Hello, and welcome to the Physics World weekly
podcast. I'm Hamish Johnston.
It's book week here at Physics World. And
over the course of three days, we're presenting
conversations with the authors
of three fascinating and fun books about physics.
First up is my Physics World colleague, Michael
Banks, whose book Physics Around the Clock,
Adventures in the Science of Everyday Living,
starts with your morning coffee and ends with
a formula for making your evening television viewing
more satisfying.
This episode is supported by the APS
Global Physics Summit,
which takes place on March
2026
in Denver, Colorado
and online.
At the largest physics meeting in the world,
you can join thousands of physicists,
students, and policy leaders for a week of
connection and collaboration.
Immerse yourself in the cutting edge science that's
shaping our shared future,
and be a part of the global physics
community
driving innovation
forward.
Explore the meeting at summit.aps.org.
Why do Cheerios tend to stick together when
floating in a bowl of milk?
Why does a runner's ponytail swing back and
forth
rather than up and down?
These might not be the most pressing questions
in physics,
but getting to the answers is both fun
and provides insights
into important
scientific concepts.
These are just two examples
of everyday physics that Michael Banks explores
in his new book,
And he joins me here in the Physics
World studio.
Hi, Michael.
Welcome to the podcast.
Hi, Hamish.
So, Michael, before we explore your book, and
some of the amazing anecdotes,
physics anecdotes that you've got in there, I
thought I'd ask you about,
the the the book itself. What motivated you?
What inspired you to write the book?
Yeah. So the book is about the the
physics of everyday life. So this is kind
of physics you might come across in your
daily routine, you know, starting off, you know,
breakfast,
all the way to bed.
And I've always enjoyed writing about topics that
you might call everyday science. You know, this
is, research about topics that you kind of
might come across, whether it's a Cheerios effect
at breakfast or the dribbling teapot
or things like that.
And so those kind of topics have always
kind of excited me and I've enjoyed writing
about. And actually regular readers of Physics World,
will may well recognize the regular column called
Quanta, you know, which features these kind of,
research stories, you know, about everyday life. Yeah.
I thought some of them looked very familiar.
That's right. Yeah.
The actual the book itself is organized into
12 chapters,
and that takes you all the way from
breakfast to breakfast itself when you look at
things like, you know, how the physics of
even the physics of eggs, cooking,
as I mentioned, the Cheerios effect, the Brazil
nut effect, Physics of Coffee as well. You
know, coffee is a favourite of, physicists, definitely
a physicist's favourite beverage.
So it starts off with the morning routine
looking at, you know, you take your dog
out for a walk,
when it gets wet and it carries out
a wet dog shake, you know, how it
manages to expel all that water
and, you know, help itself dry off.
Then it moves into the daytime, so certain
kind of daytime activities you might carry out.
So say, you know, physics of you might
come across in the garden.
If you're playing sports, for example, you know,
things like the Magnus effect or spinning balls.
And then it kind of finishes off then
into the evening. So looking at some of
the physics that you, come across,
your favorite takeaways, like pizzas,
physics of pasta. Pastas are definitely a big,
another big area of research that physicists,
work on. Physics of champagne, that might not
be a daily occurrence of champagne, but, you
know, the physics of, bubbles in certain beverages,
there's a lot of interesting physics that goes
on there.
And then, yeah, so then it basically goes,
you know, from breakfast to bed all the
way across. And it was really, you know,
interesting. And also, you know, writing the book,
it was, a lot I had a lot
of fun writing it because there's a lot
of, like, fun anecdotes in there about researchers
who come across all these different different ideas,
in their daily research.
Yeah. I mean, it really sort of,
touched a note with me. I mean, right
from the beginning because, you know, like a
lot of people, I'm pretty useless in the
morning,
without my first cup of coffee. Well, actually,
without my first two cups of coffee. You
know, as you pointed out, coffee is is
is a fascinating thing. You know, there's some
it's it's a granular material,
and you're you're sort of forcing hot water
or steam through it. There's a lot of
physics there.
I've got a steam kettle at home, and
I think you touch on,
on the physics of of the whistles in
a in a steam kettle, which I thought
was very interesting. But so so of all
the things that you've written about quite a
few aspects of of the physics of coffee,
what what do you think is the most
interesting
little tidbit that you've got in the book
about coffee?
One of the interesting aspects is actually when
you grind coffee.
Some some of us, if we have,
you know, like a bean bean to
cup type of machinery, you'll kind of put
the beans in, they'll grind, and then the
hot water presses through. I've got some people
just like to grind beans anyway.
But one interesting thing though is about,
the static electricity that's produced during the grinding
process itself.
So, actually, some researchers studied this in in
details. There's a,
Christopher Hendon,
a researcher, a geochemist in The US. He
was doing demonstrations at his university.
And,
some volcanologists, so people who study volcanoes,
were kind of intrigued by this demonstration of,
coffee grinding that he was doing.
And they kind of asked him, oh, yeah.
What do you what do you think about,
you know, teaming up to to look at
what happens when you actually grind the coffee
beans themselves, what's what the process that's, occurring
there. So we teamed up with these, researchers
and they they actually discovered that there's actually
a lot of static electricity that was produced
during the process itself. That's, like, kind of,
the metal
blade grinding up the beans at, you know,
incredibly high speed.
And what this actually happens then is basically
the coffee
actually clumps together.
So you kind of, you get the, obviously,
the very little kind of minuscule grains, but
then you also get kind of bigger clumps
due to this kind of static electricity.
But the actual implication of that is that
then the water doesn't pass fully through the
grinds properly because of this clumping.
And that actually
results in a weaker
brew.
So basically, by this static electricity from happening,
the water's kind of like looking for an
easy escape route through the beans, which or
through the through the grounds,
which it then can manage to do if
lots of it is clumped together.
So one technique actually, which is happens in
the coffee industry itself, is to add a
little bit of just a little touch of
water
to the beans themselves.
And when they're in then being ground,
they don't clump so much. So then it
kind of, like, can negate the statics in
some way. So Hendon and colleagues, they then
carried out experiments where they added some water
to it and then to then measure the
resulting static, and they found, yeah, there wasn't
very much static at all. But actually, when
they then looked at how much kind of
coffee,
kind of dissolved into the into the water
itself, they found actually there was much more
consistent
and a lot stronger brew than that resulted.
So so if you do actually manage to,
grind your beans at home, if you just
add a little kind of splash, just a
drop or so of water, you know, you
may find that,
that, you know, that the coffee is much
more consistent as a result. And of course,
like coffee
is it is a very experimental beverage. You
know, there's lots of different parameters you can
change. You know, you can change the
size of the coffee grounds,
the amount of water,
the temperature.
There's lots of different things you can,
change. So actually when I had a chat
with,
Christopher Hendon,
about, you know, about how to improve your
coffee extracting technique. You know, he was saying
that it's very difficult because there are so
many parameters to change. That basically is a
way of just, you know, you kind of
almost have to test it out yourself, you
know, you change one of the parameters,
see what the resulting
brew is like, the output is like, you
know, and then if it is something to
your taste, then, you know, maybe you can
tweak something else. You know, so it's kind
of about playing around a little bit. Yeah.
Yeah. I mean, one interesting thing about coffee,
and I suppose you allude to it in
the book, is that there's also different types
of coffee makers, isn't there? And they all
seem to work in in different ways. You
know, for example, I'm a big fan of
the, you know, the classic Melita
filter,
where you stick the filter on top of
the cup and you pour in your boiling
water,
and you get your coffee. And I think
I think I read somewhere that that's how,
you know, professional coffee tasters, that's how they
judge a coffee. They use,
like a traditional paper filter. But then you've
got,
you know, you've got your your the the
sort of espresso maker that you have at
a cafe where the the steam is forced
downwards
into the coffee. But then you've got the,
you know, the the classic stovetop.
Is it called a mocha? Moka pot. Moka
pot. Where the steam comes up.
And, you know, it's and then you've got
other
well, the press and all these other sort
of crazy ways of making coffee. And, I
mean, it sounds to me like,
you know, in a sense, if I know
how I like the coffee,
I'm not I'm not sort of being
fusty or frumpy or picky about how I
make it because there's so many different variables
that
I know how I like it, and I
know the coffee that I use, and the
the filter gives me the best
coffee possible.
Yeah. That's right. I mean, actually, Christopher Hendon,
he actually uses the, filter as well. That
was his actually preferred way of, so maybe
there is something in that. But when you
talk about the yeah. The moka pot is
an interesting one because
it's actually quite the physics that happens in
a moka pot is actually really complex,
because you have this kind of different,
you got the temperature of the water, but
then you got the the steam the pre
the pressure of the steam, kind of the
vapor pressure of the steam pushing the water
up. But then it actually changes that kind
of the granular nature of the coffee bed
itself.
So, actually, researchers have also studied, the moka
pot in in various detail.
One interesting thing about it was that when
they
so when they put cold water in the
in the bottom compartment and then put it
on the stove, then heated it, and then,
obviously what kind of the output was,
when they actually took the temperature of the
the output, it was only about the output
water as a coffee.
It was actually on it wasn't actually very
high in terms of the actual temperature itself.
And then they reflect on that that maybe
it wasn't extracting as much of the coffee
as it could have done. So instead, what
they did was they heated the temperature, the
initial temperature. So instead of putting cold water
in, they put kind of, you know,
temperature water of about 40 degrees or so.
And then they actually found that a lot
more coffee was dissolved if they did it
that way.
So one tip could be to actually don't
use cold water in a mock up, but
actually use, you know,
like lukewarm water or something like that. I
I could find it very difficult to convince
some people
about that.
Actually, you know, moving right along to tea.
And of course,
you know, I think, a lot of people
would be outraged if you put hot water
into your kettle to make a cup of
tea. You you also look at the teapot
effect, which is sort of a dribble
of of liquid that comes down the spout
and, you know, annoyingly
goes onto your countertop or tablecloth or or
whatever.
And and
one of the takeaways from from that anecdote
is that
gravity is not a factor
in the production of this dribble, and it
would happen
in outer space. I suppose, you know, the
the the English,
astronaut Tim Peake, when he has his cup
of tea, would,
would produce a dribble. And and so so
why is I I I find it difficult
hard to understand why gravity isn't involved in
this dribbling process. Apparently, with the teapot effect,
the way gravity actually
affects the teapot effect is where it actually
detaches from
the the spout itself. So it's not effect
it's not an effect of actually the teapot
effect in terms of dribbling down. It's more
about where it might detach from the underside
of the spout and then dribble down.
But, yeah, the teapot effect is something I
think it was like the nineteen fifties where
researchers started working on that.
And they discovered then it was various
effects such as, like, surface tension, etcetera,
that was behind it.
But it's only anti in recent years, I
think in the February
where,
I remember seeing the paper itself actually when
it was released where it said researchers have
come up with an ideal version of the
teapot effect where they've solved it at last.
And basically that what they found was this
the teapot effect is not so much about
gravity, but it's more about
surface tension, actually the capillary effect as well,
like another way. But, actually, the physics of
the teapot effect is actually
quite complicated, and it's,
quite involved in terms of what the actual
process processes are going on. But the but
that that they effectively found that this was
a a universal theorem
of the teapot effect and now the,
teapot effect is solved. But as you mentioned
before about,
you know, the kettles, one of the
one of the interesting aspects of the the
the noise the kettle whistle. So the noise
that's produced during the boiling during a kettle,
which is quite an interesting area. And that
was,
that was a a researcher in Cambridge, actually,
who just who who found out
that basically no one had really ever studied
how the noise in the teapot whistle was
produced. You know, it was like one of
these things that, you know, people just heard
have heard it for decades, but no one
really knew how it was actually produced. So
he looked at it in great detail with,
you know,
various,
microphones and cameras, etcetera.
And what they discovered actually was that basically
when this this kind of stream of steam
goes through the teapot whistle,
It enters the first so the teapot whistle
is basically two plates with a small circular
hole in each of the plate.
So that kind of steam initially goes through
that little hole, and then
it kind of enters into a jet basically
when he goes through this first hole. But
that's not actually the the actual sound that's
produced that you hear isn't produced in that
cavity.
What they actually found is that that when
the steam then goes through the second hole,
it kind of then turns into vortices.
So this kind of jet
powers out of the second hole, produces these
kind of vortices, and that is actually where
the noise is actually produced.
And that's a technique called vortex shedding.
So it's actually the same physics
that
happens when you have, like, telegraph wires in
the wind.
They're kind of, like, moving in the wind,
and they might be, like, producing a noise.
Oh, yeah.
So I checked exactly the same effect as
what's happening in the teapot,
whistle.
And it's the same thing that you might
hear, like, you know, if you're driving a
little motorway and you have roof bars on
top of your car, and it produces that
characteristic whistling noise from the roof bars. It's
the same, like vortex shedding. So where this
kind of kind of wind is passing over
a certain object and they're producing these kind
of vortices
that produce a noise. But it's, yeah, it's
just really fascinating kind of research. You know,
that's kind of a classic example of this
kind of everyday,
you know, phenomena. And then you kind of,
like, find new things about, you know, how
to build things like that. And and sort
of moving
on, into I suppose you've had your breakfast
and,
you know, you're moving on to shave. I
don't know. Either your face or your legs
or both.
And
you've got something
about razors. And I I can remember when
this story,
appeared,
on the Physics World website, so it made
me laugh to see it again. And,
basically, the question is, well, you know, a
razor is made out of a very hard
piece of steel.
And so how on earth do fairly soft
whiskers
or hairs
dull the razor? And I I can remember
when this story went up on the on
the on the website.
And at that time, we had comments
on the website. We don't anymore.
But,
you know, I remember this story went up,
and it gave this wonderful
explanation.
And somebody wrote in and said, oh, you're
you're being dozy.
The reason they get dull is so they
can sell more razors.
Exactly. And I thought, okay. I can't argue
with that. So,
yeah, I suppose there might be some truth
to that. But why why on earth does
a a very, very sharp,
and and,
hard piece of metal
get dulled by, you know, a fairly soft
piece of hair?
Yeah. So that's what another one of these
life's mysteries, isn't it?
Yes. A razor blade's,
mostly made of steel. And, of course, the
actual hairs on your face, they're around 50
times softer
than the materials of razor blade itself. So
you might think, how on earth does something
that's 50 times softer end
up blunting,
you know, something that's so hard such as
a razor blade?
And that's what these researchers at at MIT,
in The US,
were very keen to understand.
So they actually,
carried out a number of experiments,
in which, you know,
kind,
students plucked
individual hers
from their face, and then they got this
blade basically to cut,
just a single or very few, you know,
very few strands of of hair itself.
And they looked under that at that process
under a microscope, you know, very powerful microscope.
But they were amazed to discover that even
single hairs like that could actually end up,
chipping,
the blades themselves when you look under, you
know, very,
you know, very,
powerful microscope.
So that kind of really
surprised them about how on earth, you know,
what is the process that's happening to, you
know, such that a hair can actually blunt
a blade.
So they looked at it in much more
detail. They got, did a lot more experiments
on it. And they actually discovered there was
kind of three different types,
of process that was happening when a when
a hair was being cut.
So one example is,
where the hair
basically there's like, there may be small imperfections
in the blade itself during the manufacturing process
where you have little cracks, so it's kind
of really unavoidable that you've got these kind
of small imperfections.
So if a hair kind of then
slightly hits one was a crack, it's like
kind of slightly hits one side of it,
that is enough to eventually kind of chip
the blade if it hits if it is
a side of a crack.
Another
possibility is that in the angle itself, so
if a
if the blade is kind of perpendicular to
the hair, it should generally
avoid cracking
or chipping.
But if the angle is actually an if
the,
blade is an angle to the hair
itself, then that can actually that also could
they found it could also result in in
chipping.
And another,
the third option,
that they that they discovered was actually kind
of, inconsistencies
in the material itself. So in in homogeneities
basically where
the material was kind of softer, say, in
one part than it was in the other.
So if, again, if the hair was then
if the hair hit that softer part, it
would then basically end up chipping,
the blade.
So that's what they discovered. They discovered these
three,
these three kind of processes that are happening.
But when it comes to, as you mentioned
before about, you know, does the actual industry
want this to kind of happen,
one way of getting around it is to
have is to produce a material that's more
much more consistent. So it doesn't have those
certain kind of softer and harder regions. You
know, it's more,
more, I mean, homogeneous,
so that the that those kind of chips
won't happen. But in terms of actually for,
you know, stopping the chips altogether, it's gonna
be very difficult because you will always have
these kind of little cracks
in the material itself, which will then
even be even, you know, hers can end
up chipping, you know, end up chipping those
blades.
But actually the researchers at MIT are actually
at the moment, they're they're developing
new materials, which are,
you know, more
more consistent,
in the blade. And I think, from what
I heard, anyway, is that they, you know,
they are looking at developing products where,
you know, blades could be more consistent, and
that may end up, you know, increasing the
lifetime
of certain blades. Right. But but, of course,
that was kind of, like, you know, disposables,
etcetera. I mean, you could
you could use a blade where you sharpen
it. That's one option of getting around it,
where you would purposely sharpen the blade.
Like an old fashioned straight razor. Exactly. An
old fashioned straight razor would get around that
problem because it would then end up, you
know,
making it much more sharper each time.
But yeah, that was quite an interesting,
discovery actually, but there's kind of different ways
of
of firing us. And the same goes for
things like cutting a cutting a knife through
cheese. It's just exactly the same process, though,
where effectively cheese itself could blunt a knife.
And it's the same very same things,
but these kind of small imperfections
can result in, you know, a failure of,
even something so hard as a a steel
knife.
And and sort of staying on the, on
the subject of hair,
you you you also look at,
runners' ponytails.
And I think, you you know, if you've
seen someone with a ponytail running,
normally the ponytail sort of swings back and
forth.
And
if you think about it, a runner is
sort of going up and down.
So why isn't their ponytail
oscillating
in the horizontal
direction rather than the the vertical direction? But
the
clearly, there's some sort of coupling going on
between,
those two degrees of freedom. So what what's
happening there, Michael? Why do runners' ponytails go
side to side rather than up and down?
Yes. It's actually not only I mean, ponytails
is one possibility, but also, like, if you
have flight if you find you have, like,
a hoodie on sale, which has got
tassels, you'll find exactly the same thing. So
if you walk
at a at a brisk pace,
you might find as you're walking,
the tassels on your hoodie
are not swinging off, like, you know, back
and forth, but actually swinging from side to
side. So you're kind of thinking exactly as
you said,
how on earth,
when when your head is, you know, head
is moving up and down,
you know, why why is your your body
moving forward, and why are the tassels moving
from from side to side?
And it's actually a bit of a strange
story behind it, but it was,
it was someone who was actually working on
the kind of three body problem in terms
of the,
in terms of astronomy
and kind of the relationship between the sun,
the earth, and the moon. And it was,
various kind of equations that they produce from
that.
And somehow that could actually then be used
in terms of the physics of how a
ponytail moves. And it's to do with a
couple as you mentioned, the couplings between
the the kind of what's swinging and then
what's its kind of anchor point.
So they discovered that if there's kind of
a,
the frequency
is
a doubling.
So say if you're walking, you've got a
certain frequency that your head is moving up
and down, your,
your body's moving up and down.
If there's kind of a relationship between the
between that length and actually the length of
the ponytail itself,
then basically, if it's if it's if it's
a double wing, then basically any small
deviation
between
that up and down movement and side to
side, basically then it it expands, basically. So
if you've got a very tiny deviation
that you're slightly going to the side,
what happens is that basically then expands exponentially.
So that's what you see then, basically, is
because, you have this kind of,
coupling in your stride length and your between
the actual coupling of the of the ponytail
itself,
it's basically then it expands and you get
this kind of,
exponential increase in the swinging from side to
side. It's only a very
small thing that then results results in something
that you actually kind of see.
And so is there any up and down
motion in the ponytail?
Or is it only moving side to side?
Or is it there's is it much more
complicated than just going side to side? It's
sort of going
well, it is going up and down. Right?
Because your head's going up. Yeah. Exactly. Because
your head yeah. Yeah. Because your head's going
up and down, not the link.
So the ponytail is going, yeah, up and
down and side to side. Yeah. But
it's only that because you have that very
small,
tweak in it going side to side that
it kind of expands and ends up going
way, you know, really far side to side
because of that kind of doubling relationship between,
the frequency of the head movement and the
and and the lens of the ponytail. So,
yeah, it's kind of all in the mass
of that. But, yeah, it's quite interesting that
yeah. Something that you see when you see
someone walking around and they're or or going
for a run, you see their ponytail wafting
around. It's, you know, it is all because
of those
it came from the mathematics of working out
those relationships
between,
the sun and the moon.
And sort of moving on from running to
to driving a car, I mean, I think
one thing that's really fascinated physicists for for
years, I suppose,
ever since the first traffic jam occurred, I
think physicists have been fascinated as to why
why they occur. And then, you know, there's
a particular kind of traffic jam that I
think any driver is familiar with.
You're driving along, and the traffic suddenly comes
to a halt for no good reason.
There's no obstruction on the road, the speed
limit hasn't changed,
and then the traffic jam breaks up and
you move along for, I don't know, half
a mile, and you slow down again. And
there doesn't seem to be any rhyme or
reason to this. And I think when the
physicist looks at this,
I think they're seeing some sort
of emergent behavior,
some sort of there's gotta be a phase
transition in here and, you know, lots of
interesting mathematics.
And, I think you look at some of
that in the book, don't you? Yeah. That's
right. So one of one of the issues
is about, you know, the annoyance that you
have on daily life of coming up against
traffic jams, whether it's on the motorway or
even just, you know, out and about in
town.
And, but one of the, yeah, one of
the interesting aspects is how traffic jams can
occur.
Even when even in the absence of, you
know, anything that you may expect can cause
a traffic jam, such as, you know, roadworks,
a broken down car, for example.
You know, traffic jams can seemingly just occur
for no reason.
And the kind of
reason for it is because,
you know, even if a driver kind
of just takes her eye off the road
for a split second, for example, you know,
and then just kind of a lack of
concentration,
what what can basically happen is that that
that then has a kind of a spillover
effect. So you have someone who,
say they lack a bit of lack of
concentration,
might veer off a little bit to the
road, they break,
then the person coming behind them then has
to react to that, they break, you know,
the person behind and it's basically a domino
effect where you then you're effectively creating,
kind of a wave within the traffic itself
of people just breaking
and then accelerating off.
So yeah, physicists have studied these kinds of
waves and the physicists call these waves,
jamitons.
So basically, they just kind of happen happen
for no reason. And that was kind of
the theory about how, you know, waves could
actually form. But there were these researchers,
in Japan actually, who did an actual experiment.
Basically, they got,
a number of cars on a circular road,
just kind of a few 100 clump a
few 100 metres,
in circumference.
And they put all they told the drivers
is said, look. Just drive along the circular
road and just keep going round, you know,
as many times as you want. Just keep
going.
And they actually discovered that
traffic jams just occurred naturally. So, you know,
people would just drive along,
and then they just,
Basically, they probably just, you know, they just,
some kind of something caught their eye and
they just became distracted. They then broke and
then basically they saw this exact kind of
jamaton wave form.
Even during a very, such a simple experiment
as that way, you have basically circular road
and cars on it.
So that was basically the idea that, you
know, it doesn't take
traffic, it doesn't take a car to break
down or,
road works or anything like that. But even
on a road where there's kind of a
number once you get a certain density of
cars on the road, then you can get
these, jamaton waves forming.
And, you know, that's basically,
you know, how these waves can actually form.
And it's, it's quite interesting research, but there's
also, like, a lot of different aspects about
driving around the city,
that are interesting.
There's one one particular interesting point that's been,
covered in the book where I talk about
the physics of parking
a car on a one I mean, it's
on a one dimensional road,
which is, you know, a classic your classic
physicist,
you know, taking a problem down to its
bare minimum.
But then they look at, you know, how
how you what strategy you could come up
against. Well, if you have a one, one
basically a single road and you want to
get,
you want to park in the best spot
to get to a destination at the end
of the road, you know, what is a
certain strategy to do that?
So they looked at three possibilities. One was,
the so called meek strategy, where you basically
just go and park in the first spot,
end of
you know, but the issue with that is
that you park in the first spot, but
then you have to then walk to the
destination. So that takes time.
Another option was a prudent strategy where you
ignore that first spot.
You then see a group of cars,
in front of you, and maybe there's a
spot there.
Then you go into that spot. That's called
the so called prudent strategy.
And then a third strategy was called an
optimistic one, where you basically drive all the
way down to the,
end of the road,
turn back on yourself, and then go in
the first spot. So you kind of think,
well, surely the optimistic strategy is the best
because you basically get it's guaranteed that you're
gonna get the best spot.
But when the researchers run through the various
simulations, etcetera,
they actually discovered that the prudent strategy was
a better one,
because that got you in a spot that
maybe not was is maybe up the furthest
away,
but it's some kind of, you know, nearer
to the destination, then you can walk to
it.
And that actually has links with, the secretary
problem in mathematics.
But it's kind of the optimal,
how many how much do you kind of
ignore before you then,
park? So, yeah, if you're ever on a,
basically, if you're ever on a one dimensional
street, then the optimal solution to it is
to kind of ignore about half the spaces
and then park in the next in the
next one. Right. Then you've kind of got
about a 25 to 30% chance that you'll
actually end in the
best spot possible.
Gotcha. Yeah. And now I'm trying to imagine
how to make a u-turn on a one
dimensional road, but I'm guessing that they don't,
they don't they probably reverse bike, all the
way.
And then you've also looked at,
at other modes of of transportation.
And, you you know, you've got that classic
thing, the boarding of aircraft,
which I think,
you know, I've been I I suppose I've
been
I I've I've flown on aircraft for the
last, I don't know, four forty five years.
And I think,
you know, definitely the strategies that the airlines
take have changed over the ages, and maybe
they've refined them. Or,
and and
so you have a look at the at
the various different strategies. And then there is
a twist in the tale, isn't there? There's
a surprise,
at the end about,
a technique that you might as well use
because it's it's almost as good as,
as the actual best technique for boarding an
aircraft. So what what what have people
found when they've looked done research into,
into boarding aircraft?
Yeah. I think they can the basically the
bottom line is,
the kind of the current method of boarding
an aircraft,
you know, usually it's kind of back to
front boarding where you get all the people
on the back and then they altogether
unload, and then you get the next next
batch in. So you, you know, you board
it all the way to the front.
That kind of in terms of the speed
of boarding isn't the kind of ideal,
way. Actually, one one
possibility that is actually quite quick is actually
one that you might not think, which is
basically random. That's right. Where you basically just
randomise
people boarding and that actually in the simulations,
that people have carried out, that actually is,
quicker
than the current,
than the current method. But there is a
there is a one that a researcher,
came came up with,
which was,
the kind of the best tech best technique.
And that's kind of like splitting up the
roles itself. So you would, you know, you
go in one one, say 12, and but
then you don't board the next row 11,
you kind of skip skip one.
So it's a bit more kind of involved
than the
the way that airlines do at the moment.
But that was actually found to be the
quickest way. And they actually the researchers themselves
actually carried out an experiment where they got
a mock
aircraft, an actual
life-sized mock aircraft, and they tested out all
these various strategies,
including the, completely random one.
And then they discovered that basically, you know,
this,
this way of kind of splitting up roles,
you know, 10,
to what say, row twelve, ten, eight,
was actually
quick. But,
it seems like airlines actually are beginning to
look at the,
the procedures for for doing that. I think
there was something
about United Airlines,
in October
2023.
So they they they announced that they would
start boarding,
economy class passengers on, this method called Wilmer,
which is kind of a way of just,
you know, splitting up people in different in
different roles.
So, yeah, maybe, you know, maybe in future,
we won't be boarding just from the back
to the front, but we'll actually be using
these different different ways of boarding. You know,
because I guess, you know, at the end
of the day, you know, time is money,
isn't it? You know? So if you can
get people onto an aircraft quicker, you can
get away quicker, you know? So it does
have
financial implications. So you would think that airlines
would be more interested in,
you know, changing those boarding methods.
I mean, the interesting thing about when you
compare the, let's say, the Wilma
to the random,
is that
I mean, is there really a big difference
between those two strategies? Because when you think
about it, if you're doing it randomly,
if it is random, then if I get
on the plane, chances are the person in
front of me and the person behind me
are going to be
separated from me on the plane. So we'll
all have enough space to pack our stuff
away and quickly
get into our seats. So is I mean,
is there is there really any benefit between
Wilma and the random? Because I think I
think in the book, you say that
random is almost as Is it almost as
good as Wilma? That's right. It's almost as
effective. Yeah. That's right.
You would think in that sense
that it probably is better just to go
just to go random. Yeah. Because, I mean,
Wilma seems very
complicated and, you know, how you you know
what it's like. I mean, I just flew
back from Toronto and, you know, you always
get the situation where, you know, people who
are last to board
stand at the front of the queue
and basically block everybody else from getting on,
you know, which is very annoying. Actually, once,
I I took a flight. It was from
Toronto as well. And,
one of the gate staff actually stood there
and berated people
and said, you, what what group are you
in? Five. Oh, no. No. No. This is
three. Get out of here. Right? You know,
I was thinking, this is great. You know?
Unfortunately,
the the last flight, they they didn't have
somebody doing that.
But,
but, yeah, that is and, you know, it
is interesting because when I tell people, I
say, well, you know, physics says
that random boarding is is actually a pretty
good way of doing it. Nobody believes you.
But,
I mean, I I I don't know if
it's maybe it's because I'm a physicist. I
have a I sort of have a great,
you know, I think randomness can be very
useful. I mean, a classic one, is stuffing
a sleeping bag
into a sack. Right? Now you'd be very
tempted to think the the most efficient way
to do that is to roll up the
sleeping bag. But the problem with rolling it
up is that you're introducing
a preferred
direction.
Right? And if you don't get the the
length of that roll correct,
it you're gonna have a hell of a
time getting it into the sack. Whereas if
you just stuff it in,
it's random. And, you know, the sleeping bag
can randomly fill the space. It's a bit
like a glass, right,
I suppose.
Anyway, that's how I think of it.
But, yeah, maybe it's one of those perfect
things where randomness is. Because you wouldn't It's
your friend. You wouldn't exactly. You would not
expect a random
boarding process to be. You'd think it'd be
chaotic. You just need to be absolutely not.
But, yeah, you know, the experiments say otherwise,
and it is actually one of the best
ways to do it. So
And the the the there's a few you
you you talk about health,
in your book. And,
it's interesting that a lot of the the
things that you mentioned were were things that
came up during the COVID
nineteen
pandemic, where we were obviously very, very, very
concerned about the spread of,
virus particles.
And the the the one thing that you
looked at, and, you know, I have to
say it's made me think of toilets in
a different way ever since, is how toilets
are actually very good at
spreading aerosol
particles. So what what what exactly happens there?
This is when you flush the toilet, I
assume. That's right. Yeah. So it's when you
when you flush the toilet. And and, yeah,
this research was one of those ones that
basically came out purely from the COVID,
pan you know, pandemic where,
some researchers
researchers contacted this, a researcher in The US
who was an expert in kind of imaging,
aerosols
and, you know, the spread of aerosols.
And they were interested in, you know, looking
at the aerosol that's created when you flush
a toilet.
So there had been before then, there had
been some theoretical work done,
which showed that when you basically flush a
toilet, you know, the the water pours in.
It's quite energetic
process, and it flows into the cistern.
And then it can basically create like a
jet where,
it can basically just fling
particles out of this chaotic
vortex
like jet in the toilet in the toilet
bowl.
But that was all of that just kind
of, theoretical work. So these researchers,
were really wanting to find out whether this
is the case in terms of doing actual
experiments. So they,
basically filmed this toilet set up,
when they have all this kind of green
laser light around, so they could basically
visualise the plume that's created without actually disturbing
it. It's quite important that you don't
disturb the plume itself when you're actually
looking at it.
And what they found, you know, they told
me, you know, wrote about it in the
book, you know, really startled them,
that they literally found this kind of
these particles just shooting out,
like a rocket, basically, where you know, and
they were going basically about a couple of
meters high.
Where presumably you could breathe them in. Well,
exactly.
Contained
COVID,
COVID nineteen virus particles or some other nasty
Exactly. Basically, head height for most people,
they they could actually reach.
So, yeah, basically, by the process of flushing
itself is so you know, energetic that it
can create it can throw particles out, but
it also can create this kind of aerosol,
basically.
You know, a bunch of particles that then
can kind of spread around as well because
they kind of move around in the air.
So that's actually another another piece of work
as well looked at,
you know, where this aerosol could could kind
of how far it could travel.
And they seeded the toilet itself actually with
kind of bacteria, e coli, etcetera.
And then they flushed the toilet, and then
they took swabs,
swabs around the room.
So they kind of took samples around and
they found basically that the, that the process
of flushing an open toilet just basically
put, you know, put stuff e coli everywhere,
basically around is on the walls, on the
ceiling, it was on the floors.
So you might think because the result's that,
you might think, well, that's obvious. You just
close the lid. That's an obvious solution. You
know? What what could go wrong? But when
they actually then decided to do that and
did the experiments, they actually found that,
you know, it didn't help at all. Basically
more material was put on the floor, but
it even reached
the walls and the ceiling itself.
So basically there is no,
there's no real way to get around that
except, you know, of course, from disinfecting your
toilet quite quite often.
It's probably the best way of getting around
it.
But the researcher who did that way, you
know, they said,
they gave me a funny quote where they
basically said that after they did that research,
you know, they basically flush the toilet and
run.
That's
and also a bit and also to not
put your toothbrush anywhere near,
the toilet because basically, you know, that aerosol
is basically just going all over it. But
Right.
On that note, let let let's move on
to something much more cheery, and that's the
physics of champagne.
And,
one little tidbit. I mean, I suppose I
should have known this is is the incredible
pressure
that, champagne is under
when,
inside a bottle. It's what was it twice
the the pressure of a a car tire?
Yeah. Something like that. Car tire is around
two
bar. And, yeah, the pressure in a champagne
so the in the neck of the champagne
bottle can be around, yeah, around five bar
or so. So quite
and, of course, the pressure increases with the
temperature. So you have a chilled bottle and
the temperature's a bit lower, but it's a
warmer bottle and it's a bit higher. So
it can be quite,
actually, yeah, opening a champagne bottle can be
quite a lethal process because, you know, if
you've got all that pressure just in that
headspace,
and then it'll, you know, impact on the
cork, and the cork can and actually,
during holiday season, holiday periods, and you have
a lot of injuries,
where people, you know, hit cork. Cork to
the eye. Conte. Exactly. A cork to the
eye. Yeah. Yeah. Which you can can be
actually very dangerous. It's, you know, it's a
lot of,
you know, it's a lot of pressure that's
kind of, you know, producing that high speed
cork. Yeah. Yeah. I'm sure if it went
in the wrong direction, it could, you know,
smash a window or
take take out your your your favorite,
knickknack,
over the fireplace.
And and
one thing that you
you talk about is the that sound of
the pop, that characteristic
pop. And
am I writing in thinking it's a bit
like the a sonic boom, or maybe it
is a sonic boom? Yeah. That's right. So,
when you open a bottle of champagne,
basically, it's the the pressure,
drop that happens.
Basically, I have a huge kind of temperature
drop that happens when the champagne opens itself,
opens.
And then basically what can happen though is
you have the creation of, like, these MAC
discs
in the due to this huge temperature pressure
drop.
In temperature up, you have this, creation of
MAC discs. So, basically,
each time you are opening a champagne bottle,
you are actually kind of breaking
the soundbar in some sense by, you know,
the creation of these. It tends to be
just a single MAC disc up produced and
then basically kind of then kind of moves
up
into high you know, people have done various
high speed imagery on it. Moves up and
then it kind of moves back down into
the into the bottle itself.
But there's been various yeah. Lots of kind
of experimental work and actually theoretical work on
it as well where, you know, people,
have calculated that. You should actually be able
to see a few Mach disks created. It's
not just one.
Similar to what you see in a jet
fighter plane, where you see the creation of,
Mach disks and then Mach diamonds as well
in the exhaust plume.
A similar thing is kind of via the
physics of it. It's very similar to what's
happening when you open a bottle of champagne.
We have the
creation of these Mach disks due to that,
you know, incredible
kind of pressure drop that happens.
And the the other thing you look at
is,
bubbles
and
the bubbles that, champagne is famous for.
And the the thing that that sort of
fascinates me about beverage bubbles
is that they can vary. You know, a
a a champagne will have these tiny
bubbles.
And, I don't know,
probably get in trouble for saying this, but,
you know, a a lesser wine like, Prosecco,
you know, the bubbles will be a bit
bigger.
And by the time you get to a
fizzy drink,
you know, like,
Coca Cola or something, the the bubbles are
huge.
And, you you know, you think, well, this
is just liquid with
carbon dioxide.
Why are the bubbles
so different in size? And, you know, why
do we associate a high quality beverage with
small bubbles
and, you know, pop with with large bubbles?
That's something that's always fascinated me about
about carbonated drinks.
Yeah. Because with champagne, for example, it tends
to be, you know, a sign of the
quality. It's kind of the little kind of
bubbles that you kind of feel on your
mouth or your tongue when you're kind of,
you know, when you're drinking it.
But one one interesting thing about the the
bubbles that's create that you that you see
in champagne as compared to, as you say,
different beverages like
beer or Coke, for example.
One interesting effect with champagne is you get
these kind of bubble trains.
So you get literally
lines of bubbles, whereas in,
Coke, for example, you'll get them kind of
moving around. You know, they'll just be wavering
your own to the top.
So, yeah, you know, people have done research
about what why
does champagne have these bubble trains and other
drinks don't.
And that actually
all comes down to,
the kind of molecular makeup of the champagne
itself, and it kind of contains,
surfactants in the champagne.
And people have done various experiments where they've
kind of produced bubbles, and then they've kind
of adjusted the amount of surfactant in the
in the drink itself.
And, basically,
it's a result of champagne having more of
the surfactant molecules inside it.
That basically then is quite complicated,
you know,
bubble physics, but, basically, that results in all
these bubbles magically
kind of
forming a train
and coming one after the other.
Whereas other drinks, terra- nagal. Also has something
to do with the size of the bubbles
as well. Yeah. Mhmm. That's right. Yeah. So
that's all too, with that surface tension and
all and all those things.
But, yeah, whereas diff other drinks don't have
that level of surfactant in it,
which then, you know, results in the kind
of bubbles moving a bit more kind of
in a wavy way.
But, yeah, the bubbles is also where all
the a lot of the flavor and the
taste comes from,
in champagne. So we've got some popping of
those bubbles is what gives that kind of
magic flavour,
to champagne.
And and finally, Michael, I wanted to talk
about,
I mean, this is, I suppose, an all
time classic in terms of,
the, you know, the physics of the real
world, and that's the the broken spaghetti problem.
And essentially, the idea here is that if
you take a piece of spaghetti, dried spaghetti,
and you hold it at either end, and
you bend it until it breaks,
it almost always will break
into three pieces. No. That's right. Three or
three or more. Yeah. That's right. Into never
two. Well, yeah.
Very rarely two. And I think, you know,
when when physics students are told this,
you know, the first thing they do is
they go home and they get out the
spaghetti, and they just keep trying and trying
to break it into two pieces.
And, it's very, very difficult to do.
So
why is that? Why does why does spaghetti
break into three or more pieces and not
two?
Do we know?
Yeah. I mean, this is actually a question
that even flummoxed
Richard Feynman. He was,
he I think along with his friend, William
Hillis, they spent apparently, they spent hours,
just in the kitchen breaking spaghetti and, you
know, to desperately try and get it into
two pieces. But, you know, they always found
that it ended up breaking into three or
more.
But they never kind of come up with
an explanation of what why that kind of
happened.
But then actually some researchers,
at MIT,
they decided to look into the into that
process in a bit more detail.
And then they they came up with this
kind of elaborate setup where they had,
one end they had a spaghetti strand, one
end that was fixed.
And then it would then basically move into
another end. So it'd be basically just slowly
bend.
And they found actually that there is a
way of doing it,
that to create just two pieces, and that's
if you just do it very, very slowly.
It sometimes
can actually break into just two pieces. So
rather than just, like, you know, getting it,
but I'm just snapping it. You just do
it very slowly. But the ultimate way of
doing it actually
is to,
kind of slightly twist the spaghetti strand.
So the explanation they come with was to
do with all these kind of twisting waves
that happen
that basically dampen the effect of,
of the spaghetti wanting to break further.
So these kind of torsional waves,
by twisting it, these torsional waves stop the
spaghetti breaking.
So it only tends to break in two.
So they found that, actually, if you twist
the spaghetti on the one end,
I think it was around about two seventy
degrees, so not quite a full revolution, but,
you know, not not far off. And then
they actually did the same experiment twisted. It
always
more or less broke in two pieces.
And the reason for that is it is
because the creation of these when it breaks,
these torsional waves then dampen the effect of
further,
breakages.
And and do
do they actually know
why it breaks into three or more pieces?
The because I suppose, you know, when you
think about it naively, you think, well, you're
gonna bend it, and it will break somewhere
first.
And then why would it break again?
Or
why would it what what are the odds
of it breaking in two places
at the same time? You know, both of
those things seem
unphysical.
So are they
do neither of those things happen? Is it
something different? So So I think what happens
what they say is that when so when
you when you bend a spaghetti strand, it
creates these when they call them bending waves,
basically. And these propagate when it when it's
bent, when it snaps, these bending waves then
propagate down the strand, and then it kind
of re results in more,
breakages because of these kind of these waves
are produced. But that's then what the that
these kind of these twisting waves or the
torsional waves actually counteract the bending waves.
So that's the way then to get around
it is because,
the bending waves are dampened by this torsional
waves. Yeah. It's on, but, Well, that would
yeah. Yeah. That's interesting because, you know, it
just popped into my mind that,
if you actually take a big bunch of
spaghetti
and break it,
it will break into two. Every almost every
strand will break into two. So I suppose
when you're grabbing the spaghetti and,
sort of squeezing it together,
you're probably dampening You're probably dampening those those
breakages. Yeah. That's that's yeah. That's probably the
case, actually. Yeah. So yeah. That's one way
you're getting around it, maybe. Get the whole
packet and, bend it. Yeah. Yeah. Well, that's
I mean, I you know, I have to
say I'm I'm sure this is,
I've I've I've I've probably insulted the Italian
nation again
by saying that when I do cook spaghetti,
I do break it in half and cook
it.
Yeah, I've said
I I was planning on going to Italy
next year, but maybe Maybe not anymore.
My visa will not be granted. Yeah.
Well, thanks thanks, Michael. Thanks so much. And,
actually, there's more about, Italian cuisine in,
in your book because you've got a wonderful
section about pizza as well and how to
cook a perfect pizza. But, you know, I'm
afraid we we just don't have time to
cover that. So Michael's book is called Physics
Around the Clock, and it's out now. Is
that right, Michael? Yeah. I suppose it's out
in The UK,
now, and it will be published in The
US in April 2026.
Okay. And I'll put a link,
at least to the to The UK version
in the notes for the podcast. Thanks a
lot for coming on and talking about,
some wonderful physics, Michael. Thanks, Hamish.
That was Physics World's Michael Banks. His latest
book is Physics Around the Clock,
Adventures in the Science of Everyday Living, and
it's published by the History Press.
I'll put a link to the book in
the podcast notes.
Thanks to Michael for joining me today and
to our producer Fred Ailes.
And thank you for listening to this podcast,
which is supported
by the APS
Global Physics Summit.
To continue advancing physics beyond this podcast,
participate in the APS Global Physics Summit on
March
2026
in Denver, Colorado,
and online.
Experience
high impact
scientific sessions,
networking opportunities,
workshops,
and community building events
designed for every career stage.
Learn how you can attend and shape the
future of physics at summit.aps.org.