# String Theory and Massless Particles

One side effect of creating a consistent string theory is that it had to contain certain objects that can never be brought to rest. Because mass is a measure of an object while it’s at rest, these sorts of particles are called *massless particles.*

This would be a major problem for string theory if the massless particles predicted didn’t really exist. By 1974, bosonic string theory was quickly becoming a mathematical mess.

Overall, though, this wasn’t a terribly disturbing problem because scientists know for certain that at least one particle exists only in a state of motion: the photon. (The gluon, though not known for certain at the time, is also a massless particle.)

Under the Standard Model of particle physics at the time, it was believed that a particle called the *neutrino* might have a mass of zero. (Today we know that the neutrino’s mass is slightly higher than zero.)

There was also one other possible massless particle: the graviton. The *graviton* is the theoretical gauge boson that could be responsible for the force of gravity under quantum field theory.

The existence of massless particles in string theory was unfortunate, but it was a surmountable problem. String theorists needed to uncover the properties of massless particles and prove that their properties were consistent with the known universe.