According to Wave-Particle Duality, in a normal double-slit experiment, the quantum object behaves as wave and many stripes are displayed on the screen. But if we measure which way the quantum object passes (the measurer being the "observer"), the object becomes a particle upon being observed and just 2 stripes are displayed.

Since double-slit experiment succeeded with electron objects, other objects such as neutrons and buckyballs were also succeeded with. Theoretically, any object must be applicable, even cats.

But if we really did the experiment with cats, wouldn't the theory be inconsistent? By our perspective, there is no such thing as "observers" (just like with electrons, or just like in Schrödinger's Cat) and many stripes must be displayed. But what about in the perspective of the cat itself? Isn't the cat itself an observer (ex. it might feel collision into the plate), and must just 2 stripes be displayed? The perspectives being inconsistent?

At first, what exactly is an observer? As the plate itself consists of atoms, why doesn't the plate itself entangle with the object?

EDIT: If just as electrons, the cats are isolated systems, does this mean the cats CANNOT observe themselves? Resolving the inconsistency?

  • 1
    $\begingroup$ Can you keep a cat in a coherent state? $\endgroup$
    – valerio
    Feb 25, 2018 at 15:40
  • $\begingroup$ @valerio I can't, but eventually someone will, isn't it? $\endgroup$ Feb 25, 2018 at 15:46
  • $\begingroup$ The diffraction of cats is an idea belonging to poetry, not to physics as we know it. $\endgroup$ Feb 25, 2018 at 15:49
  • $\begingroup$ @StéphaneRollandin Sorry, I'm not familliar with English poetry. $\endgroup$ Feb 25, 2018 at 15:52
  • 1
    $\begingroup$ @DannyuNDos I have some serious doubts that someone ever will. I suggest you to read something about decoherence and macroscopic systems. You can start from here $\endgroup$
    – valerio
    Feb 25, 2018 at 16:00

1 Answer 1


The double slit experiment is a statistical accumulation of many events with the exact boundary conditions. Quantum mechanics tells us that the accumulation of observations reflects the probability distribution for such an event to happen, which probability obeys a wave function and thus, for given boundary conditions may display interference.

Here is the gradual accumulation of single photons at a time:

enter image description here

Each dot on the leftmost is one event, a single photon scattering through the specific two slits . As you can see the photon is not spread out all over the place, it leaves a dot footprint. It is the accumulation of photons that shows the interference.

Now if one tried to do the cat experiment, there is no material that slits could be designed to scatter cats through two slits and expect to see interference, because of the very small wavelengths involved due to the macroscopic mass of the cat. So let us stick to photons.

Theoretically an intact cat would hit the screen, one cat at a time.

The plate itself , the distance from the slits, is part of the boundary conditions and thus part of the quantum mechanical solution , and so entangled.

  • $\begingroup$ So the cat cannot observe itself? $\endgroup$ Feb 25, 2018 at 15:48
  • $\begingroup$ if it had a wavelength of the size to go through real slits , it would hit on the screen/detection level as the photon does, whether it were alive as to see where it hit or not would depend on biological variables. But this is not even a thought experiment for macroscopic sizes. $\endgroup$
    – anna v
    Feb 25, 2018 at 16:17

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