Hi, I'm Norman Green
and this, in case anyone having a little panic, is
the relative gravitation strand of astronomy too
good? You're there, we are.
Now there's a couple of this first lecture is all a little
bit weird because there's a bit of predatory stuff we've got to
get through about how I organise the course. And the the topics
I, I, I'm sort of gallop through in this first lecture seem a bit
disconnected and that's this are sort of disconnected. But they
are important important ideas that you've got to to get under
your under your skin really more or less over the rest of the of
the course. So you will come back to this material again and
again over the course of of these 15 lectures,
but we'll only really get properly going in the next
lecture
tomorrow.
So with that said,
that's the that's the wrong computer.
With that said, there are some things I want to make sure to
say
which are.
Um,
if you look in the middle
middle of the relevant page, you'll see that there are. I
will show you a picture of the middle
um,
just so we know what we're talking about.
Yeah,
you will see that there are.
The number of number of resources
under
under section 10 Medical section 10,
including the lecture notes
and I'd like to note
include
Introduction PDF,
axioms.pdf. And there are
some other versions of those which are the same but in a
slightly laid out slightly differently. In a way, which
might I I I think I think I aim to be more useful on a tablet.
Whichever one that's the same in both of them. Whichever one
works better for you, works best for you.
And that's a theme,
because these notes are fairly comprehensive. There's a lot of
text in there
and the way that I have ended up delivering these lectures is
that the main event is the lecture,
right? That's the the the the thing we focus on.
But
the notes others have Co main event if you like.
They are Co equal. With the lecture as the way I'm I'm
delivering this stuff to you because relativity makes a lot
more sense the second time you do it.
And if I do it in a lecture and I do it in the notes, you'd have
getting two goals at once.
And that's important, because it will only make sense the second
time you do it. Now, if you think notes, first, I'm gonna
read the notes and then just check what he says in the
lecture, that's good. If you think I'm going to sit back and
let it wash over me in the lecture and then check what's
happening in the notes, that's good too.
But you have to decide who you organise it,
OK? You have to sort of take charge of your own learning here
and work out how do I work out what's going on here?
And if someone like Sam Goldman of the of the movie mogul who
said half of the money I spend on advertising is wasted,
but I don't know which half
and half of what I say in the notes is irrelevant to you
because you don't need that bit. But the the half that's that
that is needed for you is different from for for for all
of you.
So the it's quite a lot of material here, a lot of pages
here.
Don't panic, you know it's supposed to memorise everything.
OK, now the the notes are there as a resource for you to go
revisit what happened to the lecture and make sense of it.
Second time, it follows that it's a good idea to look at the
notes before the lecture.
Now you have to understand everything, but you get an idea
of what to commit. This isn't a detective mystery. There are no
cliffhangers.
Yeah,
spoilers are good. If you know what's coming up, then you can
know that bit didn't make sense in the notes. I'll really pay
attention in the lecture because it'll be explained in a slightly
different way.
OK, I could go on at this for some length and I will post. You
know right now that that these slides are in there because for
reasons, but I'll generally post the slides after the
after the lectures. The slides unlike you know, some people do.
People do have different opinions about how to lecture
with slides. I don't like slides very much, so the slides I do
are pretty minimal. There really is something for me to stand in
front of or a diagram to point to.
There's nothing in the slides that isn't in the notes. OK, so
don't think Oh my God, if it's to work with those sides as
well.
OK. So the state-of-the-art of prop, A prop which I put, I mean
I don't think the right, I don't think there's much point in
putting them up here really but people want them, they feel
secure if they've got a copy of the site.
So I do OK, but I don't regard, I don't rate the site as a very
important part of the resource. OK
they're they're prop for me.
The E360 I I think that means this is being recorded.
I don't know where it goes after this echo through a bit of a
novel thing I'm not very sure what happens. I trust it works
and for the first couple of lectures I will do record record
them myself. And just as a bit of a backup to do we reassure
ourselves that doesn't work. Don't rely on that
because some people think ohh all the lectures are recorded,
just go listen to them afterwards. Doesn't make a lot
of sense because being in the lecture concentrating it a part
of the of the whole learning.
Umm,
the other big big big teaching resource is the exercises.
Each of the chapters in the notes has a block of exercises
which are pretty cute,
clearly linked to specific parts of the notes.
And relativity is something you only understand when you do the
exercises.
You cannot just sit back and and I think yeah, yeah, yeah, got
that.
You think you understood it when you sit in the lecture, but you
didn't.
Very probably not. OK, it's when you you you you're forced to
rethink it and and it forced to buy ahead in the table a bit
going through the exercises. That's where you understand it.
You go ohh, it's simple really
OK And that is also why it's in next semester that are released
the solutions and notes to the exercises, not the semester,
because OK is quite handy to check you've got the answer
right, but it is more useful, you know, controlling that to
bang your head on the table a bit. It's the it's the impact
that get the learning in. OK,
umm
but boom.
Um. OK,
other
Where are we?
I although it's terrifically all fashioned,
like the idea of aims and objectives, I think internally
learning outcomes are one fashion later and they're out of
fashion.
For me. The aims and objectives are very simple and are very
useful. Structuring notion
definitions differ, but for me the aims are the point.
They're why you're learning this
and why I'm teaching it. And there are things like
understand,
appreciate the the other things you'll remember
after you've done the exam in two years. You'll remember the
aims you because that that that is why you're doing this course.
And so if the exam were understand things, you went,
Yeah, then we'd all be a lot happier.
That'd be a nice exam,
but it doesn't really work in assessment terms. So
as well as the aims, there are the objectives,
and the objectives are party tricks,
the things you can do,
things that you can be assessed on.
They're not terribly exciting necessarily, but the best way of
achieving the objectives.
It's achieved the aims. If you understand something that the
the objects would become pretty easy, OK, but they are the other
other other nice concrete things which you can do. There are
things like provide descriptions of such and such,
describe as different an exam answer, blah blah blah and so
on.
So that's a very good distinction. I tend to associate
exercises with objectives. When I'm making up the exam, I look
at the list of objectives.
So that's it's not a one to one mapping, but there is a
correlation between what I think is what the objectives of what I
think is fair game in the exam.
Things which are clearly not objectives after the exam ask me
questions. But that's the that's the basic answer I'll give you
whenever you ask me that question
every time and moving on very rapidly.
Umm,
so. And my timetable for this lecture to this lecture is
timetables in a way that other ones aren't so much. It's
already all short. So
and we'll just press on.
So the
this market here because it's just backing up the the the
market made that a lot of this is for you to work out what you
need to read.
So there's there's more material than you might like
but that's because I'm sort of forcing you to to just to think.
You will probably have look look at all the words in all the
notes, but you your your study only says selected bits as there
are signpost aims and objectives are signposts. At the end of
each section, I tend to have a couple of key points, which is
also signposts is what you should have picked up from that.
You're very standard things.
So that's sort of the other thing I've got to remember.
Remember I was asked to say to you is there is no lab this
afternoon. I think you know that because I think you were told
that on Moodle. But to see you tracking up to Garscube, just
remember, no love this afternoon, OK? They start. I'm
not sure when they start. I don't have to do that, so I
don't
OK
any questions about the other sorts of organisational things?
Have I forgotten anything obvious?
Nope.
OK, so
onward to the physics bit.
I have on the next slide
put all of the physics you will learn in the next 10 weeks
because we do 10 weeks of special activity and five weeks
on general activity.
So all the physics you'll learn in the next 10 weeks,
That's it.
It's two sentences.
There's
on a national reference reference, performance of
physical experiments. It's called the principle and the
speed of light. The same constant value measured in any
inertial frame.
That's it.
Why many 10 weeks talking about that? Because those are easy to
state,
and those are the physical, the thing, the physical statements
you didn't know before. But the consequences of these are
intricate, and even understanding what those words
mean is a bit subtle,
and so there's not a lot of volume to learn in this course.
You don't memorise stuff. I have a rotten memory, so I don't
think it's fair to think to play you to memorise stuff. When I
can't, I can't.
So demonstrating that you understand these things is the
point of the objectives.
So there's not a lot to learn. And
this is not this is this. What is and isn't a mathematical
course.
The maths we we use in this course is addition, subtraction,
multiplication, division, square roots.
Later on we use differentiation,
but it's all school physics. It's all school maths.
There is no funky maths in this
course.
But in a different sense this is a mathematical course, because
the way we start things with there's there's no seat of the
pants intuitive. You know you'll get you know get this picture
and you'll understand. It doesn't there. There's no see
the fans intuitiveness in this in this course it's all about
this axiomatic approach. This is the fundamental the fundamental
insight which is expressed in logical form of feeling,
mathematical style of thinking. And then you move proceed
carefully and thoughtfully forward in a mathematical style.
So although the the arithmetic use is very simple,
the style is quite abstract and that is and and and that's I
think the first course you've done. Where that's true
First, your first year courses have tended to be more
physically
unless you know apart from say the pure pure maths courses that
you've done.
So that will be a bit of a jolt. OK.
Also, you have done a bit of special relativity before, I
think in physics one is. That's right, yeah, OK. And that's I
think 2 lectures and I'm sure the word Lorentz transformation
was mentioned and events and stuff. But that's just two
lectures. How can I spin it out to 10 lectures?
Because I want you to actually understand it,
and you can't do that without good without sort of going round
and round several times and and getting a bit confused. So this
is quite a thorough version of of special activity
that's. But these are the only
physical statements, only statements about the world. The
only thing about the world that could be otherwise that I'm
going to make all the until a bit later. All the rest is
deduction.
So
as I said, this first lecture is,
you know, dotting around a bit what what happens next is
Chapter 2, which I think is what we'll get through in one
lecture, is going through those axioms and think, what do they
actually mean? What do those words mean?
Chapter three and four, which I think is. I think it's 3
lectures basically, Roughly is but the field direct
consequences of those actions. OK, given those axioms, what
follows,
and there's quite a lot of followers pretty directly before
you get into anything, any, any, anything calculational. And
that's interesting. Those axioms, although they look very
simple, are full of physical content.
Chapter 5 is where we go onto the main apparatus.
The the main. The main, Yeah, the main international tool. Or
relatively how you how, given a A-frame, you're sending A-frame
that's moving past you. How do you say this event that happened
here happened in those coordinates, the moving frame,
blah blah. Those words will mean more in a short while
after that. That's that. That's sort of the the, the, the, the
the first pillar of the whole thing. After that we talk about
and and that we I think we'll spend 3 lectures on that. OK.
The kinematic dynamics is how do you describe motion, what how do
you talk about momentum and energy and conservation of
things in a relativistic context. You're familiar with
these things from physics. One,
What do these ideas look like in a realistic context?
And and and and that sort of thing. That's very important. If
you do particle physics for example,
and then chapter 8:00 and 9:00, the last five lectures is
general relativity. We cannot go into the maths of general
relativity because that is actually quite hard
and that's that's an honours masters course. But what we can
do is give an account of general relativity which is has much
more textured, is much more substantial because we'll spend
10 weeks, 10 lectures, sorry, and thinking through the
terminology and thought processes of special activity.
So there's a lot more substance we can say about generativity.
So it's it's beyond a popular account,
but we we can't get to the free technical account so we can get
to that intermediate range there
and that's and that's five lectures,
OK.
And
so I'm not gonna be, I'm, I'm gonna gallop through these now
really. But that's OK because I want to have said these things
to you at one point,
even though
I it doesn't sort of matter that I'm going to go through rather
too quickly because we're coming back to them again and again and
again in later weeks. So I'm sort of logging these in your
head
just just now, but you'll come back to these, you'll come back
to the notes for these bits again and again, and we will
talk over them again and again.
OK,
key thing. Number one, what is an event?
An event is
that
that happened there. It happened, you know, two metres
from the, from that door, from that, from that wall, 1 1/2
metres from the floor,
4 metres from there. And it happened
10 seconds ago
for the four numbers attached to it,
3 spatial coordinates and a time coordinate.
But notice that I said it happened from that wall. That
war, that war someone in the street
would give, would give different physical coordinates to that
event.
OK, obviously.
So the event is in some sense absolute. There's no, there's no
question that event happened, but the numbers you used to
describe it are relative. So the first one that you're going to
use that word a lot, that's the first time it comes in there.
They are basis dependent. They are coordinate frame dependent.
So my coordinate system in this room start for the order over
there say blah blah blah you you you can imagine 3 axes.
Someone standing in the street has a different coordinate
system. My watch I I said it happened 1010 seconds ago from
when it was. Someone might say it happened at rib and 11 1/2
hours from midnight.
So the zero of position and the zero of time is up to you. It's
up to the definition of the coordinate.
Another sort of event is.
I caught it.
I didn't miss it. I caught it. So how did I catch it? I I
caught it because my hand and the eraser were in the same
place at the same time
and it doesn't matter what coordinates you give to that
quote was brought to this room quarters relative to the to
buyers Rd.
They were in the same place at the same time,
and there's nothing relative about that. There's no point of
view
in which I didn't catch it. If two cars collide,
then metal is bent
and there's no point of view in which that metal isn't bent you
there's no way you can be moving in which you you look at it in a
funny way and and and they didn't. They missed each other.
So two things which happened which collide which are which
are at the same place at the same time are there. Absolutely,
there's nothing there. So why is he saying why is he saying this
again and again? Why is he making a fuss of this? It sounds
it's obvious, it's obvious, but it remains obvious if you like,
even when there's a lot of other things that you thought were
obvious aren't obvious anymore. So it doesn't need saying.
That that remains true.
So which of the following events?
Who says supernova explosion was an event
behind?
Who say it wasn't?
Who's the A concert with an event?
Who said it wasn't?
Who see the whole country clapping its hands at once? Was
an event
not
a collision between 2 particles in the LHC. Is that an event?
No,
I agree with with, with most of you, even when you you, you,
you, you disagree with each other because a concert,
OK, that is a sort of an event in the sense that it happens at
A at a place and a time.
You know you know it. It said that on a ticket you'll be here
at this time and they will start. But it also has a
duration,
and from that sense it's not an event
because it's. It's a sequence of events. It's sequence of of of
notes if you're like, which are all in the individual event. So
an event has no duration.
So the start of the concert is unequivocally an event,
but the the concept as a thing because it really and similarly
the whole kind of clapping hands at once. Well, yes, that does
that sort of has a,
you know, I definitely I I sort of definite time, but it's also
spread out in space
because it's the whole country, it's it's doing it once. There's
not happening at a place you can't see what is the place at
which the whole country does this. OK, if you stand
sufficiently far back, OK, it happened in the UK, but if
you're in the UK then because it was spread out in space, it
didn't happen at a place and that means and we'll go on to
this next time. I think that means it didn't happen at a
time.
It seemed obvious that if everyone claps that they would
see everyone claps their hands at once. That seems obviously.
Well, I know what that means.
Turns out you don't.
If something is extended in space, the question of what
tended to happen at becomes much more complicated and we'll come
to that, I hope in a moment.
It certainly will come to that again and again and again over
the next few weeks.
So,
and there's no issue about the .4, if those things were in the
same place at the same time, they decided that that that
happened at a place in the time,
you know and and the story.
So yeah,
and
what is it?
So next, how do we locate these events?
I've alluded to this,
but what we do is we have reference frames and we have
observers, and these two
fridges being something very specific, our reference frame.
They're not exotic, but we do have a very specific meaning
when we talk about these. In this context, our reference
frame is a coordinate system.
It's an origin and a set of axes in space and in time. So it is
saying that is the zero of my coordinate system and I'm
measuring all times from midnight.
I've defined A coordinate system
feel straight forward.
Someone else could have a different coordinate system, as
I've said.
So there's nothing. There's a coordinate system. Is a. There
are infinite many, many coordinate systems.
OK, everyone gets gets a coordinate system. Everyone gets
lots of coordinate systems. OK, there's coordinates from makes
more sense to you. Is the one in which you where you're standing
still at the minute. OK,
but there's nothing special about that. And that is the
thing about relativity. There's nothing special about the
coordinate system that you that you choose
within certain limits,
because there are different observers,
and observers are, well, observers.
Again, not an exotic notion, but the when we're talking about an
observer in special activity, we mean someone
with a notebook
who is very short sighted
notebook and a watch and very shortsighted and and they will
pay attention to things that happen in front of their nose
and they will write down where they are when that happens. So
OK I OK, it happened at 11:40 and I'm at this position in in
the quarter. They write that down and and and make a note
that that's their job.
If, if, if if it happened over there, they're not interested.
OK,
so observers are in a coordinate system.
They know where they are in the coordinate system, in space, and
in time. Well, they've all got to you. They know they're all
written on on the ground and they and they know what spot
they're sending on. And they've all got to watch. They're where
they are and what time it is, and they only pay attention to
things in front of their noses,
OK? They ignore things which happened elsewhere. If you want
to to to work out where an event happened, you've got to have
lots of friends.
OK, so if I want, if there was a firecracker went off in the
middle of the room,
the only person I'd be interested in Who, who, who? I
would ask where did that happen and when? If the person,
unfortunate person, who's sort of sitting on top of it when it
when it went off, everyone else ignored it but that person. I
asked Where are you? What time do you go off? No one else even
sought? OK, hold on to that thought.
So observers are very limited people
and what this is illustrating
come back to is 2 reference frames.
A reference stream S
which has an X&AY
and someone standing stationary in that reference frame with the
watch
and as someone in different reference frame it's frame which
is a different ex prime axis, a different Y prime axis and they
have a different watch and they were great. So if something
happens, an event happens and this second reference frame as
prime is moving with respect to the first one
at a speed V.
So at all times the location of the origin of this
reference frame is at X
in it. At X in the
South frame equals Vt simply because it's moving at a
constant speed VS There's a bit of school maths for you,
multiplication distance speed times time. You remember that
bit,
So what that is showing is
I'd rather elaborate way of showing a moving reference frame
S prime is moving in the frame S at a speed VS the origin is at
space at at a coordinate X at the origin. That point there is
it. Coordinate X prime equals 0 and coordinate X equals Vt
a point there is a cordon. X prime equals whatever
and it coordinate. X equals whatever plus Vt
keeps it. Nothing exotic there. OK, we'll get on to exotic
later, but there's nothing exotic there. OK,
but those two observers only pay attention to things locally.
Now I've said there's there's possibly have lots and lots of
different inertial frames and a different reference frames.
Everyone gets a, gets a reference frame, gets a
coordinate frame.
But some of them are special.
Some of them you can.
In some of them you can take it that you are not moving.
Now I'm standing here, I'm. I'm not having any difficulty
standing here. I'm not being thrown about because the
building isn't shaking. This is a good thing.
If I were in a train
and the training was just bowling along the track in a
nice calm fashion, then it could also, I could stand on the train
with the difficulty I could juggle
or a great cup of tea, whatever. Whether difficulty, because as
far as I'm concerned I can regard that steadily moving
train as being stationary
and it is stationary, that there's there's no, there's no
experiment I can do
that would tell me that that's steadily moving train isn't
stationary. I can look out of a window and see the landscape go
by, but all that tells me is I'm moving with respect to the
landscape. It doesn't mean I'm absolutely moving. I'm
absolutely moving.
OK, it might sugar, but so it might be that there there are
some vibration in the in this, but that's just OK that's OK
vibrating
and that's not that it's tricky to tell you moving. It's not
that no one knows how to
tell you movie. One of those axioms was you cannot tell
you're moving at a constant speed
and a matter of of deep physical principle, there is no physical
experiment you could do that would tell you if you're moving
at a constant speed
that you are actually the one moving as opposed to the rest of
the world moving past you. And you've seen this is happening to
image and saying in a train station, in a tree, in a train,
in a station, you're you're gazing blankly into space and
the train next to you is moving
and you don't know if you're if you're trained to start off
smoothly, whether it's that train to move at the station or
your train, you can't tell.
And there's nothing with specially processed experiments.
You could because if you had a little pendulum and you as
you're trade accelerated, you could do that little pendulum
would, would, would, would put to one side and in a way you can
imagine or or or or a ball on the table would roll off. So you
can tell you're accelerating
absolutely. And there's no doubt that you're accelerating. If you
are sitting in a train and it's accelerating at a station, you
can feel you're being pushed the small of your back. You can tell
you're accelerating unequivocally and absolutely,
but once you're not accelerating,
your frame is as good as anyone. Your your idea of stillness is
as good as anyone else, and that's to repeat myself. That's
not
a curiosity. That's a fundamental physical principle
that there are a lot of things and it's not, and it's called
the principled relativity. It's not Einstein's principle of
relativity, It's Galileo's principle of relativity. Because
he
spelled out quite like that. But it was. It was. Galilean physics
rests on that as a notion.
And so that means that those frames which are not
accelerating have a special status.
They are the ones that we're going to make all our
measurements in.
The other one will give a special name to namely inertial
reference frames, because they are inertial
and in our show in the sense that Newton's second law,
Newton's laws work in the actual frame.
If you
So you're floating out in space and we'll come back to this
later on. If you leave something where you just let go something,
it'll stay. For it is if you give something a push, it will
move in a constant, a constant speed until it hits something.
Yeah, just like you see the issues. If you are moving along
on a on a smoothly moving train, and you roll a ball across the
the, the, the, the the table, it'll move in a straight line
at constant speed until it hit something.
Different laws work while the train is accelerating. If you
try and roll the ball across the table and go rule it like that,
there's a mysterious extra force
which appears which breaks Newton's laws,
doesn't break news laws. It's called inertia. And if you are
on a roundabout, on a children's roundabout, you throw a ball to
someone across the other than their own about, then it goes
off to one side.
Is there a mysterious force that reflects it? No, it's just that
you're in a rotating reference.
Centripetal force, central centrifugal force. It's not a
force, it's a it's a
an illusion caused by being a non inertial frame. So the point
of a national frame is that nuisance laws work in it. So you
understand the physics of national frames,
Physics. No, natural frames is harder. Physical natural frames
easy. And we will always stick to natural frames, especially
activity. We will do the same thing. We get a general
activity, but we have to change our definition of what a natural
frame is. And that's exciting. But that's 10 lectures
and
I I I am
I don't feel I have to go to go through this as a as a quick
way. I have a quick question. We'll have more of these
questions and other lectures and have a think about these when
you're, when you're mulling over the the, the you know which of
these countries national frames
and
we can talk about that at some point
and blah blah.
So how do we measure times?
It's really easy. How to make, you know, we know how to measure
distances. We just draw things on the ground and and and and
you know, with
a greater things in the ground and walk away. We are in the
reference frame.
How do we talk about time? Well, look at our watch. How hard can
it be?
But when it turns out we have to be a little more specific than
that, a little more precise than that, and this remark in
Einstein's remark. It comes from I think the 1905 paper
because you know the the paper which introduced, you know,
banks especially activity in its more of its final form to the
world, all our judgments in which types apart our judgments
of simultaneous events and that were simultaneous is a key one.
If I see the train arrived at 7:00, what I mean is the point
of view of the small hand on my watch at 7
and the arrival of the train are simultaneous.
Simultaneous in the sense they happen at the same place at the
same time,
so my watch doesn't have to be right.
But when I see
my, it happened at seven of my watch. What I mean is the train
was here in front of my nose and so was my watch and they were
both doing that thing in front of my nose at the and therefore
they are. They are simultaneous, absolute and absolute. Sense
there were simultaneity gets more complicated if you're
talking about things which are separate from each other, as we
will discover.
But in the same way the if you if you have two cars crashing
because they are metal bending because they're the same at the
same time, that's absolutely simultaneous. This is absolutely
simultaneous. You can hold on to this notion,
and the reason why I'm banging on about the reason why I think
was banging on about is because that turns out to be a
clarifying way of thinking about time.
The timer event is
what happened on what happened on the clock which was at the
same place at the same time.
OK, and again, you think this sounds like it's like excess
precision. Why does this matter?
This is the this is the the list of things I'm kicking off in
this first lecture, which will come back with the significance
of which we'll come back to next time. So we're gonna see a lot
of pictures of this. These are our true observers.
Well, and and we we always have multiple observers because what
we're interested in the whole point of things like the
transformation, are
someone in that frame in that when that coordinate system says
this happened at this place and this time
I'm in this
cordon system with respect to which that coordinate system is
moving, what is the position and time of that event in my
reference frame?
That sounds, I think why would one care? But that
that's the sort of boiled down question you have to answer in
order to do things like talking about
Doppler shift about about I think the general relativity
that that that that it all boils down to that sort of question
even though it ended up being quite an abstract question. And
that in a in a way is why I I I said this was quite a
mathematical course because the idea that you take a complicated
thing you boil it down to the simplest thing it could possibly
be that still has the property and you analyse the hell out of
that. That's quite mathematical approach to this. You can
understand the rest by building up from that, but you tear down
in order to build up.
OK.
I can see some folk going, oh, this is a great some folk going,
oh, there's gonna be terrible, but it's gonna be great. Yeah,
it's, it's, it's it's a a tremendous intellectual
experience. The point is that there are multiple observers,
OK? And each of them knows where they are because they've got one
of these little spheres things which let them work out where
reference to the the, the, the, the origin of their frame they
are so they can put mark in the ground and attach a coordinated
position, Gordon, to that. And they've all got to watch,
which they can look at with their short sight.
OK, we'll see these people again and again.
So imagine you're standing on a bridge over a motorway
looking police officer standing beside the road so that you're
looking down and they're standing beside the road
and a car driver goes past them and sneezes.
Who would watch? Do you consult to find out the time of this
sneeze in your frame?
And this is all happening? Artistic speed? So so. And
let's, you know, hold on to our intuitions. Who says it's your
watch?
Who said the police officers watch?
Who said the driver's watch?
Who hasn't put the hand up yet?
OK, I'll try again. Everyone had to put their hand up. I don't
care what your answer is,
but I want to see if everyone had at least once. Just don't
you have a punted make a guess.
Who says it's your it's your your own watch.
Who's this police officer watch? Who says the drivers watch?
Talk to your neighbour.
Tell them why you're right.
Normally Adeem,
normally Odin to give you more time to to to to talk it over
than that. But folks, folks, normally I didn't give more time
to talk over than that. But you know we're going through at the
end here. So after discussion, who would say your own watch?
Who should the police officers watch?
Who would say the driver's watch?
It's the peace officers watch.
Why? Because the police officer is in your frame.
You and the police officer are both in the same frame. They're
both standing stationary in the motorway frame. Neither of you
are moving. That's the point.
Your the driver is moving.
OK, so they're not in your frame. They're not observer.
You, you, you you. You want to talk to. You only want to talk
to observers who are stationary. You are free.
Why not you? Why not your watch? Because you're short sighted
too. You can't even see this driver
and and and and and so the the driver sneeze didn't happen in
front of you.
Well, the police officer, they were standing about standing
there when the sneeze happened and the right were going past
sneezed and the and the police looked to their watch. So
they're the only person
who can see what the time of that event was in the motorway
frame.
The driver of the car, they know what time it happened. And their
friend, they look at their watch
and it turns out they'll give a different answer from the police
officer.
But we don't care, you know, to begin with and measurement terms
what they were. And they watch, we care what happened in our
frame. And the only people we talk to are the observers in the
frame. And the only one observer whose opinion matters is the
observer who was next to the event when it happened. Now, the
event didn't happen in the motorway frame rather than the
car frame. The event didn't happen in the car frame rather
than the motor we frame, it happened in all the frames. So
this is a good point of which I can say don't get into this way
of thinking of it happens in A-frame and event happens.
It's a thing in the world.
It happens in all frames
but different people describe different coordinates to it.
Meditate on that and and we have run out of time. So I will have
to to go through the last couple of bits of of of this first
chapter next time which is tomorrow which and I think we
are the Adam Smith building which is the last year that we
will swap back and forth between.