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  • In 1994, a massive earthquake shook the Northridge suburb of Los Angeles, killing 57

  • people and injuring over 5000. The cost of damages was in excess of $20 billion. It's

  • earthquakes like this one that make us question just how solid is the earth beneath our

  • feet, and what does it mean to be solid anyway?

  • At first glance, pitch looks like a solid, but it's not. It's actually a liquid at room

  • temperaturejust a very viscous one. Viscosity is a measure of resistance to flowwhat we

  • often think of as the "thickness" of a liquid. Olive oil is nearly 100 times more viscous

  • than water, and honey is about 100 times more viscous than that. Meanwhile, pitch has a

  • viscosity 2.3×10¹¹ times that of water. At the university of Queensland in Australia,

  • pitch is the subject of the world's longest running lab experiment, and it's still going to this day.

  • Back in 1927, this glob of pitch was placed into a funnel, and ever since then, in nearly

  • 90 years, it has produced only 9 dripsroughly 1 a decadeand no one has ever been in the

  • room to see a drop fall. Though in 1988, the former custodian of the experiment, John Mainstone,

  • came very close to observing a drip fall, except he stepped out of the room

  • for just a few minutes to get a cup of tea. Now, you can actually watch this experiment

  • livethere's a link in the descriptionbut since the last drop happened in 2014, I think

  • you'll probably be waiting a while.

  • Another substance that you may have heard is a very viscous liquid is glass. If you look

  • at the stained glass windows of old churches, you'll find the bottom of the pane is

  • decidedly thicker than the top, and that's because the glass has been flowing down over

  • centuries... Actually, no, it hasn't! You know, we've looked at old telescopes where the

  • optics is very sensitive to slight shifts in the lens glass, and we find they still work

  • perfectly after hundreds of years. Plus, studies of thousand year old windows find no

  • real evidence of flow. So the truth is that it's just very difficult to make glass of

  • uniform thickness, and so when the glass was originally installed thousands of years ago,

  • they would install the thickest part towards the bottom. The lead actually has a lower

  • viscosity than the glass, so if the glass had even thickened a little bit, then the lead

  • should be a puddle on the floor by now.

  • Now, glass is unusual in that it's an amorphous solid, meaning that the silica molecules

  • are not regularly arranged as in a regular crystalline lattice. Instead, they're all in a

  • jumble, and this is because the glass is cooled down so quickly from the liquid state to

  • the solid state that the molecules don't have time to arrange themselves in a nice

  • regular crystal structurebut what makes something a solid rather than a liquid is that

  • all of the atoms or molecules are so strongly bonded together chemically that they can't

  • slide past each other. So in water, or olive oil, or pitch, the molecules can slide past

  • each other, but in glass, at room temperature, they can't.

  • So what about the interior of the Earth? Beneath the Earth's crust is the mantle, which

  • is responsible for plate tectonics, and therefore earthquakes. Is it a solid or a liquid?

  • We can obviously never observe the mantle directly, but when we do see material come out

  • from underground, it is red hot rockit's lava. So you might be imagining that the mantle

  • is very similar, made up of this molten magmahot liquid rockand that would make sense,

  • because in order for it to flow, it must be a liquid, right? Actually, wrong! The mantle

  • is a solid. Under all that pressure, even though it's at very high temperature, it

  • remains in a solid state, and we know the mantle is solid because shear waves from

  • earthquakes can actually propagate through the mantle. These waves cannot propagate

  • through liquid, like the molten iron of Earth's outer core, because liquids flow in

  • response to being shearedor rubbed sidewaysand as a result, we can see the shadow of

  • the liquid outer core by measuring seismic waves from an earthquake on the other side of

  • the worldbut how exactly does this solid rock flow?

  • Well, the answer lies in the fact that crystals aren't perfect. There may be a missing

  • atom here or there, and under the high pressures in the mantle, sometimes a neighboring

  • atom will pop in to fill that gap. Now, from a human perspective it takes a very long

  • time for this to have a noticeable effect, but from the Earth's perspective, it happens

  • in no time at all. The viscosity of the mantle is similar to that of glass, to several

  • orders of magnitude greater, so it is really only over these geological time scales that

  • the mantle is fluid-like at all.

  • So pitch—a liquidcan flow so slowly as to seem like a solid, whereas the Earth's mantle

  • a solidbehaves like a fluid if you just wait long enough. As the famous American

  • geologist Grove Karl Gilbert once said:

  • "To my mind it appears that the difficulty is only imaginary and not real. Rigidity and

  • plasticity are not absolute terms, but relative, and all solids are in fact both rigid

  • and plastic... When great masses and great forces are involved ... the distinction loses value."

  • Sometimes the rigid definitions we create for ourselves can introduce misconceptions, or

  • viscous rumors, like the idea that the core of the Earth is a giant ball of magma. If

  • only we could think about liquids and solids a little bit more...fluidly.

  • Hey! This episode of Veritasium was supported by Audible.com, a leading provider of

  • audiobooks, with over 180,000 titles in all areas of literature, including fiction,

  • non-fiction and periodicals. This week I wanted to recommend to you a brand new

  • science-fiction book published in 2015. It is by Neal Stephenson, and the book is called

  • 'Seveneves', and it's about a massive catastrophe that threatens the Earth. I think that

  • if you're into science, you will really enjoy it, and you can actually download this book

  • for free by going to audible.com/veritasium, or you can pick any other book of

  • your choosing for a one month free trial.

  • So, I really wanna thank Audible for supporting me and I wanna thank you for watching,

  • and if you haven't signed up for my mailing list yet, that is something you should

  • probably do. You can click on this card here and go over to Veritasium.com, and that is

  • where I will be announcing some very cool things coming out very soon. So yeah,

  • check it out, if you haven't already. Bye!

In 1994, a massive earthquake shook the Northridge suburb of Los Angeles, killing 57

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B1 中級 美國腔

玻璃是液體嗎? (Is Glass a Liquid?)

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    Jerry shiu 發佈於 2021 年 01 月 14 日
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