Hey Crazies In a previous video, I said this:
It's physically impossible for light not to go at the speed of light.
and a bunch of you got all defensive about it.
So, today, we’re finally going to answer the question.
Can light slow down?
Well, that depends.
Say you wanted to send a laser through something like this.
It’s just a piece of glass,
nothing special, but something weird happens when this laser
passes through it.
If we measure the distance the light has to travel
and the time it takes to travel that distance OK, so we’re not measuring that time with
a stop watch, but we can measure it if we have the
proper tools.
Just go with me here.
Dividing the distance by the time gives us a speed less than the speed of light.
If you’re quick to judgement, you might be like:
See! It does slow down!”
But we haven’t necessarily shown that.
There’s a problem with the way we calculated that number.
Dividing distance by time doesn’t give you a speed.
It gives you an average speed.
And you all remember what I said about averages in the last video.
Sometimes they have meaning and sometimes they don't.
Meaning is what we're looking for in science.
We need to figure out if that average speed
has meaning, so let’s take another look at that light.
It almost looks like it has drag on it.
Kinda like a person trying to run through mud.
Except drag only happens when a solid tries to pass through a fluid.
Light is not solid and this glass prism is not a fluid.
A common explanation is that the light ceases to exist for a moment
as it’s absorbed and released by the atoms in the glass.
It’s a nice simple explanation for beginners, but it's wrong.
We’ve already seen that electrons in glass can’t absorb
any wavelengths of visible light to jump to higher energy levels.
That’s why it’s transparent.
Another common explanation is that the light doesn’t travel in a straight line.
It keeps bouncing off atoms along the way.
Also wrong!
If that were the case, different parts of the beam
would come out of different parts of the glass, randomly.
But we know the beam is very controlled and predictable.
Something else must be going on here.
We need to stop thinking of light as if it’s a stream of bullets.
Even if you imagine the light is made of photons, those photons are not bullets.
Light is a wave, which means we need to worry about wave interference.
When you have two or more waves in the same spot, they combine to form one wave.
Isn’t the original beam only one light wave though?
Well, technically, it’s a bunch of waves moving together
plus the original beam isn’t the only light in that glass.
The moment the beam comes into contact with the glass,
its atoms start to vibrate in response.
That vibration causes all those atoms to emit their own light.
Most of the light cancels except for one particular direction:
the forward direction.
The point is, when you combine all those waves together
they just look like one beam traveling slower than the speed of light.
But is the light actually traveling slower?
That depends on what you mean by “light.”
It’s a language issue.
I think everyone agrees that one single photon is “light”
but this beam has a bunch of photons in it.
If you also refer to that beam as “light” then, sure, “light” slows down in materials.
But, when I said:
It's physically impossible for light not to go at the speed of light.
I was referring to individual mass-less photons, which always go the same speed.
Beams of light don’t always have the same properties as their photons.
So would you refer to that beam as “light” or a bunch of “lights”?
Let us know down in the comments.
Thanks for liking and sharing this video and a special thanks to Patreon patrons like
Andy Kirkham, who go above & beyond.
Don’t forget to subscribe if you'd like to keep up with us.
And until next time, remember, it’s OK to be a little crazy.
In the last video, I suggested math wasn’t the language of the universe and you had a
lot to say.
frib75 and a few others took the extreme opposite position and said math wasn’t even a language,
which is an interesting perspective.
They agree the symbols we use to represent math are a language,
but math itself is something more fundamental to the universe.
Whether you’re writing math symbols or writing out the translated English sentences,
you’re doing math.
You’re just going through logical steps and those steps are the real math.
But I see two problems with this argument: One!
Some logic leads to paradoxes and the real universe doesn’t have those.
Two!
Things like photons and electrons don’t calculate the paths they travel.
They just move.
We’re the ones that do the calculating.
ScienceMaths2012 asked what might actually be a more testable question:
Can the universe be axiomatized?
In math, there are a bunch of things you can prove and a short list of things you can’t.
The things on that short list are called “axioms,” which are things we must assume to be true
for math to even work in the first place.
Like "a" times zero equals zero regardless of what “a” is.
If you can find and agree on a set of axioms for our physical laws
then you could say the universe is inherently mathematical.
The problem here is that physics can’t even agree whether or not
Newton’s laws or conservation of energy and momentum is more fundamental
let alone whether or not either of them are an axiom.
Thedarkfly said the symbols I used in the video are specifically for English.
Is that true?
I've always been under the impression that the scientific community agreed on the symbols that we use
including the parts of the community in countries with languages that aren’t latin-based.
Let me know in the comments.
Rick Clark pointed out that units of measure are completely made up by humans.
That’s true.
They’re completely arbitrary.
But a lot of theoretical work is done completely without regard for units at all
The equations can be written to show relationships between measurements that are true for any units.
So I’m not sure this piece of information works in my favor.
Ok, now onto separating the comment responses.
After counting opinions and thumb-ups on those opinions.
The consensus is to leave them as is for now.
Hopefully, in the future, YouTube will eventually extend the time for the mobile end-cards.
I also tried it on the bloopers video and I’m not sure I like how they look anyway.
Lastly, thanks to several new patrons on Patreon, we reached another goal!
You should see the pay-off at some point in the next month or so.
See you then! Thanks for watching!