Last week, part 2 of our general relativity series
went a little deeper into some aspects of flat space time
geometry.
I said some unfamiliar things, and it got a little intense.
So just like after Part 1, there was some confusion.
I answered some of your questions on the comments page.
But there's more that I'd like to clarify today, before we
move on to Part 3 in a minute.
First, I have a pretty important announcement.
I'm sad to say that I'm stepping down as the writer
and host of "PBS Space Time."
This fall, I'll be starting full-time work at the US
National Science Foundation.
And while I'd love to keep doing "Space Time,"
I just won't have the time to do it right.
Now, don't worry.
"Space Time" will continue.
It has to.
It's the basis of all reality.
And we're looking for a worthy new host
to keep that dream alive.
And we think we found one-- me.
I personally will be doing three more full episodes
after this one, plus one final challenge question,
a little harder than normal.
Keep an eye out for it in a few weeks.
You can also always keep in touch with me on Twitter.
I'm @fizziksgabe.
But sometime in August, I'm going to hand over the reins.
And I'll be doing so with mixed emotions.
For me, doing the show has been an awesome experience.
And I'm really proud of how far this show has come.
I never thought that I'd be part of a production that
has almost 100,000 YouTube subscribers and 2 and 1/2
million views.
And it's all thanks to you guys.
Without question, "Space Time" has
one of the most curious, most cerebral,
best-behaved audiences on YouTube.
And I think that our content, and you guys-- the audience--
will continue to raise the bar even higher.
Interacting with you has been, for me,
the best part of doing the show--
answering your questions, hearing your comments.
So you've made this whole enterprise
possible and rewarding.
And you really taught me a lot.
So I'm just going to end by saying thanks to all of you.
OK.
Let's do some space time questions.
A lot of you left comments saying
that you were totally confused, or felt really stupid,
watching this episode.
Well, welcome to my life.
That's what studying physics is like when you're
trying to do it at any depth, even if you're not
getting into the math.
This video that we made was not intended
to be digested in 10 minutes.
It's hard stuff.
And the idea is for it to sink in over multiple viewings
as preparation for what we're doing next week, which
I promise will be less intense.
But as John Buluba and [INAUDIBLE]
pointed out, after you view it a second time,
it starts to make more sense.
And by the third or fourth, even more.
So be patient.
Hang in there.
You'll get it.
And as a corollary to this, you should go back
and look at the comments page on the original YouTube video,
and read it.
If you have a question, a lot of other people
have probably asked the same question.
It can save you time if you look at the answers
that I've already given that person.
And then, you can go from there.
But by all means, don't sell yourself short.
Just read all the information that's in there.
There's a lot.
Group 2 asked a question that's actually going
to be important for next week.
He asked whether the inertial frames
that I discussed in the episode are the same frame so we would
ordinarily see moving with a constant three-dimensional
velocity through space.
And the answer is yes.
Inertial observers will see each other dynamically,
moving with a constant three-dimensional velocity.
But does that mean that inertial observers are really
moving with a constant three-dimensional velocity?
Remember, in Newtonian physics, there's
no unambiguous answer to that question,
because you can only talk about motion
relative to other things.
What a space time picture-- the flat space time picture-- tells
you is that in a world without gravity,
there's a way to discern who's really moving
at constant three-dimensional velocity, sort of-- which
is those observers whose world lines in space time
are straight-- i.e., that are geodesics of space time.
Those are the guys that, it turns out,
corresponds to inertial observers in Newtonian physics.
And they are distinguishable from non-inertial observers
in a geometric way in space time.
Piffrock from France corrected my French,
and also corrected an error that we had made in the space time
diagrams that we showed at Minute 504 in the video.
Some of the dots and we had placed in the two red guy
and blue guy space time diagrams were placed inconsistently.
We've produced corrected versions of those graphs
that I'm going to show you in a second.
They're going to stay on the screen for a few seconds.
And then, we're going to add an annotation and a link
in the original video that will take you over here
to see the corrected things, just so everything
is self-contained.
Finally, Tommy Taffy and Dan Fox asked a question
that's really pertinent to what we're doing in Part 3-- namely,
how would everything have looked from the frame of reference
of the accelerating car.
Dan Fox, in particular, asked whether the accelerating car's
own worldwide, wouldn't that look straight from the car's
frame of reference?
It would.
But here's what's tricky.
It is tricky to talk about the car's frame of reference
in special relativity.
It's not impossible.
It's just extremely awkward, and very, very cumbersome.
You'll notice that all the space time diagrams
that I drew in the last episode were
from the point of view of inertial observers--
Blue Gabe, Red Gabe, the pony.
All of them were inertial.
They're moving at constant spatial velocity.
If you try to draw the accelerating car's
frame of reference, things get really messy.
And this is part of what prompted Einstein to want
to develop a more general way of talking about the world that
would work for any frame of reference,
including the accelerating car's.
But here's a preview of something
that we're going to do next week that I
want you to start chewing over.
Imagine a family of inertial observers, all of whom
are moving with different speeds relative to each other.
That accelerating car, at any given moment,
from its point of view, is always
stationary instantaneously relative to one
of those inertial observers.
So even if we can't talk about the accelerating car's
entire frame of reference, we can
talk about a sequence of inertial frames of reference,
relative to which, at any given moment,
the car is instantaneously stationary.
And it might be the case that by stitching together
all of those points of view, the accelerating car
might be able to develop a self-consistent picture
of the world.
Think about it.
OK.
I hope I gave you guys a little bit more
clarity on this last episode.
Think about all these things.
You have another week, now, before we dive into Part 3--
curved space time, and, finally, an explanation
of Einstein's self-consistent view of what gravity isn't.
See you next week on "Space Time."