The quantum world is mind-bogglingly weird

If you’re interested in the smallest

things known to scientists, there’s something you should know. They are extraordinarily ill-behaved. But that’s to be expected. Their home is the quantum world.

Explainer: Quantum is the world of the super small

These subatomic bits of matter don’t follow the same rules as objects that we can see, feel or hold. These entities are ghostly and strange. Sometimes, they behave like clumps of matter. Think of them as subatomic baseballs. They also can spread out as waves, like ripples on a pond.

Although they might be found anywhere, the certainty of finding one of these particles in any particular place is zero. Scientists can predict where they might be — yet they never know where they are. (That's different than, say, a baseball. If you leave it under your bed, you know it’s there and that it will stay there until you move it.)

If you drop a pebble in a pond, waves ripple away in circles. Particles sometimes travel like those waves. But they also can travel like a pebble.

severija/iStockphoto

“The bottom line is, the quantum world just doesn’t work in the way the world around us works,” says David Lindley. “We don't really have the concepts to deal with it,” he says. Trained as a physicist, Lindley now writes books about science (including quantum science) from his home in Virginia.

Here’s a taste of that weirdness: If you hit a baseball over a pond, it sails through the air to land on the other shore. If you drop a baseball in a pond, waves ripple away in growing circles. Those waves eventually reach the other side. In both cases, something travels from one place to another. But the baseball and the waves move differently. A baseball doesn’t ripple or form peaks and valleys as it travels from one place to the next. Waves do.

But in experiments, particles in the subatomic world sometimes travel like waves. And they sometimes travel like particles. Why the tiniest laws of nature work that way isn't clear — to anyone.

Consider photons. These are the particles that make up light and radiation. They're tiny packets of energy. Centuries ago, scientists believed light traveled as a stream of particles, like a flow of tiny bright balls. Then, 200 years ago, experiments demonstrated that light could travel as waves. A hundred years after that, newer experiments showed light could sometimes act like waves, and sometimes act like particles, called photons. Those findings caused a lot of confusion. And arguments. And headaches.

Wave or particle? Neither or both? Some scientists even offered a compromise, using the word “wavicle.” How scientists answer the question will depend on how they try to measure photons. It’s possible to set up experiments where photons behave like particles, and others where they behave like waves. But it's impossible to measure them as waves and particles at the same time. 

At the quantum scale, things can appear as particles or waves — and exist in more than one place at once.

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This is one of the bizarre ideas that pops out of quantum theory. Photons don't change. So how scientists study them shouldn't matter. They shouldn't only see a particle when they look for particles, and only see waves when they look for waves.

“Do you really believe the moon exists only when you look at it?” Albert Einstein famously asked. (Einstein, born in Germany, played an important role in developing quantum theory.)  

This problem, it turns out, is not limited to photons. It extends to electrons and protons and other particles as small or smaller than atoms. Every elementary particle has properties of both a wave and a particle. That idea is called wave-particle duality. It’s one of the biggest mysteries in the study of the smallest parts of the universe. That’s the field known as quantum physics.

Quantum physics will play an important role in future technologies — in computers, for example. Ordinary computers run calculations using trillions of switches built into microchips. Those switches are either “on” or “off.” A quantum computer, however, uses atoms or subatomic particles for its calculations. Because such a particle can be more than one thing at the same time — at least until it's measured — it may be "on" or "off" or somewhere in-between. That means quantum computers can run many calculations at the same time. They have the potential to be thousands of times faster than today's fastest machines.

IBM and Google, two major technology companies, are already developing superfast quantum computers. IBM even allows people outside the company to run experiments on its quantum computer.   

Experiments based on quantum knowledge have produced astonishing results. For example, in 2001, physicists at Harvard University, in Cambridge, Mass., showed how to stop light in its tracks. And since the mid-1990s, physicists have found bizarre new states of matter that were predicted by quantum theory. One of those — called a Bose-Einstein condensate — forms only near absolute zero. (That’s equivalent to –273.15° Celsius, or –459.67° Fahrenheit.) In this state, atoms lose their individuality. Suddenly, the group acts as one big mega-atom.

Quantum physics isn't just a cool and quirky discovery, though. It's a body of knowledge that will change in unexpected ways how we see our universe — and interact with  it.

A quantum recipe

Quantum theory describes the behavior of things — particles or energy — on the smallest scale. In addition to wavicles, it predicts that a particle may be found in many places at the same time. Or it may tunnel through walls. (Imagine if you could do that!) If you measure a photon’s location, you might find it in one place — and you might find it somewhere else. You can never know for certain where it is.

Also weird: Thanks to quantum theory, scientists have shown how pairs of particles can be linked — even if they’re on different sides of the room or opposite sides of the universe. Particles connected in this way are said to be entangled. So far, scientists have been able to entangle photons that were 1,200 kilometers (750 miles) apart. Now they want to stretch the proven entanglement limit even farther. 

Quantum theory thrills scientists — even as it frustrates them.

It thrills them because it works. Experiments verify the accuracy of quantum predictions. It also has been important to technology for more than a century. Engineers used their discoveries about photon behavior to build lasers. And knowledge about the quantum behavior of electrons led to the invention of transistors. That made possible modern devices such as laptops and smartphones.

But when engineers build these devices, they do so following rules that they don’t fully understand. Quantum theory is like a recipe. If you have the ingredients and follow the steps, you end up with a meal. But using quantum theory to build technology is like following a recipe without knowing how food changes as it cooks. Sure, you can put together a good meal. But you couldn’t explain exactly what happened to all of the ingredients to make that food taste so great.

Scientists use these ideas “without any idea of why they should be there,” notes physicist Alessandro Fedrizzi. He designs experiments to test quantum theory at Heriot-Watt University in Edinburgh, Scotland. He hopes those experiments will help physicists understand why particles act so strangely on the smallest scales.

Is the cat okay?

Albert Einstein was one of several scientists who worked out quantum theory in the early 20th century, sometimes in public debates that made newspaper headlines, such as this May 4, 1935 story from the New York Times.

New York Times/Wikimedia Commons

If quantum theory sounds strange to you, don’t worry. You’re in good company. Even famous physicists scratch their heads over it.

Remember Einstein, the German genius? He helped describe quantum theory. And he often said he didn’t like it. He argued about it with other scientists for decades. 

“If you can think about quantum theory without getting dizzy, you don't get it,” Danish physicist Niels Bohr once wrote. Bohr was another pioneer in the field. He had famous arguments with Einstein about how to understand quantum theory. Bohr was one of the first people to describe the weird things that pop out of quantum theory.

“I think I can safely say that nobody understands quantum [theory],” noted American physicist Richard Feynman once said. And yet his work in the 1960s helped show that quantum behaviors aren’t science fiction. They really happen. Experiments can demonstrate this.

Quantum theory is a theory, which in this case means it represents scientists’ best idea about how the subatomic world works. It’s not a hunch, or a guess. In fact, it’s based on good evidence. Scientists have been studying and using quantum theory for a century. To help describe it, they sometimes use thought experiments. (Such research is known as theoretical.)

In 1935, Austrian physicist Erwin Schrödinger described such a thought experiment about a cat. First, he imagined a sealed box with a cat inside. He imagined the box also contained a device that could release a poison gas. If released, that gas would kill the cat. And the probability the device released the gas was 50 percent. (That's the same as the chance that a flipped coin would turn up heads.)