**Can an electron travel through two slits at the same time?**

A shorter version of this article is also published at researchgate.net (DOI: 10.13140/RG.2.2.17355.34080) and academia.edu.

Let’s go straight to the source!

Let’s go straight to the source!

In his lectures

**Richard Feynman**proposed a thought experiment with**electrons traveling through two holes**
This experiment has been realized
later by different teams, e.g. “Demonstration of single-electron buildup of an
interference pattern” / A. Tonomura, J. Endo, T. Matsuda, T. Kawasaki, and H.
Ezawa / American Journal of Physics 57, 117 (1989); doi: 10.1119/1.16104 (view
online: https://doi.org/10.1119/1.16104)

All those experiments were used to support
a “classical” interpretation of its results, i.e. “a single electron can pass
through both of the slits” (A. Tonomura et al., 1989).

Let us analyze what the author of the
experiment thought on this matter.

Feynman stated that electrons are
registered in “lumps”.

Then he stated: “

*Proposition A:*Each electron*either*goes through hole 1*or*it goes through hole 2.”
Then he arrived at: “For electrons:

*P*12 ≠*P*1+*P*2.”
And then he finishes: “… since the number
that arrives at a particular point is

*not*equal to the number that arrives through 1 plus the number that arrives through 2, as we would have concluded from Proposition A, undoubtedly we should conclude that*Proposition A is false*. It is*not*true that the electrons go*either*through hole 1 or hole 2.”
And yet, in the next chapter

he writes (all bold fonts are mine, not
Feynman’s):

1. “when there are

**two ways for the particle**to reach the detector, the resulting probability is not the sum of the two probabilities”
2. “When a

**particle**can reach a given state**by two possible routes,**the total amplitude for the process is the*sum of the amplitudes*for the two routes considered separately.
3. “we are going to suppose that the holes
1 and 2 are small enough that when we say

**an electron goes through the hole**, we don’t have to discuss which part of the hole.”
4. “the amplitude for the process in which
the electron reaches the detector at

*x***by way of hole 1**”
5. “the amplitude to go from

*s*to*x***by way of hole 1**is equal to”
6. “

**The electron goes from**.”*s*to 1 and then from 1 to*x*
7. “The electron can

**go through hole 1**, then**through hole**, and then to*a**x*; or it could go**through hole 1**, then**through hole**, and then to*b**x*; and so on.”
8. “amplitude that

**an electron going through slit 2**will scatter a photon”
9. “the amplitude that

**an electron goes via slit 2***and*scatters a photon”
10. “two factors: first, that

**the electron went through a hole**, and second”
11. “when

**an electron passes through hole 2**”
12. “when

**the electron passes through hole 1**”
Theses twelve quotes (there are more)
clearly show that the father of this experiment believed that an electron

**could**travel through one hole/slit, or through another one, but he**considered an electron traveling through***never***holes at***both***time; he never made that statement.***the same*
He wrote, for instance: “the probability
of

**arrival through**both holes”. But “arrival through” is not the same as “traveling through both at the same time”; it means rather “arrival thought a screen with two holes”.**The whole idea of a path integral is based on the assumption is that**because it is always traveling

__an electron is always located somewhere, i.e. it is always localized__,**through**and then

*this*point*this*, and then

*this*, etc.

**A**(even when a particle circles back making a loop the time keeps running ahead and on each path a particle is always located at one place at a time)

__path does__, there are no forks*not*split**, hence,**

*there are no instances when an electron is located at to places at the same time.*
A path integral was a brilliant idea of a
genius: just assign an amplitude to each possible path and add them up! So
obvious!

*After*you learn it. That is what many physicists feel - it's natural, and do not think about implications to the fundamentals of quantum mechanics, including the*interpretation*of the wave-particle duality. And the genius of Feynman was*not*inventing paths, but*assigning*to each one.__an amplitude__
A simple toy with small balls running down a set of
pins represents a good model for paths and a path integral.

When a ball drops through a
spout, its trajectory through the board is unpredictable. For each trajectory that
begins at point A and ends at point B, there is a probability that a ball will
travel exactly along that trajectory.

The key words is “probability”.
Feynman realized that in the quantum world we can use the same picture, but
instead of a probability we have to use a probability amplitude. The one who
will explain – why? – deserves the Nobel Prize.

Let us
return to our main topic. As we see, the
idea of a path integral is based on the assumption is that

__an electron is always located somewhere, i.e. it is always localized__, and, hence, there are no instances when an electron is located at to places at the same time.
This seems contradicts Feynman’s own
conclusion about Proposition A.

He wrote: “is

*not true*that the lumps go either through hole 1 or hole 2, because if they did, the probabilities should add”.
But later, as I proved using his own words,
in his further analysis he was fine with an electron traveling through one
whole or another.

So, what did he really mean?

I believe, when Feynman stated his
Proposition A, he simply did not do it as accurate as he should have done.

He should have said: “

*Proposition A:*Each electron*either*goes through hole 1*or*it goes through hole 2**– in a classical sense**”.
And

*this*statement*is*false.
Based on the next chapter (experiments
with light), we understand that when he said: “It is

*not*true that the electrons go*either*through hole 1 or hole 2 ”, he meant “It is*not*true that**we are always able to know if**the electrons go*either*through hole 1 or hole 2 -**unless the interference between the two paths is destroyed**”.
Because later he told us that an electron

*does*go through hole 1*or*it goes through hole 2 – however, in a different, non-classical sense, with the use amplitudes instead of probabilities.
If we accept that an electron can travel
through a hole – through only one hole, it is not clear yet from Feynman’s
discussion what is really happening in a two-hole experiment when no one is
watching where exactly an electron gets through the screen?

Naturally, many other physicists jumped on
this thought experiment and discussed it in great details in their books.

For example, J. D. Cresser writes (2009; http://physics.mq.edu.au/~jcresser/Phys301/Chapters/Chapter4.pdf; in the following quotes, all bold fonts are mine):

“If electrons are particles, like bullets, then it seems clear that the electrons go either through slit 1orthrough slit 2, because that is what particles would do. The behavior of the electrons going through slit 1 should then not be affected by whether slit 2 is opened or closed as those electrons would go nowhere near slit 2. In other words, we have to expect that P12(x)=P1(x)+P2(x), but this not what is observed.

In the excerpt, the author repeats arguments as old as fifty or even sixty years old – “no way to understand quantum mechanics if particles are only particles”.

“If electrons are particles, like bullets, then it seems clear that the electrons go either through slit 1orthrough slit 2, because that is what particles would do. The behavior of the electrons going through slit 1 should then not be affected by whether slit 2 is opened or closed as those electrons would go nowhere near slit 2. In other words, we have to expect that P12(x)=P1(x)+P2(x), but this not what is observed.

**It appears that we must abandon the idea that the particles go through one slit or the other.**But if we want to retain the mental picture of electrons as particles,**we must conclude that the electrons pass through both slits**in some way because it is**only by ‘going through both slits’ that there is any chance of an interference pattern forming.**After all, the interference term depends on*d*, the separation between the slits, so we must expect that the particles must ‘know’ how far apart the slits are in order for the positions that they strike the screen to depend on*d*, and they cannot ‘know’ this if each electron goes through only one slit. We could imagine that the electrons determine the separation between slits by supposing that they split up in some way, but then they will have to subsequently recombine before striking the screen since all that is observed is single flashes of light. So, what comes to mind is the idea of the electrons executing complicated paths that, perhaps, involve them looping back through each slit, which is scarcely believable. The question would have to be asked as to why the electrons execute such strange behavior when there are a pair of slits present, but do not seem to when they are moving in free space.**There is no way of understanding the double slit behavior in terms of a particle picture only**.”In the excerpt, the author repeats arguments as old as fifty or even sixty years old – “no way to understand quantum mechanics if particles are only particles”.

And then the author goes on to building an
elaborated picture of a wave packet that is a particle and a wave at the same
time, etc., etc..

And then, following Feynman, he discusses another mystery, that is - when we know through each hole an electron travelled (e.g. using flashes of light) we destroy the interference.

The answer, however, lies in the very statement used to prove that electrons cannot ravel through one hole or another one.

Let’s read it one more time.

And then, following Feynman, he discusses another mystery, that is - when we know through each hole an electron travelled (e.g. using flashes of light) we destroy the interference.

**Only when we do not know how exactly electrons travel through the holes, interference exist**. Why? No one knows.The answer, however, lies in the very statement used to prove that electrons cannot ravel through one hole or another one.

Let’s read it one more time.

“If electrons are particles, like bullets, then it seems clear that the electrons go either through slit 1orthrough slit 2, because that is what particles would do. The behavior of the electrons going through slit 1 should then not be affected by whether slit 2 is opened or closed as those electrons would go nowhere near slit 2. In other words, we have to expect that P12(x)=P1(x)+P2(x), but this not what is observed. It appears that we must abandon the idea that the particles go through one slit or the other.”

But abandoning “the idea that the particles go through one slit or the other” is

*not*only one logical solution!

Another one is

**to abandon a previous statement**, that said: “The behavior of the electrons going through slit 1 should then not be affected by whether slit 2 is opened or closed as those electrons would go nowhere near slit 2.”

Why should that behavior be

*not*affected? Because this

*is*what we would expect in the classical mechanics from

*classical particles*! But our experiment involves quantum particles! So, why should we impose on them our classical expectations?

**There is simply no logical reason to do that**. So, let’s

*not*do that and see where it will lead us.

If (a) particles do travel through one hole

*or*another (only one hole at a time), and if (b) the interference pattern exists, it means that the statement is wrong.

The statement: “The behavior of the
electrons going through slit 1 should then not be affected by whether slit 2 is
opened or closed as those electrons would go nowhere near slit 2.” Is wrong.

And

*that*means that the behavior of the electrons going through slit 1 is*affected*by whether slit 2 is opened or closed*even though*those electrons would go nowhere near slit 2.
We can make even a more general statement:

__Proposition V__**: when**

__two__slits are open, an electron (and a photon, and any quantum particle!) behaves differently than it does when__one__slit is open.
Proposition V means that when a quantum
particle travels to the screen with holes/slits it already "knows" how
many holes are open there. And under certain circumstances, some aspects of the
behavior of those particles exhibit features similar to features of classical
waves.

Particles are not waves. But their
behavior may be wave-like.

Let us step for a moment away from the
main matter and make this note on the nature of waves.

All classical waves are NOT specific individual physical
objects. A wave is a specific form/state of a substance described by a mathematical
object called “a field” (more on definitions in "On a Definition Of Science"). A field is a mathematical description of a state
of a substance distributed over a large region of space. A substance has
structure and composed of a vast number of small and usually identical
"blocks" (atoms, molecules, balls and springs). Thinking about a classical
wave as of one undivided large object is simply

**wrong**. But even an electromagnetic field has quantum structure – photons. So, when one says this word “a wave” – what does one actually mean?
Let us assume that a wave-function is an actual
physical wave. A particle is a wave-pocket traveling in space. Fine. Does it
have a definitive size; a boundary between the region filled with matter and
energy and the rest of the universe? If it does - so, it is just a large particle?
If not, if all the mass and energy asymptotically "smeared" over the
whole universe (a mathematical cut-off exists, like "effective
radius", but it is

These and other questions make this picture too complicated - it does not worth to be fought for.

But in that case one needs a different, simpler model. And that model exists - a particle is always a particle, it just is not classical, hence behaves in a non-classical way described by SchrÃ¶dinger's equation. And that behavior - statistically - resembles some elements of the behavior of classical waves.

But quantum particles are NOT waves.

*mathematical*- like a half-life for a radioactive element), how does all that mass and energy get smeared over the whole universe the moment a particle leaves an atom and then "collapses" back when it hits a screen? And if light is a composition of photons, and the double slit interference experiment for light should be explained in the same way it is explained for electrons - how to make a physical wave-pocket for it - it needs to travel at the speed of light in a non-relativistic theory.These and other questions make this picture too complicated - it does not worth to be fought for.

But in that case one needs a different, simpler model. And that model exists - a particle is always a particle, it just is not classical, hence behaves in a non-classical way described by SchrÃ¶dinger's equation. And that behavior - statistically - resembles some elements of the behavior of classical waves.

But quantum particles are NOT waves.

And the two-slit experiment does

*not*give us any proof to the statement that particles*are*also waves. The wave-particle duality is NOT about this.
What the two-slit experiment shows us is
that the configuration of the screen (one hole, two holes, three holes, etc.)

*affects*the motion of the electrons, photons, all particles traveling toward that screen.
In the classical world, a particle does
not know anything about the screen it travels to until it hits it.

But an electron “knows”/“feels” if the
hole 2 is open or closed. If we shine a light on an electron, it actually
“forgets” about the existence of another hole and travels like the only one
hole exists – hence, the destruction of interference.

The real question now is: how do quantum
particles “know” how a screen is built and react to its structure?

*That*is the true mystery of quantum mechanics.

This question requires a new discussion.

In general, the answer is – quantum particles
“know” about the features of a screen in the same way they “know” about states
of each other when they have been prepared in an entangled way.

The double-slit experiment and quantum entanglement
are two very close phenomena.

Let’s go straight to the source – the famous
EPR paper.

It has many layers, more than just the
thought experiment they use to claim that quantum mechanics is not a complete
theory (e.g. click on this link and scroll down to Appendix III). If someone talks to you about entanglement, ask if he/she rad this paper. If not - does not worth your time (more an entanglement in "On the Entanglement Between SuprFluidity, SuperConductivity and Entanglement").

The fact of the matter is that this
experiment

*does*show that quantum mechanics is different from classical mechanics (as EPR put it – “incomplete”).
When this matter is accepted, one has a
choice: (a) follow the strategy "shut up and calculate" and do not
spend any time on trying to make the theory "complete", or (b) spend
some time on trying to make the theory "complete".

In the latter case, one can be inventing
different approaches - some are mentioned in the four pieces about a cat:

But the simplest (thank you - Occame!) way to resolve all the mysteries of quantum mechanics would be to assume that - yes, "spooky action at a distance" exists, and it exists due to facter than light interactions!

Naturally, Einstein would never accepted this solution, but no one is infallible.

Naturally, Einstein would never accepted this solution, but no one is infallible.

Particles that travel faster than light
have been proposed, and named tachyons.

**Tachyons are responsible for that "spooky action at distance".**

There is a whole world of particles that
cannot travel slower than the speed of light! And that world interacts with our
world, where particles cannot travel faster than the speed of light.

Simple!

Imagine a sea of tachyons. Every known
particle can have its counterpart in that sea: tachyo-electron, tachyo-proton,
etc. Due to fluctuations, for a teeny-tiny instant of time, those tachyons may
enter our world, become a so-called virtual particle, and interact with our-world
particles. But even more interesting process happens when our-world particles
can disappear from our world and enter the world of tachyons, spend there a
teeny-tiny instant of time and come back again - but at a different location,
or with a different speed, or both, or in general in a different state.

When two particles are entangled, they
keep interacting via tachyons. And that is why making one particle to accept a certain
state (e.g. by imposing a magnetic field) it makes another particle – that one
that was entangle with the first one – to immediate accept a corresponding state

(more on entanglement in Thinking about the
origins of the Quantum Mechanics. or

Some of the entanglement experiments (thought
or real) could have been explained even without the use of tachyons. The
distances between the particles would allow photons to make the particles “feel”
each other. But tachyons are just so much cooler!

Of course, until tachyons are found, they
are just a theory, a mathematical abstract.

__But so was the Higgs boson__.
By employing tachyons, we replace several difficult
problems with one difficult problem – finding tachyons.

Tachyons, or in general the world of
faster than light particles, can also explain such intriguing quantum phenomenon
as tunneling.

A classical particle cannot escape a
potential well - when it has not enough energy. But a quantum particle can
"tunnel" through. Why? Because due to interactions with tachyons it
may "accidentally" (a scientific name – via fluctuations) gain energy
enough to get "over the well".

And, finally, back to the double-slit electron
diffraction experiment.

A screen is also made of particles. A sea
of tachyons between a flying electron and a screen makes those two objects
interact and their evolution correlate. Of course, the evolution of a screen is
simple – being there. But the evolution of a traveling electron is affected by
the structure of the screen. In a way, this picture is similar to the “pilot-wave”theory.

There is a mechanical model that may help
to visualize the phenomenon.

Imagine a small ball floating in water. It
has a little motor that spins a fan and makes it move. But it also has inside a
small of-center spinner, that makes the ball vertically oscillate in water.
Those oscillations travel away and when they reach an obstacle, for example a
screen, they get reflected and act back on the ball. Of course, the reflected
waves will depend on the structure of a screen (one hole or two). And that may
affect the motion of the ball.

The sea of tachyons should bring back some
version of a “hidden-variables” theory, because the particle-tachyon interaction
does not obey the limits imposed by the Von Neumann’s theorem
(although, some physicists claim that the theorem has flaws anyway).

The next step is the development of an appropriate
mathematical model – and the Nobel!

You’re welcome!

And finally, the answer to the question in the title - no, an electron cannot travel through two slits at the same time.

But it does NOT have to!

And finally, the answer to the question in the title - no, an electron cannot travel through two slits at the same time.

But it does NOT have to!

Disclaimer: the bulk of this post originally were published as appendixes to other posts on this page: Fundamentals of Quantum Physics

**Appendix**

When I sent a copy
of this article to arXiv, I knew it would be rejected.

My blog, as it’s said
in a title, is an experiment. This time I was wishing for reasons my article
would be rejected.

I expected to see
something like “wrong format”, “loose language”, “absence of citations/references”,
“a wrong arXiv section”.

But the result of
my fishing exceeded my expectations.

I was told, quote:
“article
does not contain sufficient original or substantive scholarly research” (the full letter
is at the end of this appendix)

It makes one put
things in perspective.

When authors
write an article where they apply a theory beyond the area of its applicability,
trying to use a standard quantum mechanical formalism to a classical system,
which is like applying Newton's 2nd law to relativistic particles, but including
references, scientifically sound terminology, and cool mathematical symbols - it
is considered a research worth to be published (“Quantum theory cannot consistently
describe the use of itself”; Appendix II of this piece
provides deeper analysis).

When an author
offers a critical analysis of a logical structure of statements made about
fundamental quantum mechanical phenomenon offering an alternative
interpretation – that’s not a “substantive scholarly research”.

This is a typical
example of a narrow-minded formatted thinking in science.

Originally, a
scientific magazine was an instrument for (1) exchange of scientific ideas, and
(2) reporting the results of a scientific research.

Of course, the format
matters! But that's what editors are for, or moderators.

I'm not sure how many
people on average read every arXiv paper, but in four days since its
publication, my article was read by more than 30 people (that doesn't count
reads on Academia and Researchgate).

************************************

A full letter from
arXiv.

Dear arXiv user,

Our moderators have determined that your submission is not of sufficient interest for inclusion within arXiv. This decision was reached after examining your submission. The moderators have rejected your submission as "unrefereeable": your article does not contain sufficient original or substantive scholarly research.

As a result, we have removed your submission.

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Our moderators have determined that your submission is not of sufficient interest for inclusion within arXiv. This decision was reached after examining your submission. The moderators have rejected your submission as "unrefereeable": your article does not contain sufficient original or substantive scholarly research.

As a result, we have removed your submission.

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Please do not resubmit this paper without contacting arXiv moderation and obtaining a positive response. Resubmission of removed papers may result in the loss of your submission privileges.

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