RECENT ARTICLES

The impossibility of building a perfect clock could help explain away microscale weirdness.

An all-encompassing framework of physics could help to explain the evolution of consciousness, intelligence, and free will.

The search is on for a fundamental framework that allows for even stranger links between particles than quantum theory—which could lead us to a theory of everything.

A radical theory replaces the cosmic crunchers with fuzzy quantum spheres, potentially solving the black-hole information paradox and explaining away the Big Bang and the origin of time.

Why physics and astronomy communities must take diversity issues seriously in order to do good science.

FQXI ARTICLE

September 19, 2018

Dissolving Quantum Paradoxes

The impossibility of building a perfect clock could help explain away microscale weirdness.

FQXi Awardees: Lídia del Rio, Renato Renner

August 31, 2018

Renato Renner

ETH Zurich

Shakespeare’s familiar line is 400 years old; it came decades before Newton, and centuries before relativity and quantum mechanics. But, the way theoretical physicists Lídia del Rio and Renato Renner, both of ETH Zurich, see it, Shakespeare may have been on to something.

Physicists traditionally like to describe the world as if they were observing it from the outside: audience members at the theater of the universe, watching the story unfold without participating in the action on stage. Call it "the God view," says Renner. It’s a formulation that works just fine in Newton’s classical mechanics. The problem? It may not actually exist.

One of the foundational insights of quantum theory is that, just by observing a system, you change it. The problem of Schrödinger’s poor, overworked cat turns on the peculiar power of the observer. As the paradox goes, a cat is trapped in a box with a vial of poison that will be released if a radioactive particle decays. But according to quantum mechanics, until the moment it is observed, the particle is in a weird state in which it has both decayed and not decayed, which means that the cat is suspended in a limbo state in which it is both alive and dead at the same time. Some thinkers say that the cat is in a "superposition" of states: that is, the cat is both alive and dead simultaneously. Others argue that the cat is in a "mixture" of states: there are equal, 50/50 odds that the cat is alive or dead. Either way, when an observer finally checks on the cat, the observation instantaneously settles the quantum stalemate, rendering the cat either 100% alive or 100% dead.

Collapsing Cats

One way to interpret this is that the observation caused the cat’s wavefunction—the mathematical expression of the cat’s possible states and how likely they are—to "collapse" into a single outcome. But today many theorists favor a different way of thinking, called the "many worlds" interpretation. In this view of things, observing the cat makes reality branch into two separate but equally valid realities. In one, the cat lives; in the other, it dies.

Which brings us back to the quantum theater, where the audience is never passive. Simply by their presence, they affect the turns the story takes, the way the characters behave, and the way the play goes. It’s a perfectly Shakespearean insight. After all, historians think that, back in Shakespeare’s day, the cheap seats (which weren’t seats at all) were filled with rowdy hecklers who were happy to interrupt the show, or even get up on stage themselves. The invisible "wall" that separates actors from audience simply wasn’t there. All the world was a stage, and the stage was all the world.

Each time I measure

time, my clock… is a

bit degraded.

time, my clock… is a

bit degraded.

- Renato Renner

Once you start thinking of an agent as a quantum system in his or her own right, says Renner, things get complicated. Imagine an experiment in which one scientist tries to use quantum mechanics to make predictions about other scientists who, in turn, use quantum mechanics to make predictions, like a play within a play within a play. Because the experimenters are all quantum systems, they must model not just the quantum state of the equipment and particles in each others’ labs, but each other as well. In certain circumstances, the result will be a contradiction, as if one member of the audience wept to see Romeo and Juliet die, while another cried happy tears when she saw the lovebirds live happily ever after.

Quantum Clock

Removing the wall between actors and audience also means eliminating the wristwatches that the audience members keep glancing at and admitting that the only time that matters is time as it passes inside the play. It’s an idea that was first introduced in the 1980s, when FQXi members Don Page, now at the University of Alberta, Canada, and William Wootters, at Williams College, in Williamstown, Massachusetts, suggested nixing the notion of a "universal" clock that exists independently of the system being studied and replacing it with a "quantum" clock. Instead of ticking away off-stage, the quantum clock is itself a quantum system. It could be as simple as a single photon or as elaborate as the movement of a Swiss clock—the more complicated the system, the more precise the clock—but whatever it is, it runs on the same quantum rules as the rest of the world.

That means that, no matter how massive or elaborate it is, a quantum clock cannot be infinitely precise. "If you measure a quantum system, you necessarily disturb it," says Renner. "Each time I measure time, my clock that gives me the time information is a bit degraded."

Time Distortion

Without an accurate clock, the difference between

various interpretations of quantum paradoxes

disappears.

"What we are now trying to do is to find out what it is that bounds the ultimate accuracy of clocks," says Renner. They are using an approach based on information theory to try and understand the physical principles that are responsible for the fundamental limits on time measurements.

"Let’s say we would like to decide whether there’s really a superposition of the dead cat and living cat," says Renner, returning to the Schrödinger’s cat paradox. "If we want to do that we have to control the system extremely well," in particular the wavefunction, says Renner. That level of control would require an exquisitely precise clock—one that might be impossible to build.

If Renner and del Rio can show that such a precise quantum clock is a physical impossibility, that would meant that there is no way to discriminate between a superposition and a mixture in a "macroscopic" object like a cat, and the difference between the two states would lose its meaning. "Then the distinction between superpositions and mixtures is just a mathematical curiosity without a ’physically existing’ counterpart," says Renner. "The paradox would dissolve."

That would also resolve the play-within-a-play problem. If there are limits on how well we can measure time, that means that there are also limits on how well quantum mechanics can describe the world. That innate inexactness might not matter on small scales, but it could compound for large systems, like scientists. "If we give up the idea that there is a perfectly precise time then the predictions that (current) quantum theory makes about scientists who use quantum theory may be wrong," Renner says.

Theorists working on quantum gravity—the problem of reconciling quantum theory with Einstein’s description of gravity—are also eager for new ways to think about time. "The goal of quantum gravity is to describe space-time itself—and thus, in some sense, the whole universe—quantum mechanically, and then there is no space for external clocks or observers," says Markus Müller, a quantum physicist at the Austrian Academy of Sciences in Vienna. "Instead, one has to treat clocks as part of the quantum-mechanical system"—just as del Rio and Renner are doing.

Matthew Leifer, a quantum physicist at Chapman University, also hopes that del Rio and Renner’s work could solve multiple longstanding theoretical puzzles at once. "The approach seems very ambitious in that they are trying to tie together several problems that are usually thought of separately, and thought to be very difficult on their own," he says. "Maybe thinking about these as instances of the same general problem will lead to new insights."

No one knows how this particular play will turn out, including del Rio and Renner. "In such conceptual work, it is always hard to say how close one is to a result," says Renner. So, for now, we’ll stay on the edge of our seats, waiting to find out what happens before the curtain falls.

Comment on this Article

Please read the important Introduction that governs your participation in this community. Inappropriate language will not be tolerated and posts containing such language will be deleted. Otherwise, this is a free speech Forum and all are welcome!

function ValidatePostText_main () {
form = document.addPostForm_main;
var recaptcha = $("#g-recaptcha-response").val();
if (recaptcha === "") {
event.preventDefault();
alert("The reCaptcha Box below must be checked before you submit the form");
}
else if (form.postText_main.value == '') {
alert ("The post contains no text");
return false;
}
else {
return true;
}
}

**Your name:**
(optional)

Recent Comments

read all article comments

Please read the important Introduction that governs your participation in this community. Inappropriate language will not be tolerated and posts containing such language will be deleted. Otherwise, this is a free speech Forum and all are welcome!

Please enter the text of your post, then click the "Submit New Post" button below. You may also optionally add file attachments below before submitting your edits.

HTML tags are not permitted in posts, and will automatically be stripped out. Links to other web sites are permitted. For instructions on how to add links, please read the link help page.

You may use superscript (10

^{100}) and subscript (A_{2}) using [sup]...[/sup] and [sub]...[/sub] tags.You may use bold (

**important**) and italics (*emphasize*) using [b]...[/b] and [i]...[/i] tags.You may also include LateX equations into your post.

Insert LaTeX Equation
[hide]

LaTeX equations may be displayed in FQXi Forum posts by including them within [equation]...[/equation] tags. You may type your equation directly into your post, or use the LaTeX Equation Preview feature below to see how your equation will render (this is recommended).

For more help on LaTeX, please see the LaTeX Project Home Page.

LaTeX Equation Preview

preview equation

clear equation

insert equation into post at cursor

LaTeX equations may be displayed in FQXi Forum posts by including them within [equation]...[/equation] tags. You may type your equation directly into your post, or use the LaTeX Equation Preview feature below to see how your equation will render (this is recommended).

For more help on LaTeX, please see the LaTeX Project Home Page.

LaTeX Equation Preview

preview equation

clear equation

insert equation into post at cursor

Attachments
[hide]

You may optionally attach up to two documents to your post. To add an attachment, use the following feature to browse your computer and select the file to attach. The maximum file size for attachments is 1MB.

Once you're done adding file attachments, click the "Submit New Post" button to add your post.

You may optionally attach up to two documents to your post. To add an attachment, use the following feature to browse your computer and select the file to attach. The maximum file size for attachments is 1MB.

Once you're done adding file attachments, click the "Submit New Post" button to add your post.

JOE WILLIAM FISHER wrote on September 18, 2018

Dear Potential Realists,

Yesterday’s Closer to Truth Facebook page contained this ridiculous statement:

“Could information be the stuff of which everything is made? Information seems so abstract, not a substance or a thing, so how could it be the building blocks of reality? There are ways and reasons how information can literally be reality, some scientists claim, and their ideas are revolutionary. Cosmologist and professor Sean M. Carroll provides his perspective on our...

Dear Potential Realists,

Yesterday’s Closer to Truth Facebook page contained this ridiculous statement:

“Could information be the stuff of which everything is made? Information seems so abstract, not a substance or a thing, so how could it be the building blocks of reality? There are ways and reasons how information can literally be reality, some scientists claim, and their ideas are revolutionary. Cosmologist and professor Sean M. Carroll provides his perspective on our...

JOE WILLIAM FISHER wrote on September 17, 2018

Dear Loyal Reality Followers,

The September 14, 2018 Closer to Truth Facebook page contained this ridiculous statement:

“Stanford University has released a five-part series that outlines the ways that theoretical physicists at Stanford University have contributed to the development of the String Theory Landscape. The series includes profiles of Closer To Truth contributors Leonard Susskind, Andrei Linde, and Alan Guth.”

Naturally, I posted this sagacious comment at the...

Dear Loyal Reality Followers,

The September 14, 2018 Closer to Truth Facebook page contained this ridiculous statement:

“Stanford University has released a five-part series that outlines the ways that theoretical physicists at Stanford University have contributed to the development of the String Theory Landscape. The series includes profiles of Closer To Truth contributors Leonard Susskind, Andrei Linde, and Alan Guth.”

Naturally, I posted this sagacious comment at the...

JOE WILLIAM FISHER wrote on September 10, 2018

Dear Georgina Woodward,

All real portions of real chemicals have a VISIBLE surface. Only one unified VISIBLE infinite surface ETERNALLY occurring in one infinite dimension that am always mostly illuminated by finite non-surface light has ever existed. Finite invisible quantum particles are delusions of unnatural human thoughtlessness.

Joe Fisher, Realist

Dear Georgina Woodward,

All real portions of real chemicals have a VISIBLE surface. Only one unified VISIBLE infinite surface ETERNALLY occurring in one infinite dimension that am always mostly illuminated by finite non-surface light has ever existed. Finite invisible quantum particles are delusions of unnatural human thoughtlessness.

Joe Fisher, Realist

read all article comments