/ THE IDEA
The arrow’s direction is called its phase. Routes with matching phase reinforce; routes with opposite phase cancel. Only routes that remain indistinguishable are combined this way. To predict one landing position, add the amplitudes for every indistinguishable route to that position, then square the size of the combined arrow. Probabilities for mutually exclusive ordinary alternatives would instead be added directly.
THE FORMAL IDEA
P(result) = |A₁ + A₂ + …|²
| A₁, A₂ … = amplitude arrows for indistinguishable routes to one result | | add the arrows before calculating probability | | |…|² means square the size of the combined arrow |
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RUN THE TINY EXAMPLE
Change only the phase
Together: A₁ = 0.5, A₂ = 0.5 → |1|² = 1 Opposed: A₁ = 0.5, A₂ = −0.5 → |0|² = 0 Same path strengths; completely different outcome probability
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These toy amplitudes isolate the idea. The relative sign determines whether the two contributions reinforce or erase each other.
/ SO WHAT?
The transferable prediction is simple: if a physical interaction leaves a usable record of which route occurred, the routes become distinguishable and the interference bands can disappear. Quantum hardware must prevent such route-revealing interactions long enough to use the relative arrow directions.
ONE CAVEAT |
| One electron still produces one definite dot. Interference predicts the distribution across many repeated trials, and the probabilities across all possible results must still total 100%. |
KEEP THIS
When indistinguishable quantum routes lead to one result, add their amplitude arrows first; their directions decide whether that result strengthens or fades.
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NEXT: The shortcut beside your data
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