What does the Double Slit Experiment tell us about the

nature of reality?

Jean-Philippe Divo

February 22nd, 2013

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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What is the nature of light?Rays of light in the sky

hint about particlestravelling in straight line

But how could particles explain rainbows and other optical

phenomena?

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A 2,000 years oldscientific 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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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 patternscould also be observed with light

3. Hence “proving” the wave nature of light.

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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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The indirect experiment:light through double split

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Young’s Interpretation1. 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 natureNote: 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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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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New dual split experimentwith 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 sourcesable to generate individual photons or other kinds of particles

Guess what happened…

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Wave or Particle – again…

Initially – random individual impacts

After some time – is there a pattern?

After longer time – interference pattern.

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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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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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Dr Quantum’s explanations

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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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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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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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Plato’s Cave

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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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Thank You

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