Ruth Kastner: Can the Future Cause the Past in Quantum Theory?
episodePreviously titled “Can the Future Influence the Past? Retrocausality in Quantum Theory | Ruth Kastner” — renamed by the publisher on Aug 3, 2026
Transcript
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What is the measurement problem and why does it matter?
It's an anomaly. Мешермент оком фейл тобі предбавинно фіри.
I met with physicist turned philosopher Ruth Kastner, who developed a formulation that claims to solve not just the measurement problem, but also retrocausality, non-locality, and the unification of quantum theory with gravity. Her transactional formulation, which builds on Kramer's work, asserts that space-time itself is not fundamental, but emerges from what she calls the quantum substratum, not a realm of probability, but a realm of possibility. Questions we explore are: what's the role of retrocausality in quantum mechanics, also known as time travel? Does consciousness play a role at all? What about free will? And can you make gravity consonant with quantum theory without so-called quantum gravity?
Ruth, I'd like you to paint a clear picture of what the motivation is behind the transactional interpretation, especially its so-called retro causality. And the way that I'd like you to do this is to pick some standard account in quantum mechanics or quantum field theory, discuss why this standard account seems to make sense to most physicists, then explain why it doesn't actually make sense, and then explain why the transactional interpretation Fixes or resolves these problems.
Sure. Okay. So um well what what uh what got me interested in the transactional interpretation is is basically my dissatisfaction with the conventional theory's inability to describe what counts as a measurement. So and th this is of course the measurement problem. of this of the conventional theory. So the problem with the conventional theory is that it does not have any uh any kind of tools or anything in the formalism that lets you um distinguish between just a kind of an interaction that would not trigger an outcome and a kind of interaction that counts as a measurement. So so So the the theory itself, the conventional theory, just doesn't have anything that lets you say that a measurement occurred and and an outcome happened.
So what what TI does is, and I can get to that in a little in a little while, but but it remedies that. And just to kind of so to to elaborate on the measurement problem, I mean it it's illustrated by the the Schrdinger cat. Experiment. And this was actually a thought experiment that Schrdinger came up with because he was dissatisfied with. The um the standard theory's inability to to explain what counts as a measurement. So the the basic um the Schrdinger-cat, you know, experiment that people are so familiar with, but that maybe perhaps don't quite understand the the the point of it, the the import of it is is what is called a reductio ad absurdum of of the standard theory, of the conventional theory that illustrates a problem
a weakness of the of the conventional theory. So the the basic you know thought experiment Starts with an unstable atom, which is a quantum system that you can represent as being in a superposition of alive of uh having decayed and having not decayed. So at some point, you know, at some time this thing's gonna send out a little decay particle from its nucleus. But its its description is a superposition of having decayed, having not yet decayed. So the standard theory. Theory, all it lets you do is create correlations between states. So if you bring in a Geiger counter, like you want to measure, well, has it decayed yet? You use a Geiger counter, but according to the conventional theory The um the Geiger counter has to be described by states that will then be linked up with the superposition of these two states of the atom.
And you can I kinda think of it as a t as the atom having like two train engines. They're the the uh having having decayed train engine and the undecayed train engine, which is like a superposition of states. So when you bring along the Geiger counter, if it's if if it's going to be correlated with these states, it then has to acquire these two states corresponding to that atom, which are uh triggered Geiger counter and untriggered Geiger counter.
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Chapters
8 chapters
1
What is the measurement problem and why does it matter?
0:00–19:10
2
How does the transactional interpretation explain measurement interactions?
19:10–35:42
3
Why distinguish between observers and measurers in quantum experiments?
35:42–51:28
4
What are emission and absorption in the transactional picture?
51:28–1:07:44
5
How does space‑time emerge from quantum entities?
1:07:44–1:26:00
6
What are the transactional axioms that replace the conventional approach?
1:26:00–1:41:28
7
Can retrocausality resolve the block‑world paradox?
1:41:28–1:58:08
8
Does the transactional interpretation say anything about consciousness?
1:58:08–2:11:36
Speakers
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