Hi there, physics fans! Over the last dozen episodes, we’ve talked about all sorts of
things that occur in the subatomic realm. But the world of the ultra-small is not the
only interesting domain to study. There’s also the big. You know…out there. That part
of the universe is governed by different rules – different forces – and that is what
I want to talk about in this episode of Subatomic Stories.
[Music] In episode six, I told you about the known
fundamental forces, specifically electromagnetism, the strong and weak nuclear forces, and gravity.
Then, in that same episode, I ignored gravity entirely. Why is that? It’s because our
understanding of how gravity affects the world is very different from how the other three
forces work. In episode five, I sketched out how forces work in the quantum realm. But
we don’t yet have an accepted theory of quantum gravity.
In >>THIS<< episode, I want to talk about gravity in the way we currently understand
it. Gravity is the most familiar of the forces. We encounter it as young children, and we
learn to feed ourselves, sometimes without success.
It was in the 1670s when Sir Isaac Newton devised his theory of universal gravity. In
his theory, the force between any two objects depends on the mass of each of the two objects
and the distance between them. He even invented a simple equation that quantified his ideas.
Newton’s ideas are still applicable even three and a half centuries later. For instance,
NASA used Newton’s law of gravity to shoot the New Horizons space probe to Pluto, travelling
3.26 billion miles over nine and a half years and hit a target ten miles across. That’s
impressive. While Newton’s law of gravity works in just
about all familiar situations, it fails in regions of very strong gravity or for very
precise measurements. That’s where Albert Einstein comes in. In 1915, he released his
theory of general relativity. General relativity was a very different way at looking at the
universe and the nature of forces. Einstein’s theory threw away the idea of
forces like most of us understand them. Instead, his theory showed that what we call gravity
is actually the bending and distortion of space and time. Locations with a concentrated
amount of energy bend space more than ones with a low concentration of energy, and mass
is a concentrated form of energy Einstein’s equation is monstrously hard
to solve, but I’ve written it down here. We see that it’s basically G with some subscripts
equals T with some subscripts times a bunch of constants.
The G side describes the shape of space and the T side describes the distribution of matter
and energy in space. The equals sign is key. It says that the distribution of mass and
energy >>equals<< the shape of space and time…well, times some constants. A famous physicist by
the name of John Wheeler summed up these equations with the phrase “Space-time tells matter
how to move; matter tells space-time how to curve.”
Now there are people who don’t believe in general relativity. But general relativity
has been tested and there is no doubt that it works very well. One prediction of general
relativity is that the path that light follows should be bent by gravity, even though light
has no mass. The second is that clocks in strong gravitational fields should run slower
than ones in weak gravitational fields. The first impactful test of general relativity
occurred in 1919 and demonstrated the ability of the Sun to bend the passage of light. Sir
Arthur Eddington took advantage of a solar eclipse to block the light from the Sun. With
the Sun blocked out, he could see the stars behind it. He knew where those stars should
be, both if relativity was or was not correct, and he saw them where they would be if Einstein
is right. There also have been several tests of how
the strength of gravity affects clocks, but I have a favorite. Researchers at the National
Institute of Standards and Technology in Boulder, Colorado had two extremely precise clocks
– so precise that they lose about one second every 3.7 billion years – that’s about
one second over the lifetime of the Earth. Gravity on Earth isn’t uniform. It’s strongest
at the surface and drops at higher elevations. Using these ultra-precise clocks, the researchers
could raise one clock by a single foot of distance, and it ran faster than the one on
the ground. This was an amazing technical accomplishment.
On a more practical level, for anybody who has been lost and had their phone save them,
they’ve got general relativity to thank. There is a constellation of satellites circling
the Earth. Each satellite contains a very precise atomic clock. Together, they are called
the Global Positioning System or GPS. A receiver in your phone listens to the signal from the
satellites and figures out your location with a precision of ten feet or so.
That precision depends crucially on Einstein’s theory of general relativity. If you didn’t
take into account the effects of relativity, they’d get your location wrong. In a single
day, GPS would say you were six miles away from where you actually are. This is the most
familiar and practical demonstration that Einstein was right.
General relativity is weird to be sure. And it is certainly counterintuitive. But it works
very well – well, except for tiny distances. But that’s a story for another episode.
In fact, I think this quick introduction to modern gravity is probably enough for one
day. Let’s see what questions our viewers had.
[Music] Hi there everyone! It’s question and answer
time, but before I get to that, I’d like to thank everyone who contributed a good (or
bad) science dad joke. I promised to call out my favorite. There were many fun ones,
but I ruled out any I’d heard before. It was very hard, but my favorite was one by
Brent Kreinop who told us that a tachyon is just a gluon that isn’t completely dry.
I liked that one. A close runner up was one by Dipsoid, who
asked which had more calories, a burger or a steak. The answer was a steak of course,
because a burger is in its ground state. Then there were a couple of jokes that when I read,
I made a face like this. One of them was so bad, I thought it deserved special mention.
Icaro Bruno asked what we called a function with a series of cat faces raised to powers.
A polinomeow function. That one hurt. Several people noted a few times where I made
some mistakes and spoke incorrectly. Emmett O’Brian mentioned both. Yep. They happened.
Sorry about that. It was mildly embarrassing, but it could have been worse. I could have
said that pineapple belonged on pizza. >>That<< would have been truly embarrassing.
Suyash Verma asks if neutrinos and antimatter neutrinos have no electrical charge, what’s
the difference? Hi Suyash. That’s a good question and harder than you think, with a
series of ever-deeper answers. The first answer is that neutrinos spin so that if they are
coming straight at you, they appear to rotate clockwise. Anti neutrinos rotate counterclockwise.
And the two particles can annihilate into a Z boson. That’s the quick textbook answer.
Now the more subtle answer is that only neutrinos and antimatter neutrinos with those spin patterns
interact with the weak force. It’s possible that there exist neutrinos and antineutrinos
with opposite spin patterns, but, if they exist, they don’t interact via the weak
force. There’s another subtlety. To keep the answer
short, let me talk only about neutrinos. Antineutrinos are just the opposite, but the idea is the
same. Neutrinos are called left handed particles because if you point your thumb in the direction
they are moving, the fingers of your left hand wrap in the direction that neutrinos
spin. And, as long as neutrinos have no mass and travel at the speed of light, this is
always true. However, if neutrinos have mass, then it is
possible to go faster than they do. This means if I saw a neutrino moving to your left, and
someone moved to the left faster than the neutrino, they’d see it moving to the right.
The spin direction wouldn’t change, but that would change the neutrino from a left-handed
particle to a right-handed one. But a right-handed neutrino doesn’t exist. Right-handed antineutrinos
do. So this is complicated. In fact, we are not 100% sure that if neutrinos
and antineutrinos are different, or the same. If a particle and antiparticle are different,
they are called Dirac particles. If they are the same, they are called Majorana. And neutrinos
could be either Dirac or Majorana. So, Suyash, the bottom line is that there is still a lot
to learn about neutrinos and antineutrinos. Rich Sposato asked about right handed neutrinos
and whether they are antineutrinos. Hi Rich. So I answered some of your question in the
previous answer, but there’s more. We don’t know if right-handed neutrinos exist, but
there is a theory that says that they do and they’re massive. Another piece of that theory
suggests that they could be dark matter. The short answer is that we don’t know everything,
but right handed neutrinos aren’t antineutrinos. Sid Gaming asks if neutrinos can explain the
matter/antimatter asymmetry. Hi Sid. Maybe. There’s a theory called leptogenesis that
ties neutrinos to the asymmetry. It also requires massive right handed neutrinos that don’t
interact via the weak force. I made a long-form video about leptogenesis and the link is in
the description. It seems that right handed neutrinos have popped up in couple of questions.
They might be the subject of a future video. Frankness777 asks if massless particles experience
time. Hi Frankness. It’s an interesting question, with answers that can be sketchy.
The answers revolve around the fact that objects without mass move at the speed of light. The
equations of special relativity work for speeds lower than the speed of light and fail when
they reach it. So, at some level, any answer I give is based on equations that we know
don’t apply However, the equations work for all speeds
less than the speed of light, say at 99.99999% or something. And, for those, as you go faster,
time slows down and objects shrink. Accepting that trend and extending it to the speed of
light, massless objects experience no time and something as huge as the universe shrinks
to having zero thickness. Accordingly, it appears that a photon crossing the universe
is everywhere at once, at least according to its perspective.
Like I said, the equations fail at exactly the speed of light, but it certainly appears
that massless particles experience no time and no distance. Weird.
DasItMane asks if spacetime is quantized or if we assume it’s smooth.That’s a great
question and I’m issuing another IOU on that one. The answer is borderline philosophical.
I’ll answer it in an upcoming video. Petr K asks about a incongruity. Scientists
say that neutrinos can pass through five light years of lead, but can only pass through a
tenth of a millimeter of neutron star stuff. How can they both be right? Hi Petr. Good
question and sharp eyes. Here’s the deal. Neutron stars have a density of about ten
to the nine power kilograms per cubic meter. Lead has a density of about ten to the fourth
power kilograms per cubic meter. Neutron stars are 100,000 times denser than lead.
So, everything else being equal, if a neutrino can pass through a tenth of a millimeter of
neutron star material, it should pass through only ten meters of lead. That all makes sense.
But it neglects something. The probability of neutrino interactions depends on energy.
Low energy solar neutrinos interact far less than high energy accelerator ones. Depending
on the energies you’re considering, high energy neutrinos can interact millions or
billions of times easier than low energy ones, or even more.
Bottom line? Neutrino interactions depend on total amount of mass encountered >>AND<<
the energy of the neutrinos themselves. If you combine both effects, the seeming incongruity
just goes away. Peter Riis asks for the logic connecting electric
potential and mass. He gets special points for invoking one of my favorite movies. Hi
Peter. Luckily, the connection is straightforward. Energy and electric potential are connected
through the charge of a particle like a proton or electron. Take a proton and release it
in a one-volt electric field and its energy will be one electron volt. Put it in a million
volt electric field, and it will gain one million electron volts of energy.
If you’re making particle accelerators using voltage, you see the obvious connection between
electrical voltage and electron volts as energy units.
The connection to mass is also straightforward and it’s Einstein’s famous E equals m
c squared. If energy has units of electron volts, then mass has units of electron volts
per c squared. Since mass units of electron volts per c squared
gets multiplied by c squared, they cancel. Since they cancel, scientists just get lazy
when they talk about mass units and simply drop the “per c squared” thing. That’s
all there is to it. OK, so that’s all the time we have for questions
today. There were some good ones. If you’re enjoying the series, please like, subscribe,
and share. I’d like to see the numbers go up. And they should go up – after all, we’re
talking about physics and – well, as you know – even at home, physics is everything.