Aephraim Steinberg: The Physicist Who Measured Negative Time
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What is the initial puzzle about negative time that sparked the discussion?
We just realized we were wrong. And that makes no classical sense.
This physicist has been asking questions we're told not to ask. What is a particle doing between measurements? Where does it go? How long does it spend there? I traveled to my alma mater of the University of Toronto to speak to Professor Steinberg, the winner of the Physics World's Breakthrough of the Year, whose lab investigates photons traveling through a barrier that apparently causes atoms to spend negative time in an excited state.
And it took us a while to appreciate it wasn't just any old negative number.
My name's Kurt Jaimungle, and on this channel I interview researchers regarding their theories of reality with rigor and technical depth. Today, we discuss negative time beyond the pop science headlines, because if you just go by them, you'll be misled. Many other YouTubers or magazines will tell you that his results are about faster than light travel, but today we go into the recondite details, exploring the truth behind these negative time results. We also talk about what weak measurements are and how they recover Bohmian trajectories in the double slit experiment, and why Heisenberg's original disturbance argument about his uncertainty principle was experimentally incorrect. Again, we're all taught that Heisenberg's uncertainty comes from literally disturbing the atoms with other measuring devices, but that's false.
We close with consciousness, quantum computing, and whether Bell's inequalities mean what we think they mean. Sir, your lab measured negative time.
How does the host introduce the lab’s measurement of negative time and its meaning?
What does that mean? You're
really gonna jump right in. That means many different things in different contexts. And we got interested in it because there's an old context in which negative times were already known, but people tended to dismiss them. They they recognize that there are certain things you can define operationally. If you define it that way, you see what number comes out, it turns out to be negative. This surprised us. I can go into more detail afterwards, but little by little we understood sort of a way to patch things up and sweep it under the rug. And more recently, What we realized is it wasn't so easy to sweep it under the rug, and that we might have been missing something, and there might be a sense in which negative times have more physical reality than we were ascribing to them.
So I I begin with that preamble because it's kind of a long story the way we stitch these different things together. But I think the simplest question to ask yourself is is the following. Suppose I fire a particle at a certain Medium, some kind of a tunnel, and the particle gets through the other side, and it is time. When does it arrive on the other side? That's an old question. It's an easy question to ask about particles. It's more complicated with waves. And of course, now we know that according to quantum mechanics, everything is both a particle and a wave in some sense. By the late 19th century, though, we already knew that light was a wave, so people were asking us. This question How does a light wave travel through a piece of glass or a cloud of atoms?
And as they began to address it, they realized that there were funny situations where if you just looked at the peak of the wave and said, well, that's a reasonable place to talk about the average position of the energy or, you know, the most likely time for a detector to fire or something like that. the time at which we predicted a detector was most likely to fire on the far side of this medium could be earlier than the time at which the detector placed before the medium would have fired. In other words, it seemed that the particles could arrive sooner than they departed. And that's what we meant by a negative time. Now that plays all sorts of havoc with ideas of causality. How can an effect precede a cause?
So Zammerfeld and Briand famously dealt with this problem and explained how, even though mathematically that's what came out, you shouldn't worry about it. No information was traveling even faster than light, let alone back to negative times, and everything was okay.
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Chapters
8 chapters
1
What is the initial puzzle about negative time that sparked the discussion?
0:00–1:33
2
How does the host introduce the lab’s measurement of negative time and its meaning?
1:33–6:18
3
What analogies (basketball and Holland Tunnel) are used to illustrate negative traversal times?
6:18–10:04
4
How do the speakers transition to defining time, measurement, and quantum versus classical descriptions?
10:04–1:07:40
5
Why does measuring a particle’s momentum disturb its position and how does weak measurement mitigate this?
1:07:40–1:25:30
6
How do weak measurements allow us to infer the average path of a photon in a double‑slit experiment?
1:25:30–1:43:14
7
What does the tunnelling‑time experiment reveal about new time scales in quantum mechanics?
1:43:14–2:00:56
8
Is time a fundamental parameter or an emergent property, and what does this mean for the arrow of time?
2:00:56–2:21:28
Speakers
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