They did not seem to have as big an influence on Einstein's thinking

as just some of the, as he termed it, the asymmetries involved.

We talked about the bar magnet and the coiled wire, and

how depending on which was considered to be moving,

physicists had one explanation for one scenario and another explanation for

another scenario, and things like that really bothered Einstein.

To the point that he described it as, after years of thinking about this by this

time being 26 years old, it was almost a state of psychic tension for him.

And then out of that,

through some conversations with his friend Michele Besso, you may remember his flash

of insight that time was suspect as he later termed it.

So, what he then did is he took two key principles that were well established in

physics, one, the principal of relativity, so we spent some time talking about

Galilean relativity and just the basic principal of motion of objects and

how you can convert from one frame of reference to another frame of

reference if they're moving at a constant velocity with respect to each other.

We talked about that a little bit and

the other principle was when he turned the principle of light constancy that

the movement of a source of light or a source of waves means

that it's not going to actually affect the velocity of the waves coming out of that.

So we spent some time just talking about how waves work to get some background

on that, and understand this principle of light constancy.

And then also the conflict between them,

because the principle of light constancy implied that it either existed and

it was just assumed, that that had to have some sort of medium for,

flight was a way, it had to have some sort of medium through which it traveled,

and the idea was that it was this thing called the ether.

So they spent a lot of time creating models of the ether,

wondering about how it might work, be put together, and so on and so forth.

And the problem with that though is that it went against the principle of

relativity because if you have it ether you can assume the ether was

the absolute reference frame and so you can have absolute motion against

measured against ether or just the state absolute rest would be ether

the underlying foundation of the universe, as it were.

And so Einstein's law is conflict here and

he said his insight was actually they are both true here.

And out of those two principles then, we showed that you get

the invariance of this speed of light that combine these two things together.

Its not just that if you have a moving source,

the light coming from that moving source has a constant invariant velocity.

But, it's any motion at all.

One observer to light or to another observer moving with respect to light.

Speed of light will always be the same to all observers.

And that came from the combination of these two seemingly

incompatible principles.

That was Einsteins key insight that set him off on this path.

And then from that, we looked at okay, what do we get out of that?

Well, one key thing was the so-called relativity of simultaneity.

That clocks synchronized in one reference frame

will not be synchronized in a moving reference frame.

We talked about the concept of a lattice of clocks for

each reference frame that would be synchronized.

And so the relativity of simultaneity, or the relativity of clock synchronization,

that was a big news for the time, because the standard assumption was,

well of course everybody measures the same time.

Time is absolute, doesn't matter how you're moving with respect to

somebody else's clock, you'll see the same time as they will.

But Einstein showed, and we sort of followed in his footsteps here,

that put these to principles together, except both of these well

accepted principles and even though they seem to be in conflict, you get,

if you accept them as true you get the relativity of simultaneity.

And from that then other results tumbled out.

We talked about time dilation using the light clock example, again,

coming from the invariance of the speed of light.

So that a moving clock, an observer who observes a moving clock will observe it to

tick more slowly than an identical clock that he or she has next to them.

So time dilation.

And from that we also got the Lorenz factor gamma, the scaling factor for

time dilation really.

And then from that we get length contraction as we analyzed it

further that moving objects contract

in the direction of motion to an observer who's observing a moving object.

If you're riding along with a moving object of course everything

in your frame of reference, everything is normal,

has this normal length because you're in it's so called rest frame.

So it's rest length or proper length.

So time dilation, the Lorentz factor gamma, length contraction,

I won't write down the formulas here because you

should know them by now of course, you can remind yourself, review them.

The invariant interval, I actually did write down this formula just to remind us

that also came out of analysis of a light clock thought experiment and

we show that for all observers this combination of things from one event,

two events and one frame versus another frame c squared t squared minus x squared

will always be constant and will allow use to do a few calculations with that and

then the phrase leading clocks lag.

We talked about the relativity of simultaneity,

relativity of synchronization.

But that as you observe two clocks go by and they're moving