Why the Sky ISN'T Blue

Happy 500,000! Thank you guys so much for subscribing to my channel and for joining

me on this scientific adventure. You know, if you got 500,000 people together and we

all held hands in a line, it would stretch from Sydney to Melbourne or from San Francisco

to San Diego or from London to Inverness.

>> Oy. What are you forming a line here for? You are going to scare off Nessy.

>> Right. So I suppose I should get down to answering your question. Go ahead. Hit me

with your best shot.

>> I understand that the sky is blue do to Rayleigh scattering. Short wave lengths get

scattered way more and blue light dominates because it is so short. If that is the case,

why isn’t the sky violet?

>> Ok, here is the thing. The sky isn't really blue. It is bluish white. So the sky is illuminated

by the sun and the sun in its all colors, but not equally. The sun doesn’t emit that

much red, nor does it emit that much violet. Most of the light it emits is kind of in the

greeny part of the spectrum. Now as you point out, due to Rayleigh scattering

more of the shorter wavelengths arte scattered and that is why the sky looks blue, because

it is basically the sun’s white spectrum shifted a little bit towards the blue. This

is the spectrum of the blue sky. And, as you can see, it is quite broad. And, in fact,

there is a fair amount of violet light in there. But there is not as much violet light

as there is blue light, because the sunlight that we started with had much more blue than

it has violet.

>> If I have a lap top on the international space station with a hard disk drive in it,

will the torque from its spinning cause the lap top to spin as well?>> Well, by the law

of conservation of angular momentum, yes, the lap top should spin. I did a quick back

of the envelope calculation and I found that if you spun up your hard disk from rest up

to 5400 rpm, then it would cause the laptop to spin in the opposite direction at a rate

hat would cause it to do about one revolution every 17 seconds.

>> What would happen if you poured liquid oxygen on a fire?

>> Kaboom?

>> Say you were able to pass the event horizon a black hole and come back, what would coming

back look like of the person and for an observer?

>> Well, to an outside observer it would look as though you never came back at all. And

to the person it would look as though you didn’t come back either, because once you

are passed the event horizon, that is it. Your entire future lies within the black hole.

And it will only be a matter of seconds before you are in the singularity, the very core

of this black hole and you are no longer. Now if you could bring a rocket back with

you and try to fight it, if you tried to accelerate away from the black hole, you would find that

you actually live for less time. You might see more space, but your time would pass more

slowly and so you would end up in the singularity sooner.

So I guess the lesson is don’t fight it. If you want to live longer, just relax and

go with it.

>> You have met a lot of interesting people on your adventures. But is there someone specific

that you would love to meet?

>> I would love to meet Bill Nye, Neil DeGrasse Tyson, Richard Dawkins and President Obama

as a non scientist inclusion. I really like his book. So if you haven’t read The Audacity

of Hope that is a good recommendation.

>> If all the galaxies are constantly accelerating away from each other, will they ever reach

the speed of light or stop accelerating all together?

>> So, yeah, all the galaxies do seem to be accelerating away from each other, faster

and faster, which makes this kind of a unique time in the history of the earth, because

right now we can still see that there are hundreds of billions of galaxies. But sometime

in the future due to the expansion of the universe, those galaxies will be so far away

and, yes, they will be receding at faster than light speed and then you might say: Well,

nothing can go faster than light. And it is kind of true in that these galaxies won’t

be moving through space at a speed faster than light, but the space in between us and

them will be expanding at a rate which means they will be moving relative to us faster

than light, if that makes any sense. So their light that they are emitting will never be

able to reach us, because it will never be able to make progress through this expanding


It is kind of interesting to think about what will happen as that time approaches. Well,

the light that is coming from those distant galaxies travels across this expanding space.

And so it becomes red shifted, cosmologically. And eventually the wave length of this light

will be so big that its wavelength is the size of the whole universe and that is impossible

to detect. So at some point, probably about two trillion years in the future we will only

see our local cluster of galaxies. Now the reason they haven’t gone out to infinity,

or very, very far away is because they are gravitationally bounded to us. So even though

space is expanding on a large scale, the gravitational force is enough to hold all of us together

so we will be in it for the long haul. But this still means that there is going to

be strange cosmology. If we are still around trillions of years from now, if we tried to

look out, we would see a very different universe, a universe which is much emptier than the

one we see today. So we live at a unique point in space time. We should appreciate that.

>> Is there such a thing as randomness in the universe? And if so, isn’t that contradictory

to science?

>> Yes, I think there is randomness in the universe and, no, I do not think this contradicts

science. Though, perhaps you are thinking along the lines of Albert Einstein when he

said: God does not play dice. He wasn’t happen when in quantum mechanics it seemed

as though some events have probabilistic outcomes. I mean, he thought that there were some hidden

variables there and we just didn’t know what was determining the outcome. So we thought

it was probabilistic. But all experiments seem to show, up to this point, anyway, that

there are some things which are randomly determined. They are probabilistic in their nature. For

example, you can get quantum, random number generators and even the FBI and CIA can’t

seem to find any pattern in those numbers. So they really, truly do seem to be random.

So, yes, it seems from all the experimental evidence that randomness is a fundamental

part of our universe, albeit mostly on the quantum scale.

>> What shape is the universe and could light from our sun go around the universe and come

back and hit the other side of earth?

>> The shape of the universe seems to be flat, which means there is no large scale curvature

of space time. And the supposed reason for this is inflation, this idea that very, very

shortly after the big bang the whole universe just started accelerating at an incredibly

fast rate. So even if it was kind of wiggly or warped or curved around on itself before

inflation, after inflation it basically would have been flattened out, just like blowing

up a balloon, that side is going to become, you know, more or less flat. So within the

limits of our observational capabilities to these days, we think that that the universe

is flat. So if you go out in one direction you probably shouldn’t return the way you

came. So, no, the universe does not appear to be closed and you would probably never

see the light coming up behind you.

>> Hey, man, I have one fantastic topic for you to make a video about, but I need your

response. You want to do it? Otherwise I contact Vsauce.

>> Hey, Michael, you want this one or shall I take it?

>> No, no, no. Go ahead, Derek, take it.

>> So we are really reaching the atomic size, but not in some exotic laboratory device.

In every device you have in your computer.

>> How many transistors are on a chip now?

>> About a billion.