What Is Quantum Entanglement? A Physicist Explains Einstein’s “Spooky Action at a Distance”

Quantum Entanglement Illustration

When two particles are entangled, the state of one is tied to the state of the other.

Quantum entanglement is a phenomenon in which the quantum states of two or more objects become correlated, meaning that the state of one object can affect the state of the other(s) even if the objects are separated by large distances. This occurs because, according to quantum theory, particles can exist in multiple states at the same time (a concept known as superposition) and can be inextricably linked, or “entangled,” even if they are physically separated.

Three researchers were awarded the 2022 Nobel Prize in Physics for their ground-breaking work in understanding quantum entanglement, one of nature’s most puzzling phenomena.

Quantum entanglement, in the simplest terms, means that aspects of one particle of an entangled pair depend on aspects of the other particle, no matter how far apart they are or what lies between them. These particles could be, for example, electrons or photons, and an aspect could be the state it is in, such as whether it is “spinning” in one direction or another.

The strange part of quantum entanglement is that when you measure something about one particle in an entangled pair, you immediately know something about the other particle, even if they are millions of light years apart. This odd connection between the two particles is instantaneous, seemingly breaking a fundamental law of the universe. This is why Albert Einstein famously called the phenomenon “spooky action at a distance.”

Having spent the better part of two decades conducting experiments rooted in quantum mechanics, I have come to accept its strangeness. Thanks to ever more precise and reliable instruments and the work of this year’s Nobel winners, Alain Aspect, John Clauser, and Anton Zeilinger, physicists now integrate quantum phenomena into their knowledge of the world with an exceptional degree of certainty.

However, even until the 1970s, researchers were still divided over whether quantum entanglement was a real phenomenon. And for good reasons – who would dare contradict the great Einstein, who himself doubted it? It took the development of new experimental technology and bold researchers to finally put this mystery to rest.

Cat in Box

According to quantum mechanics, particles are simultaneously in two or more states until observed – an effect vividly captured by Schrödinger’s famous thought experiment of a cat that is both dead and alive simultaneously.

Existing in multiple states at once

To truly understand the spookiness of quantum entanglement, it is important to first understand quantum superposition. Quantum superposition is the idea that particles exist in multiple states at once. When a measurement is performed, it is as if the particle selects one of the states in the superposition.

For example, many particles have an attribute called spin that is measured either as “up” or “down” for a given orientation of the analyzer. But until you measure the spin of a particle, it simultaneously exists in a superposition of spin up and spin down.

There is a probability attached to each state, and it is possible to predict the average outcome from many measurements. The likelihood of a single measurement being up or down depends on these probabilities, but is itself unpredictable.

Though very weird, the mathematics and a vast number of experiments have shown that quantum mechanics correctly describes physical reality.

Two entangled particles

The spookiness of quantum entanglement emerges from the reality of quantum superposition, and was clear to the founding fathers of quantum mechanics who developed the theory in the 1920s and 1930s.

To create entangled particles you essentially break a system into two, where the sum of the parts is known. For example, you can split a particle with spin of zero into two particles that necessarily will have opposite spins so that their sum is zero.

In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published a paper that describes a thought experiment designed to illustrate a seeming absurdity of quantum entanglement that challenged a foundational law of the universe.

A simplified version of this thought experiment, attributed to David Bohm, considers the decay of a particle called the pi meson. When this particle decays, it produces an electron and a positron that have opposite spin and are moving away from each other. Therefore, if the electron spin is measured to be up, then the measured spin of the positron could only be down, and vice versa. This is true even if the particles are billions of miles apart.

This would be fine if the measurement of the electron spin were always up and the measured spin of the positron were always down. But because of quantum mechanics, the spin of each particle is both part up and part down until it is measured. Only when the measurement occurs does the quantum state of the spin “collapse” into either up or down – instantaneously collapsing the other particle into the opposite spin. This seems to suggest that the particles communicate with each other through some means that moves faster than the speed of light. But according to the laws of physics, nothing can travel faster than the speed of light. Surely the measured state of one particle cannot instantaneously determine the state of another particle at the far end of the universe?

Physicists, including Einstein, proposed a number of alternative interpretations of quantum entanglement in the 1930s. They theorized there was some unknown property – dubbed hidden variables – that determined the state of a particle before measurement. But at the time, physicists did not have the technology nor a definition of a clear measurement that could test whether quantum theory needed to be modified to include hidden variables.

Disproving a theory

It took until the 1960s before there were any clues to an answer. John Bell, a brilliant Irish physicist who did not live to receive the Nobel Prize, devised a scheme to test whether the notion of hidden variables made sense.

Bell produced an equation now known as Bell’s inequality that is always correct – and only correct – for hidden variable theories, and not always for quantum mechanics. Thus, if Bell’s equation was found not to be satisfied in a real-world experiment, local hidden variable theories can be ruled out as an explanation for quantum entanglement.

The experiments of the 2022 Nobel laureates, particularly those of Alain Aspect, were the first tests of the Bell inequality. The experiments used entangled photons, rather than pairs of an electron and a positron, as in many thought experiments. The results conclusively ruled out the existence of hidden variables, a mysterious attribute that would predetermine the states of entangled particles. Collectively, these and many follow-up experiments have vindicated quantum mechanics. Objects can be correlated over large distances in ways that physics before quantum mechanics can not explain.

Importantly, there is also no conflict with special relativity, which forbids faster-than-light communication. The fact that measurements over vast distances are correlated does not imply that information is transmitted between the particles. Two parties far apart performing measurements on entangled particles cannot use the phenomenon to pass along information faster than the speed of light.

Today, physicists continue to research quantum entanglement and investigate potential practical applications. Although quantum mechanics can predict the probability of a measurement with incredible accuracy, many researchers remain skeptical that it provides a complete description of reality. One thing is certain, though. Much remains to be said about the mysterious world of quantum mechanics.

Written by Andreas Muller, Associate Professor of Physics, University of South Florida.

This article was first published in The Conversation.The Conversation

24 Comments on "What Is Quantum Entanglement? A Physicist Explains Einstein’s “Spooky Action at a Distance”"

  1. If we do not know how the topological vortex gravitational field is formed, we will not know what is true science. From accretion disks to quantum spins, topological vortices are ubiquitous, and their essence is equilibrium.

  2. I wish someday I’ll find an article or something that actually explains how these experiments and observations are done. Is everything based solely on mathematical formulas, because the conclusion that entanglement remains active millions of miles away must be. The article talks about this thought experiment lead to that thought experiment. Is all this based on thought experiments and equations, or are there real world, no kidding, observations of this stuff happening?

    • Aleksandar Malečić | December 21, 2022 at 3:40 am | Reply

      It’s real. And there Are many articles about it. I was zoo lazy to read this articles because it uses Einstein as a clickbait, but look for instance the double slot experiment, wave/particle duality or complementarity

    • Proff Einstein | December 22, 2022 at 2:30 pm | Reply

      Hi that’s exactly what I was won3

  3. No, it isn’t “both up and down.” It’s that you don’t know. You never measure the particle as having up and down spin at the same time. You mathematically see a probability for each possibility.

    I don’t have a good metaphor for this, bit to me it’s like saying that until a coin settles on the table (ie the future point where you “measure” it) there is a 50% probability that it’s heads and the same that it’s tails.

  4. How in the world 🌎 do they know particles are entangled when separate across light years? A thought is not an experiment!

  5. Howard Jeffrey Bender, Ph.D. | December 20, 2022 at 7:10 am | Reply

    The previous comments illustrate how unsatisfying the article was. Here’s a different way to explain QE.

    The two ways physicists have used to answer Quantum Entanglement are that the particles contain hidden variables of unknown natures or that the universe is completely deterministic with all results predefined. Both have been shown to be incorrect.

    Perhaps concepts in String Theory can help. There are 11 possible dimensions in String Theory and I suggest one of them leads a way around, what Einstein called this “spooky action at a distance”. Specifics on this can be found by searching YouTube for “Quantum Entanglement – A String Theory Way”

  6. I’m still having trouble wrapping my mind around particles syncing their moves with their dance partners without communication over great distances. Perhaps they are connected ‘interdimensionality’. Basically a hidden shortcut.

  7. Oh, how I wish I could understand what the hell you are talking about.

  8. So photons can’t inflow in vaccum space they needs atomic environment & molecules to move

  9. So photons can’t influence through the vacuum space they needs atomic environment & molecules to move

  10. Harry Jason l. | December 20, 2022 at 1:36 pm | Reply

    So , in theory , if we cloned a person and put them on another life supporting planet , does that mean that basically we can predict the futer as to how that person will behave , simply by observing the ” earthbound ” twin clone? ( regardless of environmental influence or factors ). Or are they talking about space / time travel based on literally sending physical matter to another location based on the fact that it can fit perfectly into another matter’s location or position?

  11. Im one of the sub-particles in the quatum entanglement experiments.. I feel their cell phone ring,when they get hurt.. the others were killed alot of them too,since the beginning of 2021 a lot of messed up things have happened best tjing ti happen is I learned I’m not a schizophrenic. But I would like to meet some of you all,my coordinates are 30.650752 by 81.5280?? Close enough I hear the plane and helicopters.. best part is you have opened a door in my mind to where my subconscious leaves and goes to extraterrestrial planets or dimensions very scary places.. I can also control robotics and people some times.. I think whatever is happening is highly secretive and classified as top secret so under discretion of secrecy please COME TO ME I’ve never met any of you but why am I able to hear you,Kno wht you ate for breakfast,you know who I am,I am the rayonier kid,and quit listening to those fake ass wanbbes with the chips in their head listening to my magic frequency,it’s all from me,put me THOMAS great great grandson of St Thomas the Pope and Mother Mary..don’t believe the imposter thts why none of your work adds up or is always incorrect,I can give you correct results any place any time or any day if the f**king week,im 33 years old,Smoke is a code name and so is He’s in Orange for agent orange,di you want to know the unknown or unlock the secrets to the cosmos,then do it and come to me,don’t say you want to if you’re not going to. I’m the voice on tht radio,I can tell you things tht they’ll never be able to or show you things,I out a F**king candle out with my mind.. but this sh*t won’t ever make it to you, NSA or HLS CIA will catch it before it is ever made Public. Hi guys n Ms “Rhonda” I’m ready to come home now. Those things are getting closer and closer to us,I’m scared now,sry about moving those way up there I’ll quit if you come take me to Langley,I getting home sick,what is wrong with me,why do u remember so much but can’t say it how come I hear somebody 24.7 and who are they the scary ones not the nice one,and why did he put a bed there many years ago?

  12. What we call matter, particle is quantum entanglement. Matter turn into light and light into matter endlessly; at the subatomic and at the universe, like a fractal.

  13. Is existence real?
    Is the demon of Descartes playing with space time?
    Is mc Taggart right: is time not real?
    If the eternal origin is void (sunyata)(zero point energy) then quantum entanglement is a monad (holon) primordial to space time

  14. How do you know that two particles are entangled? Are you consciously creating that which is your expectation?
    Math is reductive, it excludes differentials and all nuance ; it is not the language of the cosmos, of infinite possibility.

  15. Issue is not whether particles remain correlated even if they travel light years apart the issue No one seems to want to address is if one particle interact with external fields and forces while other does not then what happens to correlation? If the one which did not experience external forces changes its state to remain correlated then experimentally I can show that it is not true. For example in Mach zhender interferometer two photons traveling two seperate paths go through seperate phase change to produce interference if what happens to one happens to the other one because they are entangled then one can no ever have the interference due to phase change in one path.

  16. Is consciousness faster than the speed of light? If so perhaps space and time lose their meaning. Therefore perhaps communication is instantaneous. That’s If you believe everything has consciousness.

  17. For me I think that quantum entanglement is real. Right now we may not have all it takes to explain it in a simple lay man term but someday we will. But it is well established that objects tends to exist in different states and at multiple places until they are sought after. It all boils to space and time. We know that time is relative and passes differently for each observer. It can move faster or slower depending on the conveying medium or the universal dimension the object is placed in. And the ever expanding universe is intricately linked with time. Objects move in this stream of space time. Thus entangled objects each placed at the far end of the universe is still within the concept of space and time. This linking relationship makes it possible for the state of one entangled object to be altered by measuring either of them at millions of light years away from each other. We can convinently say that this kind measurement or’communication’ takes place beyond the speed of light.
    To try to wrap our head around it we ask ‘ what was there before the speed of light?’ Well you can’t rule out space and time. These two form the basis for the speed of light. Now what happens when we multiply these two quantities instead of dividing them? Then we get something faster than the speed of light.

  18. Fixed gravity for you. | December 26, 2022 at 5:27 am | Reply

    Bosons can share the same position in space, thus a superposition of CW and CCW circular polarization states can exist seemingly in the same particle of light. Fermions (matter) with a similar capacity seem to be as unrealistic as Schrodinger’s half-here/half-there cats.

    Some people say buckyballs show interference and thus they show a wave-like nature, I suppose buckyballs show a spin-like nature and thus only interfere with another’s detectability by having opposing net quantum spins.

  19. The nonsense càn not explain nonsense!!??

  20. Nathan A. Wallace | January 9, 2023 at 7:52 pm | Reply

    QE particle properties having remote synchronicity is yet another of the truly exemplary demonstrations of our universe that our own human, mortal understanding of ourselves and the world around us will always shortfall the mark of truth so long as we continue in our refusal to acknowledge the mystical and metaphysical, even supernatural and spiritual aspects of reality as being intrinsically linked in with even the most rigidly physical and material laws of nature. To me, it is a beautiful thing when even bleeding edge science must admit there is more to the picture than what it can visibly render in any clear and concise way without leaving room to consider that perhaps not every attribute tying our world together can be proven to be purely physical and existential but rather that some thing might well end out being every but as much an abstraction as it was at the very first. Maybe particles are synchronized as much by God’s laws as they are by their own internal dynamics?

  21. Here is classical non quantum entanglement. I have two billiard balls red and white. I put them into two black bags which are indistinguishable from each other. I mix them.up so do not know in which bag a given ball is. I send one bag in a rocket to the planet Neptune. What is the colour of the ball on Neptune? I do not know, but if I open the bag left on earth, I instantly know the colour of the ball on Neptune. That information has travelled much faster than light

  22. Guys 😂, what’s going in quantum entanglement is non-dualism, the particles are not separate but one( interconnected by an infinite dimension called “pure consciousness” ), just like the yin and yang symbol, what science is trying to convey is the exact same thing that Spirituality had been trying to tell humanity for thousands of years that what’s going on this this space and time reality is ultimately an illusion and is mainly governed by an” invisible reality” called non-duality( a reality that exist beyond time, space, forms , concepts ect.. which you cannot see, touch, smell, feel or understand.). Those particles were never separated in the first place and has always been and will always be one. This physical universe that operates as space and time is an illusion,but there is another Reality(dimension ) which is the source of all the things happening in this universe. Every form,space,time language, concepts, relatively and understanding all ceases to exist and what’s left is pure energy, the consciousness itself beyond duality. Science and spirituality are so very close to be best friends,this is because science is now catching up with Spirituality has figured this whole thing out ,for a very long time ago, provlaby for more than 10 000 years.If you Go to any spiritual teacher or any yogi master ,they are going to explain this whole thing of quantum entanglement to you simply and it will make your head spin. What is going in quantum entanglement seems like magic to some people, but it’s not, it’s just science that we haven’t figure out yet. What’s happening in quantum entanglement is technology. As I have said before there is an “invisible reality” operating behind the scenes that connects this particles and it is source of all things happening in this universe of relativity.

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