On request from a friend - a journey that starts from Young's double split experiment and ends up with fundamental questions about the nature of reality and the essence of science...

1. What does the Double Slit
Experiment tell us about the
nature of reality?
Jean-Philippe Divo
February 22nd, 2013

2. What does the Double Slit Experiment
tell us about the nature of reality?
 Fighting Theories: is light a wave or is it made of particles?
 The Double Slit Experiment: the end of controversy… for a time
 The dual nature of light: a wave and a particle at the same time?
 Relations between science and reality
 Did we learn something in the end?
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3. What is the nature of light?
Rays of light in the sky
hint about particles
travelling in straight line
But how could particles explain
rainbows and other optical
phenomena?
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4. A 2,000 years old
scientific controversy
Wave Theory
Aristotle
(384-322 BC)
Descartes
Treatise on Light (1664)
Huygens
Treatise on Light (1690)
Particle Theory
Democritus
(460-370 BC)
Alhacen
Book of Optics (1021)
Newton
Opticks (1704)
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5. The victory of the wave theory:
enters Thomas Young
 For one century, Newton’s views on physics, including
nature of light, had dominated the scientific world
 However some phenomena could not be satisfactorily
explained with Newton’s theory, such as refraction
 The wave theory eventually took over thanks to another
English brilliant polymath, Thomas Young (1173-1829)
 Young’s logical path:
1. He observed and formalized the concept of interferences
in the context of water waves
2. Then he demonstrated that such interference patterns
could also be observed with light
3. Hence “proving” the wave nature of light.
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6. The tangible experiment:
interferences in a ripple tank
 Water in a ripple tank can be excited such a way to create waves,
the interference of which can be observed. This emulates reality.
 Young generated interferences by having a plane wave front go through two
slits in a wall, with specific patterns for amplitude build-up and destruction.
Natural interferences at sea Ripple Tank Apparatus Double Split Experiment
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7. The indirect experiment:
light through double split
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8. Young’s Interpretation
1. The pattern of dark and bright fringes cannot be explained
by the particle theory of light
2. It is fundamentally similar to the highs and lows of
amplitude observed when water waves interfere together
3. Hence light must also be of wave nature
Note: Interpretation works by making an analogy between directly
visible properties (water waves) and visible effects of invisible
attributes (the interference fringes)
Young’s original interpretation drawing
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9. Wave Nature of Water,
Sound and Light
 We can simulate the behavior of water, sound and light waves
 But is it all? Can we conclude that light is a wave?
 The weakness of the wave theory is that light waves,
like sound and water waves, would need a medium for transmission…
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10. New dual split experiment
with individual objects
 One hundred years Young, scientists made progress in digging at
atomic scale and below and identified new objects:
 Discovery of electrons in 1897 (Thomson)
 Explanation of photoelectric effect through quanta of light
(Einstein & Planck, 1905)
 Demonstration of the existence of photons (Millikan, 1915)
 The experiment of Young was repeated with sources
able to generate individual photons or other kinds of particles
 Guess what happened…
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11. Wave or Particle – again…
 Initially – random individual impacts
 After some time – is there a pattern?
 After longer time – interference pattern.
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12. Interpretation: Wave-Particle Duality
 Each photon ends up as a particular hit on the screen: it must be a particle.
 But the probability of hitting the screen strictly follows the interference pattern
as predicted by Young: photons must be waves.
 However, it is possible to observe which slit a particular photon went through –
in which case the interferences disappear. So it must be a particle?
 Similar experiments have been made with other particles (electrons) and larger
molecules (fullerene C60 and beyond) and confirmed the wave-particle duality.
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13. A new theory: Quantum Mechanics
 Wave-particle duality is “explained” by Quantum Mechanics
 Quantum objects are represented by wave functions : the wave (speed and
phase) describes the propagation and interferences, and amplitude (squared) of
the wave describes the spatial probability of presence of the particle
 Observation typically collapses the wave function along the observed
dimension (e.g. momentum or position) and “resets” it from then on
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15. A debated Theory
 Although no better theory was found (yet), the debate remains intense
between scientists regarding the “true” nature of quantum objects:
 Wave-and-particle duality
 Both-particle-and-wave-view
 Wave-only view
 Neither-wave-nor-particle view
 …
 So how to conclude about reality if scientists have no common view???
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16. Science versus Reality
 Science can only describe, model and predict
 All scientific theories have been invalidated (or completed) sooner or later:
“This double nature of radiation (and of material corpuscles)...has been
interpreted by quantum-mechanics in an ingenious and amazingly successful
fashion. This interpretation...appears to me as only a temporary way out...”
Albert Einstein
 Establishing reality is out of reach of science, although it can structure our
vision of reality – the rest is about personal beliefs:
“Quantum theory also tells us that the world is not simply objective; somehow
it's something more subtle than that. In some sense it is veiled from us, but it
has a structure that we can understand. Nevertheless, all of us who work in
quantum physics believe in the reality of a quantum world, and the reality of
quantum entities like protons and electrons”
John Polkinghorne
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17. So what did we really learn?
 Reality is subject to interpretation
 Reality can be approximated to the latest working theory,
although this view may be limitative and even dangerous (scientific “truth”)
 Some interpretations of quantum physics lead to non unique reality
(multi-world interpretation, collapse of the wave function by the observer)
 Taken to the extreme, experiment tells us nothing about the nature of reality
 This is actually a millennium-long wisdom: Plato’s allegory of the cave
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18. Plato’s Cave
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19. A personal word
 We learned about science and human mind
 Since science is just an attempt to model and understand reality,
there is no absolute truth in science - always question received knowledge
 Greatest scientists have been active in multiple domains, from physics to
biology and philosophy, always trying to decipher reality (polymaths)
 Progress is obtained through doubt and cross-fertilization.
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20. Thank You
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