# B Retrocausality interpretations for fermions

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1. Mar 22, 2017

### thenewmans

This question is about experiments involving entangled electrons or any other fermion for that matter. I’ll get to that in a sec. I’ve been interested in understanding interpretations that have retrocausality. (TIQM by Cramer, Wheeler–Feynman absorber, time symmetric by Price) It’s easy to imagine 2 entangled photons getting measured with the 2 measurement events occurring completely outside of each other’s light cones. And in that case, you can’t say which event occurred first. You can always find some inertial frame of reference where the events happen in the opposite order. This avoids issues of causality. That’s one less issue for understanding interpretations that contain retrocausality. I can explain if my logic isn’t clear there.

Anyway, that logic doesn’t work as well for fermions since it’s likely the measurement events occur in the same order for every inertial frame of reference. That means there’s no getting around the retrocausality of it. And that causes all kinds of issues like causality and tachyon forces. I think that was an issue way back when Wheeler and Feynman came up with their absorber theory. Since interpretations with retrocausality are still viable to this day, I figure they must have a solution to this. I can’t quite figure out enough of these interpretations to understand what that solution could be. Any ideas?

2. Mar 22, 2017

### Staff: Mentor

Why is that?

3. Mar 23, 2017

### thenewmans

Yeah. That wasn't clear. Let me back up a bit. Let’s say I plot out 2 entangled photons on a Minkowski diagram. Their paths are right on the edge of the light cone at 45 degrees. I hope that makes sense. And let’s say I set the angle of my 2 detectors so that I can assume the results will correlate. I can place my detectors anywhere on those 2 paths and expect the results to correlate. Plus, wherever I put them, I can find an inertial frame of reference in which the 2 measurement events happen at the same time. The goal of this is to show that the 2 events are distant enough that causality is not an issue. That’s demonstrated by saying that we cannot assume the order of the events.

Now let’s do that with entangled electrons. Their speed is well short of C and therefore cannot be plotted along the diagonal. Once again, I line up the detectors for correlation and place them anywhere along the 2 paths of the electrons. But this time, I cannot assume that there exists an inertial frame of reference in which the 2 measurement events occur at the same time. I very well could have placed one of the detectors close to the emitter. If it’s close enough, then one event always happens before the other. That is to say the second event is entirely within the future light cone of the first event. That is the case for every inertial frame of reference.

The problem I’m having with this is that photons give me a convenient way of disregarding causality paradoxes and hypothetical tachyons. Electrons are not a convenient. And I haven’t quite figured out enough about interpretations with retrocausality to understand how they got around such issues.

4. Mar 23, 2017

### DrChinese

Ah, now I understand your question a bit better. Well, it turns out that your issue has been resolved experimentally. It is possible to entangle electrons that are "slow-moving" (with respect to each other, they are nearly at rest) using the magic of entanglement swapping. The electrons are 1.3 km apart at the time they are entangled. They are observed quickly, such that the observations are in distinct light cones (no signal moving at c can go either direction before they are measured). I think this meets your criteria. The experiment itself is a bit complicated, check out the link below.

The upshot is that there is no difference in considerations of entanglement and causality for fermions (electrons) as compared to bosons (photons). As with all viable interpretations of QM: there is no still way to make the statement that a measurement by Alice "causes" (or otherwise affects) the outcome for Bob, or vice versa, as relates to ordering of events in ANY reference frame. Retrocausality "could" be a factor.

https://arxiv.org/abs/1508.05949

Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km

B. Hensen, H. Bernien, A.E. Dréau, A. Reiserer, N. Kalb, M.S. Blok, J. Ruitenberg, R.F.L. Vermeulen, R.N. Schouten, C. Abellán, W. Amaya, V. Pruneri, M. W. Mitchell, M. Markham, D.J. Twitchen, D. Elkouss, S. Wehner, T.H. Taminiau, R. Hanson

For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. In his seminal work, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory. Bell showed that in any local realist theory the correlations between distant measurements satisfy an inequality and, moreover, that this inequality can be violated according to quantum theory. This provided a recipe for experimental tests of the fundamental principles underlying the laws of nature. In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments to date required additional assumptions to obtain a contradiction with local realism, resulting in loopholes. Here we report on a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell's inequality. We employ an event-ready scheme that enables the generation of high-fidelity entanglement between distant electron spins. Efficient spin readout avoids the fair sampling assumption (detection loophole), while the use of fast random basis selection and readout combined with a spatial separation of 1.3 km ensure the required locality conditions.

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