I clearly broke some of your brains with my relativity series.
You had a lot of really good comments and questions.
Let’s see what you had to say.
Why does a light wave always have to travel at the same speed?
The speed of any wave is only dependent on the properties of the stuff that’s waving.
For something like water waves or sound,
that can be things like: material elasticity and mass density.
Both of which ultimately depend on things like temperature.
Very easy to change.
Light is a wave in electric and magnetic fields.
In other words, not matter.
There’s no way to change the properties of those fields, so there’s no way to change the speed of light.
No exceptions.
Which reminds me of something I think some scientists need to learn:
If your rule has exceptions, it’s not that great of a rule.
Why is “c” the symbol we use for the speed of light?
Because the speed of light is “constant.”
Scientists aren’t usually very good at naming or labeling things.
Is there some kind of absolute universal reference frame?
No, although, they tried really hard to make this a thing back in the day.
Luminiferous aether!
But it was totally bogus!
Too many assumptions had to be made and we all know where that leads.
Brian Greene said objects always move through spacetime at the speed of light
and it's only the direction that we can change.
Well, yes, I even said something like this in a previous video,
but you have to be careful about the definitions of words.
As you can see, the word “object” has many meanings.
As long as you limit the word “object” to things with mass, then you’re fine.
Light itself doesn’t fall under Brian Greene’s statement,
but it doesn’t have mass either, so it isn’t an object.
If you want more details and have a solid math background, my book covers this.
Otherwise, you can wait for my next video.
In the spacetime diagram, how come the second time axis is diagonal or curved?
If something is stationary, then it’s moving through time and not through space.
In a spacetime diagram, that’s when your time axis lines up with your path.
It has to, by definition.
That’s what it means to be stationary.
If we were on the opposite side of the Earth, would that mean we’re upside down?
Technically, gravity defines down, so as long as that always points toward your feet,
then you’re right-side up.
What’s a geodesic?
A geodesic is the new name for an inertial reference frame.
Let’s start all the way back with Newton.
His first law defines what an inertial frame is, which is why it’s sometimes called the inertial law.
It says the acceleration is zero in inertial reference frames.
About 230 years later, Einstein says “Actually, no.”
Acceleration is equal to this.
We call it the geodesic equation and it’s like a new and improved version of Newton’s First Law.
So, when we accelerate, only time is being curved?
No, whether you’re talking about acceleration or gravity, both space & time are curved.
I left off the new space axis in the diagram for clarity.
But time does tend to be curved more than space,
so time-curvature is actually where most gravity comes from.
If you’re floating in the middle of empty space, you just sit in one place and only move through time.
Your geodesic is straight and spacetime is flat.
But if we put the Earth in there, your geodesic bends toward the Earth’s surface, so that’s where you go.
It takes something a lot bigger than the Earth to notice curvature in space.
Like a really big galaxy or a whole cluster of galaxies.
Or, you know, a black hole or something.
If I drop two coins on opposite sides of the Earth and they each think the Earth is accelerating,
then the Earth is accelerating toward both at the same time.
First of all, each coin is in a different reference frame,
so you can only be looking from the point of view of one at a time.
The Earth doesn’t accelerate toward both, it accelerates toward each.
Here’s the first coin’s frame, the second coin’s frame, and the Earth’s frame.
The distinction is very important because curvature is only defined locally.
How are inertial mass and gravitational mass the same?
It seems like they’re completely independent.
They were seen as independent for a very long time.
In fact, Newton had no idea why they were always the same.
It took General Relativity in 1915 to finally understand.
The equivalence principle says acceleration and gravity are the same thing.
So these two equations are basically saying the same thing.
So the source of gravity isn’t mass?
Nope. It’s energy.
Well, technically, it’s energy and momentum and pressure and stress.
All summarized in a quantity called the Stress-Energy Tensor.
Meaning anything with energy and momentum can curve spacetime and create gravity, including light.
Light might not have any mass, but it still has energy and momentum.
It’s all kinetic energy, but it’s still energy.
It still has the ability to do stuff.
How can gravity be considered a fundamental force when it isn’t even a force
and how does the graviton fit into all of this?
This is tricky because the other three fundamental forces aren’t really forces either.
We don’t even call them the fundamental forces.
We call them the fundamental interactions
and understanding them requires a type of physics called quantum field theory.
and gravity doesn’t fit into that very well.
I need to do way more research into QFT before I’m ready to say anything more.
Thanks for commenting. Keep them coming.
Some of the big questions can even inspire whole videos.
Also, thanks for liking, sharing, and subscribing.
And until next time, remember, it’s OK to be a little crazy.