Laplace’s demon is very different from last Halloween’s demon (Maxwell’s demon with his tiny trapdoor)! This brainy demon was born in 1814 from the mind of French scientist Pierre-Simon Laplace. Laplace had been studying classical mechanics; the laws of forces and motion as formulated by Issac Newton. He realized that if a vast intelligence existed – so vast that it could know the positions and motions of every particle in the universe at one moment in time – then it would also know the future. No surprises, all set in stone. Does quantum physics forbid the demon from existing?

The author Jennifer Rittenhouse West is a postdoctoral fellow in theoretical physics, studying theoretical particle and nuclear physics in the Lab’s Nuclear Science Division. She discussed the creation of this *Gedanken* demon – our second Halloween physics demon – and what we can learn from contemplating this spooky creature with an enormous brain.

**Q: How did high school physics lead Laplace to his demon?**

**A: **In a classical mechanics physics class we learn about the laws of motion – simple and powerful laws, like force equals mass times acceleration or *F=ma*. In homework problems, we are given initial conditions such as “a person on the surface of the earth throws a baseball with speed* X* in direction* Y”* and then asked where the baseball will land and when it will land there. The beauty of classical mechanics is that when you know the initial conditions and the laws of physics you know exactly what will happen in the future. The baseball will land exactly where you calculated it, at exactly your calculated time. The future is determined. This is known as scientific determinism. Laplace expanded this idea to the entire universe – if some creature knew everything’s position and motion at one moment, then the laws of physics would give it complete knowledge of the future. That creature is Laplace’s demon. Laplace called it a “vast intelligence” because it would take so much brain power to hold all the knowledge of that one instant in time.

**Q: So the demon predicts the future?**

**A:** Yes, it would know the future of every person, planet and particle. But imagine the computing power this demon requires – there are 10^23 atoms of carbon in 12 grams of carbon, for example. Twelve grams of carbon is a tiny amount of stuff, you could put it in a spoon. The demon has to know the positions of every single atom, and every single electron, proton and neutron inside, and the quarks inside the nucleons, and their speeds! When Laplace said “vast” he really meant it.

We think Laplace’s demon can’t exist even in principle but the question of his existence is very subtle and worth contemplating. Even in classical mechanics there are certain physical setups with initial conditions that do not allow us to determine the future exactly. But they are very special and highly unlikely scenarios. The most important obstacle for Laplace’s demon is quantum mechanics.

**Q: How is quantum mechanics an obstacle for the demon?**

**A:** The demon’s job is to determine the future of the entire universe from initial conditions and the laws of physics and he’s got the brains to do it. Quantum mechanics throws a wrench into this because it only allows us to calculate probabilities for outcomes. The laws of quantum mechanics are perfectly rigorous and deterministic until it comes to measuring things (like we measured where the baseball would land and when). If I throw an electron at a wall, I can only say things like: there is a 90 percent chance it will land right where I’m aiming, a 5 percent chance it will end up in the corner, 1 percent chance it will end up in a further corner, etc. It is still rigorous but is not deterministic. The demon cannot predict outcomes the way he could when he only had to deal with classical physics. Our all-knowing demon would not know it all anymore, he’d have to start rolling the dice!

In fact, when quantum mechanics was formulated during the 1920s it took quite a few years, and many experimental and theoretical clues had to be gathered. The probabilistic nature of this new theory was shown to match experiment incredibly well; many physicists including Albert Einstein had a terrible time accepting that we could no longer predict exact outcomes. Einstein famously said, “God does not play dice!” He came up with thought experiments to test the implications of quantum mechanics and to prove that there is a hidden determinism within quantum mechanics. But it looks like nature does play dice.

**Q: Why is he still interesting?**

**A:** The question Laplace’s demon poses – whether or not the universe is deterministic or probabilistic – is interesting because we are testing the implications of the probabilistic nature of quantum mechanics in experiments even today. The Nobel Prize this year was awarded for experiments – some carried out here in Berkeley! – that tested quantum mechanics for the hidden mechanisms (called hidden variables) that Einstein was so intrigued by. The results all point to a probabilistic universe. This is bad news for our demon, but good news for us. Quantum physics is one of the hottest topics in science today – quantum computing research is just one example of using the power of its probabilities to solve specific problems that are too difficult for classical computers. Our Lab is heavily involved in the quantum frontier!

Interested in learning more? Check out this brief reading list:

Author Sean Carroll on probability, determinism and Laplace’s demon

“Where Quantum Probability Comes From”

Quantum mechanics, spooky action-at-a-distance & the 2022 Nobel Prize

“Physics Nobel recognizes Berkeley experiment on ‘spooky action at a distance’”

Stephen Hawking on Laplace’s demon

Carl Sagan’s 1995 book “The Demon-Haunted World: Science as a Candle in the Dark” is perfect for every Halloween!

The Nobel Prize in Physics 2022 press release

## 3 Comments

Happy November everyone!! I wanted to note that Laplace’s demon is so vast that in our illustration of the demon, a spiral galaxy nearly 55,000 light years across would fit on the tip of one of his horns. This much computing power would need a ton of space, the demon has to be gigantic – at least, according to what we know today in physics. We should have put a galaxy in for scale 🙂 I hope everyone had a good spooky Halloween, thank you for reading!!

Love it!! That spiral galaxy is named NGC 4144 (strange name, ours is nicer, the Milky Way 🙂 ), about 60 million light years from Earth and about 55,000 light years across. The image is from the Hubble Heritage team.

Laplace’s demon could put it in his hand like a grain of sand. That’s how big he would have to be to be able to hold his knowledge of everything all at one time and his calculations to know the future. Thank you for posting little NGC 4144, Will!!