I Is it a fact that vacuum energy exists?

[Suekdccia]

TL;DR Summary

Is a fact that vacuum energy exists? Has it been ever proven to exist experimentally? If it has been, does the energy remain constant as space expands? Does that violate the law of energy conservation?

I have always read that vacuum energy and zero point energy are established facts of physics supported by various observations of their effects both indirectly and even directly. But I have also read some comments from various physics discussion sites where they say that it is not a fact that vacuum energy exists, that it does not have conclusive evidence of its existence and that its effects can be ignored or replaced by other explanations in the theories that they contribute to.

So, do vacuum energy and zero point energy definitely exist? Or is their existence not clear enough...?

 

[vanhees71]

The absolute energy level, and thus the energy value you associate with the ground state is arbitrary. Observable are only energy differences (at least as far as we neglect gravitation and GR). What is an established fact, which is often described as "vacuum energy" in popular-science writings are quantum fluctuations. In the connection of the Casimir effect it's the quantum fluctuations of the charges and the electromagnetic field that lead to the famous Casimir force between uncharged bodies. The usual introductory-textbook calculation where one considers two plates and just calculates the em-field-zero-point energy difference between the situation with the plates and without the plates, is an approximation. For a correct understanding from QED, see

https://arxiv.org/abs/hep-th/0503158

 

[Vanadium 50]

would suggest Casimir effect

This has nothing to do with vacuum energy. I wish people would stop saying there is.

 

[PeterDonis]

I have always read

Where?

I have also read some comments from various physics discussion sites

Which are not valid references here. You need to be looking at textbooks and peer-reviewed papers.

 

[Suekdccia]

Where?Which are not valid references here. You need to be looking at textbooks and peer-reviewed papers.

Even though I have not seen these at peer reviewed papers I think one may take them into account. In fact some of them come from this site:

https://www.physicsforums.com/threads/why-do-we-assume-that-zero-point-energy-is-infinite.386326/

In there, users Born2bwire and Pythagorean both say that there is no experimental proof for vacuum energy.

Also, I have seen that some people explain the Casimir forces in terms of van der Waals forces and therefore argue that this is more proof that vacuum energy does not exist

So I am confused because I see these comments in serious physics discussions while a vast amount of papers and articles keep saying that vacuum energy is completely real

 

[PeterDonis]

Even though I have not seen these at peer reviewed papers I think one may take them into account. In fact some of them come from this site:

Actual links are very helpful, yes. But even better than comments by individual users are Insights articles, where we have them. Fortunately we have them for this topic. I suggest reading these:

https://www.physicsforums.com/insights/physics-virtual-particles/

https://www.physicsforums.com/insights/misconceptions-virtual-particles/

https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

https://www.physicsforums.com/insights/vacuum-fluctuations-experimental-practice/

 

[Demystifier]

So, do vacuum energy and zero point energy definitely exist?

All I have to say about this, I said here:
https://arxiv.org/abs/1702.03291

 

[CoolMint]

I cannot, but it doesn't contradict any of my other statements.

Virtual particles have a necessary contribution that cannot be avoided. Virtual particles are a model, like everything else.

 

[Demystifier]

Virtual particles have a necessary contribution that cannot be avoided. Virtual particles are a model, like everything else.

Did I say that it isn't so?

 

[weirdoguy]

Virtual particles have a necessary contribution that cannot be avoided. Virtual particles are a model, like everything else.

That has been discussed here milions of times, a good start are those Insight articles:

https://www.physicsforums.com/insights/misconceptions-virtual-particles/
https://www.physicsforums.com/insights/physics-virtual-particles/

I think he is talking about the inherent energy-time uncertainty

I know, it's still nonsense to apply it to virtual particles.

 

[CoolMint]

That has been discussed here milions of times, a good start are those Insight articles:

https://www.physicsforums.com/insights/misconceptions-virtual-particles/
https://www.physicsforums.com/insights/physics-virtual-particles/
I know, it's still nonsense to apply it to virtual particles.

Anyway, what you gave us is some pop-sci article, which is not a good reference per PF rules.

You typed that post via exchange of virtual particles. QFT requires virtual particles, electromagnetism requires virtual particles. The strong nuclear force requires them.
Unless you want to throw half of physics out and propose some novel level of physics, it's better to stick to what is already working. The perturbation model is working.

 

[weirdoguy]

QFT requires virtual particles, electromagnetism requires virtual particles. The strong nuclear force requires them.

Yes, but that does not mean what you think it does. Just read the articles I gave.

 

[Suekdccia]

Actual links are very helpful, yes. But even better than comments by individual users are Insights articles, where we have them. Fortunately we have them for this topic. I suggest reading these:

https://www.physicsforums.com/insights/physics-virtual-particles/

https://www.physicsforums.com/insights/misconceptions-virtual-particles/

https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

https://www.physicsforums.com/insights/vacuum-fluctuations-experimental-practice/

Frankly I am utterly confused.

These articles argue that vacuum fluctuations and vacuum energy are just mathematical concepts with no existence in reality. How can there be then a huge vast amount of physicists who say that vacuum energy is real? I mean if it's obvious that it is a mathematical artifact, why do all these physicists say that it is real?

The cosmological constant is an observed parameter and every article and physicist I can find treat it as vacuum energy (https://ned.ipac.caltech.edu/level5/Carroll2/Carroll1_3.html). If vacuum energy is not real how can all these physicists have ignored that and treat it as real? If vacuum energy is not real, why is the energy density of the universe constant as it expands (which has been observed experimentally as far as I know)?

No offense but it seems like as if this topic in physics is just philosophy and there is no one unique truth but different equally valid contradicting interpretations and theories :s

 

[CoolMint]

Frankly I am utterly confused.

These articles argue that vacuum fluctuations and vacuum energy are just mathematical concepts with no existence in reality. How can there be then a huge vast amount of physicists who say that vacuum energy is real? I mean if it's obvious that it is a mathematical artifact, why do all these physicists say that it is real?

The cosmological constant is an observed parameter and every article and physicist I can find treat it as vacuum energy (https://ned.ipac.caltech.edu/level5/Carroll2/Carroll1_3.html). If vacuum energy is not real how can all these physicists have ignored that and treat it as real? If vacuum energy is not real, why is the energy density of the universe constant as it expands (which has been observed experimentally as far as I know)?

No offense but it seems like as if this topic in physics is just philosophy and there is no one unique truth but different equally valid contradicting interpretations and theories :s

They don't like the word 'real' as it implies that virtual particles have all the properties of 'real' particles. Which they do not. But it's still impossible to remove them from theoretical and experimental physics. This should answer your question.

 

[Suekdccia]

They don't like the word 'real' as it implies that virtual particles have all the properties of 'real' particles. Which they do not.

But there is a difference between saying that virtual particles exist but they are not 'real' in the sense of having other properties compared to 'normal' particles (just as demystifier's article said that zero ppint energy is unphysical in the sense that although it exists it is the ground energy state and therefore there cannot be a gradient that would be observed) and saying that vacuum fluctuations sre just mathematical expressions and do not exist by any means

 

[CoolMint]

But there is a difference between saying that virtual particles exist but they are not 'real' in the sense of having other properties compared to 'normal' particles (just as demystifier's article said that zero ppint energy is unphysical in the sense that although it exists it is the ground energy state and therefore there cannot be a gradient that would be observed) and saying that vacuum fluctuations sre just mathematical expressions and do not exist by any means

It's more complicated than it looks. And there is no test to measure the virtual particles existence. For most layperson accounts, it's better to treat them as mathematical entities than to enter unprepared with wrong convictions in debated issues.

 

[f95toli]

But there is a difference between saying that virtual particles exist but they are not 'real' in the sense of having other properties compared to 'normal' particles (just as demystifier's article said that zero ppint energy is unphysical in the sense that although it exists it is the ground energy state and therefore there cannot be a gradient that would be observed) and saying that vacuum fluctuations sre just mathematical expressions and do not exist by any means

Is there? I think that depends on who you ask.
As was mentioned above, this is mostly a philosophical question; there isn't necessarily a right or wrong answer.

 

[martinbn]

Summary:: Is a fact that vacuum energy exists? Has it been ever proven to exist experimentally? If it has been, does the energy remain constant as space expands? Does that violate the law of energy conservation?

What do you mean by "energy (of any kind) exists"?

 

[PeroK]

Virtual particles have a necessary contribution that cannot be avoided.

You can evaluate the "Dyson Series" integrals directly. You don't need to consider virtual particles at all.

 

[PeroK]

But it's still impossible to remove them from theoretical and experimental physics.

Which is false, especially in regard of experimental physics, given that virtual particles are not detected or part of the experimental results.

You can't say that you detected only one virtual photon exchange in an electron-electron scattering, for example.

 

[Vanadium 50]

Also, I have seen that some people explain the Casimir forces in terms of van der Waals forces and therefore argue that this is more proof that vacuum energy does not exist

Who? Certainly not me. The point is the Casimir Effect is irrelevant to the question at hand.

 

[PeterDonis]

Some prominent physicists disagree.

Scientific American is not a peer-reviewed publication, it's pop science. The fact that professional scientists write articles for it does not change that; even professional scientists will say things in pop science articles that they would never say in peer-reviewed papers because they know they wouldn't get away with it. For this topic, you need to be even more careful than usual to avoid pop science sources.

 

[PeterDonis]

Frankly I am utterly confused.

That's because, first, you are trying to understand this topic in terms of vague ordinary language instead of math, and second, because unfortunately the vague ordinary language that gets used when discussing this topic, even by professional physicists, is even more vague and confusing than usual.

In terms of the actual math, the distinction between what is "real" and what isn't is fairly clear: if something is a direct observable, or appears in all mathematical models that predict a direct observable, then it's "real". But if something isn't a direct observable, and only appears when you do the math in one particular way, not in other ways, then it's not "real".

By this criterion, the nonzero cosmological constant is "real" because it's closely linked to a direct observable: accelerated expansion of the universe. Every mathematical model we have that predicts this has a nonzero cosmological constant, or something that acts, mathematically, just like one.

To the extent that the term "vacuum energy" is used to mean a nonzero cosmological constant or something that acts mathematically just like one, "vacuum energy" would also be "real".

However, "virtual particles", "vacuum fluctuations", etc. are not "real" by the above criterion, because they aren't directly observable and they only appear in the math when you do the math one particular way: perturbation theory. There are other ways of doing the math in which they don't appear at all.

And to the extent that "vacuum energy" is used to mean virtual particles, vacuum fluctuations, etc., "vacuum" energy would also not be "real".

So to even answer the title question of this thread at all, you first need to answer the question: what do you mean by the term "vacuum energy"? And that's a matter of choice of words, not physics. The physics is the same no matter what you choose to mean by "vacuum energy"; all that changes is which part of the physics you are using "vacuum energy" to refer to.

No offense but it seems like as if this topic in physics is just philosophy and there is no one unique truth but different equally valid contradicting interpretations and theories :s

This is not correct. The actual physics is clear and well tested. The issue is that different people use the same vague ordinary language to describe different parts of the physics.

 

[PeterDonis]

The perturbation model is working.

Perturbation theory works for some phenomena, but there are plenty of phenomena that cannot be explained by perturbation theory. Also, even the phenomena that can be explained by perturbation theory can also be modeled in other ways that don't include things like "virtual particles" that perturbation theory includes. Perturbation theory is used because it's mathematically convenient and intuitively easier for many people in the field to grasp, not because it is ultimate truth.

 

[PeterDonis]

I am reading the top of Wikipedia article

Wikipedia is not a valid reference for this topic. It's not a good primary source in general, but especially not for this topic.

All thread participants, please: only references to peer-reviewed papers (and PF internal sources, such as other PF threads or Insights articles) will be allowed in this thread. This topic is confusing enough without adding pop science sources into the mix. Any further invalid references will receive a warning.

 

[mitochan]

All thread participants, please: only references to peer-reviewed papers (and PF internal sources, such as other PF threads or Insights articles) will be allowed in this thread.

I found an article with names of contributors and a curator on cosmological constant in scholarpedia which is referring vacuum energy. A layman like me could read it though far from full understanding. Is such an article worth passing your threshold of reference ?

 

[PeterDonis]

I found an article with names of contributors and a curator on cosmological constant in scholarpedia which is referring vacuum energy. A layman like me could read it though far from full understanding. Is such an article worth passing your threshold of reference ?

If it's a peer-reviewed paper, it might be, but it doesn't sound like one. You can PM me a link.

 

[otennert]

The absolute energy level, and thus the energy value you associate with the ground state is arbitrary. Observable are only energy differences (at least as far as we neglect gravitation and GR). What is an established fact, which is often described as "vacuum energy" in popular-science writings are quantum fluctuations. In the connection of the Casimir effect it's the quantum fluctuations of the charges and the electromagnetic field that lead to the famous Casimir force between uncharged bodies. The usual introductory-textbook calculation where one considers two plates and just calculates the em-field-zero-point energy difference between the situation with the plates and without the plates, is an approximation. For a correct understanding from QED, see

https://arxiv.org/abs/hep-th/0503158

In fact, most of what needs to be said in response to the original question is in the paper just cited by @vanhees71, and in the one from Nikolic cited by @Demystifier above.

Funnily enough the fact that the Casimir forces as treated in most books and articles are essentially to be interpreted as van der Waals forces between 2 giant molecules (the metal plates) was even known back then to Casimir. But just as in other parts of physics, the same phenomena can be seen from different angles. Thus the Casimir effect can also be seen as resulting from the difference in the energy fluctuations between the space in between the plates, and the space outside. A good overview on this is e.g. the book by Peter Milonni "The Quantum Vacuum: an Introduction to QED".

However, what surely makes no sense, and I am still wondering why this has actually happened at all back in the late 80s and 90s, is to calculate the vacuum energy in the usual, naive, way, and to think that this is to be seen as a measureable physical quantity and relate it to the cosmological constant. Even the late Steven Weinberg himself shared this view once in his well-known review on the cosmological constant (I've found a link here: "https://isidore.co/misc/Physics papers and books/Recent Papers/Dark Energy Reviews/1. Weinberg (1989).pdf". I cannot remember who said it first, but it has once been called the worst prediction ever in physics, and is off by actual cosmological measurements by a factor of about 10¹²⁰.

The vacuum energy as calculated by just adding up the contributions from all modes (summation over momentum, polarization $k,\lambda$) gives an unrenormalized quantity and cannot be seen as an experimental quantity. Moreover, after renormalization it is exactly zero by definition, which leads to the result that the Casimir effect is actually depending on energy differences, and not absolute values.

All in all, I recommend to study the references given above, to get a clearer insight into this.

 

[anuttarasammyak]

Moreover, after renormalization it is exactly zero by definition, which leads to the result that the Casimir effect is actually depending on energy differences, and not absolute values.

Say Casimir effect comes from energy differences of "vacuum energy", anyway Casimir effect has something to do with vacuum energy, doesn't it ?

 

[Demystifier]

In fact, most of what needs to be said in response to the original question is in the paper just cited by @vanhees71, and in the one from Nikolic cited by @Demystifier above.

All in all, I recommend to study the references given above, to get a clearer insight into this.

I couldn't agree more.

 

[otennert]

Say Casimir effect comes from energy differences of "vacuum energy", anyway Casimir effect has something to do with vacuum energy, doesn't it ?

No, because as I said the Casimir effect relates to "energy differences", not to "vacuum energy differences". There is an effective potential energy as a function of the distance $d$ between the plates, leading to an effective force, that's the main point. But taking the Casimir effect as "proof" for the existence of a vacuum energy (as e.g. hinted at in Weinberg's paper just cited) is plainly wrong.

In my opinion the only thing that is safe to say is that the relation between the cosmological constant and potential quantum effects is unclear.

 

[Suekdccia]

In my opinion the only thing that is safe to say is that the relation between the cosmological constant and potential quantum effects is unclear.

Does that mean that it is unclear to say that the cosmological constant is the same as vacuum energy?

 

[otennert]

Does that mean that it is unclear to say that the cosmological constant is the same as vacuum energy?

Well, this is why it is called the "cosmological constant problem", isn't it? Again, I am referring to the Weinberg review from 1989. There are also tons of more recent literature on this.

Actually I think the Wikipedia entry is also a good entry point into this matter: https://en.wikipedia.org/wiki/Cosmological_constant_problem

 

[anuttarasammyak]

No, because as I said the Casimir effect relates to "energy differences", not to "vacuum energy differences". There is an effective potential energy as a function of the distance $d$ between the plates, leading to an effective force, that's the main point. But taking the Casimir effect as "proof" for the existence of a vacuum energy (as e.g. hinted at in Weinberg's paper just cited) is plainly wrong.

In my opinion the only thing that is safe to say is that the relation between the cosmological constant and potential quantum effects is unclear.

Thanks. I found a web article to explain it with a key word of "field radiation pressure".
"At a cavity-resonance frequency the radiation pressure inside the cavity is stronger than outside and the mirrors are therefore pushed apart. Out of resonance, in contrast, the radiation pressure inside the cavity is smaller than outside and the mirrors are drawn towards each other." https://physicsworld.com/a/the-casimir-effect-a-force-from-nothing/