The quantum world is mind-bogglingly weird | Science News for Students

The quantum world is mind-bogglingly weird

No matter how hard physicists probe, they still puzzle over this, the universe's deepest secret
Sep 14, 2017 — 7:16 am EST
quantum atom

The quantum world is the world that’s smaller than an atom. Things at this scale don’t behave the same way as objects on the scale that we can see.


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.

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.)

ripples pond
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.

“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. 

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

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?

quantum newspaper
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.)

Schrödinger cat
This is a diagram of the Schrödinger's cat thought experiment. The only way to know if the poison was released and the cat is dead or alive is to open the box and look inside.
Dhatfield/Wikimedia Commons (CC-BY-SA 3.0)

To check the status of the cat, you open the box.

The cat is either alive or dead. But if cats behaved like quantum particles, the story would be stranger. A photon, for instance, can be a particle and a wave. Likewise, Schrödinger’s cat can be alive and dead at the same time in this thought experiment. Physicists call this “superposition.” Here, the cat won’t be one or the other, dead or alive, until someone opens the box and takes a look. The fate of the cat, then, will depend on the act of doing the experiment.

Schrödinger used that thought experiment to illustrate a huge problem. Why should the way that the quantum world behaves depend on whether someone is watching?

Welcome to the multiverse

Anthony Leggett has been thinking about this problem for 50 years. He’s a physicist at the University of Illinois at Urbana-Champaign. In 2003, he won a Nobel Prize in physics, the most prestigious award in his field. Leggett has helped develop ways to test quantum theory. He wants to know why the smallest world doesn’t match with the ordinary one we see. He likes to call his work “building Schrödinger’s cat in the laboratory.”

Leggett sees two ways to explain the problem of the cat. One way is to assume that quantum theory will eventually fail in some experiments. “Something will happen that is not described in the standard textbooks,” he says. (He has no idea what that something might be.)

The other possibility, he says, is more interesting. As scientists conduct quantum experiments on larger groups of particles, the theory will hold. And those experiments will unveil new aspects of quantum theory. Scientists will learn how their equations describe reality and be able to fill in the missing pieces. Eventually, they will be able to see more of the whole picture.

multiverse universe
Today, you decided to wear a certain pair of shoes. If there were multiple universes, there would be another world where you made a different choice. Today, there is no way to test this “many-world” or “multiverse” interpretation of quantum physics, however.

Simply put, Leggett hopes: “Things that right now seem fantastic will be possible.”

Some physicists have proposed even wilder solutions to the “cat” problem. For example: Maybe our world is one of many. It’s possible that infinitely many worlds exist. If true, then in the thought experiment, Schrödinger’s cat would be alive in half the worlds — and dead in the rest.

Quantum theory describes particles like that cat. They may be one thing or another at the same time. And it gets weirder: Quantum theory also predicts that particles may be found in more than one place at a time. If the many-world idea is true, then a particle might be in one place in this world, and somewhere else in other worlds.

This morning, you probably chose which shirt to wear and what to eat for breakfast. But according to the many worlds idea, there is another world where you made different choices. 

This weird idea is called the “many-world” interpretation of quantum mechanics. It is exciting to think about, but physicists have not found a way to test whether it’s true.

Tangled up in particles

Quantum theory includes other fantastic ideas. Like that entanglement. Particles may be entangled — or connected — even if they’re separated by the width of the universe.

Imagine, for instance, that you and a friend had two coins with a seemingly magical connection. If one showed up heads, the other would always be tails. You each take your coins home and then flip them at the same time. If yours comes up heads, then at the exact same moment you know your friend’s coin has just come up tails.

Entangled particles work like those coins. In the lab, a physicist can entangle two photons, then send one of the pair to a lab in a different city. If she measures something about the photon in her lab — such as how fast it moves — then she immediately knows the same information about the other photon. The two particles behave as though they send signals instantaneously. And this will hold even if those particles are now separated by hundreds of kilometers.

Story continues below video.

Quantum entanglement is really weird. Particles maintain a mysterious link that persists even if they are separated by light-years.

As in other parts of quantum theory, that idea causes a big problem. If entangled things send signals to each other instantly, then the message might seem to travel faster than the speed of light — which, of course, is the speed limit of the universe! So that cannot happen.

In June, scientists in China reported a new record for entanglement. They used a satellite to entangle six million pairs of photons. The satellite beamed the photons to the ground, sending one of each pair to one of two labs. The labs sat 1,200 kilometers (750 miles) apart. And each pair of particles remained entangled, the researchers showed. When they measured one of a pair, the other one was affected immediately. They published those findings in Science.

Scientists and engineers are now working on ways to use entanglement to link particles over ever-longer distances. But the rules of physics still prevent them from sending signals faster than the speed of light.      

Why bother?

If you ask a physicist what a subatomic particle really, truly is, “I don’t know that anyone can give you an answer,” says Lindley.

Many physicists are content with not knowing. They work with quantum theory, even though they don’t understand it. They follow the recipe, never quite knowing why it works. They may decide that if it works, why bother going any further?

Others, like Fedrizzi and Leggett, want to know why particles are so weird. “It’s far more important to me to find out what’s behind all of this,” Fedrizzi says.

Forty years ago, scientists were skeptical that they could do such experiments, notes Leggett. Many thought that asking questions about the meaning of quantum theory was a waste of time. They even had a refrain: “Shut up and calculate!”

Leggett compares that past situation to exploring sewers. Going into sewer tunnels might be interesting but not worth visiting more than once.

“If you were to spend all your time rummaging around in the bowels of the Earth, people would think you were rather strange,” he says. “If you spend all your time on the foundations of quantum [theory], people will think you’re a little odd.”

Now, he says, “the pendulum has swung the other way.” Studying quantum theory has become respectable again. Indeed, for many it has become a lifelong quest to understand the secrets of the tiniest world.

“Once the subject hooks you, it won’t let you go,” says Lindley. He, by the way, is hooked.

Power Words

(for more about Power Words, click here)

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

entanglement    (in quantum physics) A concept in quantum physics that holds that subatomic particles can be linked even if they are not physically near one another. Quantum entanglement can link the properties of things at great distances — perhaps at opposite ends of the universe.

equation     In mathematics, the statement that two quantities are equal. In geometry, equations are often used to determine the shape of a curve or surface.

field     An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.

laser     A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

matter     Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."

microchip    A tiny wafer of semiconducting material (the chip), often silicon, which holds tiny electronic parts and the "wiring" needed to connect them to an electric circuit. Or a small computer chip that is implanted in goods or animals and acts like a tag. It holds information that can be retrieved as needed (such as an animal's name or the inventory lot for commercial products.

multiverse    A term to connote the idea that our universe may be one of many (perhaps an infinite number of alternative universes) and that different things may happen in each.

Nobel prize     A prestigious award named after Alfred Nobel. Best known as the inventor of dynamite, Nobel was a wealthy man when he died on December 10, 1896. In his will, Nobel left much of his fortune to create prizes to those who have done their best for humanity in the fields of physics, chemistry, physiology or medicine, literature and peace. Winners receive a medal and large cash award.

particle     A minute amount of something.

photon     A particle representing the smallest possible amount of light or other electromagnetic radiation.

physics     The scientific study of the nature and properties of matter and energy. A scientist who works in such areas is known as a physicist.

probability     A mathematical calculation or assessment (essentially the chance) of how likely something is to occur.

proton     A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

quantum     (pl. quanta) A term that refers to the smallest amount of anything, especially of energy or subatomic mass.

quantum mechanics     A branch of physics dealing with the behavior of matter on the scale of atoms or subatomic particles.

quantum physics     A branch of physics that uses quantum theory to explain or predict how a physical system will operate on the scale of atoms or sub-atomic particles.

quantum theory     A way to describe the operation of matter and energy at the level of atoms. It is based on an interpretation that at this scale, energy and matter can be thought to behave as both particles and waves. The idea is that on this very tiny scale, matter and energy are made up of what scientists refer to as quanta — miniscule amounts of electromagnetic energy.

radiation     (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

satellite     A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

science fiction     A field of literary or filmed stories that take place against a backdrop of fantasy, usually based on speculations about how science and engineering will direct developments in the distant future. The plots in many of these stories focus on space travel, exaggerated changes attributed to evolution or life in (or on) alien worlds.

sewer     A system of water pipes, usually running underground, to move sewage (primarily urine and feces) and stormwater for collection — and often treatment — elsewhere.

skeptical     Not easily convinced; having doubts or reservations.

smartphone     A cell (or mobile) phone that can perform a host of functions, including search for information on the internet.

subatomic     Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

superposition      (in quantum physics) The ability of some minute subatomic-scale particle to be more than one place at the same time. It has to do with particles in the quantum world having the weird capacity to exist in all possible states (or positions) at once. (in geology) An understanding that unless subsurface strata of soil and rock have been disturbed somehow, the age of the materials will get successively older with depth.

theory     (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

thought experiments    Mathematical analyses of ideas, situations or events. They are not based on real-world tests in a lab or the environment. They instead use numbers and relationships between mathematical operations to test whether something can or will happen. This is also known as theoretical research.

transistor    A device that can act like a switch for electrical signals.

universe     The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

verify     (n. verification) To demonstrate or confirm in some way that a particular claim or suspicion is true.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

wave-particle duality    The concept that a subatomic particle can exhibit properties of a wave and a particle. But at any one time it will only show attributes of being either a wave or a particle.

wavicle     A term invented in 1928 by the British physicist Arthur Stanley Eddington to convey the duality of light and radiation as being both waves and particles, although they never appear to be both at the same time.


Journal: J. Yin et al. Satellite-based entanglement distribution over 1200 kilometers. Science. Vol. 356, June 16, 2017, p. 1140. doi: 10.1126/science.aan3211.

Journal: M. Ringbauer et al. Measurements on the reality of the wavefunction. Nature Physics. Vol. 11, March 2015, p. 249. doi: 10.1038/NPHYS3233.

Book: D. Lindley. Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science. Anchor Books, 2008, 272 pp.

Journal: C. Liu et al. Observation of coherent optical information storage in an atomic medium using halted light pulses. Nature. Vol. 409, January 25, 2001, p. 490. doi: 10.1038/35054017.

Book: D. Lindley. Where Does The Weirdness Go? Why Quantum Mechanics Is Strange, But Not As Strange As You Think. Basic Books, 1997, 268 pp.