# Odd delayed choice interferometer interpretations

1. Jun 11, 2015

### Thomasphysicist

This thread is about my not understanding the conclusion that scientists came to that the photon makes a 'decision' on how to travel.

In a simple interferometer experiment, a photon displays interference fringes only when a second beam splitter is present. According to Wiki, this led scientists to assert that the photon 'decides' to travels as a particle when there is no beam splitter and a wave when there is a beam splitter.
The delayed choice version of the experiment demonstrates that the photon shows interference fringes, even if the second beam splitter is placed after the photon is already on it's way. Given the initial idea that the photon decides to travel as either a particle or wave depending on the apparatus, this led some scientists to conclude that the photon must act retroactively- if a 2nd beam splitter is placed, the photon goes back in time and decides to travel as a wave, whereas it was initially travelling as a particle (before the 2nd beam splitter was placed).

Now, either I'm missing something very simple or these scientists were- I see no reason to assert that the photon makes any kind of decision in the first place. Going back to the simple interferometer experiment, I understand that the photon must be said to travel as a wave when the beam splitter is present- how else do the interference fringes appear- but I do not see any reason that it must be said to travel as a particle when the beam splitter is not present. Why not posit that it travels as a wave in both instances? I mean, in the simple interferometer exp, without the 2nd beam splitter you wouldn't expect interference fringes from a wave as the waves are travelling in perpendicular directions. If this is the case, then you can say that the photon travels as a wave in both instances and therefore there is no photon 'decision'. In the delayed choice version, this means there is no need to posit retroactivity.

Where have I gone wrong?

2. Jun 11, 2015

### Staff: Mentor

Seriously, kidding aside, there are some good wikipedia articles and some not so good ones. Without a pointer to the specific article (which is why PF has a rule that references must be cited!) we can't tell whether you're misunderstanding one of the good ones or being misled by one one of the not-so-good ones.

The idea that a particle can be either a particle or a wave, let alone that it "decides" which, is no part of the modern understanding of quantum mechanics. It's one of those things that drifted into the popular imagination decades ago when the theory was less well-developed, and like an invasive weed, once established it's very hard to get rid of.

Last edited: Jun 11, 2015
3. Jun 11, 2015

### Staff: Mentor

Just to elaborate on what Nugatory said you may find the following useful:
http://arxiv.org/pdf/quant-ph/0609163.pdf

I shouldn't have to say this but from discussions about that paper in the past, it clearly states its using wave in the sense of wave-function which is different from what is usually thought of as a wave.

Thanks
Bill

4. Jun 11, 2015

### Thomasphysicist

Good point. http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment

In short it says experimenters considered the photon to travel in a particle like fashion in the first instance but wave like in the second. The last line in that section says that the delayed choice results led experimenters to conclude the photon reacts retroactively to the apparatus being changed.

However, since the experimenters would have known that with the 2nd beam splitter, the photon exhibits wave like nature, followed by particle like nature upon detection (i.e. the way it behaves apparently alters), why then could they not conclude that similarly, the photon exhibits wave like nature, followed by particle like nature upon detection without the 2nd beam splitter. Instead they insist the photon exhibits particle like nature throughout, leading to the need to introduce 'decision making' and retrocausality.

That is, what forces them to consider the photon as exhibiting particle like behaviour in the 'no-2nd beam splitter' instance?

5. Jun 11, 2015

### Staff: Mentor

The real answer is its neither wave or particle - the question is meaningless.

If you want a correct analysis of the experiment here it is:
http://quantum.phys.cmu.edu/CQT/chaps/cqt20.pdf

But you need beyond a pop-sci understanding to get to grips with it. For that studying the whole text its from is really the only solution:
http://quantum.phys.cmu.edu/CHS/histories.html

Thanks
Bill

6. Jun 11, 2015

### Thomasphysicist

I seem to have trouble getting my point across as everyone seems to think I'm asking how to correctly interpret QM. Which I'm not. What I'm doing is trying to understand the evolution of thought/theory over the last century and how it has been affected by various experimental results.

"Observing that photons show up in equal numbers at the two detectors, experimenters generally say that each photon has behaved as a particle from the time of its emission to the time of its detection" - From the wiki article

"in the first case the photon is said to "decide" to travel as a particle and in the second case it is said to "decide" to travel as a wave" - From the same article

What I'm wondering is why, back then, these scientists regarded the photon as behaving as a particle (rather than a wave) prior to detection, given that it was both unnecessary and introduced further complications such as 'decision making' and retro causality.

Thanks for the 'myths' paper btw, very interesting.

7. Jun 11, 2015

### Thomasphysicist

A better way of putting it is why were they happy to concede that the photon travels down both paths of the interferometer in one instance, but not happy to concede that is does so in the other instance?

8. Jun 11, 2015

### Staff: Mentor

Its a hangover from its history (particularly De-Broglies hypothesis) and the way its usually taught. Modern textbooks like Ballentine that is my bible on QM are carefull to tell it correctly - but its at graduate level. And the correct explanation for things like Schroedinger's equation via symmetry is rather involved and not really suitable for undergraduates who may get bogged down in the math.

Thanks
Bill

9. Jun 11, 2015

### Staff: Mentor

Sorting out QM took time. Even now our understanding is evolving eg the PBR theorem.

Thanks
Bill

10. Jun 11, 2015

### zonde

I have not tried to find source of this "photon decides to be particle or wave" model but my bet would be that it's straw man. As you say It's unnecessary complicated.

11. Jun 11, 2015

### Staff: Mentor

You've found one of the not-so-good wikipedia articles. Take a look at the Talk page associated with the article (always wise and often entertaining when dealing with wikipedia) you will see that the article is not derived from Wheeler's serious scientific publications, but rather from the popularizations that he wrote for a non-technical audience. The author of that part of the wikipedia article is probably unaware of the distinction.

Some of the experimental results that led scientists to question the neat classical physics of the 19th century did indeed suggest that something could behave as a wave one moment and a particle the next; and that's how the notion slipped into the discussion. But the experiment being discussed in that wikipedia article is not one of the convincing ones, and you are right to be question it.