We're a bit tighter in this room than where we are in the other
room, so we will, well, we'll all be friends.
I think we're alternating. We're more or less alternating between
this room and the boys room from from day-to-day. So keep track
of the application to where you are
after the confusion, technological confusion of the
beginning of last time didn't get all the way through chapter
one. So what I'm going to do is quickly touch on the last bits
of chapter one.
I'm going to go over them quite quickly,
not talking them in as much detail as they as they would
would deserve, because as I said last time, there are sections
that you'll want to go back to in in coming weeks when it when
the the point of you know why do we fight about time, why do we
make a fuss about clocks becomes a little clearer. So I'm
introducing these ideas to you, what you to have them in your
head at this point. But the the texture of them will will arrive
later, I think
so, so quite promptly I I I expect to go on to chapter 2 in
this section. So, so
I'm just warning you that this will feel a bit rushed because I
I expect you to be going back to this later.
The last thing we did more or less last time was talk about
this quick question of measuring times. And
rather counterintuitively, I said that the only
observer of the three involved whose time we are interested in
right now, we put up with answering the question what is
the time of the event in this frame? The only observer the
relevant is the observer who was stationary in our frame and Co
located with the event.
OK, the driver has a can describe a time to that event
because they were Co located with the event within the
sneezing
and they were stationary in that frame. They were sitting
stationary in the car. So there are two frames here just to
drive this point home, 2 frames here, the motorway and the car.
They are both perfectly good frames.
Different people are stationary in them and and different
people. And there are two people Co located with the event.
The policeman and the driver are both Co located with the event,
even though they're in different frames. OK, so we're imagining
the sneeze, the driver and the people all in the same the same
spot of the same location and location. Even though you know,
because it's a car driving past, they're not literally in the
same spot. But for the purposes of this, they are the same
sport.
I will watch up. You signing somewhere else?
Um,
watching this happen is not
a way in which we measured the time of that event.
Partly because if we see that event happen, things like we've
got to wait for the light to get to our eye, so there's an extra
correction would have to apply. But also because as woke become
clear, the question of what is the time, what is the time?
Becomes a complicated question
and we are simply avoiding that question. You know in a good way
by saying the only time that matters is a time of person
collocated with an event. So I'm seeing this again and again and
again just to drive it home that the time at which only happens
at the time of an observer Co located with the event.
So you'll forget that and you get confused. But I've said to
you several times, so bank those up and listen to them in your
head,
intended to come,
so that how we measure time,
we make a big fuss about it.
How do we measure distance? How do you measure lengths? And this
also is something that will
become
a bit more complicated
that you think that you expect.
If I wanted to measure the length of this bench here,
then there's a variety of where they could do it.
I could get a a tape measure
or he'll get better stick and lay it out. And there's also the
ways I can think of doing it.
But the way we are, you will think of of measuring. That
is if I get me and a lot of friends line it up along the
front here in previous years I've made this an audience
participation but
activation front bench you if this is a a coordinate frame, it
is room, it's a coordinate frame.
Then this observer here has you know, you know your coordinates,
that you're this far from this wall, this far from this wall,
this far from the ground. You know the same.
Everyone knows their coordinates in the room,
and there's another observer at the other end of the bench
who knows their coordinates. And you know and the timings are
one.
And the way we measure the length of the bench is we
arranged beforehand that everyone will walk. It will will
look at the bench at a certain time
and everyone I said tape ever looks at the bench and the two
people who are at the end knew it down. I was at the end of the
bench
and then we subtract the coordinates.
OK and the length of the bench is.
This was every coordinates mine, this was average coordinates,
but that seems a very long winded way of doing it. But it's
very precise. It's very and it's precise enough that we can
really think, think through what what happens here.
It's over. It's over complicated for this further set of
situation where the bench is just sitting there. There are
all sorts of other ways we could do that.
What about the case where the bench wasn't just sitting there,
it was flying through the room.
OK, so it is zooming along
at some relatively speed.
How then
do we measure the length? What is the length of the bench in
that sense?
And there's a variety of ways you could think of doing that.
But the way that hangs together, and the way that it is
productive for relativity is this we do basically the same as
we did before.
So imagine you're all sitting where you are
and the bench is moving, moving past you.
OK, so you're standing clear.
And as before we say, OK, everyone,
get you synchronise your watches.
At a certain time we're going to look in front of our, in front
of our noses
and as this bench, you fly through the room,
everyone goes
do, do I see a bench in front of me?
And depending on where the bench is, some foot wouldn't.
Some thought would
so we see that the first person who spotted where we saw the
bench in front of them and we see the last person who saw the
bench in front of them and we surprised those coordinates
and that's the length of the bench in this frame
seemed over complicated.
But the point that the crucial thing is at the same time,
OK, so we prearranged what time the observation were going to be
made. Everyone made an observation just in front of
their nose. Do I see the bench or do I not and we collected the
data afterwards
and that the the the reason why we're doing it that way is
because it makes it clear where time comes into it.
Everyone made the observation at the same time in the with their
synchronised watches and everyone in the in the room
has synchronised watches,
OK and and we can talk about the the, the, the process of of
doing that. It's interesting, but
play parenthesis
and that and that of the very other other options you might
think of. That is the way we measure the length.
What we'll discover is that gets a bit more that that that that
has interesting consequences when it comes to
thing moving at Russia 6 speed. So I'm not going to explore
those consequences right now. I'm going to explore them
shortly in probably chapter three or more Lecture 3,
but this is the picture I want you to have in mind.
We have among the people standing on a platform that we
could hear an awful lot of trains going through stations.
People are standing on a station platform. They've marked off
the coordinates along the platform. So there's the origin
of the X axis is at one end of the platform, and there's a grid
on the platform. There are two observers are stationary.
They're standing on the platform and the train goes past at a
certain time. The same time
they observe where the train is, you know, where trained and the
and the two extremes 2. At the end of the train, subtract the
coordinates and that's the length. That's what we are
taking to be the length of the train in the in that in that
frame.
We're going to explore that more in a bit, but I want to log that
with you right now as what we mean we have, we have a we we do
we we mean a very specific thing. We'll talk about
measuring a length.
Any questions about that?
And
and and and and sneak preview. The reason why we are making a
fuss about this
is because it turns out that the question of of of simultaneous
is what is what turns out to be more complicated than we
thought. We're measuring the end of this, of this train end to
this bench, whatever at two points for just simultaneous in
our frame.
It turns out that isn't A-frame dependent thing. That that is a
is A-frame dependent thing. It's not a frame independent thing.
Different frames
have different notions of what is simultaneous
and that's where a lot of the there's a lot of stuff about
length contraction possibly comes from.
So that's a picture of a, I think an engine change odometer
that I I talked to the the
observers and their
distance measuring sticks. I think that was a sort of
imperial distance measure in
available now. The other thing I want to talk about is clocks.
Clocks are nice and simple things. Of course you have one
on your phone, on your wrist, or on the wall.
We're going to abstract the notion of a clock. A clock is a
thing which tells you the time and nice and complicated way.
What is the time? It's a
um,
this is where you know the whole thing can start to go off rails
and people get off go. What is time and all that stuff? Time.
Time is a distance. Time is a distance through time. We're
going to be very simple minded about what? What time is it?
It's. It's
how far from in time from something you have moved. And a
useful image, I think is this thing and this thing is it's
called
Ohh God, what is
And
a tough real log.
If you are on a sailing boat and you want to know how far you've
sailed, then one way you can do that is to a propeller behind
you.
And as it pulled through the water it turns and you discover
how much water you've you've moved through and you can use
that to navigate your position if you have GPS or whatever. And
that's a picture of old fashioned taffrail log, and I
think it is useful to have in mind the idea of a clock as a
taffrail log. It a clock tells you how much time you have gone
through.
And again, this sounds as if I'm making things over complicated.
You know how How is that a complicated notion?
It's. It turns out to be important, and it turns out that
we have to be precise about it
because at the
the question of how much time have I moved through?
Is very straightforward from the point of view of being a clock.
It gets more complicated when you want to step back and talk
about things moving around at speed.
So again I want to lodge that thought in your head I
in in the next chapter and one after that we'll we'll we'll
start to use that
and and see why we have to be so precise about that. So I'm I I'm
this this this whole first chapter is a whole a whole bunch
of of of sneak previews
and you may see mention of the clock hypothesis. The clock
hypothesis sounds grand is just clocks are unison simple the the
they they don't they don't malfunction you we we assume
that clocks are working that they don't get suddenly break
when you got when they got high speed or with their accelerators
That's obviously not true all clocks but the the the clocks we
imagine our heads for the purposes of this of this whole
study are uncomplicated and and and people are careful enough
about this whole area people need to see that
OK
I think almost you're getting good progress here
then one of the one of the last points is as you saw that same
are we in the right place.
No we are not
and I was all I was saying
Yeah
that we can talk of
frames yes see X&Y and a
observer
stationary in that frame
with an order order there and coordinates X&Y.
And we could talk of
another frame,
it's primed with coordinates X prime, Y primed and Z prime and
T primed. And we presume there will be observer, one or more
observers stationary
in that frame
and that frame
is moving at speed V in
with respect to the frame S along the X axis.
And that is that's a setup we're going to, we're going to see
that diagram again and again and again in one other variant. OK.
So get used to it.
That setup is called standard configuration.
So when I say 2 frames are in standard configuration, that's
the picture that jumps into your head,
plus
the constraint that at time zero
time t = 0. That's the time on the watch of the observer in S
the
the origin of the moving frame.
At the origin of the
stationary frame,
the origin of the
moving frame is there as well. So sorry that that makes our
over complicated. The origins coincide at time t = 0 we we we
set the clocks and set the the coordinate systems so that the
origins are the same. At time t = 0. So at time t = 0, T prime
is equal to 0 as well and the true origins are at the same
place.
OK, so that that, that the, the, the, the setup, that everything,
all the equations we end up using not too many of them and
presume. OK. And so
and
there is
basically there's approximately in round numbers, there's
approximately 1 relativity question and it comes to the
exam.
It's Here's a puddle
cast into standard configuration.
Here are some coordinates you know extract from from from the
the some the corners of the various events. Identify the
events. Work out what they're they're those coordinates are in
the other frame.
That's what so many of the exercises about is basically
what the what the all of the exam questions are about. You'll
see that again and again and again
and and and it sounds, you know, why is that important? You you
might ask him. We would. Do we have to worry about the
coordinates of this event in of in two different frames?
Specific that specific problem? No. But that's the the tool that
we use, that the mental tool we use to work out things like the
the the way that Russia's momentum works, relativistic
force, Russia energy and and so on. So that that toy problem in
a sense is the one that drives all the all the all the other
things.
So step one of all of those
that variance of that more same puzzle is draw diagram, put it
inside configuration.
OK. So again, you'll see this lots and lots of times I'm
seeing it now to say it's important make sure you're
you're more or less have that more or less right way up in
your head.
OK.
Yeah.
Um,
what's the definition of that?
I'm also going to mention,
as I said, I I hope I made clear last time the notes and the
lectures are the same material, just delivered in different
ways.
I hope the lecture is a bit more vivid than the notes, but the
the notes are more, you know, more careful than the the the
the the the precise wording is quite carefully thought through
and revise from year to year based on what people have found
puzzling.
So there's a sort of fast and slow stream of the material, but
another source of material is books.
The library has books. Wonderful thing.
The web has stuff stuff on it,
not always equally good, but I will mention a couple of books
which are on the which, some of which are on short loan in the
library and are available for as ebooks.
So if you go I I see that there's a link in the middle to
the A book list which in the notes to a booklist which it was
actually turns out to be out of date last year's. But on the
Moodle you'll find a link to the book list books at the library
or something. It's called and that's a link to recommended
books and including electronic texts.
The Colonoscopy is the A2 course recommended book. It talks
relativity. It's not wrong. I don't find it terribly exciting
the way it talks relativity. And there are and it makes some
slightly
aspect notational changes, but still worth reading to get a
different way of of making sense of this.
Tyron Wheeler is an excellent book. It's a book though you
it's worthwhile looking looking through that. It's a book you
read all of the more or less. I think it's a great book but a
bit of a proposition
that book Rindler if you're getting if stuck in puddles and
but deep subtleties here you know what is time and and so on
then I might well you say go and have a look at that I might in
the notes points will bits of of of renderer could render is very
careful about things like that
it's a book that goes on to well beyond this course but for for
deep things that that that's useful. The book called French,
which is very old fashioned but in portable, quite a good way
and I think quite different from from me. There's a long way of
saying read other things.
Don't know,
I think. I think the way I teach relativity is the best way,
but that's the way it makes more sense to me and I am not you.
I hope you will agree that the way I teach relativity is a good
way, but other views are possible
so I don't I I encourage you to to to to find other ways of
thinking it through. Be aware though, there are some there.
There's more than one routine, not all of them are completely
compatible, and there are a few rotational differences, so just
be a bit alert to that when reading other other sources.
OK, that's the end of last week's lecture
a while ago into Chapter 2. Are there any questions?
Everyone has
any worries,
he thoughts.
OK, I'll take that as a I mean, my essay isn't terribly good,
but I don't think I would put hand up. I'll show, I'll show
it. I don't mind audience participation within limits.
OK,
so
I mentioned
and the last time that the entire physical physical content
of this course is comes into two axioms.
And by physical content by I mean things about our universe
that could be otherwise.
The mathematical statements are, and we admit, a bit of the
philosophy of mathematics here. Mathematical statements are true
if the things that the
are based on are true. So there's a there's a logic to
maths. You can't. If you agree with this statement, then this
deduction from it, you can't disagree with. So those are
mathematical statements.
Physical statements are things about our universe that could be
otherwise.
So gravity, you know,
if he goes me, for example, you can imagine a universe where
that wasn't true. And before Newton, Aristotle did imagine a
universe where that wasn't true and everyone went, oh, that's
fair enough. That's clearly how it works.
So Newton saying
using first law in second and third law, we're making a
statement about the world. He using a statement. This is about
how our universe is,
although it could be otherwise.
And the two axioms are
relatively interesting because the the the physical content of
it is so tiny
it is a large chunk of
of relatively going to learn. For the first five chapters, C
consist of just these two bits of physical information.
And that's amazing. I mean, relatively is is weird
and the most so parenthesis and most physical theories version
one of them when they were first produced by by Newton or Galileo
or Maxwell or whoever.
Yeah, I'm largely an intelligible and if you go back
to the original papers you know you they don't make sense. You
know because folks since then have gone. I know that's a
really weird weird way of explaining it. He had a much
better way of explaining it that that turns out to be a much more
productive way of doing so. So the original version of
Microsoft equations which underlie all of electromagnetism
are unintelligible. You can't read that paper unless you're a
historian in physics basically because it just doesn't make
sense. Why would you even think of it that way?
Relativity is,
I think, almost unique in the Einstein 1905 paper is still
basically readable,
and that's 120 years ago.
Einstein's version. One overactivity will basically
that it you know that's it done
and that's that's very strange. I mean that and and and and that
in a way what made Einstein weirdly good that you got the
right answer from the very beginning
and all this stuff but trains as well everyone talks about trains
through relativity because it's a very convenient. It's a great
example to use. Actually did that first in a popular book
talking about relativity.
It's enough enough
fandom,
SO2 axioms and the consequences.
Objectives, power, blah. And
I want to see
that we're talking about Einstein, Special relativity.
The principle of relativity, which is actually one, was not
Einstein's, but Galileo's,
and in the notes I quote a passage from
and from from Galileo's account which Book Awards and which is a
long party. We describes imagining
being below decks in a ship with all sorts of stuff.
Both. You could roll, roll around small animals, whatever,
and and they all do their thing
and if the if the boat then starts sailing out so it's
moving
at a constant speed,
everything works as before,
Agario said. Ohh, that's actually quite significant.
That's telling something about the world that you can't tell
you're moving
and that is the the the the that that Galileo is relatively
principle is
axiom one. You're like of Einstein's special relativity
that you can't tell you're moving. And so if we say the the
if we see that same thing and we pinned down your intuitions in
the the, the, the, the terminology we're just starting
to use here.
We're saying that if you are,
if you if you describe physics,
describe a physical process like someone throwing the ball
using the coordinates
of some understanding on on the harbour,
the ship goes past someone on the board could be throwing a
ball from hand to hand and it's uses laws work perfectly well.
You know things weren't going to parabola blah blah blah blah
blah. You know how to do that sort of stuff. So you could
describe it perfectly successfully using the X
coordinate along the harbour or or the train platform and the
time when you watch
and the person in the porch or on the train platform or on the
train
throwing the ball from hand to hand could do exactly the same
thing. They could also describe what's happening using Newton's
laws and, but they would be using the coordinates X prime,
which are the coordinates attached to the boat.
And both of these are good descriptions and it's very easy
if you know one of these
to to work out the other one. Because
for this boat instant configuration,
because the frame is moving at speed V along the X axis
at any for any event, the ex prime coordinate of that event
see the observer clapping the hands.
The ex prime coordinate of that is the X coordinate of that
minus the distance that the thing has travelled. This sounds
insultingly obvious. OK, I'm. I'm not telling you anything
sophisticated here. I'm telling you something you are very
familiar with.
I'll say that a lot, A lot of times as well.
A lot of one of the ways people get confused about relativity is
they they hear something like that said and they think, Oh my
God, that much more complicated than I thought.
It's not what we're doing, What I'm doing there in describing
something you do understand that you understood when you were in
school.
But I'm using the terminology of this rather elaborate
terminology of changes of frames and stack configuration to do
it. So what I'm doing is I'm, I'm letting you translate, I
mean you already understand, into slightly different
conceptual rotation. OK, so I'm not telling you anything funky
there.
OK.
And this transformation of coordinates
is known as the Galilean transformations. Galileo didn't
call that, and no one called it that until after people talked a
little bit relativity. In fact, they had to give a name to that
perfectly obvious thing, that perfectly obvious transmission
coordinates. OK, so I talk about the Guardian transformation. I
just mean that the obvious thing
and that is the guardian transformation written out
again. It looks like I'm making a meal of this,
the X coord X prime coordinate. So the coordinate of this event
in the moving frame is. Is that the why part? Prank coordinates,
how high up it was? Blah, blah, they're the same time as the
same the speed of the ball being thrown.
If I threw a ball from the back of the ship to the front or the
back of the train to the front, then it speed in the train and
its speed as viewed by the station platform are going to be
different.
Not surprisingly and and the different by that amount you
know the, the, the, the, the, the, the, the speed on the train
is the speed on the platform minus the speed of the of of the
train. Not a big surprise.
Nice and simple. Too simple even did writing down
sue relative principle. Is is actually one
quick question.
Suppose I have a fancy new cosmological theory that says
the special point in the universe. See halfway between
here and Andromeda.
And the gravity should constant G changes depending on how far
you are
from that point there. So there's a sort of a sort of, I
don't know what you call it, but that that my wonderful wonderful
idea. This solves dark matter. I I see if the graphic content
varies according to distance from the special point
to that idea of a chance of being right,
who is the chance being right?
Who didn't have a chance to be right?
Who had put the hands up yet?
How about we chat between you and I
whichever way you think that could be. Why?
OK,
how many folk would say that idea? Yeah, is a is a gore.
How may we see it? Couldn't work work.
OK,
that the reason it can't work
because if that worked it would violate the relativity
principle.
So that Richard principle that that that that that axiom you
can't tell you're moving had already done some work.
Why did it violate the right principle? Because if
if you if that theory were true, then what you could do is
measure. Measure G, Measure big. Not very easy. You could measure
big and and find a value for it and then you sort of think what
was a wonder about a bit and you measure again. I think I didn't
get right the first time. You measure it again and it's
changed.
You know, Either you made a mistake in your measurement or
you've got closer to this, this magical point in half. In other
words, you'll be able to tell you we're moving
so, so, so there are two principle, says
Ohh. There are multiple ways of of reframing the relativity
principle, and one of the books I point to in the in the
biography is a book about the relativity principle about, you
know, how you can think about this and what and what its
consequences are. Just that one axiom.
So it's already doing work just by by saying that because it
effectively says all coordinate frames are equal, you can't pick
one that's special, and you can't pick one that's absolutely
moving.
So it's useful to think back on that and think
and I think that's that's a very that looks rather silly question
but it's very productive question.
We've got two things puzzle about and if you make sure you
understand why they're principle is it's getting there
and the other thing that I'll that's important is the idea of.
Ohh go this is not I'm gonna get through all through all the
changing have to have to go faster and is
I, I, I I said that we can analyse physics in one frame and
and and equally well in another we'll get different numbers
the the velocity of the ball of the throwing ball will be
different in in in in these two cases but it would be the
there'll be the same you know but there'll be the same
physics. It would be the same equation.
For example, the constant acceleration equations are just
X prime equals
UT plus half a t ^2
as you don't recall,
and
the executive square
even apply the
the
gully and transmission to that
I will get.
So that's X = X X prime equals X
-, b T and we end up with. What is it?
And the X
goes P minus
then we say that X = X prime plus
PT equal to UT primed plus
half
a T primed squared. Because remember the the T prime equals
T was one of the other.
You need to go in transmission or X prime is equal to U -, v
she primed plus
half a
T prime squared.
So what I've done there
is I have changed frame,
I change coordinates using the Galilean transformation
equations. I've gone from the description of this accelerated
motion
in the platform frame
or the harbour frame whatever, to the description of the same
motion in the
train frame and I've got an equation which is different
because it has a different speed. Here I have the same
form.
In other words, the explanation of the acceleration acceleration
equations work just as well in the 2 frames. It's as if I just
swapped added primes to everything
and that looks what you're going. You know what the why
that surprising.
It's not surprising because you you've been you have you intuit
quite a lot of the principle. But the point being that the is
important here is that the description of of the physics
that's happening as described by an equation in the platform
frame
and the description of the physics that's happening as
described in the
train frame
are the same.
The equation has the same form, the number of different, because
the velocity has picked about a term from the speed of the
frames.
But the form of the equation is the same, and that turned out to
be a. That's your first look at a deeply important principle if
you're going to do general activity in 4th, 4th or 5th
year. If you stick with this area,
then that becomes
the the the principle that guides essentially all of
general relativity, The fact that that the form of the
equations has to say the same in different frames.
The court is basically we are seeing the coordinates you've
picked. The coordinates you pick don't matter,
they're just a calculation or two.
So the next thing that's interesting here is
you've heard of Maxwell's equations. Is that right? Yep.
Is that is that, is that right or is that wrong? I saw some
notes there, but there were. I'm not, I'm not. I'm not going to
ask you questions about them but you've heard of them and you
you very good and they they describe electromagnetism. So
they they unify all of the the electric and magnetic laws that
were developed through the century and that's a way of of
of writing them down. I'm not going to you know that that's
math beyond portable point but they are they are nice macro
creations are great. They were developed during the next
century and they were found very quickly to be, OK, that's the
answer. That's that's how electromagnetism works.
But the problem
which is that the speed of light,
it's sort of built into those equations
and people discovered that if you do
the Galilean transformation
on macro equations and that's a mathematical quite intricate
thing to do. But that's not the point. If you do this
transformation
that what you get afterwards
is microaggressions.
In other words, what that appear to be saying was the macro
equation that didn't work when you were moving.
But what that means is that you can tell if you're moving
because if that were true.
Then you could just do an electrical experiment. The
experiment and if maximum equations work then you're
stationary. If they don't work, then you know you're moving and
that breaks. Breaks the relative. France
and folk were disturbed by this because that's clearly
not right, because a radio still works when you're moving
light still works removing that electronically radiation.
So there was a big problem
at the end of the next century,
and this was highlighted in the very first sentence of of of
Einstein's 1905 paper.
It is known that Maxwell's equations, as usually understood
at the present time, making five would apply to moving bodies,
least asymmetries who should not appear to be inherent in the
phenomena. That's a long way of saying they break
the IT looks like Maxwell tweeted don't work is the
problem.
So at this point there are were four possibilities.
Either
Maximilian is wrong,
perhaps
could try. It didn't work.
One possibility.
Perhaps the relativity principle is wrong. Perhaps you can tell
you're moving in some circumstances. But that seems
very that's not comfortable motion because that seems so,
so, so fundamental to our idea of how the universe works,
pops the Galilean transformation is wrong.
Perhaps the the the this
thing here, perhaps that perhaps there's more to it than that.
Perhaps there's some new physics happening here,
and the answer you will not be surprised to discover it.
Migrations are right.
The relativity principle is right. Is the Galilean
transformation. That's wrong.
And special activity is the new physics that comes out of this.
So this, it turns out, is this bit that's wrong or assumption
that that was how you went from one frame to another. That's the
bit that broke
you know to everyone surprise and horror and and it it
although it's relativity was adopted as the answer and it was
agreed organically the right answer in a remarkably short
time. It was about a decade it took for basically everybody to
to be on board with saying ohh right that that solves the
problem. There were holdouts were a long time and there's an
interesting story to talk about about the whole thoughts, but
the the acceptance of what was in fact faced with
and in that paper.
I think it was on to talk about
example of the sort together with unsuccessful attempts to
discover any motion of the earth related to the light medium.
And this is the idea of the ether, because with one of the
ways of making sense of Maxwell equations and undergoing
transformation would see our maximum speeds only work in
this. There is a reference in an apparent reference frame, sort
of which is this idea of the ether, which is the the thing
that that electromagnetism wiggles in, in the same way that
water we wiggle and water
we waves on wiggle in ether and had strange properties. We
didn't really make a lot of sense. But there's another long
story there. But it was experiments such as the famous
Michael Small experiment, which we could, which you may have
heard of but we're not going to talk about. We did try to try to
actually detect that movement in the ether and failed
and failed again. And failed again and everyone was going
well. This can't fail. This can be wrong, but that is what is
being referred to here
and
and what
Einstein. Einstein then in the paper upgrades this principle,
relativity,
to not being just a statement about mechanical motion, but to
say all the national frames are equivalent for the performance
of all physical experiments,
so that it absolutely nothing you can do. It's not just that
this works from mechanics, but for trains going through
stations, it works for all of physics. There's no physical
experiments there, no chemistry experiment, no biological
experiment, no social experiment that you can do that works
differently when you're moving. And that's a very bold
statement, is saying we're not talking about Marxism here.
We're not talking mechanics here. We are talking about all
of physics.
Umm,
this. This will be a point to to to to to, to, to finish on.
So I'm
I'm standing here with a friend moving past me in a rocket. So
some some huge speed. Let's see.
Very high speed.
And I observe her watch to be ticking slower than mine.
Let's see it's a good it has magically broken because of the
rocket. But I unless don't worry how we observe that that watch
taking slower because that also the complicated thing. But one
way or another observed that what should be moving more
slowly than mine,
you just I'm going to tick, tick, tick. She's going to tick
take, take.
No.
These windows are too easy.
She could look and see my watch,
OK, and she would she see my watch moving faster than hers,
or slower than hers?
Who says you might watch moving faster than hers?
Who says you see my watch moving slower than hers?
Who had put the hand up yet?
I'm not. I'm not taking notes of Hubert the hands up the the
reason I was able to put the hand. But I want you to guess
when we're another just just to write the company self. So in
the last couple of minutes,
just talk to your neighbours about what that should be.
Why does it bother?
Yeah.
OK.
Think of the question again.
Whoops,
books.
I watched the movie slower than mine.
Who would see? She sees my watch moving slower than hers.
Who sees? You see my watch moving faster than hers,
fairly evenly split. There
the answer is
that although there's clearly quite quite interesting to
analyse
and
the watch must be moving slower.
So I see her watch moving through the main,
and she must see my work moving slower than hers, because if she
didn't,
then
one of us would know which one was moving
and that we find the right principle. And that makes no
sense. I mean, come on, how can my watch moving faster than
hers? And how Watch moving faster, faster than mine
That's so. So we don't. We have. We haven't even got the second
action yet.
Already the
first. The first action was telling us this extraordinary
thing about time
that that we can, the two of us can make observations of each
other, other clocks, and come to opposite conclusions. But what's
happening
and this sort of thing is why people go no brothers can't be
right. That doesn't make any sense. Oh my God, it does. But
you have to be quite careful. But what questions you're
asking? Because I said let's not worry about how we measure the
measure. How her watch.
There's a lot of missing out there.
OK
and and and so it's it's when we ask the question what do you
actually mean by measuring the other person's watch
that when you discover you can step through this and you can
come to something that does make sense and hangs together and and
that's what we're going to be doing in starting in chapter 3.
I have the other half of this chapter to go, but I will be, I
think displaying myself about taking more. So I'll see you
next Wednesday, next Tuesday.