B Can Particles Really Pop Into Existence from a Vacuum?

[NewToThis]

Why do some physicists talk about particles popping into existence from nothing. The nothing they refer to is a vacuum, but the vacuum exists in this universe so must contain the fabric of the universe, it must contain gravity, so obviously it is a something rather than a nothing.

Kaku and Krauss are 2 I can think of that have talked about this.

 

[Vanadium 50]

Why do some physicists talk about particles popping into existence from nothing.

Because Kaku will say anything if he thinks it will bring in a buck. Real physicists say no such thing.

 

[NewToThis]

So do you think most physicists would say that these particles DO come from something?

 

[phinds]

So do you think most physicists would say that these particles DO come from something?

You misunderstand. He is saying that there ARE NO "particles" of this sort.

You are talking about the so-called "quantum foam", which is a highly speculative extension of quantum mechanics.

Also, there is no such thing as "the fabric of space". This is a made-up pop-sci concept that evolved from something casual that Einstein said.

No, it is NOT true that "it must contain gravity". Space-time contains gravity only if there is matter present. There are models that describe a universe with no matter but they are not the ones we live in. In the one we live in, there is nowhere that has no gravitational field but the existence of a gravitational field has nothing to do with the quantum foam concept.

DO NOT think that you are learning actual physics by reading pop science books, ESPECIALLY ones by Kaku.

 

[OCR]

Because Kaku will say anything if he thinks it will bring in a buck.

Well, Kaku "might" say nothing... but he'd still want 50 cents....

 

[NewToThis]

You misunderstand. He is saying that there ARE NO "particles" of this sort.

You are talking about the so-called "quantum foam", which is a highly speculative extension of quantum mechanics.

Also, there is no such thing as "the fabric of space". This is a made-up pop-sci concept that evolved from something casual that Einstein said.

No, it is NOT true that "it must contain gravity". Space-time contains gravity only if there is matter present. There are models that describe a universe with no matter but they are not the ones we live in. In the one we live in, there is nowhere that has no gravitational field but the existence of a gravitational field has nothing to do with the quantum foam concept.

DO NOT think that you are learning actual physics by reading pop science books, ESPECIALLY ones by Kaku.

But I assume what they are talking about has been observed in labs and in those labs there is gravity, there is the universe and what we exist in is made from something so it's not possible for there to be nothing in the universe. Or is just a theory?

Im not a fan of Kaku, and other physicists talk about it too.

 

[phinds]

... so it's not possible for there to be nothing in the universe.

"nothing" is an English language word, not well defined in physics. You need to be more specific.

 

[phinds]

But I assume what they are talking about has been observed in labs ...

Why do you assume that? If it had been observed, do you think I would have said it is highly speculative?

 

[NewToThis]

Why do you assume that? If it had been observed, do you think I would have said it is highly speculative?

Thought you meant quantum foam is speculative.

Aren't they referring to virtual particles which have been observed?

They argue these particles come from nothing. But something must be causing them so they can't be coming from nothing.

 

[phinds]

Thought you meant quantum foam is speculative.

Yes, that is what I meant because that is what you were talking about.

Aren't they referring to virtual particles which have been observed?

virtual particles are a mathematical fiction and have no physical existence so it would be REALLY hard to observe them.

They argue these particles come from nothing. But something must be causing them so they can't be coming from nothing.

SInce they don't exist, it's meaningless to argue about what the come from. Also, you need to stop saying "they ... ". You are making factual claims with no citations other than mention of useless pop-sci books.

 

[houlahound]

In fact nothing has never been observed.

Physics ultimately comes down to observation and measurement, in what units would nothing be measured and if you did measure a lump of nothing you then have a measure of something, ie a nothing measured is by definition a something.

 

[vanhees71]

Why do some physicists talk about particles popping into existence from nothing. The nothing they refer to is a vacuum, but the vacuum exists in this universe so must contain the fabric of the universe, it must contain gravity, so obviously it is a something rather than a nothing.

Kaku and Krauss are 2 I can think of that have talked about this.

Where did they do this? I guess some popular-science book, and that's bad, because I'm sure both of them know better! There's nothing popping in and out of existence from the vacuum. By definition the vacuum is stable under time evolution, and it's Poincare invariant.

 

[A. Neumaier]

about particles popping into existence from nothing

To understand the psychology and the physics behind this, read https://www.physicsforums.com/posts/5453034
and https://www.physicsforums.com/insights/misconceptions-virtual-particles/

 

[mikeyork]

Here is a simple (and grossly oversimplified) picture of how you get something from nothing. Take a vacuum state vector |0> and project it onto a basis that permits multi-particle states |s_i> where the state s_i consists of multiple particles in specific states. Then we get

|0> = sum(s_i) |s_i><s_i|0>.

(<s_i|0> is essentially a vector-coupling coefficient analagous to a Clebsch-Gordon coefficient.)

An observer looking for s_i will then find it with relative frequency |<s_i|0>|^2. And this can be non-zero whenever the net quantum numbers of s_i all add up to those of the vacuum. For example, a particle/anti-particle pair with opposite momentum and vanishing net angular momentum. (Note that s_i is a non-space-time state, whereas a space-time observer will have to project s_i into a space-time frame.)

 

[A. Neumaier]

where the state s_i consists of multiple particles in specific states.

This implies already that the inner product with the vacuum state is zero. Thus the observer will find s_i with probability zero.

Note that a particle-antiparticle pair cannot decay into the vacuum or be created from it, because of energy conservation.

 

[mikeyork]

This implies already that the inner product with the vacuum state is zero. Thus the observer will find s_i with probability zero.

Not so. It's a standard vector-coupling to a composite state that is indistinguishable from the vacuum (by which I mean it has the quantum numbers of "nothing") when seen only as a composite state.

Note that a particle-antiparticle pair cannot decay into the vacuum or be created from it, because of energy conservation.

That assumes positive energy states only. That may be true for what an observer sees, but they may be seeing only part of s_i and the negative energy part may not be accessible to the observer, just implied by the vacuum requirement.

Anyway, let's not take this notion too literally in a context of current theory. I was only trying to give an idea of how the concept of "something from nothing" could arise. Clearly, it is a challenging idea for our current knowledge.

 

[A. Neumaier]

Not so. It's a standard vector-coupling to a composite state that is indistinguishable from the vacuum (by which I mean it has the quantum numbers of "nothing") when seen only as a composite state.

That assumes positive energy states only. That may be true for what an observer sees, but they may be seeing only part of s_i and the negative energy part may not be accessible to the observer, just implied by the vacuum requirement.

A k-particle state is orthogonal to j-particle state unless k=j. Since the vacuum is a 0-particle state it is orthogonal to any multiparticle state!

And in relativistic quantum field theory, all states have nonnegative energy.

 

[mikeyork]

A k-particle state is orthogonal to j-particle state unless k=j. Since the vacuum is a 0-particle state it is orthogonal to any multiparticle state!

Only if they are states in the same basis. Nature knows nothing about an observer's basis. Any vector in one basis can be projected to a superposition in another. I made this projection explicit in my first post.

And in relativistic quantum field theory, all states have nonnegative energy.

Yes, but if we are going to imagine "something from nothing" we will inevitably run up against existing theories that will have to be modified. We already know this from trying to bring gravity into QFT.

Even if we require that any observer can see only nonnegative energy, we must still understand that a universe with net vacuum quantum numbers has no observer except as an internal part of the system.

 

[Nugatory]

Anyway, let's not take this notion too literally in a context of current theory. I was only trying to give an idea of how the concept of "something from nothing" could arise. Clearly, it is a challenging idea for our current knowledge.

This might be a good time to mention the Physics Forums rule restricting discussion to currently accepted science.

Also, please do remember that is a B level thread started by a poster who has been misled by the pop-sci treatment of QFT.

 

[A. Neumaier]

Any vector in one basis can be projected to a superposition in another.

But inner products are independent of the basis. And zero inner product in one basis (hence in all) implies zero probabilities.

 

[mikeyork]

According to your logic, then, the coupling of two spins to a single (composite) spin state is impossible, since the singlet state is orthogonal to the 2-particle state. It also contradicts your own acknowledgment of the coupling of particle/anti-particle pairs to vacuum quantum numbers (although you conceded this only for virtual states). I think the problem you are finding lies with your limiting your Hilbert space to a fixed number of particles. But Hilbert spaces are defined by states, single or composite, not by particles and we couple singlet states to multiplet states all the time (using vector coupling coefficients which are inner products).

As far as I can see, your only relevant criticism relates to the requirement of nonnegative energy states and I think I responded to this with a viable suggestion.

However, in deference to Nugatory's comment regarding speculative theories I suggest we close this topic.

 

[maxverywell]

Why do some physicists talk about particles popping into existence from nothing. The nothing they refer to is a vacuum, but the vacuum exists in this universe so must contain the fabric of the universe, it must contain gravity, so obviously it is a something rather than a nothing.

Kaku and Krauss are 2 I can think of that have talked about this.

You are right for asking this. The problem here is in semantics rather than in physics. What they (physicist in popular science books) mean by "nothing" is absence of matter/fields from space, but not the space and time themselves. Moreover, this absence of matter/fields is observer dependend (check e.g. https://en.wikipedia.org/wiki/Unruh_effect). So in my opinion this is a bad term to use in this context.
On the other hand, the philosophers by "nothing" mean the true, absolute nothingness (which you have in your mind), i.e. the absense even of the space and time. It seems that our universe emerged from such a true nothingness, but we don't know how ("what caused the Big Bang?" kind of question).

 

[bhobba]

Aren't they referring to virtual particles which have been observed?

No.

Its usually invoked to explain the Casmir force but its a crock of the proverbial - its actually a Van Der Walls force.
http://folk.ntnu.no/nordam/080826/intro.pdf

It is very sad fact that anything you read outside a QFT textbook is likely wrong. That's because its very very mathematical so to get a handle on it physicists invent a pile of rubbish like virtual particles to aid getting a bit of a physical grip. Trouble is they don't tell that to the lay public that believer all sorts of rubbish that actual physicists know is simply a fiction that helps in intuition.

Wikipedia explains what virtual particles are:
https://en.wikipedia.org/wiki/Dyson_series

Virtual particles are pictorial representations of teems in a mathematical series. It helps doing the math, intuition, etc etc but its WRONG,

Thanks
Bill

 

[ObjectivelyRational]

You are right for asking this. The problem here is in semantics rather than in physics. What they (physicist in popular science books) mean by "nothing" is absence of matter/fields from space, but not the space and time themselves. Moreover, this absence of matter/fields is observer dependend (check e.g. https://en.wikipedia.org/wiki/Unruh_effect). So in my opinion this is a bad term to use in this context.
On the other hand, the philosophers by "nothing" mean the true, absolute nothingness (which you have in your mind), i.e. the absense even of the space and time. It seems that our universe emerged from such a true nothingness, but we don't know how ("what caused the Big Bang?" kind of question).

1. The concept of "coming from", presupposes causation, causation is a phenomenon linking existents in the universe and is inapplicable to the universe as such or to attempts of showing that existence had an origin which was "preceded" by nothing.

2. Our universe did not emerge from true nothingness, because by definition, nothing can emerge from true nothingness. As soon as you try to allege anything came from X, that X must be something.

3. Whatever you initially define as the universe, if you conclude that it came from something else, by definition you have contradicted your initial identification of the universe. The universe includes the something else and what you originally identified as the universe.

If any A emerges from B ( A is not B), you can be sure B must exist, and both A and B are in the universe.

 

[StandardsGuy]

Why do some physicists talk about particles popping into existence from nothing. The nothing they refer to is a vacuum, but the vacuum exists in this universe so must contain the fabric of the universe, it must contain gravity, so obviously it is a something rather than a nothing.

Kaku and Krauss are 2 I can think of that have talked about this.

The vacuum has energy. It may be against the rules to post a link to Wikipedia, so just Google vacuum energy and click on the Wikipedia link. It talks about virtual particle popping into and out of existence with a timespan too short to observe. Most likely the "particles" are too small to observe anyway. Some people would say that anything not observed doesn't exist.

 

[Nugatory]

The vacuum has energy. It may be against the rules to post a link to Wikipedia, so just Google vacuum energy and click on the Wikipedia link. It talks about virtual particle popping into and out of existence with a timespan too short to observe. Most likely the "particles" are too small to observe anyway. Some people would say that anything not observed doesn't exist.

Wikipedia is not an accepted source here because it is not peer-reviewed and its quality is spotty - some articles are excellent, some are awful, and there is no way for a non-expert to know which are which. (We do allow links to wikipedia at the mentors' discretion, to take advantage of the good ones).

This particular wikipedia article is one of the not-so-good ones. Instead of relying on it, you may want to take a look at the Insights articles linked further up in this thread. A forum search here wil also find some good threads. However, there's really no substitute for working through a real textbook on quantum field theory - Lancaster's "Quantum field theory for the gifted amateur" is a good starting point if you have the necessary background.

 

[Nugatory]

We seem to have run out of ways of answering the original B-level question (I just found myself referencing Lancaster, a serious I-level book) so this thread is closed.