Are Virtual Particles A New Layer of Reality?

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Let me tell you a story about virtual particles. It may or may not be true. Out there in the emptiest places of the universe, phantom particles appear and vanish again out of nowhere. They borrow the energy for their existence so briefly that they cheat the watch fly of the universe. Near black holes, virtual matter and antimatter pairs are separated by the event horizon to create Hawking radiation. And every time two particles interact, an infinite number of virtual particles mediate infinite versions of that one interaction. Virtual particles sound pretty cool, I guess, but is this really how they work? Seriously, what are virtual particles? Sometimes our mathematical hacks point to strange new aspects of reality. For example, Max Planck used a quantization trick to figure out the spectrum of light emitted by hot objects. The quantization part of his math trick was meant to disappear in the final form of the equation, but it remained. It proved fundamental. That math hack turned out to represent the very real quantum nature of the photon. This insight led to the discovery of all quantum physics. A more recent mathematical hack is the virtual particle. They started out as a trick to make impossible calculations in quantum field theory possible-- possible, at least, for the sort of people who can do quantum field theory. So will virtual particles also prove to represent a new underlying aspect of reality? Now, we're going to go pretty deep in this one, but it will bring us closer to a better understanding of the quantum nature of reality. So stick with me. First, let's get to the origin of virtual particles. So quantum field theory is the machinery behind the standard model of particle physics. In it, particles are excitations in fundamental fields that exist everywhere in space. In particle interactions, packets of energy are exchanged between these fields. For example, two electrons-- excitations in the electron field-- will repel each other by exchanging energy through the electromagnetic field. That process is kind of a mess. Each electron jiggles the electromagnetic field, and those jiggles have a back reaction that jiggles each electron, which in turn affects the way the electrons jiggle the EM field ad infinitum. It's a hopelessly tangled feedback cycle of reaction and back reaction, and it's impossible to calculate it perfectly. But it is possible to approximate it-- in fact, with astonishing accuracy. That's where perturbation theory comes in. It's our mathematical hack. The hack is to approximate this single multilayered mess of an interaction by adding together a set of much simpler idealized interactions. Those interactions are mediated by virtual particles. In that sense, virtual particles are the building blocks of our approximation of the behavior of quantum fields. Let's do an example. In the case of the interacting electrons, you start by saying each electron interacts once with the EM field, transferring between them energy momentum and one photon worth of quantum properties in a single packet that we call a virtual photon. Then you add the effects of doing this transfer in two, three, four packets, as well as every other idealized field interaction that you can imagine. Every one of these interactions is described with a simple excitation and transfer of particles-- virtual particles. The hope is that by adding together the contributions of enough of these, you can approximate the messy state of the field in the true interaction. We call these idealized interactions intermediate states or virtual states of the field. But in reality, the field never exists in these states. The virtual particles never exist independently. Instead, virtual particles are the mathematical building blocks we use to approximate the complex states of interacting fields. Maybe you recognize these things-- Feynman diagrams. We've definitely talked about them before. Richard Feynman came up with them as a bookkeeping tool to keep track of which intermediate states are important in your calculation of an interaction and which you can ignore. Particles that either enter or leave these diagrams are our real particles. All those that both start and end within the diagram are virtual particles. Feynman diagrams are an absolutely essential tool in most modern quantum field theory calculations, but they also add to the misconception about virtual particles. They sure make it look like virtual particles are doing regular particle stuff like traveling through space but that's just not the case. Virtual particles share some properties with their real counterparts-- in particular, quantum numbers like charge and spin, but they don't need to obey Einstein's relationship between energy mass and momentum. In fact, they ignore a lot of the physics of real particles. They can have any mass. And they can even travel faster than light or backwards in time. This isn't because they're magic. It's because they aren't physical. Virtual particles are our mathematical representation of the quantum mechanical behavior of fields, and that behavior is weird. Here's a really good illustration of this weirdness. In our first example, we looked at two electrons repelling each other. One electron throws a virtual photon at the other one causing them to be deflected from each other like a game of quantum dodgeball. But what about attractive forces? What about an electron and a positron? How can throwing photons between particles cause them to be drawn together? Let's look at the fine and diagram of a single virtual photon passing from electron to positron. To calculate the effect of this, you add together the possible effect of every possible virtual photon being emitted by the electron and absorbed by the positron. Bizarrely, that includes photons that are pointing in the wrong direction to even make the journey. Their momenta are pointing from positron to electron rather than electron to positron. And you also count photons emitted by the positron but pointing away from the electron. These are the virtual photons that ultimately provide the attractive force. But how do they make the journey between the particles? Uh, they don't-- there is no journey per se. These virtual particles sort of exist everywhere at once, which is confusing. Each one of these infinite possible virtual particles represents a quantum of energy in a single possible vibrational mode of the underlying quantum field. In a way, a virtual particle represents a pure excitation of the field, an idealized case of perfectly defined momentum. The Heisenberg uncertainty principle tells us that the perfectly defined momenta of virtual particles means completely undefined position. In contrast, real particles are mixed up combinations of many excitations, many different momentum modes. And that uncertain momentum gives them real locations, real trajectories through space. Our virtual photon doesn't have a location, so it doesn't travel a real path. It can move between our electron and positron even if its momentum is pointing in the wrong direction. It's a bit like the photon starts out moving in the wrong direction and then quantum tunnels between the particles, kicking them towards each other like a teleporting boomerang. But even that description is way too physical and Australian. No individual virtual photon can be credited with producing the attractive force. In fact, you only see that force in the sum of all possible virtual photons over all possible Feynman diagrams. Bizarrely, you also have to include the case where the electron and the positron totally ignore each other to even see an attractive force. Did I mention that quantum mechanics is weird? So that's the deal with virtual particles in particle interactions, but we also hear about the role of virtual particles in a complete vacuum. You might have heard the quantum vacuum described as his roiling ocean of virtual particle-antiparticle pairs popping into and out of existence, the so-called vacuum fluctuations or zero point fluctuations. What's the deal with that? So the quantum fields are composed of these vibrational modes of all different frequencies/momenta that can be excited to become particles. These modes also exist in the vacuum just without the excitations. Each mode should have 0 energy in a vacuum, but in quantum mechanics, nothing can be so exact-- thanks, again, to the Heisenberg uncertainty principle. In order to remain "uncertain," there has to be a slight chance that when you look at a vacuum, any given mode will have non-zero energy. This leads to a non-zero average energy called, confusingly, the zero point energy. So there's a chance that when measured the vacuum will appear to have energy and so have particles. But the key word here is "measured." Do those particles exist if you're not looking? Or more to the point, do vacuum fluctuations produce actual particles when there's nothing else around? The answer seems to be no-- at least, as far as you can answer such a question. In the math of QFT, the perfect vacuum is a steady state. It doesn't change over time. It has a constant zero point energy. Regarding its particle content, it remains in a steady state of uncertainty. It's a quantum state in a superposition of "yep particles" and "nope, no particles." The quantum state is not fluctuating on its own, but it will randomly collapse into one of these possibilities when something interacts with the vacuum. This is a pretty subtle point, but it makes a huge difference in how we think about the vacuum. Virtual particles are not popping into and out of existence in the absence of any else. Instead, they're fanciful way of talking about what might happen if something interacts with the vacuum. The classic example of this is Hawking radiation. Stephen Hawking himself was the first to use virtual particles as an intuitive way to describe his radiation. He painted a picture of virtual matter-antimatter pairs being separated by the black hole event horizon, allowing one of the pair to escape to beautiful freedom and reality. But Hawking himself also cautioned against taking this picture too seriously. In his actual mathematical derivation, he instead talks about vibrational modes of the quantum vacuum being cut off by the event horizon. This disturbance of the vacuum generates particles. But without the black hole, the vacuum stays a vacuum. A similar perturbation of the quantum vacuum is also seen in the Casimir and Unruh effects. We also did episodes on these, and just like with Hawking radiation, you don't need for virtual particles to have an independent existence to explain these effects. So to recap, virtual particles are best thought of as a mathematical device to represent the behavior of quantum fields. The original idea of virtual particles came about as a calculation tool in perturbation theory as we tried to approximate the behavior of quantum fields. Now, in the case of Max Planck discovery of the quantum nature of photons, it turned out that a mathematical artifact represented new real physics-- the quantum nature of the photon in that case. Planck knew that he was onto something because there was no way to express his Planck law without an artifact of that quantum nature-- namely, the Planck constant. So what about virtual particles? If they represent a physical reality, then there should be no way to do quantum field theory calculations without them. It turns out there is a version of quantum field theory that doesn't use virtual particles at all. That will be the family of lattice field theories in which space-time itself is defined on discrete grid. It doesn't rely on perturbation theory, and so it doesn't use virtual particles while ultimately giving the same results. Ergo, virtual particles are probably just a mathematical artifact. There is no good reason to believe that virtual particles exist outside the math we use to approximate the behavior of quantum fields. At best, they can be interpreted as a small component of possibility space for a quantum field doing something real. That said, for something that doesn't exist, they're surprisingly useful for describing the weird underlying machinery in our quantum space-time. Last week we talked about the latest thinking on the Fermi paradox and what conclusions we can really draw about this persistently annoying lack of aliens. Let's see what you had to say. A few people say that the Fermi paradox isn't really even paradoxical with some common objections about the behavior of aliens, saying things like the window of strong radio emission for any civilization is too short to overlap with us or aliens probably don't build Dyson swarms or aliens probably don't build Von Neumann probes or aliens probably don't colonize widely and, in general, that aliens probably stay quiet or use technologies undetectable to us. Any and, perhaps, all of these may be true but that misses the point. The point is that the overwhelming majority of every civilization that ever developed would have to observe all of these conditions in order for us to see nothing. Remember, guys, it only takes a single rocket happy individual to execute one of these projects, a single Musk or Bezos or Branson or Milner from potentially thousands of generations and potentially millions of civilizations to play their hands, tentacle, pseudo pods, whatever to a big space project. All of these arguments of aliens probably don't do this or that are what we call a soft filter. Something that may be unlikely for any given civilization but given enough civilizations that thing becomes inevitable. These arguments on alien psychology are soft filters. The Fermi paradox must be explained by a very long chain of soft filters that add up to extremely small probability or one or more hard filters. A hard filter is a step in development that's so unlikely it can cut off nearly all progress for development of life or civilization. Paper Dragon points out that not enough attention is paid to one particular possible great filter. That's the transition from what we'd call not intelligent to intelligent life. It's a good point because we have very little capacity to assess the likelihood of that transition. Humans are the only species to have ever built a society with anything close to the technological capacity to be seen by other species. Was there some extreme fluke in the development of Homo sapiens and our technological ascendance that was an extreme fluke? Maybe, though, it's not right to say that it took the span of life on Earth for this fluke to happen-- as if we were rolling the dice every Millennium and only now came up sixes. A lot of that time, life was evolving towards the point that it could become intelligent. There are now many tool using species, including primates, birds, octopuses, dolphins. And we aren't the only species to have evolved language, art, and imagination. Our extinct Homo-cousins, Neanderthals, Denisovans, et cetera had various levels of the above. But again, we just don't know. It really could be that modern Homo sapiens underwent an improbable genetic or cultural evolutionary movement that qualifies as a great filter. A few of you pointed out that our depiction of the binary star system was wrong. They were apparently orbiting a center of mass that wasn't even between the stars. So that was actually a three star system. One star had collapsed into a black hole, which is why you couldn't see it. It was totally there though-- try increasing your screen resolution.
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Channel: PBS Space Time
Views: 531,695
Rating: 4.9048405 out of 5
Keywords: quantum, theory, max, planck, quantization, virtual, particles, light
Id: ztFovwCaOik
Channel Id: undefined
Length: 17min 13sec (1033 seconds)
Published: Wed Oct 31 2018
Reddit Comments

If the virtual particle are not real, then which is the phisical origin of Hawking radiation? And can be the Hawking radiation the origins of dark matter?

👍︎︎ 3 👤︎︎ u/Zurich18 📅︎︎ Nov 01 2018 🗫︎ replies

I can watch any of these videos 10x each and still not understand half of the subject. :(

👍︎︎ 1 👤︎︎ u/mcbwaa 📅︎︎ Nov 01 2018 🗫︎ replies
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