\n<\/p>\n
Every now and again I find myself pondering the scientific veracity of pop songs. I\u2019m a nerd, and I like music, so sometimes those worlds collide. It can be interesting to think about, and the investigation itself can be instructive\u2014even fun!<\/p>\n
Consider, for instance, the song \u201cI Melt with You,\u201d by the new wave band Modern English. It was arguably the group\u2019s biggest hit, and it\u2019s still in rotation on classic rock radio stations.<\/p>\n
The song isn\u2019t about science at all, yet it has these remarkable lyrics:<\/p>\n
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I\u2019ll stop the world and melt with you
You\u2019ve seen the difference
And it\u2019s getting better all the time
There\u2019s nothing you and I won\u2019t do
I\u2019ll stop the world and melt with you<\/p>\n
So is it right? If we stop the planet (let\u2019s assume this means halting Earth\u2019s spin), will it melt? Amazingly, we can figure this out. The key here is the amount of energy it would take to literally stop the world.<\/p>\n
The energy of motion is called kinetic energy. You can think of it as how much energy must be imparted to a massive object to get it moving at a certain speed. You already have an intuitive feel for this when it comes to familiar objects; throwing a baseball at 100 kilometers per hour is a lot easier (takes less energy) than getting a car up to that same speed. And, of course, for a given object, the higher the velocity, the more energy needed to get it up to that speed.<\/p>\n
Now, this is for linear, straight-line motion. There\u2019s also rotational kinetic energy, and again, this is intuitive: it\u2019s easier to spin a basketball than a car. In this case, though, you also have to consider the size of the object. The larger it is, the more rotational energy it has for a given mass and spin.<\/p>\n
Calculating these numbers isn\u2019t all that hard. (It could be a high school homework problem.) The real difficulty is in knowing what numbers to use. We have to make a lot of simplifying assumptions or else this quickly burgeons into a Ph.D. thesis. For example, Earth isn\u2019t a solid, homogeneous sphere but is instead layered. It has a dense core, a lighter mantle, and so on, each with a different composition, which all affect its total spin energy. Still, we\u2019re not going for pinpoint accuracy here, just a very rough number to see where we stand.<\/p>\n
Working through the equations, we find that Earth\u2019s rotational energy is about 200,000,000,000,000,000,000,000,000,000 (2 \u00d7 1029) joules, the metric unit of energy. A single joule isn\u2019t much\u2014it takes about 300,000 joules to raise a liter of water from room temperature to its boiling point\u2014but 2 \u00d7 1029, or more than anoctillion of them, is a lot.<\/p>\n
For comparison, it\u2019s about the same as our current annual global energy use\u2014over the course of roughly half a billion years.<\/p>\n
That energy is stored in Earth\u2019s spin, which was gained as our planet formed 4.6 billion years ago. Because physics is generally okay with doing some operations forward or backward, this means we\u2019d also need that much energy to stop our planet from rotating.<\/p>\n
This presents two problems, as I see it. One is how to do it and the other is what happens when you do.<\/p>\n
The how is not trivial. That\u2019s a fantastic amount of energy. Think of it this way: 66 million years ago an asteroid 10 km wide\u2014and several times the volume of Mount Everest\u2014slammed into Earth at a speed 20 times faster than a rifle bullet. Its immense kinetic energy was instantly converted into heat, creating a colossal explosion that carved out a crater 200 km across, wiping out the nonavian dinosaurs and creating a global ecological catastrophe that took millions of years to recover from. And yet the total energy released in that event was about 1023 joules, or about a millionth of Earth\u2019s rotational kinetic energy.<\/p>\n
In other words, if you want to stop our planet\u2019s spin by hitting it with asteroids (aimed just so to provide maximum braking power), you\u2019d have to do the dinosaur-killer impact again, then repeat it 999,999 times.<\/p>\n
I\u2019m not a biologist, but it seems like this process might be detrimental to life on Earth.<\/p>\n
But this asteroidal solution to our thought experiment brings up a good point: changing the energy of an object usually leads to heating it. Try to stop a spinning basketball using friction from your hand and it\u2019ll noticeably heat your skin. Doing this for Earth would rapidly dump all that heat into the planet itself (and have other important catastrophic consequences). You may see where this is going.<\/p>\n
So how much energy would it take to melt Earth? This is very difficult to calculate, but happily physicists have done some of the work. In The Encyclopedia of Volcanoes, the energy to melt just Earth\u2019s mantle\u2014which is in fact a solid and not a liquid\u2014is shown to be about 3 \u00d7 1030 joules. (Interestingly, it would take about the same amount of energy to melt Earth\u2019s solid inner core.) That\u2019s more than a factor of 10 greater than Earth\u2019s rotational energy, so right away we have our answer: stopping the world won\u2019t melt it\u2014at least, not entirely.<\/p>\n
Well, what about if we limit it to just Earth\u2019s crust because that\u2019s where we all live? Making some quick assumptions (such as that it\u2019s a 10-km-thick granite shell), I calculate that about 1030 joules of heating would be needed to completely liquify the crust (though I\u2019ve seen somewhat lower estimates). So melting the crust is iffy, though it\u2019s not like Earth will be habitable after; the oceans would easily boil away with that much energy dumped in them.<\/p>\n
Is it possible to despin Earth without overly heating it? Perhaps, if you do it slowly: for example, you could mount rocket engines with their business ends facing east, then ignite them. A back-of-the-envelope calculation indicates a Falcon 9 rocket can generate about a trillion joules of energy, so if you set up a million of them and let them burn continuously for a few million years you can stop Earth\u2019s rotation. Again, there might be some negative environmental impacts from this (as well as fueling issues). Better check with the appropriate federal regulatory agencies.<\/p>\n
Still, perhaps we need to expand what we mean by \u201cstop.\u201d If Modern English meant stopping Earth in its orbit around the sun, how much energy would that take? This is actually much easier to calculate because we know Earth\u2019s mass (6 \u00d7 1024 kilograms) and orbital velocity (30,000 meters per second): a whopping 3 \u00d7 1033 joules. That\u2019s not only enough to melt Earth through and through but it\u2019s also enough to vaporize it! Literally blowing up the world, Death Star\u2013style, \u201conly\u201d takes about 1032 joules, so stopping the planet cold in its orbital track would indeed make our world very, very hot. In that scenario, the song is actually understating the case.<\/p>\n
Conclusion: Depending on how you interpret the song, you can indeed melt the world by stopping it. At the very least, it\u2019ll do serious damage\u2014or, as Modern English put it in their hit song, \u201cI saw the world crashing all around your face.\u201d<\/p>\n
I\u2019ve seen the difference, but it\u2019s definitely not getting better all the time.<\/p>\n
My thanks to my friend Michael Walter, director of the Carnegie Science Earth & Planets Laboratory, for his help with melting Earth\u2019s mantle.<\/p>\n","protected":false},"excerpt":{"rendered":"
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