Category: quantum gravity

Can We Test Gravitational Waves For Wave-Particle Duality?

“Although we have every reason to believe that gravitational waves are simply the quantum analog of electromagnetic waves, we have, unlike the electromagnetic photon, not yet risen to the technological challenges of directly detecting the gravitational particle that’s the counterpart of gravitational waves: the graviton.

Theorists are still calculating the uniquely quantum effects that should arise and are working together with experimentalists to design tabletop tests of quantum gravity, all while gravitational wave astronomers puzzle over how a future-generation detector might some day reveal the quantum nature of these waves. Although we expect gravitational waves to exhibit wave-particle duality, until we detect it, we cannot know for certain. Here’s hoping that our curiosity compels us to invest in it, that nature cooperates, and that we find out the answer once and for all!”

One of the revolutionary discoveries of the quantum world was that every particle that we know of also behaves as a wave. Photons are the quanta associated with light, and every light wave is made up of a discrete number of photons. Particles like electrons also can behave as waves; if you send them through a double slit, even one-at-a-time, they’ll produce an interference pattern.

So what about gravitational waves? We’ve seen the wave part; could we ever test them for the “particle” part of that? Find out today!

Ask Ethan: What Does It Mean That Quantum Gravity Has No Symmetry?

“What does it mean that quantum gravity doesn’t have symmetry?“

Last month, a scientific paper was published in a prestigious journal, entitled: Constraints on Symmetries from Holography. It states that three long-standing conjectures about quantum gravity, including one stating that quantum gravity does not allow global symmetries of any type, have just been proven if the holographic principle, and by extension the AdS/CFT correspondence that’s fundamental to string theory, is correct.

This is really, really big news. If there are no global symmetries, then there are no absolute conservation laws. Energy is not conserved, momentum and charge are not conserved, and additionally all sorts of things that are not forbidden but not observed, like magnetic monopoles, must necessarily exist. 

Paradoxically, if string theory is right, our expectations about hidden symmetries revealing themselves at a more fundamental level are not only wrong, but that nature has no global symmetries at all.

Come get the full, fascinating story, with all the deep implications that come along with it, today.

This Is Why Quantum Field Theory Is More Fundamental Than Quantum Mechanics

“But the motivation for quantizing the field is more fundamental than that the argument between those favoring perturbative or non-perturbative approaches. You need a quantum field theory to successfully describe the interactions between not merely particles and particle or particles and fields, but between fields and fields as well. With quantum field theory and further advances in their applications, everything from photon-photon scattering to the strong nuclear force was now explicable.”

What’s wrong with quantum mechanics? It might surprise you to hear that the answer is, “it isn’t quantum enough.” The enormous differences between the quantum and the non-quantum Universe are so striking, as we replace:

* continuous matter with discrete particles,
* ideal points with dual-nature wave/particle quanta,
* and observable properties like position and momentum with quantum mechanical operators containing an inherent uncertainty.

But it’s still not enough. For one, the original (Schroedinger) equation of quantum mechanics doesn’t play nice with relativity, but even the relativistically invariant versions don’t describe reality fully. Why not? Because only the particles are quantized in quantum mechanics. To reveal the full behavior, you need to quantize their fields and interactions, too.

Here’s how quantum field theory succeeds where quantum mechanics fails, and why Einstein’s dreams of unification were abandoned upon his death.

Could An Incompleteness In Quantum Mechanics Lead To Our Next Scientific Revolution?

“But in the quantum Universe, this notion of relativistic causality isn’t as straightforward or universal as it would seem. There are many properties that a particle can have — such as its spin or polarization — that are fundamentally indeterminate until you make a measurement. Prior to observing the particle, or interacting with it in such a way that it’s forced to be in either one state or the other, it’s actually in a superposition of all possible outcomes.

Well, you can also take two quantum particles and entangle them, so that these very same quantum properties are linked between the two entangled particles. Whenever you interact with one member of the entangled pair, you not only gain information about which particular state it’s in, but also information about its entangled partner.”

One of the most important ideas in classical physics are those of locality and causality: that objects in close proximity can affect one another through the forces they exert on one another, which are limited by the speed of light. But quantum mechanics turns much of that on its head, where locality doesn’t appear to be a fundamental property of reality at all. Yet one of the most remarkable ideas of all out there conjectures that quantum gravity, which contains fundamental non-localities, could be described by variables that completely explain what we view as non-locality in standard quantum physics.

Could this be right? Lee Smolin is giving a talk on that today, and I’ll be live-blogging it as it happens. Don’t miss this one-of-a-kind event!

Could An Incompleteness In Quantum Mechanics Lead To Our Next Scientific Revolution?

“But in the quantum Universe, this notion of relativistic causality isn’t as straightforward or universal as it would seem. There are many properties that a particle can have — such as its spin or polarization — that are fundamentally indeterminate until you make a measurement. Prior to observing the particle, or interacting with it in such a way that it’s forced to be in either one state or the other, it’s actually in a superposition of all possible outcomes.

Well, you can also take two quantum particles and entangle them, so that these very same quantum properties are linked between the two entangled particles. Whenever you interact with one member of the entangled pair, you not only gain information about which particular state it’s in, but also information about its entangled partner.”

One of the most important ideas in classical physics are those of locality and causality: that objects in close proximity can affect one another through the forces they exert on one another, which are limited by the speed of light. But quantum mechanics turns much of that on its head, where locality doesn’t appear to be a fundamental property of reality at all. Yet one of the most remarkable ideas of all out there conjectures that quantum gravity, which contains fundamental non-localities, could be described by variables that completely explain what we view as non-locality in standard quantum physics.

Could this be right? Lee Smolin is giving a talk on that today, and I’ll be live-blogging it as it happens. Don’t miss this one-of-a-kind event!

Ask Ethan: Are Gravitational Waves Themselves Affected By Gravity?

“Are gravitational waves themselves subject to gravity? That is, if a gravitational wave were to pass by a galaxy cluster, would its form get distorted (even though the wave, itself, is a distortion of space-time)? One side of me says gravitational waves are a form of energy so therefore must be affected by gravity. The other side of me says “Nah – that just doesn’t make sense!"”

Think about the fabric of space itself. All the masses and forms of energy in the Universe cause space itself to curve, while the curved space itself alters the path along which any matter or form of energy will travel. Massless particles, like photons, are bent by the fabric of space itself. But what about gravitational waves? Are they also subject to this, or does gravitation lack a self-interaction that it would require for this to be possible?

For a very long time, this was a question that was theoretical only. But over the last three years, we’ve observed a slew of gravitational waves, allowing this idea to be tested for the first time.

What were the results? Gravitational waves are affected by gravity, in at least three different observable ways. Come find out how today!

This Simple Thought Experiment Shows Why We Need Quantum Gravity

“The description that General Relativity puts forth — that of matter telling space how to curve, and curved space telling matter how to move — needs to be augmented to include an uncertain position that has a probability distribution to it. Whether gravity is quantized or not is still an unknown, and has everything to do with the outcome of such a hypothetical experiment. How an uncertain position translates into a gravitational field, exactly, remains an unsolved problem on the road to a full quantum theory of gravity. The principles that underlie quantum mechanics must be universal, but how those principles apply to gravity, and in particular to a particle passing through a double slit, is a great unknown of our time.”

Perhaps the greatest holy grail in theoretical physics is the quest for a quantum theory of gravity. For all the gravitational phenomena we’ve ever measured, observed, or subjected to a test, General Relativity has come through with predictions that match what we’ve seen exactly. For all the other physical phenomena in the Universe, the rules of quantum field theory and the Standard Model of particle physics match up perfectly. But what would happen if we tried to apply General Relativity to an inherently quantum phenomenon? In particular, what happens if we fire a single particle, like an electron, through a double slit? What happens to that particle’s gravitational field?

Believe it or not, measuring that (or something analogous to it) would tell us whether gravity is a fundamentally quantum force or not! Come learn why this is arguably the most important, first stop on the road to quantum gravity.

Are Space And Time Quantized? Maybe Not, Says Science

“Incredibly, there may actually be a way to test whether there is a smallest length scale or not. Three years before he died, physicist Jacob Bekenstein put forth a brilliant idea for an experiment where a single photon would pass through a crystal, causing it to move by a slight amount. Because photons can be tuned in energy (continuously) and crystals can be very massive compared to a photon’s momentum, it ought to be possible to detect whether the “steps” that the crystal moves in are discrete or continuous. With a low-enough energy photon, if space is quantized, the crystal would either move a single quantum step or not at all.”

When it comes to the Universe, everything that’s in it appears to be quantum. All the particles, radiation, and interactions we know of are quantized, and can be expressed in terms of discrete packets of energy. Not everything, however, goes in steps. Photons can take on any energy at all, not just a set of discrete values. Put an electron in a conducting band, and its position can take on a set of continuous (not discrete) values. And so then there’s the big question: what about space and time? Are they quantized? Are they discrete? Or might they be continuous, even if there’s a fundamental quantum theory of gravity.

Surprisingly, space and time don’t need to be discrete, but they might be! Here’s what the science has to say so far.

This Is Why Physicists Think String Theory Might Be Our ‘Theory Of Everything’

“String theory offers a path to quantum gravity, which few alternatives can truly match. If we make the judicious choices of “the math works out this way,” we can get both General Relativity and the Standard Model out of it. It’s the only idea, to date, that gives us this, and that’s why it’s so hotly pursued. No matter whether you tout string theory’s successes or failure, or how you feel about its lack of verifiable predictions, it will no doubt remain one of the most active areas of theoretical physics research. At its core, string theory stands out as the leading idea of a great many physicists’ dreams of an ultimate theory.”

You don’t have to be a fan of string theory to understand why it’s such a promising area of scientific research. One of the holy grails of physics is for a quantum theory of gravitation: that describes gravity on the same footing as the other three forces, in very strong fields and at very tiny distances. Surprisingly, by looking at analogies between gravity and field theories, replacing particles with strings might be the answer.

It’s an incredibly difficult concept to understand why this would be the case without a slew of advanced mathematics, but in 2015, the world’s leading string theorist, Ed Witten, tried. That is to say, he wrote a piece for other physicists entitled, “What every physicist should know about string theory.” 

But what if you want to understand it and you’re not a physicist? Then you should read this.

Ask Ethan: Which Fundamental Science Question Is The Most Important?

If you could have a complete answer to one of these 5 questions what would it be?

1.) Did cosmic inflation happen or was there another process?
2.) Is earth the only place in the cosmos with life?
3.) How [can we] merge general relativity and quantum mechanics?
4.) What is dark energy and dark matter?
5.) How did life begin on Earth?

There are a very large number of unsolved mysteries in the Universe, many of which would revolutionize our understanding of what it all is… and what it all means. If you could know the answer to only one of these questions, but know it immediately and fully, which one would you pick? Would you want to know more about the origin of the Universe, pushing things back before the Big Bang? Would you want to know about life elsewhere in the Universe, far beyond Earth? Would you choose quantum gravity, or how to merge our two great, incompatible theories of how everything works? Would you want to know what dark matter and dark energy truly are? Or would you go for the origin of life on Earth?

There’s no right answer, which is to say there are five great answers, but not every answer is equal. Come take a deep consideration on this week’s Ask Ethan!