Sunday, September 17, 2017

Are Virtual Particles Real?


One of the strangest features of our universe may be entities that scientists call "virtual particles," fundamental particles that apparently come into existence from nothing for a brief period of time, then disappear. When I talk about virtual particles I usually get questions like: "Do these virtual particles really come into existence from nothing? How do you know they exist if they can never be seen? Do they do anything?" Because these virtual particles are a huge part of my scientific life, and because they invoke so many questions, I'm going to discuss what they are, their importance in our understanding of nature, and even some remarkable fine-tuning noticeable in their behavior.

Before discussing virtual particles we need to lay a foundation by discussing, first, what it means to "see" an elementary particle, and second, the relationship between mathematical theory and observational physics in a field such as elementary particle physics. It is impossible to visually see any fundamental particle studied by particle physicists including electrons, protons, and the constituents of protons, quarks. Instead, we see an effect in some kind of macroscopic detector. For instance, in older CRT television sets (before flat screen televisions), electrons would be fired toward the front screen and when they hit the phosphorescent screen they would create a picture. A person could never see the actual electrons but could see their effect when the screen glowed. All of our information about the fundamental structure of matter comes from the interaction of particles too small to see with our eyes because they interact with some kind of macroscopic detector.

Since we can't see these subatomic particles, but can only detect their effect, how do we know they even exist? Basically, we believe they exist because we have mathematical theories that predict certain outcomes for our experiments. When the experimental outcome is accurately described by the theory we usually claim the theory is valid and postulate that the entities implied by the theory really exist. Because the Standard Model of Particle Physics makes accurate predictions about the outcome of our experiments we say that the Standard Model, with its electrons, quarks, and other particles, is correct. It is, in principle, possible that there are other mathematical models that would also accurately predict the outcome of experiments that were different than the Standard Model. The only way to determine which is more accurate is to find an experiment that can differentiate between the different models and then see if the outcome of the experiment agrees with any prediction. It is often most thrilling for a scientist when the outcome doesn't agree with any model, meaning there is something going on that we don't yet understand.

What does this have to do with virtual particles? Of course, we can't see them since they are infinitesimals small fundamental particles, but also because they exist for a very brief period of time, usually about a trillionth of a second or less. However, we can perform mathematical calculations to determine what the outcome of our experiment would be if virtual particles exist and what the outcome would be if they don't exist. All experiments performed only match the theoretical predictions when virtual particles are included in the math. Thus, scientists conclude that these virtual particles are real and that they do actually exist for these extremely brief fractions of a second.

But what are these virtual particles? In my post on "A Universe from Nothing" I wrote,

"The elementary particles and forces in the universe are described by a relativistic quantum field theory (QFT). Within the framework of QFT, our quantum vacuum can be thought of as a bubbling sea of many different kinds of fields. These fields have the potential of creating particles out of nothing. For instance, an excitation, or perturbation, of an electron field can produce an electron. Within QFT a particle and its antiparticle (e.g. an electron and a positron) can come into existence from "nothing" where the "nothing" is the underlying quantum vacuum. Except for near a strong gravitational field like a black hole, these particles from nothing will quickly annihilate each other and cease to exist, and are thus called "virtual" particles."

So virtual particles are all the usual particles of nature, like electrons or quarks, when they are created in special circumstances so that they exist only momentarily due of a feature of nature called "Heisenberg's Uncertainty Principle." When dealing with virtual particles, you can think of the Uncertainty Principle as allowing the principle of conservation of energy to be violated very briefly, as long as it is done in a extremely short enough period of time. Recall that Einstein's famous equation E=mc2 tells us that mass is just one form of energy. Because energy can change from one form to another, we can take one form of energy, (say kinetic energy from motion), and transform that into another form of energy, mass. This happens all the time in nuclear reactors, stars, and other natural phenomena.

But Heisenberg's Uncertainly Principle allows us to momentarily create more mass than the energy we started with should allow. As an example, let's say I had enough energy to create a certain particle with a mass of 1 gram. (Way too much mass really, but we're just using this for illustrative purposes). Because of the Uncertainty Principle I might actually be able to create two particles, each with a mass of 2 grams, even though I only have 1 gram's worth of energy. However, these two particles, with a total mass-energy of 4 grams would quickly annihilate since energy must ultimately be conserved and I would return to the state where I have the same energy I started with, 1 gram's worth. (Putting in real numbers, the Uncertainty Principle would allow the creation of 3 extra grams worth of energy for a time of about a 10-48 seconds which is shorter than the Planck time which may be the smallest possible quantity of time, so the amount of mass created in this example could never really happen.)

If you're still following this you may be able to now surmise exactly what virtual particles are. They are the usual particles of nature that are created for a brief moment, but must quickly be annihilated or conservation of energy would be permanently violated. Nature allows a quick violation of conservation of energy, but not a permanent violation.

As stated, our mathematical calculations only agree with experimental results when we include these virtual particles in the calculation. In addition, our current theory states that certain processes, like natural radioactive decay, only occur through virtual particles. It seems these particles must actually exist in nature and perform valuable functions like radioactive decay.

An interesting application of these virtual particles is that we can infer certain particles might exist in nature even if we can't directly detect them. For instance, if the results of our experiment were to disagree with the mathematical calculations, it could be because the process we are measuring involved virtual particles that we have not included in our calculation. So the difference between our experiment and the theoretical calculation could be due to the effect of virtual particles we haven't yet discovered. Scientist refer to this as a way of "indirectly" discovering new particles since we would see their effect as virtual particles and infer that an undiscovered particle must have contributed to our experimental result, but not actually detect any new particle.

Finally, let me describe a property of nature that, to me, shows remarkable design and fine-tuning. We know that the mass of the proton seems to be finely tuned. If it were to change slightly there would be many life-destroying consequences. Stars like our sun that burn stably for a long time would not be possible. The ratio of the mass of the proton to the electron is fine tuned to allow appropriate neutron decay. Neutron decay and other factors would radically affect chemistry and biology if the proton's mass were to change slightly. At a very simplistic level every proton is made of 3 quarks. But actually the proton is very complicated and is made of not just 3 quarks, but also gluons and even virtual particles. Very rarely a top quark and a top-antiquark will momentarily be created and destroyed in the proton as virtual particles. This occurs even though the top quark is the heaviest known fundamental particle and about 175 times heavier than the proton itself. Its like two pickup trucks being created and destroyed inside your bicycle while you're riding it. So ultimately, the mass of the proton is partially determined by the mass of the virtual top quarks that momentarily and very rarely are created then destroyed inside the proton. Consequently, if the mass of the top quark were to change by a few percent, virtual top quarks in the proton would change the mass of the proton as well, ultimately leading to a universe in which life could not exist. So the exact mass of the top quark and its virtual existence inside the proton is required for our existence in this universe.

Let's summarize with the answers to the questions I posed in the first paragraph.
1) Do these virtual particles really come into existence from nothing? They actually come into existence from the underlying fields in the space-time fabric of the universe.
2) How do you know they exist if they can never be seen? Our experimental results only agree with theoretical calculations when virtual particles are included in the theory.
3) Do they do anything? Yes, they are responsible for phenomena like radioactive decay, and their existence in the proton slightly affects the proton's mass which puts the proton mass into the appropriate range to allow life-necessary features of our universe.

Isn't the universe an amazing place?

The opening picture shows a Feynman diagram of a an electron scattering off of another electron. The circular loop in the middle illustrates the creation and destruction of a virtual particle and its anti-particle. Physicists use these types of diagrams, invented by Richard Feynman, as a tool to help calculate the experimental result of a measurement of the characteristics of the illustrated process.

13 comments:

  1. It would be interesting if readers knew when the full compliment of particles real or virtual began to exist. Say when from the big bang forward were all the processes and particles necessary to have the universe support life that is become the universe we see and experience.Perhaps the quantum vacuum with all the possibilities you describe existed before the BB.

    ReplyDelete
    Replies
    1. Hi Keith. Thanks for your questions. All of the observational and theoretical evidence points to the space-time fabric of our universe coming into existence at the Big Bang. Consequently, there would have been no quantum vacuum before that. At least not the quantum vacuum that makes up our universe. Lawrence Krauss has speculated that there was a different type of vacuum that brought the universe into existence, but he even recognizes that the virtual particles of our universe are a result of the quantum vacuum of this universe. (See my post on A Universe From Nothing). In any case, fundamental quarks, electrons and neutrinos were formed somewhere in between about a trillionth and a millionth of a second after the Big Bang. Of course it took much longer for atoms to form (about 380,000 years after the Big Bang), and longer still (200 million to a billion years after the Big Bang) for stars and galaxies to form. A planet like the earth takes about 9 billion years after the Big Bang to form (See my post on A Small Big Universe).

      Delete
  2. Of semi related note to particle physics and the inference to design:
    Richard Feynman, in his role in developing Quantum-Electrodynamics, which is a mathematical theory in which special relativity and quantum mechanics are unified,

    Theories of the Universe: Quantum Mechanics vs. General Relativity
    Excerpt: The first attempt at unifying relativity and quantum mechanics took place when special relativity was merged with electromagnetism. This created the theory of quantum electrodynamics, or QED. It is an example of what has come to be known as relativistic quantum field theory, or just quantum field theory. QED is considered by most physicists to be the most precise theory of natural phenomena ever developed.
    http://www.infoplease.com/cig/theories-universe/quantum-mechanics-vs-general-relativity.html

    ,, Richard Feynman was only able to unify special relativity and quantum mechanics in quantum electrodynamics by quote unquote “brushing infinity under the rug” by a technique called Renormalization

    THE INFINITY PUZZLE: Quantum Field Theory and the Hunt for an Orderly Universe
    Excerpt: In quantum electrodynamics, which applies quantum mechanics to the electromagnetic field and its interactions with matter, the equations led to infinite results for the self-energy or mass of the electron. After nearly two decades of effort, this problem was solved after World War II by a procedure called renormalization, in which the infinities are rolled up into the electron’s observed mass and charge, and are thereafter conveniently ignored. Richard Feynman, who shared the 1965 Nobel Prize with Julian Schwinger and Sin-Itiro Tomonaga for this breakthrough, referred to this sleight of hand as “brushing infinity under the rug.”
    http://www.americanscientist.org/bookshelf/pub/tackling-infinity

    In the following video, Richard Feynman rightly expresses his unease with “brushing infinity under the rug.” in Quantum-Electrodynamics:

    “It always bothers me that in spite of all this local business, what goes on in a tiny, no matter how tiny, region of space, and no matter how tiny a region of time, according to laws as we understand them today, it takes a computing machine an infinite number of logical operations to figure out. Now how can all that be going on in that tiny space? Why should it take an infinite amount of logic to figure out what one stinky tiny bit of space-time is going to do?"
    - Richard Feynman – one of the founding fathers of QED (Quantum Electrodynamics)
    Quote taken from the 6:45 minute mark of the following video:
    Feynman: Mathematicians versus Physicists - video
    https://www.youtube.com/watch?v=obCjODeoLVw

    I don’t know about Richard Feynman, but as for myself, being a Christian Theist, I find it rather comforting to know that it takes an ‘infinite amount of logic to figure out what one stinky tiny bit of space-time is going to do’:

    “Why should it take an infinite amount of logic to figure out what one stinky tiny bit of space-time is going to do?"
    - Richard Feynman

    John1:1
    "In the beginning was the Word, and the Word was with God, and the Word was God."

    of note: ‘the Word’ in John1:1 is translated from ‘Logos’ in Greek. Logos is also the root word from which we derive our modern word logic
    http://etymonline.com/?term=logic

    The reason why I find it rather comforting is because of John 1:1, which says "In the beginning was the Word, and the Word was with God, and the Word was God." ‘The Word’ in John1:1 is translated from ‘Logos’ in Greek. Logos also happens to be the root word from which we derive our modern word logic.

    ReplyDelete
  3. One aspect of QM is so called Spooky Action at a Distance, that is real and demonstrable and understood by no one. But I ecounter it nearly daily and perhaps even have an explanation. It's called personal private prayer, the communication is instantaneous and assurance is the same.

    ReplyDelete
  4. So fascinating! So many questions! What happens if the law of conservation is broken and we figure out a way to keep those particles in existence longer? Could energy be harnessed from that? Theoretically of course. Where does the law of entropy come into the virtual particle theory? As the universe "cools", disperses and becomes more chaotic, will it be easier to detect these subatomic particles? And to bring God into it, why must we wait to discover these things? What would be God's purpose for having us come into this knowledge one generation at a time? Sorry for my elementary questions.

    ReplyDelete
    Replies
    1. Actually, near a black hole it is possible that one of the particles is devoured by the black hole and the second particle remains in the universe. This was postulated by Stephen Hawking and is called "Hawking Radiation." But energy must still be conserved overall, so the particle falling into the black hole has "negative" energy and the energy/mass of the black hole gets smaller with each radiation event. To the outside observer, the black hole has emitted a particle and lost mass. Eventually, the black hole radiates away to nothing. So energy can't be harvested in any way that we know of.

      Scientists have discussed whether this vacuum energy can be captured and used, but so far no concrete ideas have surfaced that I know of. Entropy still increases as with any process so the universe continues to "wind down."

      I don't know God's purposes, but I do know that as I said in my last sentence, the universe is an amazing place. It is also well designed and surprising. I think that the universe reflects the Soul of the Artist (http://www.michaelgstrauss.com/2017/01/the-soul-of-artist.html), so we do see God's amazing character in the amazing universe.

      Delete
    2. Mike My only experience was with im aginary numbers that crop up in EE impedance math and power ...are virtual and imaginary related.

      Delete
    3. No, imaginary numbers are those that include an imaginary part (defined as the square root of -1). There is no connection between the math of imaginary numbers and virtual particles. Although both are useful, one as actual particles in nature, one in the world of mathematics, but still very useful for calculating natural phenomena.

      Delete
  5. Dr. Strauss,
    Here is a new video upload that you may appreciate.

    Multiverse Mania vs Reality - video
    https://www.youtube.com/watch?v=nQJV4fH6kMo
    Paper:
    https://docs.google.com/document/d/1K1zhcd3ElF-3G_DYlMYBR4qaZrvU09lg0MqmhvN2Sw8/edit

    ReplyDelete
  6. Virtual particles are fascinating! I have also found the following article to be helpful: https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/

    ReplyDelete
  7. Mike is it really that for a short period conservation of energy is violated or is it that a small amount of energy is pulled out of the quantum vacuum which is briefly expressed as a virtual particle then quickly goes back to the quantum vacuum

    ReplyDelete
  8. Dr. Strauss,
    You may be interested in this new video upload. There is a short audio clip of you in it talking about photons.
    Quantum Mechanics, Special Relativity, General Relativity and Christianity - video
    https://www.youtube.com/watch?v=gKggH8jO0pk
    Paper:
    https://docs.google.com/document/d/1nRZECqs8Iqeqv0GzP5lV6et_K9_rYrz06Tchoa4U0Rw/edit

    ReplyDelete