Because strings are so tiny, we cannot physically measure how stiff they are, but we can use an indirect technique to determine their tension. Scherk and Schwarz used this kind of method to predict the tension of the graviton string. They found that, given the string's pattern of vibration, its tension is inversely proportional to the strength of the force it carries. Since gravity is a relatively weak force (i.e., it takes a great deal of matter to create much gravity) they calculated that the tension is very high, about a thousand billion billion billion billion (10) tons, called the Planck tension.

Keep in mind that nothing is pulling the string and holding it down to give it its tension, as in the case of a piano string. The tension of strings is what makes them so small. The Planck tension gives them an average length of 10 cm, the Planck length. The extreme tension leads to the strings usually having very high amounts of energy since it takes more energy to pluck something that is tight. This leads to the conclusion that a string's energy depends on two factors: the manner in which it vibrates and the tension it is under. According to quantum mechanics, a string's energy comes in units. The size of these units is proportional to the tension of the string and its vibrational pattern. The whole number multiple of the energy units a string carries depends on the string's amplitude. Because the tension on strings is so high, so is the size of the energy units, which are multiples of the Planck energy. If you convert the Planck energy into mass using E=mc², you get the Planck mass which is ten billion billion (10) times the mass of a proton. This is about the mass of a speck of dust, huge by microscopic standards.

So how, you might ask, can multiple strings make up a proton if each has a mass of ten billion billion times that of a proton? The answer has to do with quantum jitter. According to the uncertainty principle in quantum mechanics, nothing is completely at rest. Quantum jitter actually has negative energy that cancels out much of a string's mass. In the case of the graviton, the cancellation is perfect, yielding a particle with zero mass. This is what was predicted since gravitons travel at the speed of light.

There are an infinite number of possible string vibrational patterns. Thus you may conjecture that there are an infinite number of elementary particles. This is true, but our current particle accelerators can only reach energies at the level of a thousand times the proton mass so we are unable to detect the heavier predicted particles if they exist. We may be able to prove their existence another way though. There may be heavy particles left over from the big bang that we can identify, but this is doubtful because very heavy particles are inclined to be unstable and transform into many lighter particles.

You may be wondering if strings are truly fundamental or if they're just another constituent in a seemingly endless line of smaller and smaller elements. They may not be, but as far as many physicists are concerned, strings are the end of the line. There is much evidence to support this.