Being a layman, I can only assume this Delayed-Choice Experiment is something that most physicists consider textbook stuff (my apologies if I assumed wrong). Therefore, I hope the experts on this board can help me with my questions. This is what I know about it: in a conventional two-slit experiment, electrons are aimed at a barrier containing 2 slits. If both slits are opened, the electrons act like waves and go through them at once, forming interference pattern. If one slit is closed at a time, the electrons passes through the opened slit like simple particles. In the Delayed-Choice Experiment, the decision on whether to leave both slits open or only one open is made after the electrons have already passed through the barrier. This is the part I don't quite get. If the decision is made after, then what is the condition of the barrier as the electrons are going through it - are both the slits closed? If so, how does the electrons penetrate the barrier? From what I read, this experiment is supposed to demonstrate the nature of the quantum phenomena. The results of the delayed-choice experiment is the same as that of the conventional experiment.
The point of the experiment was to show that it doesn't actually matter which slit we open - we get the same result. When you fire a single photon at two slits an interference pattern emerges on the screen behind it - as if the photon went through both slits simultaneously. Now do the experiment again and wait until AFTER the photon passes through the slits and THEN close one of slits. Only THEN can we now conclude that the photon went through 1. Slit A 2. Slit B 3. Slit A & B The point is, the history of the photon depends upon how we choose to make a measurement. We can open both slits and let the photon through and THEN close slit A and say it went through slit B or both. OR we could have closed slit B and say it went through slit A or both. By doing this we alter the history of the photon as it passed through the slits. Once you fire the photon, it's wavefunction is such that the photon could be travelling along all possible trajectories at once. Only until you decide to measure it's trajectory do you collapse all other possibilities. So by Quantum Mechanics the photon passes through both slits at the same time. Then when you close a slit AFTER it passes through you are creating an experiment in which it passed through the OTHER slit. In essence you are creating experiments based on the measurement you made after someting happened. "...the past has no existence - except in the way it is recorded in the present" implies history has no meaning. Other may be inclined to use this information to say that we are CREATING the Big Bang by simply observing background radiation! What if you chose to ignore it? Would the Big Bang still have existed? What if we chose to close slit A? Would the photon still have passed through A? Who knows?
oxymoron, I am vaugely familiar with this experiment and its arguements but I find it un-impressive. The reason goes like this. If I back away from light, Relativity, QM, etc., and their unorthodox quirks and I run a simular experiment using water in a tank with two spouts. When I fill the tank with both spouts open the water will spill out and the streams can cause interference patterns where they merge. If I close one spout, I continue to get flow through the one spout left open with no interference pattern but I have not altered the history of the flow that went through both spouts. Now I understand the claim is different because we are talking about a single photon in the light experiment. But it seems to me that the problem vanishes if we merely assume we have not isolated a single photon but may have shown that what we call a single photon is in reality composed of a finer grain which when permitted to do so remains clumped but will biforcate when allowed to do so. This all seems related to the EM wave front associated with moving particles. I don't see how it supports any claim of changing history. I would see this differently if for example a test could be done where the same interference pattern of two slits was produced with one slit by closing one slit after the photon was projected but before it reached the slit. That is if one could still cause an interference pattern through one slit, showing that the photon had made its decision to go through two slits but only had one upon arrival. In such a case it would appear they had alatered the future however, not the past.
1) In this experiment, is the single slit closed after the photon passes both slits before hitting the screen or after the photon has hit the screen? 2) What is the result on the screen when one of the slits is closed AFTER the photon has left the barrier - does the photon hit the screen as if like a particle? 3) What constitutes measurement in this experiment - the photon hitting the screen or the photon going through the slits? 4) when the photon goes through both open slits, it goes through behaving like a wave, am I right? If so, how does closing off one of the slits after it has left the barrier affect the wave-nature of the photon since there is no longer any interaction between the slits and the photon? 5) what happens if in this experiment, both slits are closed AFTER the photon has gone through? Does the photon still reach the screen? What implication does this have on affecting the photon's history?
As I understand it, the mere possibility of detection is enough. Humans do not necessarily have to witness. So in your example it doesn't matter if humans detect it, only the possibility of detection matters. Another conceivable scenario is that we detected CMBR but thought it to be something else, or that we detected it indirectly without knowing.
As far as I am aware it is only a thought-experiment. I will do some more investigation into this matter.
I read that this experiment was carried out in the early 1990's and confirmed the thought experiment's prediction. Unfortunately I could not get the particulars nor the specifics of the experiment from the same source. Looks like I'll have to do some investigation myself. Anyway, thanks for the input, Oxymoron. At least I have a slightly more elaborate picture now than before.
The possibility of detecting an object is proof of existence? Sounds a bit backwards to me. Anyway, if that was true, then we can say that: It is possible to detect aliens who use the EM spectrum to communicate. Therefore aliens exist.:bugeye:
Two questions guys: a)What is the definition of 'detecting' something and b) have virtual particles ever been 'detected' (defined in a) above)
