Warped Passages - Lisa Randall [114]
When all the contributions from all possible paths are added together, the calculation shows that the vacuum dilutes the message that the photon carries from the electron. The intuitive explanation for the dilution of the electromagnetic interaction is that opposite charges attract and like charges repel, and therefore, on average, virtual positrons are closer to an electron than are virtual electrons. The charges from the virtual particles therefore weaken the full impact of the initial electron’s electric force. Quantum mechanical effects screen the electric charge. Electric charge screening means that the strength of the interaction between a photon and an electron decreases with distance.
The true electric force at long distances appears to be smaller than the classical short-distance electric force because a photon that communicates a force over short distances more frequently takes a path that doesn’t involve virtual particles. A photon that travels a short distance wouldn’t have to travel through a big, weakening cloud of virtual particles, as the photon that was communicating a force far away would have to do.
Not just the photon, but all force-carrying gauge bosons interact with virtual particles en route to their destination. Virtual particle pairs, the particle and its antiparticle, spontaneously erupt from and get absorbed by the vacuum, affecting the net strength of an interaction. These virtual particles temporarily waylay the gauge boson transmitting the force and change its overall interaction strength. Calculations show that the weak force’s strength, like electromagnetism’s, decreases with distance.
However, virtual particles don’t always put the brakes on interactions. Surprisingly, they can sometimes help them along. In the early 1970s, David Politzer, who was then a Harvard graduate student of Sidney Coleman (who suggested the problem), and separately David Gross and his then student, Frank Wilczek, who were both at Princeton, as well as Gerard ’t Hooft in Holland, did calculations that demonstrated that the strong force behaves in precisely the opposite way from the electric force. Rather than screening the strong force at long distances and thereby making it weaker, virtual particles actually enhance the interactions of the gluons (the particles that communicate the strong force)—so much so that the strong force at long distances deserves its name. Gross, Politzer, and Wilczek won the 2004 Nobel Prize for Physics for their critical insight into the strong force.
The key to this phenomenon is the gluons themselves. One big difference between gluons and photons is that gluons interact with one another. A gluon can enter an interaction region and turn into a pair of virtual gluons which then influence the force’s strength. These virtual gluons, like all virtual particles, exist only momentarily. But their effects pile up as you increase the distance, until the strong force is indeed extraordinarily strong. And the result of a calculation is that virtual gluons dramatically enhance the strong force’s strength at larger particle separations. The strong force is much stronger when particles are well separated than when they are close together.
Compared with electric charge screening, the increase of the strong force with distance is a very counterintuitive result. How can it be that an interaction gets stronger when particles are further apart? Most interactions subside over distance. We would really need a calculation to show this, but there are examples in the world of such behavior.
For example, if someone sends a message through a bureaucracy whose importance some middle manager doesn’t understand, the middle manager might blow up what should have been an ordinary memo into a critically important directive. Once the middle manager modified the message, it would have a far greater impact than if the original author had communicated it directly.
The Trojan War is another example in which long-distance forces were more powerful than those at short distances. According to the