Re: Does the delayed choice quantum eraser indicate time-reversed causality?

Hi Bill, sorry about the slow response. This was a tough question, and it took me a while to figure out the right approach. The details of this paper still go a bit over my head---but I think I can clear up the causality question.

From your question you seem to understand ordinary quantum entanglement experiments. In these experiments, you can take two entangled particles and separate them by any distance---in practical terms, say, you could send two entangled photons down a kilometer-long fiber optic cable. A measurement done on one photon has the effect of "collapsing" the wavefunction of the other photon, instantaneously. The finite speed of light does not come into play. However, it turns out that these experiments cannot actually transmit any information, so you can't use them for faster-than-light communication. The "recipient" physicist can't tell the difference between random photons and "signal" photons, until he or she talks to the "sender" and correlates data from both stations. Check out Nick Herbert's book "Superluminal loopholes in physics" to learn more about this.

Now, consider the experiment I just described---two entangled photons, collapsing simultaneously a few kilometers apart---and suppose that you watch it happen from a spaceship moving at 1/2 of light speed. The wavefunction-collapse events which the (stationary) scientists would call "simultaneous" are not simultaneous when seen by the spaceship. The spaceship, depending on its trajectory, might see the "recipient" making measurements before the "sender" generates them. Since there is no information being exchanged, the spaceship should not see anything unusual going on. A real information transfer (at the speed of light or below) can never be seen as acausal, for any real spaceship (at the speed of light or below).

Since quantum entanglement experiments always involve this "wavefunction collapse" which seems to move faster than light, it will always run into odd behavior where "effects" happen before their "cause". If it doesn't look odd to a stationary observer, it will look odd to a moving one. This paper seems to be an experiment which manages to look odd to observers at rest---maybe it would look normal to someone moving very fast!! But, again, no information is transferred and causality is maintained.

It's pretty weird stuff, though, and I enjoyed thinking about it---thanks for asking! In retrospect, I guess we should not be surprised that such experiments are possible!

-Ben