MATT STRASSLER
Of Particular Significance

Tag: QuantumGravity

Quantum Interference 2: When Does It Happen?

Last time, I showed you that a simple quantum system, consisting of a single particle in a superposition of traveling from the left OR from the right, leads to a striking quantum interference effect. It can then produce the same kind of result as the famous double-slit experiment.

The pre-quantum version of this system, in which (like a 19th century scientist) I draw the particle as though it actually has a definite position and motion in each half of the superposition, looks like Fig. 1. The interference occurs when the particle in both halves of the superposition reaches the point at center, x=0.

Figure 1: A case where interference does occur.

Then I posed a puzzle. I put a system of two [distinguishable] particles into a superposition which, in pre-quantum language, looks like Fig. 2.

Figure 2: Two particles in a superposition of both particles moving right (starting from left of center) or both moving left (from right of center.) Their speeds are equal.

with all particles traveling at the same speed and passing each other without incident if they meet. And I pointed out three events that would happen in quick succession, shown in Figs. 2a-2c.

Figure 2.1: Event 1 at x=0.
Figure 2.2: Event 2a at x=+1 and event 2b at x=-1.
Figure 2.3: Event 3 at x=0.

And I asked the Big Question: in the quantum version of Fig. 2, when will we see quantum interference?

  1. Will we see interference during events 1, 2a, 2b, and 3?
  2. Will we see interference during events 1 and 3 only?
  3. Will we see interference during events 2a and 2b only?
  4. Will we see interference from the beginning of event 1 to the end of event 3?
  5. Will we see interference during event 1 only?
  6. Will we see no interference?
  7. Will we see interference at some time other than events 1, 2a, 2b or 3?
  8. Something else altogether?

So? Well? What’s the correct answer?

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POSTED BY Matt Strassler

ON March 20, 2025

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Quantum Interference 1: A Simple Example

A very curious thing about quantum physics, 1920’s style, is that it can create observable interference patterns that are characteristic of overlapping waves. It’s especially curious because 1920’s quantum physics (“quantum mechanics”) is not a quantum theory of waves. Instead it is a quantum theory of particles — of objects with position and motion (even though one can’t precisely know the position and the motion simultaneously.)

(This is in contrast to quantum field theory of the 1950s, which [in its simplest forms] really is a quantum theory of waves. This distinction is one I’ve touched on, and we’ll go into more depth soon — but not today.)

In 1920s quantum physics, the only wave in sight is the wave function, which is useful in one method for describing the quantum physics of these particles. But the wave function exists outside of physical space, and instead exists in the abstract space of possibilities. So how do we get interference effects that are observable in physical space from waves in a weird, abstract space?

However it works, the apparent similarity between interference in 1920s quantum physics and the interference observed in water waves is misleading. Conceptually speaking, they are quite different. And appreciating this point is essential for comprehending quantum physics, including the famous double slit experiment (which I reviewed here.)

But I don’t want to address the double-slit experiment yet, because it is far more complicated than necessary. The complications obscure what it is really going on. We can make things much easier with a simpler experimental design, one that allows us to visualize all the details, and to explore why and how and where interference occurs and what its impacts are in the real world.

Once we’ve understood this simpler experiment fully, we’ll be able to discard all sorts of misleading and wrong statements about the double-slit experiment, and return to it with much clearer heads. A puzzle will still remain, but its true nature will be far more transparent without the distracting cloud of misguided clutter.

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POSTED BY Matt Strassler

ON March 18, 2025

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A New Interview

A pause from my quantum series to announce a new interview on YouTube, this one on the Blackbird Physics channel, hosted by UMichigan graduate student and experimental particle physicist Ibrahim Chahrour. Unlike my recent interview with Alan Alda, which is for a general audience, this one is geared toward physics undergraduate students and graduate students. A lot of the topics are related to my book, but at a somewhat more advanced level. If you’ve had a first-year university physics class, or have done a lot of reading about the subject, give it a shot! Ibrahim asked great questions, and you may find many of the answers intriguing.

Here’s the list of the topics we covered, with timestamps.

  • 00:00 Intro
  • 00:40 Why did you write “Waves in an Impossible Sea”?
  • 03:50 What is mass?
  • 09:03 What is Relativistic Mass? Is it a useful concept?
  • 17:50 Why Quantum Field Theory (QFT) is necessary
  • 23:50 Electromagnetic Field, Photons, and Quantum Electrodynamics (QED)
  • 36:17 Particles are ripples in their Fields
  • 38:47 Fields with zero-mass particles vs. ones whose particles have mass?
  • 46:49 The Standard Model of Particle Physics
  • 52:08 What was the motivation/history behind the Higgs field?
  • 1:02:05 How the Higgs field works
  • 1:05:33 The Higgs field’s “Vacuum Expectation Value”
  • 1:12:02 The hierarchy problem
  • 1:24:18 The current goals of the Large Hadron Collider

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON March 17, 2025

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Did the Particle Go Through the Two Slits, or Did the Wave Function?

What is really going on in the quantum double-slit experiment? The question raised in this post’s title seems to lie at the heart of the matter. In this experiment, which I recently reviewed here, particles of some sort are aimed, one at a time, at a wall with two slits, and their arrival is recorded on a screen behind the wall. As a parade of particles proceeds, one by one, past the wall, an interference pattern somehow appears, emerging gradually like a spectre on the screen.

Interference is a familiar effect, commonly seen in water waves and sound waves. If water waves passed through a pair of slits in a wall, interference would be observed and no one would be surprised. But here we have one particle passing through the wall at a time; it’s not at all the same thing. How can we explain the interference effect in this case?

It’s natural to imagine that somehow either

  • each particle acts like a wave, goes through both slits, and interferes with itself, or
  • the quantum wave function that describes each particle (or all the particles [?]) goes through both slits and interferes with itself.

So… which is it? Did the particle go through both slits, or did the wave function?

In 1920s quantum physics, there is a very simple answer to this question.

The answer is,…

“No.“

No — neither the particle nor the wave function [not its wavy pattern or its peak(s) or any other part of it] goes through the two slits.

(more…)
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POSTED BY Matt Strassler

ON March 13, 2025

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Do Quantum Wave Functions Collapse?

In the last post, I showed you how a projectile in a superposition of moving to the left or of moving to the right can only be measured to be doing one or the other. But what happens to the wave function of the system when the measurement is made? Does it… does it… COLLAPSE!?

Sounds scary. But it is only scary when it is badly explained.

Today I’ll show you what wave function collapse would mean, what it would require, and what a couple of the alternatives are. Among other things, I’ll show you that:

  • The standard way of explaining wave function collapse, which argues collapse is required to avoid a logical problem, is not legitimate;
  • If the Schrödinger wave equation is correct, then wave function collapse can never happen (and anything resembling “collapse” is viewed not as a physical effect but as a user’s choice);
  • Therefore, if wave function collapse really does occur, then the Schrödinger equation is wrong;
  • But if the Schrödinger wave equation is correct, an understanding of why quantum theory predicts only probabilities for multiple possibilities, rather than definite outcomes, is still lacking.

Today’s post uses several previous posts and their figures as a foundation, so I’ll start with a review of the most recent one, with links to others of relevance.

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POSTED BY Matt Strassler

ON March 10, 2025

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Can a Quantum Particle Move in Two Directions at Once?

So far, in the context of 1920s quantum physics, I’ve given you a sense for what an ultra-microscopic measurement consists of, and how one can make a permanent record of it. [Modern (post-1950s) quantum field theory has a somewhat different picture; please keep that in mind. We’ll get to it later.] Along the way I’ve kept the object being measured very simple: just an incoming projectile with a fairly definite motion and moderately definite position, moving steadily in one direction. But now it’s time to consider objects in more interesting quantum situations, and what it means to measure them.

The question for today is: what is a quantum superposition?

I will show you that a quantum superposition of two possibilities, in which the wave function of a system contains one possibility AND another at the same time, does not mean that both possibilities occur; it means that one OR the other may occur.

(more…)
Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON March 6, 2025

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