is the existing long form video that might tide you over until then.
The bottom line is that there are many cosmic and quantum mysteries involving gravity.
General relativity is an excellent theory and it’s definitely on the right path, but
we certainly have more to learn.
Okay, so that’s all the time we have for questions.
You know what to do.
Like, comment, share, and spread the word about awesome physics.
Because, and well, you know this of course, even at home, physics is everything.