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You Have Never Actually Seen Light — The Most Disturbing Idea in Physics

Blue Pale Signal

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You cannot see light. A beam passing right in front of your face is completely invisible to you — and that single fact is the first link in a chain of reasoning that ends at the multiverse.

This is the many-worlds interpretation of quantum mechanics, argued by physicist David Deutsch — the inventor of quantum computing — in his landmark book "The Fabric of Reality." Starting from how photons actually behave, we trace a path through shadow photons, the double-slit experiment's real implications, and the staggering computational demands of quantum algorithms to reach a conclusion Deutsch insists is not interpretation but inevitability: parallel universes are the simplest explanation of observed reality. The final act turns the argument on time itself — and asks why "the past" and "the future" may be every bit as real as "here" and "there."

If you've ever wondered whether parallel universes are real, whether time is an illusion, or where a quantum computer actually does its work, this is the deep dive.

Chapters:
0:00 A flashlight in an infinite dark room
1:06 You have never actually seen light
2:55 Who is David Deutsch?
4:51 Light comes in packets: the quantum
8:16 The most important experiment in physics
11:36 Firing photons one at a time
13:46 The measurement problem and "don't ask"
15:26 Deutsch's answer: shadow photons
19:00 Shadow matter and parallel universes
23:18 The machine: quantum computing and Shor's algorithm
28:45 Occam's razor, inverted
30:24 Turning the argument on time
33:52 The block universe meets the multiverse
37:41 The delayed-choice quantum eraser
42:52 Hugh Everett and the objection everyone has
45:54 Why you can't feel the earth turn

In this video: the many-worlds interpretation, quantum interference, the double-slit experiment explained at the single-photon level, shadow photon theory, Shor's algorithm and the quantum computing resource paradox, the block universe, and the relationship between time and the multiverse. David Deutsch's work connects quantum physics to the philosophy of knowledge and explanation in ways rarely covered on YouTube.

Based on "The Fabric of Reality" by David Deutsch, with additional material from "The Beginning of Infinity" (Deutsch, 2011) and the original delayed-choice quantum eraser experiments.

If this changed how you think about what's real, subscribe for a new physics deep dive every two weeks.

#quantumphysics #multiverse #daviddeutsch #paralleluniverses #manyworlds #science

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Image credits:

Hugh Everett III
Photo from U.S. Department of Defense archives
Public Domain
https://en.wikipedia.org/wiki/File:Hu...

David Deutsch
Photo by Simon Benjamin
Licensed under Creative Commons Attribution 3.0 (CC BY 3.0)
https://commons.wikimedia.org/wiki/Fi...

World Line
Created by Inductiveload
Licensed under Creative Commons Attribution-ShareAlike 3.0 (CC BY-SA 3.0)
https://commons.wikimedia.org/wiki/Fi...

Bryce S. DeWitt with Cécile DeWitt
Photo by Brandon dinunno
Licensed under Creative Commons Attribution-ShareAlike 3.0 (CC BY-SA 3.0)
https://commons.wikimedia.org/wiki/Fi...

Oxford – Bodleian Library and Radcliffe Camera
Photo by DAVID ILIFF. License: CC BY-SA 3.0
https://commons.wikimedia.org/wiki/Fi...

iPhone First Generation
Photo by Carl Berkeley
Licensed under Creative Commons Attribution-ShareAlike 2.0 (CC BY-SA 2.0)
https://commons.wikimedia.org/wiki/Fi...

Scientific paper:

Interference experiment with electrons (Fig. 1–3)
Illustration from The Feynman Lectures on Physics, Volume III, Chapter 1
Authors: Richard P. Feynman, Robert B. Leighton, Matthew Sands
Copyright © California Institute of Technology
Source: https://www.feynmanlectures.caltech.e...

Double-slit experiment with single photons (Figure 3)
Source: Anton Paar Wiki
Title: Double-slit experiment
Copyright: Anton Paar Wiki (see website terms)
https://wiki.anton-paar.com/en/double...

Quantum theory as a universal physical theory
David Deutsch
International Journal of Theoretical Physics, Vol. 24, No. 1 (1985)
Source: https://www.daviddeutsch.org.uk/wp-co...

20 Comments

@kalimike3248 4 days ago (edited)

1. With X-rays instead of light / electrons?

It still interferes, you just need much, much smaller slits.

Interference only happens when slit width and slit separation are comparable to the wavelength.

- Visible light: wavelength ∼500 nm - you can cut slits with a razor.
- X-ray: wavelength ∼0.01 to 10 nm. You can't mechanically make two slits that small and keep them coherent.

So what physicists do is use a crystal lattice as the slits. Atoms in a crystal are ∼0.1 nm apart - perfect for X-rays. That's exactly what X-ray diffraction is: a double-slit experiment with millions of slits. You still get bright and dark fringes.

There is one difference: an X-ray photon is much more energetic. It interacts more strongly with matter, so it's much harder to keep it from leaving a "which-path" trace. If anything in the environment can tell which slit it went through, even in principle, the interference washes out. That's why X-ray interferometers have to be perfect crystals in vacuum.

But if you preserve coherence, you get the same rule:
I = I_1 + I_2 + 2\sqrt{I_1 I_2}\cos(\Delta\phi)
Wave-particle duality doesn't stop at visible light.

2. With 3 slits instead of 2?

Yes, and this is where it gets really interesting.

With 2 slits, intensity at a point on the screen is from 2 waves adding:
\psi = \psi_1 + \psi_2
With 3 slits:
\psi = \psi_1 + \psi_2 + \psi_3
I = |\psi_1 + \psi_2 + \psi_3|^2 = I_1+I_2+I_3 + \text{(pairwise interference terms)}
What you see:
- The main bright fringes get sharper and brighter
- You get small secondary maxima between them
- It's essentially the start of a diffraction grating. With 2 slits you get soft sine-wave fringes, with 3 you get narrower peaks, with 1000 you get the sharp lines of a grating spectrometer.

Why physicists care: The triple-slit experiment is a direct test of the Born rule, $P = |\psi|^2$.

If quantum mechanics had true 3-path interference, there would be a term that only appears when all 3 slits are open that isn't just the sum of the 2-slit combinations. Physicists define the Sorkin parameter:
\epsilon = I_{123} - I_{12} - I_{13} - I_{23} + I_1 + I_2 + I_3
If $\epsilon \neq 0$, quantum mechanics is wrong and we need higher-order interference.

Every experiment so far with photons, electrons and molecules finds $\epsilon = 0$ within experimental error. Interference is always just pairwise. Three paths interfere by interfering as pairs.

So: X-rays = same physics, harder engineering. 3 slits = same physics, sharper pattern, and a beautiful proof that quantum mechanics stops at second-order interference.

@stuartmitchell3739 3 weeks ago

So do I need to get up for work?

@kalimike3248 4 days ago

I want yo see the doublet slit experiment done with x-rays...or infrared, or radiowaves, or individually with Every frequency beyond visible light...

@yengEEs 3 months ago

What’s the final equation to put on a bumper sticker?

@zzaa1756 2 months ago

The stars shining in the night sky teach us that the past and the present exist without distinction.

@senserave 1 month ago

During my college days same question what u say arrived to mind. But others told don't confuse.

@kalimike3248 4 days ago

Shor's algorithm to factor a number:
1. Put a quantum register into a superposition of ALL numbers at once. Imagine opening not 2 slits, but $2^{n}$ slits at once.
|\psi\rangle = \sum_{x=0}^{2^n-1} |x\rangle
2. Compute $f(x) = a^x \mod N$ in superposition. Now each slit gets a label $f(x)$. Many different x's have the same label because $f(x)$ is periodic. That periodicity is what encodes the factors.

3. Now you apply the Quantum Fourier Transform. The QFT is just the mathematical lens that makes all those $2^n$ paths interfere on the screen.

Just like with 3 slits you got sharper principal maxima, with $2^n$ slits you get extremely sharp peaks. The peaks only survive where the phases line up constructively. Where they survive tells you the period $r$ of $f(x)$.

From $r$ you get the factors: $\text{gcd}(a^{r/2} \pm 1, N)$.

So what if you apply Shor's to the double-slit?

You already are. Shor's says: if I can maintain coherence across an insane number of paths - and not let the environment measure which path - the interference pattern will compute something no classical computer can.

Why we don't do Shor's with X-rays and real slits: you'd need a screen with $2^{1000}$ positions and perfect stability. Instead we use qubits, which are slits in Hilbert space, not in physical space. The physics of erasing which-path information is identical.

In short: 2 slits proves quantum mechanics allows interference. Shor's algorithm uses that interference to break RSA.

@Queeney59 2 months ago

I just love this stuff. I'm wondering if the dark photons would appear as light in their universe and our light would appear as dark photons.

@ChrisWilson-wn4qc 1 month ago

Its like going crosseyed and knowing only one of the imagines are tangible in our physical reality. And the other is the split refraction of itself. Perhapes like looking in at the screen of the mind.

@MichaelGreene-l8i 2 months ago (edited)

If "time is a feature of the map, not the territory the map sits in", then what aspect of the territory is being expressed by time? And more importantly what is the territory being mapped? If the territory being mapped is space, what purpose does time serve in the territory of space? Are there other features in the territory of space besides time we can observe?

@katon1872 2 months ago

PLATO had the same ideas more than 2000 years ago. Especially in his ALLEGORY OF THE CAVE 😅

@DragulaAD 2 months ago

This video was so good. I wish it was longer.

@AliránG.M 2 months ago

Thank you for sharing this theory detailed...😊

@Noro-i9i 2 months ago

If the could factor large numbers with quantum comps, we would know for sure.

@ConstantinVibe 3 months ago (edited)

But you can feel. The problem is that we expect to feel something more vivid or more valid or more painful but it is a feeling. It's called anticipation. It's called visualisation. It's called perspective. It's also called potential

The fact that we dont feel that exists until we actually have it makes me think that we actually are moving true static universes and we just chose whether to go ( and we call that planing and patience we just need enough slides of universes to pass true our destination)

@tonynikolaos3527 2 months ago

It’s a play of words and semantics to make you “realize” something that looks perplexing… going for cheap effects!

@justtrolin 3 months ago

It is telling when a speculation, an attempt, an explanation takes on the form of several statements. For my attempt I used the metaphor of a pendulum: the weight of the pendulum and the length of the string swinging from a fulcrum. It's the illustration of the tactile senses, specifically the olfactory one, for you can have similar-shaped things but the aforementioned setup determines how the mind experiences the difference. This simple principle can be applied across all of our senses. The question then becomes: how do we set up a frame of reference that translates this phenomenon in a way that is universal or at least reductive enough to teach in a classroom in ways we can understand? In my personal speculations in these meditations it seems that the sine wave necessary is one of body, which suggests that such ideas can be so delicate that the moment they are examined they literally fall apart because of the rigid preconceptions and the systems they answer to. The artistic rendition becomes one of a jungle with scattered greenhouses protecting such delicate forms. This could also mean that there might be brains out there that have yet to be cataloged by the sciences and hence the best we can do is medicate them in order to align them to what we describe as reality.

@superbahmindcontrollers179 2 days ago

I am 68 years old. The last two years I have had a real feeling that everything that I experienced, throughout my life needed to happen exactly as it did or I would not be who I am today. Really quite magical.