I had about a dozen other ideas for this week but then Stephen Hawking died, soā¦Hereās an explanation of one of his very cool theories, formed largely on the work of Jacob Bekenstein.
Physics, especially quantum physics, is a bit out of my depth so I hope any physicists or physics enthusiasts will forgive any over-simplifications or slight misrepresentations. This page offers a really intelligible explanation of field theory, how things can be both particles and waves, and how all this creates virtual particle pairs.
Hawking radiation is a type of radiation predicted to be released by smaller black holes, eventually leading to a black holeās demise.
One theory of our universe is that everythingĀ physical is produced by non-physical fields that can ripple and move. When sufficient energy is put into the field, it can produce a real, physical particle****. The field also ripples naturally without any energy input. These natural fluctuations can produce a virtual
particle pair ā that is, a particle and an opposing antiparticle that donāt have enough energy to be real physical particles. Most of the time they blip into existence, immediately rejoin, and disappear. Sometimes the particles donāt touch and instead separate. They can then become real physical particles.
****A wild E = mc^2 appears! Well, thereās actually a more complex version of the formula for this kind of situation. But the gist remains: the energy put into the field produces a corresponding amount of particle mass.
Hawking calculated that when virtual particles form on the edge of a black hole, they could become real particles (usually photons or neutrinos) due to the black holeās gravitational energy. One particle could then fall into the hole but the other particle could escape. This escaping particle then appears to be emitted from the black hole, creating Hawking radiation. The particle pair was created by the black holeās gravitational field, so the loss of one of the particles causes the black hole to lose mass.
As the black hole continues to shrink, it gets hotter and more energized until
it explodes in a burst of energy and radiation.
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Particle physicists sometimes talk about virtual particles.Ā The word āvirtualā evokes images of online avatars or holograms: things that are in some sense false, which causes virtualĀ particles to sound fake. Yet physicists also say things likeĀ āParticle A and Particle B exchange a virtual particle and experience a force.ā Well, if it makes a real force, then how is the exchanged particleĀ virtual? Letās get into it.
Modern particle physics is described viaĀ ~*~Quantum Field Theory~*~, which associates each kind of particle with a quantum field. For instance, an electron is a type of particle, and so there exists an electron field.
You can think of a quantum field like a sprawling network of guitar strings across all of space and time. Like guitar strings, you can pluck the quantum field andāif you do it just rightāyouāll hit a note that resonates across the whole network of strings. (A note that resonates across all of space and time!) These metaphorical notes correspond to physical particles.
It turns out the fieldās resonant notes donāt occur randomly. In fact, each field is associated with a numberācalled the mass m of that fieldāand the fieldās resonant notes all satisfy the following expression:
where E and p are the energy and momentum of the resonant note/particle, and c is the speed of light. (You might recognize this formula as Einsteinās Mass-Energy Equivalence Formula. Quantum fields automatically encode Einsteinās theory of special relativity!)
These resonant notes are called real particlesĀ and are said to be on-shell because they satisfy Einsteinās Formula. If it can avoid interacting or decaying, a real particle has the potential to travel forever. Thus, real particles can travel far enough to reach our detectors and theyāre Pretty Important as a result.
So real particles appear when we pluck the field and hit a resonant note. However, nothing prevents us from hitting a non-resonant note.Ā These result in particles that do not satisfy Einsteinās Formula:
That means these ill-formed particles can have any mass theyād like, unlike real particles. They also wonāt travel forever. Instead, they quickly lose effectiveness and decay into other particles. Therefore, they donāt reach our detectors. These are what we call virtual particles. Because of their unrestricted masses, theyāre said to be off-shell.
But just because we canāt detect virtual particlesĀ doesnāt mean theyāre unphysical. See, our experiments often involve throwing real particles at each other (such as in a collider), and oftentimes when these particles make contact, theyāll pluck a field via an interaction. When they do this, they mightĀ hit a resonant note of that field, but thereās also a strong chance that theyāll hit a non-resonant note. Really, anything is fair game when particles collide!
Furthermore, any particleāthat means real or virtualācan decay or interact with any other particles. So, yeah, virtual particles wonāt live long enough to see our detector, but their energy and momentum has to go somewhere, and so theyāll often decay into other particles. Those resulting particles might be real, and so might reach our detectors. Alternatively, a virtual particle might dump itās energy/momentum into some other particle, and that lucky particle might be real enough to be detected. Either way, virtual particles yield physical consequences.
Therefore, despite their name, virtual particles are as real as any other particle. We just donāt directly observe them.
Thank you for reading! Follow sineofpsi.tumblr.com for more physics posts. Have questions about QFT or particle physics? Send me an ask! Best wishes.
