What specifically are “virtual particles” and “negative energy” in Hawking radiation?


I’m listening to an audiobook of A Brief History of Time, bopping along, and when I get to the chapter on black holes I’m suddenly “hold up, whaaa? . . .” And then I learn that “virtual particles” can somehow become “real particles” through “negative energy” and I’m thinking it sounds like a comic book where Batman saved the day by “reversing the polarity.”

In: 5

Hi /u/tamsui_tosspot!

Virtual particles are the source of some of the most pervasive misunderstandings in pop-physics. I would argue their conception has done more harm than good…

Virtual particles are not real. They are a mathematical book-keeping device, nothing more. Please don’t ascribe any physical meaning to them.

Similar things are true about negative energy.

Any explanations of Hawking radiation that involve virtual particles and negative energy try to paint a picture that is somewhat intuitive in order to render this effect more accessible. However, neither virtual particles nor negative energy have a whole lot to do with the proper mathematical description of Hawking radiation.

There are some areas of physics where it is useful to use a sort of placeholder thing to explain something; some mathematical construction to fill in the gaps for what is actually going on.

For example, when particles interact, if they are interacting at a distance (as most interactions are), it can be useful to throw into the model a pretend particle that moves from one to the other, carrying energy, momentum, charge etc.. This pretend particle isn’t really real, but fills in the gap between the two real particles and makes the maths work nicely. It doesn’t matter that the pretend particle isn’t actually real as it is only going to “exist” in our model for a very short time.

With negative energy, again, while energy normally is going to be positive, there may be some situations where it is convenient to temporarily assign something a negative energy value, just to make the maths work neatly. It’s not a problem overall as it will be for a short time, and it will be balanced out by some positive-energy thing, and will average out to a positive value on large scales.

Except it turns out that when you get to quantum mechanics, the line between “physical reality” and “mathematical model” becomes a lot more blurred than we are used to. Your “just a mathematical tool thrown in to help make the model work” thing ends up having actual physical implications, and being a better model of reality, giving us more accurate answers.

With Hawking Radiation (very, very roughly), we can get virtual particles being “created” (i.e. the maths saying it would be convenient to have them here), but one gets drawn into a black hole, while the other doesn’t. Energy cannot be created out of nothing(ish), so if one of the particles has some energy (i.e. mass), the other must have negative energy (to balance it out). Normally, for virtual particles, this wouldn’t be a problem as they are just mathematical tools to help understand things. But if one gets drawn into the black hole and the other doesn’t, the virtual particles end up separating and kind of becoming “real.” And the one drawn into the black hole ends up having to be the one with negative energy. If we add something of negative energy to the black hole, its total energy will decrease. Meaning that eventually the black hole can evaporate.

In theory.

Virtual particles are particles that appear to existence and then dissapear, they are fluctuations on the quantum field. Since you are talking about hawking radiation, it is probably about the pair of virtual particles that pop to existence near the black hole event horizon. His explanation is that what usually happens is that a virtual particle and antiparticle pair pop up, and then anhilate each other, however at the event horizon if this happens, one particle is sucked to the black hole, and the other goes away, effectively making the black hole evaporate over time.

That being said, this explanation for hawking radiation is wrong. This is S. Hawking way of ELI5 his paper about black holes, because the principle is similar enough.

Negative energy is just a negative value for some quantum field.

Virtual particles are a small perturbation in the quantum field. You would see a particle as a wave packet in this field.

It is possible to use a squid, which in this case is like a tiny mirror that oscillates back and forth at about 1/3 speed of light. When placing the squid in a vacuum it can transfer its kinetic energy to virtual particle and a detectable photon will be born.

Using it as an analog to Hawking radiation the black hole creates a condition that imparts some of its mass/energy into a photon that zooms away.

It is fundamentally a condition that within a quantum field there is no such thing as static zero energy state when there are no particles around. The perturbations in field are constant and they interact with matter and energy. In the case of a black hole it is the result of redshifted light and a slight amplification of that light. How does a wave get amplified? By another wave, of course. It is that amplifying wave that is able to extract some of the black hole mass and impart it into the photon that speeds away, thus Hawking radiation.

To understand it will be a good idea to backtrack in Science, and start wih Field Theory.

Back in the day, it was a big mystery, as to how light, could travel as a wave, through empty space. Because a wave needs a medium to travel through, you can’t have a wave in a vacuum. But empty space was much like a vacuum, there was no media to propagate the wave through.

Scientists used very creatively Schroedingers uncertaincy complex, to make a little fictive square in the universe. As a particles place and speed/direction can not be determined, if you determine one, the other becomes unknown. The smaller the fictive square was made, the more certain you knew where the particle was. And on the other hand, the speed and direction became more variable. That is to say, the potential energy of the particle becomes higher and higher, as you make your little square smaller.

Untill you square is small enough, that the potential energy is high enough, to conjure the particle from empty space out of nothing.

And voila, particles were popping up here there and everywhere in space, and that is how light as a wave could travel through space.

Problem was, that the particles weren’t there to be seen. So they got creative, they had some bits of a math, in a famous mathematicians equations, that noone knew what actually were or described. This math it was deduced, described the anti particle to a particle.

And so when you conjured a particle in empty space by pinpointing it’s place, an antiparticle would also be conjured. They exist just long enough for light waves to travel through, before they crash into each other and disappear again.

The math they had found, didn’t actually mathematically describe a complete opposite entity, to the particle, which it should. But then it was stated, that slight error, is why some particles dont collapse and disappear, and that’s why there are particles and matter in the universe in the first place. Because there is a slight discrepancy, between matter and anti matter.

Hohoho, I bet they ordered 10 shots in the bar after that one.

Is this utter idiocy? Yes most likely, recent findings would suggest it is. Observations by Physisicsts are never what they expect according to the theories they use. For example finding out there is exactly as much anti matter in the universe as matter, and so no slight discrepancy that could explain why matter is abundant and anti matter is not.

Hawkins takes this “thinking”, and puts it at the very edge of a black hole. And he says that in some rare cases, the 2 particles conjured will not crash into each other and disappear. Because one is right where it gets dragged into the black hole, while the other is just where it doesn’t.

The one that stays in known space, radiates energy. So to keep net energy at 0, the one that fell into the black hole must have had negative energy. Adding negative energy, means that the black hole is essentially getting dissolved. It will “evaporate” according to this theory.

He calls them virtual particles, because we apparantly can’t see this conjuring of particles, that makes light travel through empty space. They don’t exist, unless one of them falls into a black hole, so that they can’¨t collide.

It’s all made up nonsense. A good physicists has 2 conversation subjects that interest him; wormholes, and how utterly wrong physics is.