"In the classical physics of Newton and Einstein, the vacuum of space is entirely empty, but the theory of quantum mechanics assumes something very different.
According to quantum electrodynamics (QED) - a quantum theory that describes how light and matter interact - it’s predicted that space is actually full of 'virtual particles' that pop in and out of existence and mess with the activity of light particles (photons) as they zip around the Universe."
That's not quite accurate. Local hidden variable theories have been proven wrong, but pilot wave theory is a nonlocal hidden variable theory. Bell himself was a proponent of pilot wave theory.
Pilot wave theory, if simulated, still requires you to run all the same calculations as (say) the Many-Worlds interpretation, in order to know what the pilot wave is doing.
In fact, the pilot wave will still contain everything that MWI does: Multiple timelines, vast assemblies of nearly-independent universes, Earths beyond counting, etc etc. What PWT does is add in some nearly-epiphenomenal particles, and declare that only these count; that any observer described by the pilot wave is a philosophical zombie.
No. Quantum field theories and their vacuum state has nothing to do with pilot wave theory (PWT).
PWT is oddly popular amongst internet people, but is just a mathematical transformation of the Schrodinger equation, with some associated interpretation. It makes no additional predictions.
I wonder. In the Em-drive paper the NASA scientists seem to point to PWT for a possible explanation. If it didn't make a difference, then why should they have done that?
If anything the virtual particles are an alternative to the pilot wave theory. Instead of having quantum effects mediated by virtual particles the pilot wave theory uses a 'pilot wave'. Now as far as I know most phenomena caused by virtual particles can be explained by the pilot wave, but I'm not sure how easily.
Either way, results like these which seem to be in favour of the existence of virtual particles are weak evidence against the pilot wave theory.
No. Vacuum birefringence is understandable from a perfectly conventional understanding of quantum field theory, and does not remotely require anything about the pilot wave idea to be true. Every quantum field theory, including QED, has a roiling vacuum of virtual particles, whether you hold an orthodox view or not.
Can you give me some hints on what virtual particles actually are if its not too complicated? I keep reading that they are not actually real particles, only useful for calculations/as a simple way to understand what is going on. How does something like Hawking radiation work if they are not real particles?
The main challenge in understanding QM is the fact that we are all intelligent animals whose brains developed in what is for all practical purposes a classic newtonian universe. When you were one year old, crawling on the floor at your parents' house, your brain was learning to operate within the realm of classic physics.
In such a universe, things are very clear-cut. An object must be either here, or there. Something must be either in this state, or the other. Particles behave like billiard balls, and waves behave like, well, waves. Things either exist, or they do not. Space and time have perfect accuracy, there's no "smearing" of coordinates either in space or in time.
The problem is, that is not at all the case at microscopic levels. You've learned what a marble is as a kid, and you think of electrons as little marbles racing around the nucleus - but that's utterly wrong. You've watched water waves on a pond and you think that's how a beam of light behaves, but that's ridiculously simplistic.
The fundamental reality at the quantum level is the awkwardly named wave function (awkward because there's no wave proper there). That's what electrons, and photons, and neutrons, etc, are. It's a function that describes a probability amplitude as a complex number. That's it. They're not little marbles - although sometimes they kind of behave like that. They're not little waves - although they kinda-sorta look like that. They are bundles of probability amplitudes smeared out over space and time intervals. This is not a metaphor. This is stark reality.
The familiar classic universe with crawling floors and marbles and water waves is just an epiphenomenon. It's a collateral that emerges on top of the quantum world if you zoom out a trillion-fold and ignore all the little details.
The "smearing out" of space, time, momentum and energy parameters at the quantum level simply means these are not the truly fundamental things you think they are on that level. Take energy and time; in a quantum system, you could measure energy with infinite precision, but then you can't say anything about the time intervals. Or you could measure time very precisely, but then you don't know anything about energy. That's not a limitation of your equipment. It simply means neither energy nor time are the precious snowflakes you thought they are - but their combination is.
A particle that does not exist has exactly zero energy and exists for exactly zero seconds. But wait, this is the quantum world. That means a deviation from zero energy is permissible, as long as it does not take place longer than a correspondingly short interval. The higher the energy deviation, the shorter the duration of the blip. That means, in completely empty space, you will be able to measure that particles seem to appear out of nowhere, exist a very short time (the more massive the particle, the shorter the time), then vanish.
But do those particles actually exist? In QM, that's meaningless. Remember, there's no such thing as a particle? Only bundles of probability density? Remember that? And those bundles obey the basic relations about energy/time and momentum/location. As long as you're talking about fluctuations the size of an electron, then yes, pure vacuum would seem to produce electrons continuously, but they only blip in and out of measurement for a very short time.
But are they "real"? If you can measure it, it's real. Full stop. But classic categories such as "it exists" or "it does not exist" get blurred at the quantum level. Anything could potentially, and does actually, exist - provided it does not violate basic QM relations.
---
So that "quantum fog" exists in a vacuum all the time. Usually it's completely chaotic. But magnetars have such strong magnetic fields, they impose a structure over those short-lived virtual particles. And then when light passes thr...
Thanks! That's a really nice explanation. I keep forgetting that particles don't really exist so much as a wave function does in the Copenhagen interpretation, so that gives me a new way to think about virtual particles, and helps tie it in with the article. Also a lot of other nice tidbits in this explanation for me to mull over...
>That's what electrons, and photons, and neutrons, etc, are. [...] They are bundles of probability amplitudes smeared out over space and time intervals. This is not a metaphor. This is stark reality.
Aren't you describing a representation of atomic particles, not the particles themselves?
Edit: I'm not looking to get into a discussion about what constitutes "existence". I am just wondering if you are conflating a scientific-mathematical model for a thing with the thing itself.
You mix facts about the mathematics with opinions about interpretation and philosophical positions pretty freely, and that equivocation in science communication is what puts a lot of people off QM.
Scientists are too comfortable presenting philosophical positions or opinion as fact just because it's consistent with a model.
See the split between Copengahen, MWI, and Bohmian views about how to interpret the underlying calculations.
In Many Worlds virtual particles are simply other universes (other parts of Hilbert space) that have electrons in different positions. They are nearby enough to contribute some amplitude to have an observable effect. Edit a word.
(Apologies to Quantum Field Theorist for the gross simplification below.)
I'm going to assume you know about basic calculus and Taylor series; if not, I'll explain what these are in a subsequent answer if you are interested.
In Quantum Field Theory, we cannot do exact calculations ... only (better and better) approximations. The way we do these calculations is very much similar to doing a Taylor series in Calculus. However, the calculations involved many complicated multi-dimensional integrals
Feynman found out a way to represent these complicated mathematical expressions that occured in those approximate calculations as pictures (known as Feynman diagrams). In these pictures, one could represent (real) particles interacting as lines (straight, curvy, etc., depending on the rules) that end in open space..
As we increased the order of expansion in the approximation (think of powers of the variable in a Taylor series), the terms corresponded to more and more diagrams, each increasing in complexity with the order of expansion.
Well, it turned out that, using the Feynman rules for translating mathematical expressions into parts of a pictures, that lines (straight, curvy) similar to that of "real" particles appeared inside the diagrams; however, these lines were all connected to others and did not end in open space: they do not correspond to any particles that we can measure directly.
However, we can think of these lines as representing "virtual" particles, that we cannot probe directly.
One thing to keep in mind (and that many that mention the "virtual particle" explanations often forget to mention): these diagrams are, in a very strong sense, an artifact of the way we do calculations using the best approximation techniques we know. These are useful computational tools. But individual diagrams should not be thought of as representing an actual precise interaction that is taking place. And virtual particles are just a way to make a connection between what we think we know ("real" particles interacting) and what our approximative way of calculating things give us.
Thanks! That's an interesting fact about Feynman diagrams. If we had a hypothetical machine which could perform these calculations with infinite precision, would the "virtual particles" disappear from the diagram, or is it impossible to know this?
Let me answer your question first very obliquely. We know that we can represent the sine function near x=0 as the following Taylor series:
sin(x) ≈ x - x^3 / 6 + x^5 / 120 - ....
With more and more terms, we can can calculate it more and more precisely. Does this means that this function really contains all of these powers of x?
Well, suppose we want to calculate sin(pi/2) (angle in radians, which is required for the Taylor series). We know that it is exactly 1. There are no powers of (pi/2) appearing in "1". The Taylor series gives us a convenient way to calculate the value of the function (technically: inside a radius of convergence), but it is not the same as the function itself.
Now, to give an example from Quantum Field theory: let me quote from a wikipedia page [1]:
_In a perturbative approach to quantum field theory, such interactions may require the calculation of hundreds of Feynman diagrams. In contrast, twistor theory provides an approach in which scattering amplitudes can be computed in a way that yields much simpler expressions._
So, these hundreds of Feynman diagrams would lead us to think of many virtual particles describing an interaction. The Amplituhedron type of calculation is done completely differently, in a way that does not lead one to think of interactions through virtual particles.
Now to answer your question: I do believe that, if we were smart enough to find a way to compute things exactly, "virtual particles" would not appear as intermediate steps of computations. But, then again, to be perfectly honest, the same would likely be of "real" particles which are just a convenient way of thinking of excitations in quantum fields ...
I see by "first" they meant the first evidence of vacuum birefringence. Quantum vacuum fluctuation is by itself well established. E.g., Casimir effect.
Thanks for the reference. Just to be clear, his point is made with regard to the reality of vacuum zero point energy, esp. whether it should affect the cosmological constant. However quantum vacuum fluctuation (which I read as meaning the virtual particles) is essential to QED whether zero point energy is real or not. As far as I can tell, quantum birefringence also does not refer to the zero point energy.
I'm not a physicist but I wonder how is it possible to disambiguate this effect from classical "Einsteinian" gravitational deflection? Neutron stars must have strong gravitational effects on light as well.
We can predict the effects of gravitational lensing and note a difference (if applicable and there are any) which must then be explained by some other mechanism. That could be anything from a misestimation of the metric in the region of the body to quantum effects as this observation posits.
Suppose someone gives you a dollar, and then someone gives you another dollar. Now you have two dollars, which is more than a dollar. Disambiguating two dollars from one dollar is fairly easy, because one is more than the other.
// Edit: And maybe someone can explain this to me, but the "polarization in -> polarization out" artist's rendering isn't in their paper. Where did it come from? Is it showing the example of what would be happening if the effect was happening at 100% ?
Misleading article. Einstein is responsible for a lot of quantum statistics and quantum mechanics.
Also, this isn't really anything new from some new theory.
It's all contained within QFT and the uncertainty principle.
More or less, this happens at the LHC when protons are smashed together, forcing momentum energy to be converted into rest energy, leading to the creation of short-lived particles from the vacuum (provided that quantum numbers, energy, momentum, and angular momentum are conserved).
35 comments
[ 6.7 ms ] story [ 94.0 ms ] threadAccording to quantum electrodynamics (QED) - a quantum theory that describes how light and matter interact - it’s predicted that space is actually full of 'virtual particles' that pop in and out of existence and mess with the activity of light particles (photons) as they zip around the Universe."
So, does this mean that this explanation for Quantum mechanics https://www.youtube.com/watch?v=WIyTZDHuarQ has been vindicated?
i.e. the double slit with the fields being the cause of the oddity instead of... something that is hard to explain.. including the observer effect
Can someone weigh in and put this article in the context of the pilot wave theory?
(Not a physicist!)
Source: check wikipedia.
In non-relativistic situations, Bohemian mechanics (pilot wave theory) makes the same predictions as more mainstream interpretations of QM.
Pilot wave theory, if simulated, still requires you to run all the same calculations as (say) the Many-Worlds interpretation, in order to know what the pilot wave is doing.
In fact, the pilot wave will still contain everything that MWI does: Multiple timelines, vast assemblies of nearly-independent universes, Earths beyond counting, etc etc. What PWT does is add in some nearly-epiphenomenal particles, and declare that only these count; that any observer described by the pilot wave is a philosophical zombie.
I have objections to this concept. :P
PWT is oddly popular amongst internet people, but is just a mathematical transformation of the Schrodinger equation, with some associated interpretation. It makes no additional predictions.
If anything the virtual particles are an alternative to the pilot wave theory. Instead of having quantum effects mediated by virtual particles the pilot wave theory uses a 'pilot wave'. Now as far as I know most phenomena caused by virtual particles can be explained by the pilot wave, but I'm not sure how easily.
Either way, results like these which seem to be in favour of the existence of virtual particles are weak evidence against the pilot wave theory.
Source: am physicist ;P
In such a universe, things are very clear-cut. An object must be either here, or there. Something must be either in this state, or the other. Particles behave like billiard balls, and waves behave like, well, waves. Things either exist, or they do not. Space and time have perfect accuracy, there's no "smearing" of coordinates either in space or in time.
The problem is, that is not at all the case at microscopic levels. You've learned what a marble is as a kid, and you think of electrons as little marbles racing around the nucleus - but that's utterly wrong. You've watched water waves on a pond and you think that's how a beam of light behaves, but that's ridiculously simplistic.
The fundamental reality at the quantum level is the awkwardly named wave function (awkward because there's no wave proper there). That's what electrons, and photons, and neutrons, etc, are. It's a function that describes a probability amplitude as a complex number. That's it. They're not little marbles - although sometimes they kind of behave like that. They're not little waves - although they kinda-sorta look like that. They are bundles of probability amplitudes smeared out over space and time intervals. This is not a metaphor. This is stark reality.
The familiar classic universe with crawling floors and marbles and water waves is just an epiphenomenon. It's a collateral that emerges on top of the quantum world if you zoom out a trillion-fold and ignore all the little details.
The "smearing out" of space, time, momentum and energy parameters at the quantum level simply means these are not the truly fundamental things you think they are on that level. Take energy and time; in a quantum system, you could measure energy with infinite precision, but then you can't say anything about the time intervals. Or you could measure time very precisely, but then you don't know anything about energy. That's not a limitation of your equipment. It simply means neither energy nor time are the precious snowflakes you thought they are - but their combination is.
A particle that does not exist has exactly zero energy and exists for exactly zero seconds. But wait, this is the quantum world. That means a deviation from zero energy is permissible, as long as it does not take place longer than a correspondingly short interval. The higher the energy deviation, the shorter the duration of the blip. That means, in completely empty space, you will be able to measure that particles seem to appear out of nowhere, exist a very short time (the more massive the particle, the shorter the time), then vanish.
But do those particles actually exist? In QM, that's meaningless. Remember, there's no such thing as a particle? Only bundles of probability density? Remember that? And those bundles obey the basic relations about energy/time and momentum/location. As long as you're talking about fluctuations the size of an electron, then yes, pure vacuum would seem to produce electrons continuously, but they only blip in and out of measurement for a very short time.
But are they "real"? If you can measure it, it's real. Full stop. But classic categories such as "it exists" or "it does not exist" get blurred at the quantum level. Anything could potentially, and does actually, exist - provided it does not violate basic QM relations.
---
So that "quantum fog" exists in a vacuum all the time. Usually it's completely chaotic. But magnetars have such strong magnetic fields, they impose a structure over those short-lived virtual particles. And then when light passes thr...
Aren't you describing a representation of atomic particles, not the particles themselves?
Edit: I'm not looking to get into a discussion about what constitutes "existence". I am just wondering if you are conflating a scientific-mathematical model for a thing with the thing itself.
Bohm would say yes. Others would say no.
Scientists are too comfortable presenting philosophical positions or opinion as fact just because it's consistent with a model.
See the split between Copengahen, MWI, and Bohmian views about how to interpret the underlying calculations.
Yes, I do take sides - but only for ease of explanation. The "sides" I'm taking are not even the ones I actually prefer.
Get people started first, then they can read rigorous analyses on their own.
I'm going to assume you know about basic calculus and Taylor series; if not, I'll explain what these are in a subsequent answer if you are interested.
In Quantum Field Theory, we cannot do exact calculations ... only (better and better) approximations. The way we do these calculations is very much similar to doing a Taylor series in Calculus. However, the calculations involved many complicated multi-dimensional integrals
Feynman found out a way to represent these complicated mathematical expressions that occured in those approximate calculations as pictures (known as Feynman diagrams). In these pictures, one could represent (real) particles interacting as lines (straight, curvy, etc., depending on the rules) that end in open space..
As we increased the order of expansion in the approximation (think of powers of the variable in a Taylor series), the terms corresponded to more and more diagrams, each increasing in complexity with the order of expansion.
Well, it turned out that, using the Feynman rules for translating mathematical expressions into parts of a pictures, that lines (straight, curvy) similar to that of "real" particles appeared inside the diagrams; however, these lines were all connected to others and did not end in open space: they do not correspond to any particles that we can measure directly.
However, we can think of these lines as representing "virtual" particles, that we cannot probe directly.
One thing to keep in mind (and that many that mention the "virtual particle" explanations often forget to mention): these diagrams are, in a very strong sense, an artifact of the way we do calculations using the best approximation techniques we know. These are useful computational tools. But individual diagrams should not be thought of as representing an actual precise interaction that is taking place. And virtual particles are just a way to make a connection between what we think we know ("real" particles interacting) and what our approximative way of calculating things give us.
sin(x) ≈ x - x^3 / 6 + x^5 / 120 - ....
With more and more terms, we can can calculate it more and more precisely. Does this means that this function really contains all of these powers of x?
Well, suppose we want to calculate sin(pi/2) (angle in radians, which is required for the Taylor series). We know that it is exactly 1. There are no powers of (pi/2) appearing in "1". The Taylor series gives us a convenient way to calculate the value of the function (technically: inside a radius of convergence), but it is not the same as the function itself.
Now, to give an example from Quantum Field theory: let me quote from a wikipedia page [1]:
_In a perturbative approach to quantum field theory, such interactions may require the calculation of hundreds of Feynman diagrams. In contrast, twistor theory provides an approach in which scattering amplitudes can be computed in a way that yields much simpler expressions._
So, these hundreds of Feynman diagrams would lead us to think of many virtual particles describing an interaction. The Amplituhedron type of calculation is done completely differently, in a way that does not lead one to think of interactions through virtual particles.
Now to answer your question: I do believe that, if we were smart enough to find a way to compute things exactly, "virtual particles" would not appear as intermediate steps of computations. But, then again, to be perfectly honest, the same would likely be of "real" particles which are just a convenient way of thinking of excitations in quantum fields ...
[1] https://en.wikipedia.org/wiki/Amplituhedron
The only thing it contradicts was already contradicted by Quantum theories which were some of the more classical Newton and Einstein's theories.
So this is exciting 'only' because it is a larger version of a man made in lab phenomenon, and seen occurring naturally.
Again, thanks
Same principle.
https://arxiv.org/abs/astro-ph/9505100
prepublish link: https://arxiv.org/abs/1610.08323 pdf: https://arxiv.org/pdf/1610.08323v2.pdf
// Edit: And maybe someone can explain this to me, but the "polarization in -> polarization out" artist's rendering isn't in their paper. Where did it come from? Is it showing the example of what would be happening if the effect was happening at 100% ?
http://cdn.sci-news.com/images/enlarge3/image_4410_2e-Vacuum...
Also, this isn't really anything new from some new theory.
It's all contained within QFT and the uncertainty principle.
More or less, this happens at the LHC when protons are smashed together, forcing momentum energy to be converted into rest energy, leading to the creation of short-lived particles from the vacuum (provided that quantum numbers, energy, momentum, and angular momentum are conserved).