Virtual Particles and Radioactivity

  • #1
Is it possible for a virtual particle consisting of a proton/antiproton pair to "pop" into existence close enough to the nucleus of a helium atom (two protons and two neutrons) such that the antiproton of the virtual particle annihilates one of the protons of the helium atom leaving Tritium (H3) while the a virtual proton finds itself just far enough away to escape capture by the tritium nucleus (like the way the Hawkings Radiation claims one member of a virtual pair might escape a black hole)?

Do virtual particles cause some forms of radioactivity in this fashion?
 

Answers and Replies

  • #2
fzero
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A real process has to conserve energy. In this case, the final state of proton + tritium nucleus has about 20 MeV more energy than the helium nucleus that we started with. This is neglecting any momentum that the proton or tritium nucleus might have. This energy needs to be provided somehow, such as by a background electromagnetic field or by colliding a beam of particles onto the helium nuclei.

The fact that the helium nucleus is a lower energy state than any of the things it could split into is the reason why helium is stable. In the absence of external stimulus, the helium nucleus does not transform or decay.
 
  • #3
Thanks for your quick response!

So does this mean that if we had a large quantity of Helium in a powerful electric field that there is the possiblity that we might see an He4 atom changed to H3 and H+ as a result of an interaction with a virtual particle?
 
  • #5
Ken G
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I am not a particle physicist, but I do have some well-considered opinions about "loose language" in physics, and its benefits and pitfalls. In general terms, I agree that we must distinguish between a prediction made by a theory, which is the "hard" science, and the pictures we invoke to understand those predictions, which we might term "pedagogy", yet all the same, physics is rife with pedagogy. It really couldn't get by at all without a good pedagogy, so the discussion should never be about "is that a true description of what is happening," because it never is, the discussion should be about "how does that language help us obtain a useful picture, and what are the pitfalls it invokes that can lead us astray." Examples of that are as dense as physics itself, starting right at Newton's laws, what is potential energy, and what are inertial and fictitious forces in the first place.

In particular, I would point to this quote from your link:
"But it should be clear, that this is a purely symbolic metaphor that
hasn't anything to do with what happened in the laboratory - where
(as the abstract says), colliding photons create an electron-positron
pair."
Although I certainly do not disagree with the statement itself, I take issue with the implication that there is anything in physics, other than its equations, that are not "purely symbolic metaphors." In the above, "colliding" is a symbolic metaphor, "create" is a symbolic metaphor, and "electron-positron pair" is a symbolic metaphor. Physics is a set of equations combined with purely symbolic metaphors that are useful for finding the correct application of the equations, and the ability to correctly use the equations is always only as good as the purely symbolic metaphors. What's more, the general public never gets the equations, they pretty much only get the metaphors, and that just comes with the territory.
 
  • #6
A. Neumaier
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In general terms, I agree that we must distinguish between a prediction made by a theory, which is the "hard" science, and the pictures we invoke to understand those predictions, which we might term "pedagogy", yet all the same, physics is rife with pedagogy. It really couldn't get by at all without a good pedagogy
I fully agree.
In particular, I would point to this quote from your link:
"But it should be clear, that this is a purely symbolic metaphor that
hasn't anything to do with what happened in the laboratory - where
(as the abstract says), colliding photons create an electron-positron
pair."
Although I certainly do not disagree with the statement itself, I take issue with the implication that there is anything in physics, other than its equations, that are not "purely symbolic metaphors." In the above, "colliding" is a symbolic metaphor, "create" is a symbolic metaphor, and "electron-positron pair" is a symbolic metaphor.
There are degrees of metaphors. One can say in equations what it means to collide and to have an electron-positron pair. The latter are described by a 2-particle state, and the collision process is given by the scattering matrix. To say that ''colliding photons create an electron-positron pair'' is concise language that tells something about in-states and out-states, which are supposed to be real, preparable things.

But one cannot prepare or measure Feynman diagrams and/or their internal lines. What they mean depends on the perturbative formalism used and except at tree level usually only an infinite sum of them makes any computational sense. Thus virtual particles have a far less tangible relation to reality as colliding photons creating an electron-positron pair.
 
  • #7
No. They don't cause anything. See Chapter A7 ''Virtual particles and vacuum fluctuations'' of my theoretical physics FAQ at http://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html#A7
Wow, I really found Chapter A7 enlightning! Thanks for the link, I plan to spend some time studing it! You say there that:

"There is plenty of evidence that sums of Feynman diagrams, interpreted
as renormalized multidimensional integrals, correctly predict many
phenomena. But to interpret this as evidence for the existence of
virtual particles manifesting themselves in space and time is
stretching the interpretation too far -- something perhaps acceptable
at the at the layman's level to provide some sort of intuition for
otherwise too abstract things (which is what one can find in
popularizing accounts by some well-known physicists), but unacceptable
on a more scientific level.

Indeed, there are strong arguments that loudly speak against assigning
reality to virtual particles. If virtual particles were real, they
would leave their trace in all methods of predicting certain phenomena,
and they would assign the same properties to the virtual particles no
matter which approximation method is used. "

I read in "How to Teach Your Dog Physics" that "virtual particles" affect the motion of an election in an electric field, from your writing I understand that this is just a descriptive way to visualize a well measured phenomena. Not evidence supporting the existance of such particles then.

Is that correct?

As you can see from the above, the following line from your text is refering to me :)

"Thus from the scientific point of view, the concept of virtual
particles is quite shallow. The latter is not the level of science but
the level on which scinece can be presented to laymen."

I must apologize for my ignorance and am most thankful for folks such as yourself who help me understand even a moon cast shadow of what is going on in reality.

This answer brings up a bunch of other questions, please be patient as I am sincere in asking the following as my ignorance is truly great in this area as you shall shortly see.

When I was young I was told that Santa Claus was not real. I was heart broken but immediately I had to ask about of the Easter Bunny, the Tooth Fairy, God, and Disneyland (I had never been to Disneyland). Now that I have learned that virtual particles are not real. I need to ask where does reality start in all this? Are any of the following real?

quarks? antineutrinos? quarks? bosons? photons? electrons? neutrons? protons? atoms? hydrogen? water (the H2O kind not the wet kind)?

Where is the line of reality drawn in all this?

Sincerely,

Blair Nelson
 
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  • #8
Ok I got to the part in you book that says:

"But one can easily write down state vectors for the usual, real objects, such as
quarks, nuclei, electrons, or photons."

So let me rephrase. Is it just the virtual particle concept that has no real counterpart in this context?
What does this imply for "Hawkings Radiation"?
 
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  • #9
A. Neumaier
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I read in "How to Teach Your Dog Physics" that "virtual particles" affect the motion of an election in an electric field, from your writing I understand that this is just a descriptive way to visualize a well measured phenomena. Not evidence supporting the existance of such particles then.

Is that correct?
Yes.
As you can see from the above, the following line from your text is refering to me :)

"Thus from the scientific point of view, the concept of virtual
particles is quite shallow. The latter is not the level of science but
the level on which science can be presented to laymen."
Long ago, I also didn't understand the nature of virtual particles. It was a long way before I understood how things make sense on a deeper level.
When I was young I was told that Santa Claus was not real. I was heart broken but immediately I had to ask about of the Easter Bunny, the Tooth Fairy, God, and Disneyland (I had never been to Disneyland). Now that I have learned that virtual particles are not real. I need to ask where does reality start in all this? Are any of the following real?

quarks? antineutrinos? quarks? bosons? photons? electrons? neutrons? protons? atoms? hydrogen? water (the H2O kind not the wet kind)?

Where is the line of reality drawn in all this?
Well, the standards for reality are a matter of one's philosophy. For me Santa Claus and virtual particles are unreal, God and quarks (the latter understood as fields) are real.
But God is outside of physics.

In physics, I think the best way to draw the line is given by what you already noticed:
Something is considered as real (given a particular context of time and depth of modeling) if it can be described by a state in a mathematical model fitting the data and decribing the dynamics of the state (which might be trivial - in equilibrium).

Ok I got to the part in you book that says:

"But one can easily write down state vectors for the usual, real objects, such as
quarks, nuclei, electrons, or photons."

So let me rephrase. Is it just the virtual particle concept that has no real counterpart in this context?
What does this imply for "Hawkings Radiation"?
it is not from my book but from my FAQ.

I know about Hawking radiation only on the layman's level (or slightly better, since I understand the context on a deeper level). So I can't tell, and I am not interested enough to find out, since apparently none of its consequences has been observed so far.

My interest is to understand things in the range from QED to the sun, and understand that really well. I leave speculations about cosmology and grand unification to others.
 
  • #10
I am very impressed at how many FAQs you have taken the time to write. Definately a link near the top of my favorites list. There is some really good stuff there on reality and Hawking's Radiation.

Have you written any books a layman might nibble at?
 
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  • #11
Ken G
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There are degrees of metaphors.
Yes, that seems unavoidable-- "degrees of metaphors." That means the terrain is not black-and-white, and I think in the final analysis, anything that is not a direct consequence of some mathematical prediction, but rather is a choice of language, we have to say it is ultimately up to the individual making the prediction to decide if it was a helpful metaphor, or a metaphor that is close to the physics, or a misleading metaphor, or one that is so divorced from the physics that it is no more than analogy. That doesn't make you wrong to point out weaknesses in the virtual particle metaphor-- that's a valid thing that helps people learn the pluses and minuses of any metaphor. I'm just saying we should all recognize we are up to our ears in metaphors, and there's nothing wrong with that.

One can say in equations what it means to collide and to have an electron-positron pair.
But if what it means to collide just comes from the equations, then why use the word at all? The only reason we have words in the first place is how they evoke images from our own experience, experience that might have nothing at all to do with what is "actually going on in the lab" when we choose to picture something we call particles doing something we call colliding. It is the same with "virtual particles"-- as you say, they are nothing but terms in a diagram, so calling them anything will be evoking a metaphor. Some metaphors might be more problematic than others, but at some level, it's not deciding between what is purely metaphor and what isn't, it is merely understanding the limitations we accept whenever we use language instead of equations.

Thus virtual particles have a far less tangible relation to reality as colliding photons creating an electron-positron pair.
I'm skeptical about the tangible relation to reality of any of these metaphors. Scientific reality is just the readings on our instruments, and the rest is interpretation-- when the correct predictions are made for the outcomes, the physicist can claim to success in his/her interpretation, even if they are also served by understanding the limitations of that interpretation.
 
  • #12
A. Neumaier
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Scientific reality is just the readings on our instruments, and the rest is interpretation
No. Scientific reality is (as nonscientific reality) just interpretation. Even the reading of an instrument must be interpreted - and reading of instruments only takes a small fraction of the time scientists (even experimantalists) spend on their work.
 
  • #13
Ken G
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No. Scientific reality is (as nonscientific reality) just interpretation. Even the reading of an instrument must be interpreted - and reading of instruments only takes a small fraction of the time scientists (even experimantalists) spend on their work.
Yet one could take that same stand you took above-- there are degrees of interpretation. We can trust our instruments at the lowest degree of interpretation, and after that, one is wading into the land of metaphor. At that point, the dscourse shifts to what might be a better or worse metaphor, not to what is or is not metaphor. Virtual particles may be farther from the reality than an instrument reading, with a real photon being somewhere in the middle, but the territory differs only in degree-- if the interpretation is used successfully, then it is at some level correct. What can be incorrect about an interpretation other than leading to false predictions?
 
  • #14
A. Neumaier
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Yet one could take that same stand you took above-- there are degrees of interpretation. We can trust our instruments at the lowest degree of interpretation
But we are trusting our theories more than any instrument reading! If something doesn't fit, we first check whether everything was set up correctly, there is no bug in the instrument, etc....
 
  • #15
Ken G
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Yes, that's a valid point, our first suspicion when we get surprised is we did something wrong. But ultimately, observation always wins over theory.
 
  • #16
A. Neumaier
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Yes, that's a valid point, our first suspicion when we get surprised is we did something wrong. But ultimately, observation always wins over theory.
No. Theory always wins. It accounts for observations by fitting them into its conceptual framework, but only if the latter are accountable for. Unaccounted observations simply survive as anomalies until the theory gets stronger.
 

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