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.”
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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.
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