Of Particular Significance

Tag: measurement

What Is a Measurement?

Nature could be said to be constructed out an immense number of physical processes… indeed, that’s almost the definition of “physics”. But what makes a physical process a measurement? And once we understand that, what makes a measurement in quantum physics, a fraught topic, different from measurements that we typically perform as teenagers in a grade school science class?

We could have a long debate about this. But for now I prefer to just give examples that illustrate some key features of measurements, and to focus attention on perhaps the simplest intuitive measurement device… one that we’ll explore further and put to use in many interesting examples of quantum physics.

(more…)

POSTED BY Matt Strassler

ON February 27, 2025

The Particle and the “Particle” (Part 2)

In my last post, I looked at how 1920’s quantum physics (“Quantum Mechanics”, or QM) conceives of a particle with definite momentum and completely uncertain position. I also began the process of exploring how Quantum Field Theory (QFT) views the same object. I’m going to assume you’ve read that post, though I’ll quickly review some of its main points.

In that post, I invented a simple type of particle called a Bohron that moves around in a physical space in the shape of a one-dimensional line, the x-axis.

I discussed the wave function in QM corresponding to a Bohron of definite momentum P₁, and depicted that function Ψ(x₁) (where x₁ is the Bohron’s position) in last post’s Fig. 3.
In QFT, on the other hand, the Bohron is a ripple in the Bohron field, which is a function B(x) that gives a real number for each point x in physical space. That function has the form shown in last post’s Fig. 4.

We then looked at the broad implications of these differences between QM and QFT. But one thing is glaringly missing: we haven’t yet discussed the wave function in QFT for a Bohron of definite momentum P₁. That’s what we’ll do today.

(more…)

POSTED BY Matt Strassler

ON February 25, 2025

The Particle and the “Particle” (Part 1)

Why do I find the word particle so problematic that I keep harping on it, to the point that some may reasonably view me as obsessed with the issue? It has to do with the profound difference between the way an electron is viewed in 1920s quantum physics (“Quantum Mechanics”, or QM for short) as opposed to 1950s relativistic Quantum Field Theory (abbreviated as QFT). [The word “relativistic” means “incorporating Einstein’s special theory of relativity of 1905”.] My goal this week is to explain carefully this difference.

The overarching point:

in QM, an electron really is a particle, at least to the limited extent that QM can handle that notion;
in QFT, an electron is at best a “particle”, with the word in quotation marks
- (and I personally prefer the term wavicle.)

I’ve discussed this to some degree already in my article about how the view of an electron has changed over time, but here I’m going to give you a fuller picture. To complete the story will take two or three posts, but today’s post will already convey one of the most important points.

(more…)

POSTED BY Matt Strassler

ON February 24, 2025

It was the Double Slit Experiment All Along!

Yesterday I posted an animation of a quantum wave function, and as a brain teaser, I asked readers to see if they could interpret it. Here it is again:

*Yesterday’s wave function, showing an interesting interference phenomenon.*

Admittedly, it’s a classic trap — one I use as a teaching tool in every quantum physics class. The wave function definitely looks, intuitively, as though two particles are colliding. But no. . . the wave function describes only one particle.

And what is this particle doing? It’s actually in the midst of a disguised version of the famous double slit experiment! This version is much simpler than the usual one, and will be super-useful to us going forward. It will make it significantly easier to see how all the puzzles of the double-slit experiment play out, both from the old, outdated but better known perspective of 1920’s quantum physics and from the modern perspective of quantum field theory.

You can read the details about this wave function — why it can’t possibly describe two particles, why it shows interference despite there being only one particle, and why it gives us a simpler version of the double-slit experiment — in an addendum to yesterday’s post.

POSTED BY Matt Strassler

ON February 21, 2025

Can You Interpret This Quantum Wave Function?

A scientific brain teaser for readers: here’s a wave function evolving over time, presented in the three different representations that I described in a post earlier this week. [Each animation runs for a short time, goes blank, and then repeats.] Can you interpret what is happening here?

The explanation — and the reasons why this example is particularly useful, informative, and interesting (I promise!) — is coming soon [it will be posted here tomorrow morning Boston time, Friday Feb 21st.]

[Note added on Thursday: I give this example in every quantum mechanics class I teach. No matter how many times I have said, with examples, that a wave function exists in the space of possibilities, not in physical space, it happens every time that 90%-95% of the students think this shows two particles. It does not. And that’s why I always give this example.]

(more…)

POSTED BY Matt Strassler

ON February 20, 2025

Some Facts About Wave Functions

What is a wave function in quantum physics?

Such a question generates long and loud debates among philosophers of physics (and more limited debate among most physicists, who tend to prefer to make predictions using wave functions rather than wondering what they are.) I have a foot in both camps, even though I have no real credentials among the former set. But no matter; today I won’t try to answer my own question in any profound way. We can debate the deeper meaning of wave functions another time.

Instead I just want to address the question practically: what is this function for, in what sense does it wave, and how does it sit in the wider context of physics?

(more…)

POSTED BY Matt Strassler

ON February 19, 2025

Buy The Book

Reading My Book?

Got a question? Ask it here.

Media Inquiries

For media inquiries, click here.

Tag: measurement

What Is a Measurement?

POSTED BY Matt Strassler

The Particle and the “Particle” (Part 2)

POSTED BY Matt Strassler

The Particle and the “Particle” (Part 1)

POSTED BY Matt Strassler

It was the Double Slit Experiment All Along!

POSTED BY Matt Strassler

Can You Interpret This Quantum Wave Function?

POSTED BY Matt Strassler

Some Facts About Wave Functions

POSTED BY Matt Strassler

Recent Posts

Buy The Book

Reading My Book?

Media Inquiries

Order Waves In An Impossible Sea