Virtual Fluctuations
The Heisenberg uncertainty principle states that there are certain pairs of variables whose values cannot be known simultaneously. As mentioned earlier, an example of such variables is the pair momentum and position. However, this pair is not the only one which obeys the uncertainty principle. Another such a pair is energy and time:
𝚫𝐄 ⋅ 𝚫𝐭 ≥ ħ/𝟐
Let us say, for instance, that we have a measuring device around which we send a photon. We want to measure the energy of the photon and the time in which the photon has passed the measuring device. However, every particle obeys the uncertainty principle for time and energy, so the more precisely we measure the energy of the photon, the greater uncertainty there is about the time it passed the measuring device.
But what happens if we apply this uncertainty to vacuum? Vacuum is by classical physics defined as empty space (space where there are no particles), therefore, its energy should be zero. However, the uncertainty principle for time and energy states that there is always at least a tiny amount of uncertainty regarding the energy of every system, which means that one can never be sure that the energy of vacuum is truly zero. This means that even vacuum itself can obtain non-zero amount of energy for short periods of time. These deviations in the energy of vacuum are called vacuum fluctuations. The question is: What is this temporary energy caused by vacuum fluctuations used for?
It turns out that it is used to create a peculiar new type of particles – virtual particles. These virtual particles of vacuum fluctuations are created spontaneously everywhere in the universe and usually exist for very short periods of time. Each virtual particle may never be created by itself – it is always created in pair together with its antiparticle. As one might expect, they annihilate after a short period of time.
The equation above shows that the greater the uncertainty in energy, the smaller the uncertainty in time. This means that the more energy a given virtual pair “borrowed”, the sooner the particles of the pair have to annihilate. When a virtual pair annihilates, no energy is created, the law of conservation of energy (energy cannot be created out of nothing) is thus not violated. Virtual particles and antiparticles simply “borrow” energy which they soon return.
Virtual particles might not always have the same properties as their classical counterparts. A virtual electron, for instance, might not have the same mass as a classical electron. Moreover, virtual particles cannot be observed directly. We can, however, observe their impact on the environment around them. Under certain conditions, they can even be transmuted into classical particles, as we shall see in the following chapters.
Very nice