Phil Plait's Bad Astronomy: Bad News: World Jump Day

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Jump to Conclusions

July 17, 2006

Have you gotten an email recently from someone claiming that July 20, 2006 is World Jump Day? The email links to the World Jump Day website (let's call it WJD for short), which claims that they want to get 600 million people to jump up at the same time. Why? So that the Earth will move in its orbit and stop global warming.

Yes, seriously. At least, that's what the site claims. I think it's an elaborate joke, because it makes no sense. There are signs all over the site that's it's a hoax. For one, they interview "Professor Hans Niesward", who they say is a scientist at Germany's Department of Gravitationsphysik. Problem is, all the web pages I see when I search on his name go to... pages about World Jump Day! There is no mention of him in any real science capacity; tellingly, searching on "Hans Niesward -world -earth -jump" returns almost no sites written in English. WJD also links to a video of an interview with him with a wrinkly map of the Moon on the wall and some old books on a shelf next to him. Inspire trust and confidence he does not.

Update (about two hours after I wrote this page): BA Blog reader Mark Martin mentioned on the blog that this is indeed a hoax: he found a site that says WJD is akin to performance art. Still, the debunking here may prove useful if anyone ever asks you what would happen if all the Chinese people jumped at the same time!

Normally, I'd simply laugh at this since it's a funny idea. But as with most pseudoscience, there is no idea so outrageous that you won't get some people to take it seriously. I'll be honest and admit I have no evidence anyone is, but I do know that people took Nancy Lieder seriously. And of course kids may not be able to understand right away why this is a joke.

But I'll be honest: I'm a huge geek and it's fun to think about stuff like this.

So why won't it work? I need to break this into two sections: the WJD site's claims of how to move the Earth won't work, and what they claim will happen if they do move the Earth are wrong as well.

Why this won't work

First things first: why won't the Earth move if we get 600,000,000 people to jump up at the same time? There are three reasons, actually.

Mass delusion

First, there's the problem with mass. 600 million people sounds like a lot, but the Earth is big. Really, really big. Let's say each person weighs 100 kilograms to make it easy (that's 220 pounds, so we're already being generous). 600 million people times 100 kg = 60 billion kilograms. That's a lot of meat! But the Earth masses 6 x 10²⁴ kg, or 6,000,000,000,000,000,000,000,000 kg. In other words, the Earth weighs (well, masses) 100 trillion times the total mass of all those people!

Put it this way: a single cell weighs about 1 billionth of a gram. Compared to the weight of a human, that's the same ratio as 600 million people compared to the Earth. Now imagine a single cell jumping up and down on your head. How much do you move? That's the scale we're talking here.

Even a major asteroid impact won't move the Earth much in its orbit. So people-- even all of us, all six billion or so-- won't make any difference in the Earth's orbit. I actually calculated the energy of 600 million people jumping at once: I get about the equivalent of 50 tons of TNT exploding. Since we blow up bombs much larger than that, why hasn't the Earth been jerked around its orbit thousands of times?

Where in the world

The second problem is one of placement. Even if 600 million people could move the Earth, they're located all over the world. The Earth being a ball, that means that people in, say, northern Spain will be perfectly canceled out by people in New Zealand. You'd need to get all 600 million people in one place on Earth to do this. Not only that, you'd have to find the right place so that when the Earth moved, it went in the right direction. Global warming won't be stopped if you accidentally move the Earth closer to the Sun. That also means timing the jump perfectly, since the Earth rotates.

Newton's Third Law

Finally, there is another basic reason this won't work, even if everyone on the Earth weighed 100 trillion times as much. The problem is that we're a closed system. If we get everyone to jump up, they'll fall back down. So even if we were able to push the Earth in one direction by jumping, when we come back down we'll move the Earth back to where it was!

Think of it this way. Stand up. Now bend over and grab the tops of your shoes. Now pull up. Are you flying? No, duh, because as you pull up on your shoes, your shoes pull down on you by the same amount. You won't go flying around, no matter how much you believe Wile E. Coyote cartoons are actually documentaries. You're attached to your shoes, just like we're attached to the Earth.

Because we are gravitationally bound to the Earth, no matter how hard we jump, when we fall back down the Earth will move back into its old position. The energy you got from jumping up is returned to the Earth when you fall back down, and the net effect is zero.

I'll mention that you can move the Earth if you really needed to, and you had several million years to do it. You could turn a bunch of rockets upside-down and fire them so they push down on the Earth. The energy of the exhaust comes from chemicals burning, so in a sense it's not a closed system. But even with powerful rockets it would take essentially forever to move the Earth by any measurable amount.

Why nothing would happen even if this did work

So okay, World Jump Day won't work. But even if it did, the claims they make as to why we should do it are wrong as well. They want to lower the Earth's average temperature to counteract global warming. A noble idea, but I don't think they did the math. You'd have to move the Earth out from the Sun to lower its temperature, of course. It turns out that the temperature of the Earth depends roughly on the square root of the distance. The Earth's temperature is about 280 Kelvin (about 7 Celsius), so to drop the temperature one degree you'd need to move the Earth out from the Sun by about square root (1/280) = 6% of its current distance. The Earth is 150 million kilometers from the Sun, so we're talking moving the Earth 9 million kilometers farther out. That's 22 times the Moon's distance from the Earth. According to the WJD website, they want to drop the temperature about 2 degrees, which means moving the Earth more like 13 million kilometers.

Conclusions

Still wanna jump?

Again, I think this is all a joke. It has all the signs: a dodgy "Professor" with no apparent history, weird claims with no basis in physics, math, or reality, and no real substance on their site. And, as I write, they claim they have 598 million people signed up. What a coincidence, three days before J day, and they are almost at their goal!

Maybe the WJD site designers will never admit it. Maybe they'll get a kick out of a scientist taking the time to debunk their joke. Maybe they're serious; after all, it's not hard to find even wackier claims out there. As Robert Heinlein once said, "Never underestimate the power of human stupidity." Of course, he also said, "No statement should be believed because it is made by an authority."

Hmmph.