Hi there, physics fans. In the last episode,
we learned about some common misconceptions about the Big Bang. It’s some crazy stuff,
right? One thing we didn’t learn about was the first moments which the universe began. That’s
a big topic and sounds like a great subject for this week’s episode of Subatomic Stories.
When people hear about the Big Bang, they often think it means more than it really does.
And we scientists have to take some responsibility for that, as we often are a tad sloppy in our
language. But let’s be a bit more precise now. The Big Bang theory really only states that the
universe was once smaller and hotter. It is currently expanding, and the expansion has been
going on for about 14 billion years. But the theory of the Big Bang says
nothing about what caused the universe to expand. It says nothing about the moment of creation.
Technically, it doesn’t say that there was a moment of creation. And it certainly says
nothing about before the universe began, or even if words like time or before even mean
anything in such circumstances. Indeed, the version of the Big Bang that relies on general
relativity says that such ideas are meaningless. All the Big Bang theory really says is that the
universe is expanding from a smaller and hotter state. That’s it. But that’s also
an unsatisfactory state of affairs. We’d like to know why and how the universe began
expanding and whether it was inevitable. We may never know the answer to the why
question. Maybe an upcoming video will explore some of the ideas that have been proposed.
But when we turn our attention to the question of how the expansion began, our understanding
is on somewhat better footing. And the answer arises from some unexplained observations.
So, what are those observations? There are two big ones. The first is what is called
the horizon problem. The basic idea is simple. It boils down to the fact that there is a spot over
that way which is 14 billion light years away. That ignores the expansion of the universe,
which we shouldn’t do, but the expansion doesn’t change my core point. So, I’m ignoring it.
Okay, it has taken fourteen billion years for light to travel those fourteen billion light
years from that point to us. Since the universe is 14 billion years old, what we see is essentially
the birth of the universe. Now let’s take a point fourteen billion light years that way.
The situation is the same, with the light from the birth of the universe just getting to Earth.
But, if light from the birth of the universe coming from that direction
is just getting to Earth, then it means that the light hasn’t had time to get that distant location
over there on the other side of the universe. Essentially, two places we can see have never
seen each other. They have never interacted. Yet they look exactly the same. The microwaves
we see from one side of the universe are indistinguishable from the other side.
The only way that this makes any sense at all is if the two sides of the universe,
which the standard Big Bang theory and limitations of the speed of light say were
never in contact, were once in contact. This is a real problem for the Big Bang.
The other issue has to do with the shape of space. It’s flat. Now flat, in the sense I mean it,
is that it is like the space you learned about in geometry, where the sum of the angles of a
triangle add up to 180 degrees. It might surprise you, but that’s only true if space is flat. If
space were two dimensional, it would be like the surface of an infinite table.
How we know space is flat is a bit complicated, but the clearest evidence
has to do with the size of slight variations in the cosmic microwave background radiation.
If space were curved in any way, the size of the variations would be bigger or smaller.
Now space didn’t have to be flat. Again, using a two dimensional stand in for the universe,
space could be like the surface of a sphere, or the surface of a saddle.
It could be the surface of a torus, which is a fancy word for donut shaped. In fact,
space could have any sort of crazy shape. But it’s flat.
And that’s kind of weird. Flat is but one shape of an infinite number of possibilities,
and yet it is somehow special. Just why space should be flat begs for an explanation.
In 1980 or so, a physicist by the name of Alan Guth came up with an idea called inflation
to explain both the uniformity and the flatness problem. His idea is subtly different than the
traditional Big Bang theory. When the universe was very small and very young – at about a time
of ten to the minus 36 seconds, the universe began to expand at speeds faster than light.
The inflation period lasted for a very short period of time – only until about ten to the
minus 33 or 32 seconds or so. But, in that brief time, the universe grew enormously. At the end of
the period, the universe was at least ten to the 26 times bigger than it was at the beginning.
This proposed expansion explains those residual mysteries in the universe. If you take the most
complicated shape imaginable and blow it up by a factor of ten to the 26, the small bit you can
see will look flat. And two pieces of the universe that originally were connected could be separated
to distances that are so far apart that light can’t yet have travelled from one to the other.
The inflation idea explains both why the universe appears flat and why parts of the universe that
aren’t in contact according to general relativity can look so similar. It’s because they once were
in contact, and inflation ripped them apart. It also changes a little our idea of the early
moments of the universe, which we can break into three parts – the moment the expansion began,
the inflation period of superluminal expansion, and finally the traditional Big Bang era,
where the universe is expanding and governed by the theory of general relativity.
Of course, we’re not sure if the inflation idea is right. It offers a solution to these mysteries,
but there has been no independent confirmation yet. So you should regard it as a promising
idea that is well regarded, but not one that has been proven to be true.
I’m sure there will be questions about inflation and I look forward to them. Speaking of questions,
what questions did our viewers have, motivated by our episode about the Big Bang?
Before I answer questions, I thought I’d take a moment to acknowledge some unsung heroes.
I mean, I talk about all sorts of physics, but that physics would have never been discovered,
and I wouldn’t have known anything about it without great high school physics teachers,
who spend a lot of time trying to teach very complicated concepts
to some very young and distracted minds. For me, it was the indomitable Mr. G.
This comes to my mind because I got a letter from a physics student from Australia, named Oscar.
He and his class sung the praises of their teacher, a man by the name of Mr. Lambden.
Your class thought enough of him to email me and tell me how great he is, and I thought it was
the least I could do to let him know that they all think that he made a difference for them.
So, to you Mr. Lambden, and all physics teachers out there, without which there would be no
research to talk about, thank you all. Okay, so now for questions.
Audiman Audiman asks if it is just sophistry to say that a particle is its own antiparticle.
Hi Audiman. Kind of, but not really. Think about numbers. Negative one is the opposite of one.
Negative two is the opposite of two. But what is the opposite of zero? Negative zero,
of course. But negative zero is the same as zero, so the negative and positive are the same thing.
The same is true for certain subatomic particles, for which matter and antimatter is the same.
Oliver Twist Kostanza asks how it is that we can see an object that is 1/10,000th the size of a
proton? Hi Oliver. Well, there are a couple of ways to answer that question. Perhaps the easiest
one is to remind us of how we can see anything. We do so by having light interact with matter. Light,
of course, has a wavelength, with higher energy light having a shorter wavelength.
Now one of the things about wave theory is that only objects bigger than a wave’s wavelength can
be seen by that wave. The shortest wavelength light is about 400 nm, which means that the
smallest thing we can see with light is 400 nm. If you need to see something smaller than that,
you need to use ultraviolet, and then eventually x-rays and gamma rays. That’s how it works.
Now in quantum mechanics, it’s true that all objects have both a wave and particle nature,
and the wavelength is inversely proportional to the object’s energy. So here’s what is going on.
The highest energy particle beam in the world is at the Large Hadron Collider. It has an energy of
6.5 trillion electron volts. If you convert that to wavelength, it is…at least in round
numbers…about 1/10,000th the size of a proton. Consequently that is the smallest size we can
resolve using data from the LHC. Good question. Mister, uh… I don’t know how to pronounce that-
asks what is the difference between preons and strings? Hi mister. There are a couple things.
To begin with, preons are thought to be the next smallest particle after quarks and leptons. In
contrast, strings are thought to be the smallest objects possible. There could be a lot of levels
between the two. There could be preons, pre-preons, pre-pre-preons, and so forth.
The second thing is that preons are thought to be pointlike particles, just like quarks are,
but smaller. Furthermore, they have the same sort of wave/particle duality of all known particles.
Strings are thought to be extended objects that vibrate, with the different tones of vibration
corresponding to the different particles. And, it’s also important to remember that both
strings and preons are currently unconfirmed ideas. They both could be false leads.
Ca24 tamie asks if we could build an antimatter particle beam weapon to
vaporize everything. Hi Ca24. Yes. Yes, we could. The laws of physics allow it. But
before you get too excited, there are some real limitations. For those of you who took chemistry,
there’s a number called Avagadro’s number. It is six times ten to the 23. That’s the number
of atoms required to make a mole of something. And a mole of hydrogen weighs a single gram.
The most productive antimatter production facility on the planet was at Fermilab.
And, at its peak, it could make about ten to the eleventh power antiprotons
every hour. So that means to make a gram of antiprotons, it would take about a trillion hours.
In round numbers, that’s about a hundred million years of running, assuming nothing broke down.
In fact, the antimatter production facility ran for about two decades and, during that time,
it made about enough antiprotons to warm a five gallon urn of coffee from room temperature up
to a drinkable temperature. That’s about 40 liters for the metric crowd. The bottom
line is that this idea, while physically possible, is not even remotely realistic.
Isaac Williams asks if there is any connection between dark matter and dark energy.
Hi Isaac. The short answer is I don’t think so. They’re different things. The name Dark
Matter was coined back in the 1930s by Fritz Zwicky and the original term was in German.
The term dark energy was coined by Michael Turner, an emeritus professor from the University
of Chicago, before he moved to sunny Southern California. He admitted it to me when we were
on a radio show together. The only link between the two is they’re both really mysterious.
Samuel Machado asks why when hydrogen and anti-hydrogen annihilate, the outcome is
two gamma rays and not other stuff. Hi Samuel. I’m sure you got that from a previous video,
but I’m afraid that your understanding isn’t quite accurate. Hydrogen consists of a proton and
electron. Antihydrogen consists of an antiproton and a positron. When hydrogen and antihydrogen
hit, a bunch of things happen. The proton and antiproton annihilate,
and what comes out of that is a mess. All sorts of things can happen. When the electron
and positron annihilate, what happens is two gamma rays come out. It’s the annihilation of
electron and positron that I was talking about. Two come out instead of one because otherwise you
can’t conserve momentum. You need at least two. You could have more, but the emission
of two gamma rays totally dominates. Since the electron and positron aren’t
moving very fast, and photons move at the speed of light, conservation of energy and momentum
require that two photons are released, both with exactly 511 thousand electron volts of energy.
That’s the signature we look for when we look for lots of hydrogen/antihydrogen annihilation
and we just don’t see it. So that’s it. No antimatter galaxies.
Okay, so that’s all the time we have for questions today. If you enjoyed the video,
please like, subscribe, and share – and be certain to hit the little bell icon to get
notified about future videos. I hope your week is awesome and be sure to spend some
time thinking about amazing physics because, after all, even at home, physics is everything.