Does a particle really try every possible path?

[Mark Harder]

In the path integral are there ftl paths?

"ftl" ?

 

[StevieTNZ]

"ftl" ?

faster than light

 

[Mark Harder]

I agree w/ all of bhobba's statements but I think I do see what you are getting at and yes, other than your mis-statement about your being able to travel at the speed of light, you are right. These weird things are possible (but one at a time, not all together ... it's a statistical thing) but almost all paths are so utterly improbably that they can be ignored for all practical purposes. I'm not sure that changes the weirdness of QM but it's also clear that you have a distorted view of QM so it may well change what YOU view as the weirdness of QM.

It seems to me, a non- physicist with an admittedly rudimentary knowledge of, but some experience using, QM, that these "weird" notions in QM are really physical interpretations, as in the "Copenhagen Interpretation", of an essentially mathematical framework for predicting physical observations. Intuitive understandings of classical mechanics were relatively easy to make. Waves and fields could be pictured. Even spacetime curvature can be pictured. But these intuitive pictures couldn't be extended to include quantum realities, so new physical explanations had to be invented. The Copenhagen interpretation is one of these. Perhaps out of ignorance, I am skeptical of these pictures. Someday, I will learn more QM and perhaps I will arrive at a physical intuition that satisfies me.

 

[bhobba]

As for "not real", either a path is in the integral or it isn't. What does "real" have to do with it?

Hmmmm. Actually that's a good point.

Thanks
Bill

 

[bhobba]

I agree w/ all of bhobba's statements

I suspect I have goofed here. Peter has corrected me, and its exactly in the area I often stress - namely what is real.

Even in QFT you include FTL and other normally not allowed things - that they are not classically allowed and in that sense are not real doest mean didly squat as far as what is included in the path integral.

Thanks
Bill

 

[Mark Harder]

How do you define PARTICLE?
How do you understand the FTL transmission of information observed in recent entanglement experiments?

Does entanglement require the transmission of information, or is it a statement about what happens when 2 quantum systems originate from the same event? My understanding is that the experiments that demonstrate entanglement involve particles that interact locally, then each carries information depending on the state of the other as the partners move apart, a process that does not occur ftl.

 

[bhobba]

Does entanglement require the transmission of information

No it does not.

Its a statement about correlations like if you have a pair of different coloured socks - put on one and you know what colour the other is automatically - Google Bertlmann's socks.

Thanks
Bill

 

[Mark Harder]

No it does not.

Its a statement about correlations like if you have a pair of different coloured socks - put on on and you know what colour the other is automatically - Google Bertlmann's socks.

Thanks
Bill

Thanks, Bill. BTW, what does "Ballentine" refer to, a textbook?

 

[phinds]

I suspect I have goofed here. Peter has corrected me, and its exactly in the area I often stress - namely what is real.

Even in QFT you include FTL and other normally not allowed things - that they are not classically allowed and in that sense are not real doest mean didly squat as far as what is included in the path integral.

Thanks
Bill

I vaguely remember reading that early in my readings on QM but it seemed unreasonable and didn't stick. Thanks for pointing that out

 

[bhobba]

Thanks, Bill. BTW, what does "Ballentine" refer to, a textbook?

Yes.

IMHO the finest textbook on QM available - but of course that is just an opinion - although many on this forum also hold it in high esteem:
https://www.amazon.com/dp/9814578584/?tag=pfamazon01-20

Be warned however - it's advanced.

If you are just starting out I recommend Susskinds book:
https://www.amazon.com/dp/0465036678/?tag=pfamazon01-20

Thanks
Bill

 

[brianhurren]

That's not how the double-slit experiment works; the particle is never either a particle or a wave. Search this forum for some discussion of why "wave-particle duality" is misleading for more informatiuon.

The sum-over-all-paths approach produces the right answer when you include all the possible paths through both slits and include none of the paths that are blocked by the screen.

this works if you model the double slit experiment using boolean logic as well. I was curious as to weather I could treat a double slit as an OR gate. the output of the OR gate is the sum of the two binary streams entering each input (which there are two of). if you plot the relative positions of the '1's in the output in relation to each other you get something resembling an interference pattern. So it is the sum of the superposition of the two inputs. It seams that the universe is very 'logical'

 

[Ken Natton]

To the OP. Hi, gerbilmore, I operate very much at your level and not at the level of most of the contributors to this thread in terms of my understanding of this stuff, but I have a perspective to offer that might assist you. It isn’t specifically about what you raised in your original post, rather more general about understanding this probabilistic – formally it is called ‘stochastic’ – approach employed by these chaps.

So one of the best, easiest to grasp examples I encountered was the matter of electrical resistance. The phenomenon of electrical resistance was known and fairly well understood before the QM guys put a deeper explanation on exactly what causes it. So you can get tables that will tell you what the resistance of a given length of copper wire of a given gauge will be. Quantum physics can explain the phenomenon in terms of the interactions between the electrons flowing through the wire that constitute the electrical current and the electrons of the atoms that actually make up the piece of wire. Now, it is impossible to exactly predict every interaction that will take place, not just because of the limitations of human science or the sheer scale of the exercise, but because, at a fundamental level, there is a degree of a random nature to it. But it is possible to take the probabilistic approach and come up with a theoretical calculation that matches the already known experimental data. And that really is the heart of the point about the probabilistic approach. It is all about coming up with theoretical methods of calculating that which can be experimentally verified. In many cases, the experimental results were known first and the challenge was to come up with a theoretical method of achieving the same results. That, I think is what lies behind the infamous maxim of theoretical physicists – ‘shut-up and calculate’. And perhaps this is the point about a particle taking every path between two points. It isn’t that it does, only that we don’t know and can’t know what path it does take, but we can calculate the probabilities and come up with a worthwhile result by so doing.

 

[my2cts]

Not having read but a few of the previous 82 posts I will give my answer the original question.
The quantum particle is described by a wave function which obeys a wave equation.
That wave function will use every 'path' to get to its destination. The amplitude there is the sum of all amplitudes reaching the position.
This is just a form of the Huygens principle.
That is not to say that the wave function IS the particle, so it is inaccurate to say that the particle uses or even tries every path.

 

[Atomic squire]

No, they don't. The particles that create the field are virtual particles (more precisely, in the appropriate approximation, the field can be viewed as being mediated by virtual particles--but there are field phenomena that cannot be modeled in this approximation), and virtual particles have a nonzero amplitude to travel faster than light. (They also have a nonzero amplitude to move slower than light even if they are massless--for example, virtual photons have a nonzero amplitude to move slower than light. Virtual particles that move in a way that violates the usual energy-momentum relation for their particle type are called "off-shell", and they must be included to get the right answers out of the path integral.)

I certainly have to improve my overly generalized approximation. Also, what is meant by a "nonzero amplitude"?

 

[bhobba]

IAlso, what is meant by a "nonzero amplitude"?

In Quantum Field Theory an important object is the propagator which is a complex number whose square gives the probability of something happening - such is known as an amplitude. Non zero amplitude means there is a probability of that happening.

Thanks
Bill

 

[Atomic squire]

In Quantum Field Theory an important object is the propagator which is a complex number who square gives the probability of something happening - such is known as an amplitude. Non zero amplitude means there is a probability of that happening.

Thanks
Bill

An excellent explanation, but how do scientists observe that the nonexistent "particles" can move faster than the speed of light if they are, by definition, nonexistent? Do they measure fluctuations in the field associated with them, or do they arrive at the conclusion with mathematics?

 

[PeterDonis]

how do scientists observe that the nonexistent "particles" can move faster than the speed of light if they are, by definition, nonexistent?

Who said they were nonexistent? The quantum fields exist, and as I said before, the "particles" are just particular states of the quantum fields.

As for measuring them moving faster than light, we don't, and nobody has said we do. What we have said is that, in order to get the right answers out of the path integral, you have to include states of the quantum field that, under the particle interpretation, correspond to particles moving faster than light. But that is an interpretation, and that's all it is. It doesn't mean you can actually measure a particle moving faster than light, and it doesn't mean information can travel faster than light.

Do they measure fluctuations in the field associated with them, or do they arrive at the conclusion with mathematics?

Yes. ;) Two important measurements of "field fluctuations" of this type are the Lamb shift and the Casimir effect. The mathematical argument is basically what I said above, about what is required to get the right answers with the path integral.

 

[Joel A. Levitt]

A particular type of excitation of a quantum field.
Reference, please?

 

[Joel A. Levitt]

Peter Donis:

Would you please provide a more detailed response to "How do you define PARTICLE?"

Requested Reference Re: Recent experimental demonstrations of the FTL transmission of information between entangled entities --

Challenging preconceptions about Bell tests with photon pairs
Authors: V Caprara Vivoli, P Sekatski, J -D Bancal, C C W Lim, B G Christensen, A Martin, R T Thew, H Zbinden, N Gisin, N Sangouard
Journal: Phys. Rev. A 91, 012107 (2015)

 

[naima]

Not having read but a few of the previous 82 posts I will give my answer the original question.
The quantum particle is described by a wave function which obeys a wave equation.
That wave function will use every 'path' to get to its destination. The amplitude there is the sum of all amplitudes reaching the position.
This is just a form of the Huygens principle.

I completely agree with you.
When we have a non relativistic lagrangien with energy = $$m v^2 /2$$ we have no problem to integrate on all paths. Things become more difficult if energy = $$\frac{m c^2}{\sqrt{1-v^2/c^2} }$$ . The integral is ill defined for paths on which there is a point where v = c.
A naive solution would be to say: avoid those paths because a massive particle cannot reach v=c. We could also avoid path where speed is not continuous and so on.
Feynman found the good answer. If we Fourier transform this integral we go from space time to momenta energy and proposes a contour around the poles. This is very technicall but it succeeds.

So we need no cut off , no v < c.

A problem still remains. Suppose yhat we have a particle in a box. its wave function is null outside the box. At time 0 i destroy the box. The propagator is not null outside the future cone of the box But relativity says that a particle which was in a region will remain in the future cone of this region.
Read faster than light? in wiki to solve te problem

 

[Nugatory]

Requested Reference Re: Recent experimental demonstrations of the FTL transmission of information between entangled entities --

Challenging preconceptions about Bell tests with photon pairs
Authors: V Caprara Vivoli, P Sekatski, J -D Bancal, C C W Lim, B G Christensen, A Martin, R T Thew, H Zbinden, N Gisin, N Sangouard
Journal: Phys. Rev. A 91, 012107 (2015)

You are seriously misunderstanding this paper if you believe that it suggests that FTL information transfer is possible. It is discussing some of the mathematical niceties around the well-known fact that spacelike-separated measurements of entangled pairs will demonstrate non-local correlations; it says nothing to challenge the equally well-known fact that these correlations cannot be used to transmit information.

 

[bhobba]

Would you please provide a more detailed response to "How do you define PARTICLE?"

Loosely it's an excitation in a quantum field. The sense that is meant is made rigorous in books on Quantum Field Theory.

Quantum Field Theory is notoriously difficult and challenging, usually requiring a course in advanced Quantum Mechanics.

However books have started to appear at that can be tackled with less preparation:
https://www.amazon.com/dp/019969933X/?tag=pfamazon01-20

Suitable preparation would be Susskinds text:
https://www.amazon.com/dp/0465036678/?tag=pfamazon01-20

It ends where the QFT book starts at the harmonic oscillator.

Thanks
Bill

 

[Nugatory]

Peter Donis:
Would you please provide a more detailed response to "How do you define PARTICLE?"

PeterDonis is using the generally accepted definition that you will find in any textbook on quantum field theory. Two that I recommend are https://www.amazon.com/dp/0521670535/?tag=pfamazon01-20 (challenging, to put it gently) and Quantum Field Theory for the gifted amateur (like the title says, suitable for someone who has made it through an undergraduate physics degree program).

 

[Joel A. Levitt]

You are seriously misunderstanding this paper if you believe that it suggests that FTL information transfer is possible. It is discussing some of the mathematical niceties around the well-known fact that spacelike-separated measurements of entangled pairs will demonstrate non-local correlations; it says nothing to challenge the equally well-known fact that these correlations cannot be used to transmit information.

Any measurement of one of a pair changes its state and therefore the state of the other. This is the FTL transmission of information. Unfortunately, it isn't all that useful, because the prior state of the second member wasn't known.

It is to be noted that we both have avoided using the word particle. I assume that this is because we both know that this word was originated from our macro sensory experience and is only some sort of metaphor when applied to almost all quantum phenomena. It's unfortunate that the use of this word confuses so many. This is also the case when, in discussion with the general public, the Schrödinger-motivating elements of classical mechanics are introduced without detailed explanation.

By the way, all three volumes of Weinberg's "The Quantum Theory of Fields" are available in relatively inexpensive soft cover.

A second matter, you wrote to me, " You are talking too much and listening too little." Having spent almost 50 years as a frequently published applied physicist and having successfully nurtured 7 PhD candidates, I find this amusing. You seem to believe that this site is you very own sandbox. Forgive me for intruding.

Bye!

 

[bhobba]

Any measurement of one of a pair changes its state and therefore the state of the other.

Entangled particles do not have a state for each particle - that's part of what entanglement means.

What the measurement does is break entanglement and when that is done we see the outcomes are correlated.

Locality in QM is a bit different - its the so called cluster decomposition property:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

It does not apply to correlated system. Entangled particles are correlated so locality doesn't apply.

Thanks
Bill

 

[macrobbair]

it is a matter of probability of being in a location, some are much more likely than others...

 

[Rishabh Sharma]

When we say that a particle "tries" every path, we don't mean that it actually has a choice at the given point in time. What it means is that we don't know which path it will take because we don't know the actual events that cause the particle being where it is now.
This is called a probabilistic model. The opposite (or rather its perfect form) is a deterministic model.

 

[brianhurren]

double slit seems to act like an OR gate. look at the truth table for an OR gate.

 

[vanhees71]

If this idea makes sense, then it's rather a "quantum or gate". That's the very point of the discussion of the double-slit experiment! The particle distribution behind the double slit is not the naive sum of the particle distribution behind each single slit, but there's an interference term. In the former days this was taken as a hint for what the physicsts called "wave-particle duality", which is a highly misleading concept, but in this case it's a good buzz word to describe what's really happening in the mathematical description of particles going through a double slit: It shows some analogy to the behavior of classical waves (no matter which ones you consider, e.g., water waves or the electromagnetic field/light) running through openings.

Of course, the meaning of the waves is quantum theory completely different from the classical analoga: It describes a probability amplitude. Shooting a single particle through a double slit will never result in an extended interference pattern at the detection screen but a single point. You cannot predict with certainty, where such a particle will hit the screen, but shooting many single particles through the double slit, however, reveals a distribution resembling the interference pattern of waves' intensity. Mathematically the analogon is quite direct: The probability amplitude is described by the Schrödinger equation which leads to wavelike solutions, and its modulus squared is the probability distribution where the particle will hit the screen.

 

[brianhurren]

If this idea makes sense, then it's rather a "quantum or gate". That's the very point of the discussion of the double-slit experiment! The particle distribution behind the double slit is not the naive sum of the particle distribution behind each single slit, but there's an interference term. In the former days this was taken as a hint for what the physicsts called "wave-particle duality", which is a highly misleading concept, but in this case it's a good buzz word to describe what's really happening in the mathematical description of particles going through a double slit: It shows some analogy to the behavior of classical waves (no matter which ones you consider, e.g., water waves or the electromagnetic field/light) running through openings.

Of course, the meaning of the waves is quantum theory completely different from the classical analoga: It describes a probability amplitude. Shooting a single particle through a double slit will never result in an extended interference pattern at the detection screen but a single point. You cannot predict with certainty, where such a particle will hit the screen, but shooting many single particles through the double slit, however, reveals a distribution resembling the interference pattern of waves' intensity. Mathematically the analogon is quite direct: The probability amplitude is described by the Schrödinger equation which leads to wavelike solutions, and its modulus squared is the probability distribution where the particle will hit the screen.

can the double slit be thought of as a 'phase filter'. the distance between the slits determans what phases it will filter?

 

[Quds Akbar]

An electron is a quanta meaning it can behave like a wave or particle, this was first observed in the micro-slit experiment, search it, it is really interesting. Basically, if you have a point A and point B, the electron would theoretically takes every possible path from point A to point B.

 

[vanhees71]

As stressed before, an electron is one quantum (formally it's a one-particle Fock state). This means it's neither a classical particle nor a classical wave but can only described by quantum theory. There's no simpler way to describe it that is entirely correct. The wave-like and particle-like properties are only consistently described by quantum theory, and you cannot describe it in some simpler way.

You cannot say, the electron takes a certain way or that it takes every possible path at once. What you calculate with the Schrödinger equation or, equivalently, with the path integral is a socalled propagator, which is a mathematical description how the state (a highly abstract mathematical object) evolves in time, given the state at some initial time and the interactions (forces) of the particle with the experimental setup (in this case with the double slits). The result is a probability distribution that the electron makes a mark on the detection screen. You an make this probability distribution visible by performing the experiment very often with the same initial state of each electron and the same experimental setup. All we can say is that up to know the predictions of quantum theory are confirmed by the so made observations. You cannot expect more from the natural sciences than such a successful description of objectively observable facts about (certain aspects of) nature. Particularly, it never answers and also never aims to answer the question, "what's really going on". The reason is, that you cannot even precisely define, what you mean by this question. It's highly subjective, depending on your personal experience in life. It takes time to get used to the very unfamiliar way of thinking when it comes to the realm of nature requiring quantum theory to describe it. The intuition is due to quite abstract ideas, and you can only grasp its meaning by looking at it in different applications to get a kind of intuition for these highly abstract ideas.

 

[Joel A. Levitt]

As stressed before, an electron is one quantum (formally it's a one-particle Fock state). This means it's neither a classical particle nor a classical wave but can only described by quantum theory. There's no simpler way to describe it that is entirely correct. The wave-like and particle-like properties are only consistently described by quantum theory, and you cannot describe it in some simpler way.

You cannot say, the electron takes a certain way or that it takes every possible path at once. What you calculate with the Schrödinger equation or, equivalently, with the path integral is a socalled propagator, which is a mathematical description how the state (a highly abstract mathematical object) evolves in time, given the state at some initial time and the interactions (forces) of the particle with the experimental setup (in this case with the double slits). The result is a probability distribution that the electron makes a mark on the detection screen. You an make this probability distribution visible by performing the experiment very often with the same initial state of each electron and the same experimental setup. All we can say is that up to know the predictions of quantum theory are confirmed by the so made observations. You cannot expect more from the natural sciences than such a successful description of objectively observable facts about (certain aspects of) nature. Particularly, it never answers and also never aims to answer the question, "what's really going on". The reason is, that you cannot even precisely define, what you mean by this question. It's highly subjective, depending on your personal experience in life. It takes time to get used to the very unfamiliar way of thinking when it comes to the realm of nature requiring quantum theory to describe it. The intuition is due to quite abstract ideas, and you can only grasp its meaning by looking at it in different applications to get a kind of intuition for these highly abstract ideas.

Excellent!

 

[bhobba]

can the double slit be thought of as a 'phase filter'. the distance between the slits determans what phases it will filter?

Thinking back to when I was into electronics I would say no.

I have noticed a pattern in your posts - trying to reduce things like the double slit to something else. It can't be done - QM is QM - its analogous to just one thing - QM. The wave particle duality is an attempt at such an analogy left over from the early days of QM - but it's wrong. In popularisations and beginning texts so students can get an initial grasp they do such things, but as you advance they are abandoned. Personally I am not a fan of that method - I would prefer facing it head on from the start as an extension of probability:
http://www.scottaaronson.com/democritus/lec9.html

But I must emphasise my background is applied math and that's how someone like me imbued with the spirit of mathematical modelling would approach it. Those into physics normally take the route of building intuition by a semi-historical approach.

Thanks
Bill

 

[bhobba]

Excellent!

As usual.

I have learned so much from reading Vanhees posts.

Thanks
Bill