A colossal jerk

The term 'jerk' has a special meaning in physics.

The rate of change of an object's position is called its velocity.

The rate of change of an object's velocity is called its acceleration.

Newton realized that to understand how objects move, you need to focus on their acceleration. If you push on an object, it accelerates. Knowing exactly how much it accelerates, you can work backward and figure out its velocity. Knowing that, you can work backward and figure out its position.

But why stop with acceleration?

The rate of change of an object's acceleration is called its jerk.

For example, if you suddenly slam on the brakes, your car suddenly accelerates backwards - we call that 'deceleration', but it's a form of acceleration. Since the acceleration is changing, this counts as a jerk. So you say to the person whose car stopped in front of you: "What a jerk!"

The universe is expanding. It's not expanding into anything - the picture here is misleading - but the distance between any two faraway galaxies is increasing.

The Big Bang happened about 14 billion years ago. Since then, the universe has been expanding. For a while the expansion was slowing down, or decelerating. That's because matter in the universe attracts other matter, thanks to gravity.

But 5 billion years ago, something interesting happened. The expansion started to speed up, or accelerate. We believe this is because dark energy has a repulsive effect... and while matter spreads out and becomes less dense as the universe expands, dark energy does not! So, the repulsion of dark energy eventually overwhelmed the attraction of matter.

In short, about 5 billion years ago, the acceleration of the universe's expansion changed from negative (deceleration) to positive. We know this from carefully looking at the redshifts of distant galaxies... and from other methods, too.

The acceleration is continuing to increase, too. So, we are experiencing a jerk on a colossal scale.

But why stop there?

The rate of change of an object's jerk is called its snap.

The rate of change of an object's snap is called its crackle.

The rate of change of an object's crackle is called its pop.

Before this week, I had never heard anyone use any of these terms except as jokes. These concepts just aren't very important. But then someone pointed out this paper to me:

• Matt Visser, Jerk, snap, and the cosmological equation of state, https://arxiv.org/abs/gr-qc/0309109.

He shows that if we know the jerk and snap of the universe, we can figure out stuff about matter and dark energy. He also brings in experimental data. According to one analysis of the data, there's a 92% chance that the jerk is positive. That is, the scenario I explained is actually right.

Unfortunately the experimental data is not good enough to measure the snap.

So we're stuck with a colossal jerk, and we may never snap out of it.

Puzzle: who first introduced the terms snap, crackle and pop?

Alex Klotz, who pointed out Matt Visser's paper, said:

The earliest reference I can find is a 1996 USENET discussion in which you and Bill Jeffreys both list them as the accepted terms, and this discussion was later cited in an Am.J.Phys paper. The discussion is here: https://groups.google.com/d/msg/sci.physics/eQLC6Tv_IVM/X0n8bkMc4EYJ

Anyway, I was wondering if you knew how those terms were initially used in physics and who used them first. Did you invent them?

I did not invent them.

Here is another reference from 1996, written by +Philip Gibbs:

http://math.ucr.edu/home/baez/physics/General/jerk.html

It's a lot of fun to read, but I'll only quote the end:

Momentum equals mass times velocity!
Force equals mass times acceleration!
Yank equals mass times jerk!
Tug equals mass times snap!
Snatch equals mass times crackle!
Shake equals mass times pop!!

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