Federica, thank you for your elaboration. Yes, the points that you have mentioned about superposition and the half-silvered mirror are taken for already understood in Sabine's video.Federica wrote: ↑Sun Oct 01, 2023 9:50 am So, if the photon is detected in A, the photon was observed on the upper path, it was forced to collapse. And only an actionable bomb can have forced the collapse, simply by monitoring the only alternative possible. So it’s not weird. An active bomb has the quality of an observer no matter what, and observation creates wave collapse.
Now, the 'bomb' in this experiment is used only to make it more dramatic. As you noticed, it's the same as simply placing any barrier which can absorb the photon and thus acts as an observer/collapser. A live bomb is simply something which stands on the way of the photon and can absorb it. This absorption which can trigger an explosion is only for drama. A dead bomb is the absence of a barrier. I think this part is somewhat confusing because my first guess would be that a dead bomb would still absorb the photon but will not explode. It turns out that a dead bomb is the same as the absence of a bomb or that it is transparent for photons. I don't think Sabine made that clear enough either.
There's a fun cartoonish game that exemplifies quantum optical effects. Many famous experiments are modelled there. Here's the bomb experiment.
The presence of the bomb can be controlled with the Y/N switch. The link above opens the experiment in 'beam' mode where light looks like continuous laser and it shows what percentage of its energy reaches where. There's also 'waves' mode (although I think it would have been more appropriate to call it 'photons' mode) which can be chosen through the slider in the upper left corner. Now with the 'play' button, single photons can be emitted.
So we have two cases. One is:
I understand why you say that the above is not weird. Actually it's the more understandable of the two because we can follow it completely with classical logic. We imagine a single photon emitted. The first half-silvered mirror sends it with 50% in each direction. Thus 50% of the time the bomb is triggered. The other 50% it goes on the other path and unsurprisingly is split once more in the upper mirror, landing with 25% in each detector. We don't even need quantum science here. We can do with completely classical and local reasoning.
The other case is actually the weirder:
It is not that weird if we imagine light as continuous energy beams/waves (not made of discrete particles). In that case we imagine that the energy is smoothly split in two at the first half-mirror, then recombines and interferes at the second. The two beams that go upwards at the upper half-mirror (not drawn on the above illustration but they would be the reflected upper beam and the part of lower beam that passes through) are out of phase thus they interfere destructively. They become out of phase because passing through the half-mirror (not reflecting) shifts phase with 90°. It is as if the wave is slowed down while it passes through the glass. Here's a visual cue about what phases mean.
On the other hand, reflecting at the surface (without entering the glass, this holds for both half- and full silvered mirrors) shifts the phase 180°. If we count the reflections/refractions, for the upper beam we have 180° at the first half-mirror (because it is only reflected), 180° at the full mirror in the upper-left and 180° at the other half-mirror. So we have 180° + 180° + 180° = 360° + 180°. But 360° is full rotation so it brings the phase to its initial state and is the same as 0° - not shifted at all. Thus effectively, the upper beam arrives at the upper detector at 180° phase.
The lower beam passes through the lower half-mirror getting shifted 90° as it passes through the glass, then it is shifted 180° in the mirror down right and finally shifts another 90° while passing through the glass of the upper half-mirror. Thus we have 90° + 180° + 90° = 360° or effectively that beam doesn't have phase shift. The final result is that the beams are 180° out of phase and thus they interfere destructively.
Analogously, for the right detector. For the upper beam we have 180° + 180° + 90° (since it passes through the glass of the upper right half-mirror). The final phase shift is 90°. The lower beam has 90° + 180° + 180° so it is also at 90° phase. That's why both beams are in phase at the right detector.
Now all this is understandable through the reasoning above but there's one problem. The above reasoning would suggest that the detector at the right should receive only 50% of the initial power of the laser. Let's follow it again: the first half-mirror splits the beam in two each carrying 50% of the initial power (thus the two split beams are dimmer). If we think about the two split beams separately, they are once gain split at the upper half-mirror, so they become even dimmer (25%). If we now take again the two initially split beams together, it seems that we have 2x25% that go up and 2x25% that go right. Those that go up are out of phase and annihilate each other. What goes right is in phase so adds up to 25% + 25% = 50%. So we should be left only with the 50% energy that goes right, right? Yet the experiments show otherwise. The whole 100% of the power goes right. This is already quite difficult to comprehend classically. It certainly doesn't make sense if we imagine it with water waves. It's like the energy of the waves that annihilate each other is magically transferred to the other waves that interfere constructively.
But things become even more strange when the intensity of the laser is lowered to such an extent that only individual packets of energy (photons) are emitted at a time. This is what really shatters our classical intuition. If we imagine a photon as some energy ball that travels along one or the other path, there could be no notion of interference (there's nothing to interfere with). Photons would have to land in either of the two detectors with 50% chance.
So if we reason classically with waves, it seems that we should find 50% powered laser in the right detector (the other 50% energy are lost in the beams that go up and annihilate themselves). If we reason classically through particles sent one by one we should see each particle arriving at one of the detectors with 50% chance (no energy is lost in this case). But in reality, neither of this is observer experimentally. No matter if we have beams or emit photons one by one, in all cases 100% of the energy is received at the detector on the right. As said, this is somewhat disturbing when we think about continuous beams but it is downright illogical when thinking about individual particles. If photons are balls that travel through one or another path, what prevents a single photon to ever land in the upper detector?
If we contemplate something like the above and allow it to work its way to its full consequences, we should realize how much of our classical intuition (based on throwing apples, for example) is not valid in these domains. The above setup is effectively similar to the double-slit experiment for which Richard Feynman said:
In the light of all the above, I agree that Sabine got it somewhat upside-down. She takes for a clear fact that if there are no barriers, light should go 100% in the right detector. Then she thinks how when a barrier is placed, 25% of the time a photon will be detected in the upper detector. This isn't weird if we think classically. It's what is to be expected if there was no quantum mechanics but only photons flying as apples."a phenomenon which is impossible […] to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics].
So to summarize. Let's say that we don't know if there's a barrier at the upper beam. If we think classically, when we emit photons one by one we can imagine them as apples flying through specific paths. If we receive a photon at the upper detector this doesn't really tell us anything about whether there's a barrier on the path. In the photons-as-apples world (thrown one by one, thus no interference effects) we can always expect some photon to be detected at the upper detector. If there's barrier we'll receive only 50% of the photons that go through the non-barriered path and which are further split to 25%-25% by the upper half mirror. Half of the time we'll receive nothing at the detectors because it would have been absorbed by the barrier/triggered the bomb. If there's no barrier, then we'll always detect something at one of the detectors with 50% chance. Thus in a classical world we can know that there's a barrier only if we don't receive anything at the detectors (thus the bomb must have been triggered).
The weirdness comes when we already know that these photons act according to quantum rules (confirmed experimentally, not simply a floating theory). Then we know that without barrier we have 100% at the right detector.
If we ever detect something at the upper detector and we hold on to our quantum understanding then we know, it is guaranteed, that there's a barrier. This is the weirdness. That a photon arriving at the upper detector is only possible if there is something which could have absorbed the photon, although it didn't (in this particular case).
So in a nutshell, the bomb experiment is somewhat convoluted because it presents as a mystery something which appears as such only when we have already gotten used to the initial mystery (of which Feynman speaks) and the 100% going right. When we have become numb to that initial mystery and have accepted is as a matter of fact, then in the bomb experiment it simply reemerges in a more dramatic form and we forget that it is no more mysterious than the first mystery that we have gotten numb to. But it is essentially the same fundamental mystery of which Feynman speaks. And of course, it only looks like a mystery because for a long time we've become used only to thrown apples that fly in specific paths. We have forgotten that, for example, in the astral world there could be 'two churches in the same place'.