[ExI] Google’s Willow Quantum Chip: Proof of the Multiverse?

[John Clark]

On Mon, Oct 13, 2025 at 10:12 AM Adrian Tymes via extropy-chat <
extropy-chat at lists.extropy.org> wrote:

*> Even MWI would not seem to explain it, as I read it, though I may
> be misunderstanding.*


*The thought experiment is that you are the foreman in a factory that makes
bombs that are so sensitive if a single photon hits the fuse it would go
off, but your manufacturing process is not perfect and sometimes you make a
defective fuse, however your customers demand perfection, they insist that
each fuse be tested so it could be guaranteed to work. This would seem to
present an impossible problem because if you test it with a photon and it
goes off you know it did work but now you don't have the bomb any longer so
you can't sell it. But actually there is a way to do this and it involves a
Mach–Zehnder interferometer (which has half silver mirrors) and whether or
not which-way information has been preserved because if you have which-way
information about a photon in a Mach–Zehnder interferometer it will behave
differently than it would if you had no information about which path the
photon took. The device looks like this:*

[image: CC42E819-DD07-404F-91BE-7FA8ADF3CC84.png]

*I then got lazy and asked GPT the following question:*

*"How could the Many Worlds interpretation explain how the Elitzur–Vaidman
bomb tester could work?" And this is what GPT said:*
*===*

"The Elitzur–Vaidman bomb tester is a quantum thought experiment using an
interferometer to identify live bombs without detonating them. In a classic
Mach–Zehnder setup, a single photon is sent into a beam splitter so that it
travels along *both* paths in superposition. A sensitive bomb sits on one
path: if a photon hits it, the bomb explodes (destroying the photon); if
not, the photon continues. Remarkably, there are cases where a detector
click reveals the bomb is live even though it did *not* explode – seeming
to detect the bomb “without touching” it. As Elitzur and Vaidman explain,
this is a new nonlocal effect of quantum mechanics: one can “ascertain the
existence of an object in a given region of space without interacting with
it” In practice the outcomes are: the bomb explodes (50%); the photon
emerges at one detector (25%); or the photon emerges at the other detector
(25%). A click at the normally silent detector (say “D”) certifies the bomb
is live even though it has not been triggered. This counterfactual result
relies on quantum interference and poses a conceptual puzzle: *how can the
photon learn about the bomb’s presence without interacting with it?*
Many-Worlds Interpretation (MWI) of the Bomb Tester

In the Many-Worlds Interpretation (MWI) of quantum mechanics, the photon’s
superposition leads to *real branching*: every possible outcome occurs in
some “world” of the universal wavefunction, with no collapse. Applied to
the bomb tester, the incoming photon at the first beam splitter creates two
coherent branches (or worlds). In one branch the photon travels along the
lower path toward the bomb; in the other branch it takes the upper path. If
the bomb is live, the *lower-path branch* causes the bomb’s detector to
trigger and explode. In MWI this branch becomes a “world” where the photon
and bomb are both destroyed (a macroscopic outcome). Meanwhile, the *upper-path
branch* is a world where the photon avoided the bomb entirely and continues
on. Crucially, these branches *initially interfere* until a measurement or
irreversible event occurs. Because the bomb exploded in the lower branch,
it decouples (or decoheres) that branch from the upper branch. The photon
in the upper branch then reaches the second beam splitter without its
partner, so interference is *lost*. So whenever the photon hits the first
mirror two worlds are effectively created and they continue to interfere until
a photon measurement is made in either of them. In practice, this means our
branch (photon on the upper path) is still influenced by its counterpart
that went toward the bomb. If that counterpart is absorbed by the bomb the
link between our world and the parallel one is disrupted and interference
ceases, and it is possible for our own photon to hit the normally dark
detector.

   -

   *Branch 1 (Bomb branch):* Photon hits the bomb. The bomb’s detector
   absorbs the photon and explodes. This is a measurement event that yields
   one real outcome in that world (bomb destroyed, photon gone)
   -

   *Branch 2 (Safe branch):* Photon takes the alternate (upper) path.
   Because the twin photon in Branch 1 was absorbed, there is no two-path
   interference. The surviving photon now behaves like a classical particle at
   the second beam splitter, reaching detectors C or D with equal probability.
   If it hits Detector D (the normally “dark” port), this *in our branch*
   signals “live bomb”.

>From the perspective of *our branch*, the photon never touched the bomb yet
we infer its presence. In MWI this is not magic but the result of the
parallel world’s event. As Vaidman notes, “obtaining information about the
region without anything being there” is only possible because “the photon
which we sen was there, but in another world”. In fact, three worlds emerge
in the experiment: one with explosion, and two where the photon is detected
at the two detectors. In the world where detector D clicks (informing us
the bomb is live), the bomb did not explode *in that world*. But that
world’s information is possible because in the explosion world the photon
*did* touch the bomb. In Vaidman’s words, “we get information about the
object without touching it in one world but we ‘pay’ the price of
interacting with the object in the other world”. In short, under MWI the
counterfactual measurement simply means the photon’s interaction happened
in a parallel branch, not in our own.
Branching Structure with a Live Bomb

The presence of a *working* bomb changes the branching structure of the
quantum state. If the bomb were a *dud* (transparent), the two photon paths
recombine and interfere perfectly at the second beam splitter, always
yielding one deterministic detector (say C) and a single effective branch
(the photon’s amplitude for Detector D cancels). But a *live* bomb
introduces irreversible events that split the worlds:

   -

   *Splitting at the First Beam Splitter:* When the photon hits the first
   beam splitter, it enters a superposition of “lower path” and “upper path.”
   In MWI this creates two coherent branches (“worlds”).
   -

   *Bomb Interaction (Conditional):* In the lower-path branch, the photon
   encounters the bomb. If the bomb is live, the photon is absorbed and the
   bomb explodes. This is a macroscopic interaction that effectively
   “measures” that branch. In MWI terms, that branch now has a definite
   outcome (explosion) and becomes decohered.
   -

   *Surviving Branch:* In the upper-path branch, the photon never interacts
   with the bomb. However, because the other branch just exploded, the
   upper-branch photon no longer has an interference partner. At the second
   beam splitter it thus has a 50/50 chance of emerging at either detector C
   or D, just like a classical particle.
   -

   *Outcome Worlds:* If Detector D clicks, that branch (our branch) knows
   “bomb is present.” If Detector C clicks (or if the bomb explodes), those
   are the other branches. Importantly, whenever Detector D fires without
   explosion, *another world* witnessed the explosion.

Thus, a live bomb guarantees a *multi-world outcome structure*. The bomb’s
“measurement” of the photon forces branching into at least three final
outcomes: (i) explosion (no photon detected), (ii) photon detected at C,
and (iii) photon detected at D. In the exploded world the photon was
absorbed; in the detected worlds the photon survived. If the photon is
detected at D (a sign of the bomb), our branch succeeded in “testing” the
bomb without destruction. But the logical cost is borne by the explosion
branch: the interference link was broken by the photon’s absorption there. Our
photon’s otherworldly shadow photon carried information; if the “OK”
interference signal does not come, it means the shadow photon has fallen,
that is was absorbed in the parallel world.
“Detecting” the Bomb Without Detonating in MWI

In practice, when a detector click (e.g. at D) tells us the bomb is live
yet it hasn’t exploded in our world, what does this mean philosophically in
MWI? It means that the measurement result we see is correlated with an
event in another branch. In our branch the bomb remains intact, but the
detection of the photon at D happened *because* in the exploded branch the
photon was absorbed. We are thus inferring the bomb’s existence *indirectly*,
via the missing interference. There is no mystery communication: the
“signal” comes from the fact that the branches’ evolution was linked until
the bomb’s interaction. Once the explosion occurred (in the other branch),
the branches decohered and our photon’s behavior changed.

In short, *“interaction-free” becomes almost a misnomer: globally, there
was an interaction (in another world), but locally in our branch the bomb
was not touched. MWI resolves the paradox by distributing reality: the
photon did interact in the universe, but not in the branch we experience.*
There is no nonlocal action at a distance in the many-worlds view – just
the usual quantum evolution with branching. All the possible outcomes
“happened” somewhere, so detecting the bomb unscathed simply means we find
ourselves in the branch where the interaction happened elsewhere."

==

*John K Clark*

>
>
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