In the field of quantum physics, various claims are made concerning the detection of particles or waves in the famous double-slit experiment. Many respected commentators have put forth the idea that the mere process of ‘detection’ (undefined) is sufficient to affect the outcome of the experiment. The concept has spawned the idea that it may be human consciousness that is doing the ‘detection’ and that therefore such consciousness is able to have causal effects in the physical world.
Quite aside from the fact that the logic here is quite laughable and that there is no scientific definition of consciousness anyhow, it seems that there is no credible empirical evidence whatsoever for this effect.
A search for empirical evidence for the phenomenon of ‘detection’ included:
- Asking an AI engine to provide evidence
- Reading the experiments listed in the Wikipedia article on the double-slit experiment
No evidence was found that anything other than purely physical processes were able to affect the outcome of an experiment and no empirical evidence for a connection to human consciousness was apparent in any of the results.
I asked an AI engine “In the famous double slit experiment, a device is set up to detect which slit a photon passes through. What is the nature of this device and where is the relevant experiment described?“
The answers range from evasive to deranged. Nothing sensible at all is said and certainly no actual experiment involving such a device is presented.
The AI engine is strangely reluctant to give a physical description of an instrument, preferring instead to resort to abstract language and theoretical constructs that only make sense if the basic tenets of quantum physics are already accepted.
What is the nature of a which-slit detector?
Crucially: it is not a little camera watching the photon go by.
A which-path detector is any physical system that becomes entangled with the photon in a way that, even in principle, allows you to distinguish the paths.
The nature of a detector is described by its function, which is to ‘distinguish’, or ‘detect’. This gives no information and tells us nothing about the physical construction. What does ‘becomes entangled with the photon‘ mean? How do we know that such an ‘entanglement’ has taken place, apart from the fact that our experiment no longer works? This is not an answer.
More concretely, such a device must:
- Interact locally with the photon at one slit or the other
- Leave a different physical state depending on which slit was taken
- Store that information somewhere in the universe (even if nobody reads it)
Once that happens, interference is lost.
This is better, it is acknowledged that there must be some sort of physical process that does the detecting. However, although no specific process is singled out, the claim is made that whatever process is used, interference is necessarily lost.
Why should this be so? Why should it be that any specific physical process that is designed for detection somehow ends up destroying the interference pattern? No reason is supplied and no evidence available.
Examples of real which-path devices
1. Recoiling slit or screen (Feynman’s version)
If the slit apparatus can recoil differently depending on which slit the photon passes through, then:
- The apparatus carries path information in its momentum
- Even if you never measure the recoil, interference disappears
Richard Feynman, The Feynman Lectures on Physics, Vol. III, Chapter 1
This is a thought experiment, not a lab setup—but it’s foundational.
This is a thought experiment and yet is considered ‘foundational’!
The experiment is said to prevent interference even when when no ‘detection’ takes place. Therefore, the disappearance of the interference pattern cannot be said to have been caused by ‘detection’ itself but only by the physical mechanics of the instrument.
The experiment is imaginary and the outcome invented.
2. Polarization tagging (real experiments)
You place a polarizer at each slit:
- Slit A → photon becomes horizontally polarized
- Slit B → photon becomes vertically polarized
Now the photon’s polarization state tells you the path.
Result:
- No interference pattern
- Even if you don’t look at the polarization
This is one of the cleanest and most common laboratory realizations.
The experiment was changed with the addition of polarisers and no longer demonstrates interference. This is a different experiment to that which was originally described and the result is therefore irrelevant.
Even if we like the result, we did not do any ‘detecting’.
3. Atomic “which-path markers” (most famous real experiment)
This is the big one.
Instead of disturbing the photon’s momentum, they:
- Let an atom emit a photon
- Arrange things so the atom’s internal state records which slit the photon came from
- The photon itself is barely disturbed
M. O. Scully, B.-G. Englert, H. Walther (1991) “Quantum optical tests of complementarity” – Nature, 351, 111–116
Interference still disappears.
This demolished the old idea that interference loss is caused by “measurement disturbance.”
“The photon itself is barely disturbed” and yet “Interference still disappears”. Maybe the two are connected?
No pure ‘detection’ took place here. This is not the same experiment as that which was originally described.
The deep takeaway (this is the part most people miss)
A which-path detector is not defined by what it looks like
It is defined by whether path information exists in principleInterference is destroyed when:
- The photon becomes entangled with anything that carries path information
- Even if that information is never read
- Even if the detector is microscopic
- Even if the detector is later destroyed
No conscious observer required. No collapse button pressed.
I really would like to know what such an instrument looks like, but the engine isn’t going to tell me. A ‘detector’ is defined by “whether path information exists in principle“. However, the existence or otherwise of path information, in principle or practice is surely independent of the existence of an actual physical detection device whether or not it is destroyed later on.
This is, of course, complete gibberish, but must have come from somewhere and does in fact resemble the linguistic acrobatics performed by human-composed texts on the subject. We ask for an actual experiment that demonstrates a scientific principle and instead what we get is a theoretical lecture on why the results would certainly have been produced if only the experiment had only been performed in principle.
The definition here pretty much defines a detector according to whether or not the interference pattern is destroyed, thereby providing a circular definition, using the term ‘entanglement’ as a linguistic MacGuffin.
Furthermore, versions of the experiment that include detectors at the slits find that each detected photon passes through one slit (as would a classical particle), and not through both slits (as would a wave). However, such experiments demonstrate that particles do not form the interference pattern if one detects which slit they pass through. – Wikipedia
This, again, refers to photons that are ‘detected’, in an abstract sense with no physical mechanism described and yet the experiments contained ‘detectors’, which are presumably actual physical instruments. This is supposed to be an article on physics for heaven’s sake!
The phenomenon of ‘detection’ is surely a concrete physical process performed according to a specific measurement protocol and yet the authors seemingly want to ignore the physical processes to concentrate only on the abstract concept of ‘detection’. There seems to be a determination here, and in other areas of physics, to describe the world in purely abstract philosophical terms as opposed to measurable physical processes.
The phrase ‘versions of the experiment’ is misleading. If the parameters of the experiment have changed significantly then we have a different experiment and not a ‘version’ of the same experiment. For two different physical set ups to be regarded as essentially the ‘same’ then they should demonstrate both theoretical and practical equivalence. If they are giving two different sets of experimental outcomes then how can they be said to be the same experiment?
Wikipedia on detectors
The Wikipedia article gives several references describing ‘detectors’.
However:
“…if in a double-slit experiment, the detectors which register outcoming photons are placed immediately behind the diaphragm with two slits: A photon is registered in one detector, not in both…” – Introduction to Quantum Mechanics: Schrödinger Equation and Path Integral – Müller-Kirsten, H. J. W. (2006).
No physical description of a ‘detector’ here.
“It seems that light passes through one slit or the other in the form of photons if we set up an experiment to detect which slit the photon passes, but passes through both slits in the form of a wave if we perform an interference experiment.” Rae, Alastair I.M. (2004). Quantum Physics: Illusion Or Reality?
“It seems that..” – he has not performed this experiment himself nor witnessed a demonstration. He does not make reference to a first hand account of such an event and in all probability has not read a description of one.
Inherently probabilistic
Other atomic-scale entities, such as electrons, are found to exhibit the same behaviour when fired towards a double slit. Additionally, the detection of individual discrete impacts is observed to be inherently probabilistic, which is inexplicable using classical mechanics. – Wikipedia
What does ‘inherently probabilistic‘ mean? There is no such thing. This is a meaningless phrase from the realm of philosophy with no physical definition and consequently does not belong in a theory of physics. If it is undefined then of course it is inexplicable from the point of view of classical mechanics.
The phrase ‘inherently probabilistic’ has no sensible definition in mathematics, physics or philosophy.
If the phrase somehow refers to a mathematical or philosophical construct then we can ask “How does a mathematical or philosophical construct have causal effects in physical reality?” If the phrase is somehow interpreted as a physical process then such a process needs describing and its relationship to the rest of reality needs clarifying.
‘Randomness’ is an outcome pattern and not a generative mechanism: Random events
Conclusion
They haven’t performed this experiment in the way claimed, haven’t persuaded anybody that there is such a thing as ‘detection’ which is somehow independent of physical processes and have not provided any evidence for the involvement of human consciousness with physical reality.
All that happens is that they recycle the same misleading narrative phrased in evasive and deceptive language. The impression is given that there are many important experiments demonstrating the abstract phenomenon of ‘detection’, but no specific instance of this is ever fully described.
If no specific form of a detector is described, then the phenomenon of ‘detection’ is undefined for practical purposes and if this is so, then no downstream deduction can be made.
If no physical form of a detector is described, then we cannot assess in what way the ‘detection’ process might be responsible for the absence of the interference pattern.
However, in those documented cases where the physical form of a detector is described, it is obvious that it is the physical properties of the detector that is upsetting the results and not the abstract process of ‘detection’ itself.
The assertion that only physical processes can affect the physical world seems obvious to most people, but time and again we are asked by physicists to believe that the opposite is true and that physical events are actually driven by abstract philosophical ideas.
Quantum physicists would have us believe that the foundations of physical reality consist merely of statistical laws and that any perceived physical laws are merely some emergent product of such an abstraction.
“Physical laws rest on atomic statistics and are therefore only approximate” – Schrödinger
We have no proof of such an assertion.