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  • 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 space.

  • 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.

Happy 500,000!

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為什麼天空不是藍色的 (Why the Sky ISN'T Blue)

  • 3 0
    林宜悉 發佈於 2021 年 01 月 14 日
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