String Theory: Branes are Required by M-Theory
Joe Polchinski’s work proved that D-branes weren’t just a hypothetical construct allowed by string theory, but they were essential to any version of M-theory. Strongly motivated by Edward Witten’s proposal of M-theory, he began working intently on D-branes. Furthermore, he proved that the D-branes and p-branes were describing the same objects.
In a flurry of activity that would characterize the second superstring revolution, Polchinski showed that the dualities needed for M-theory only worked consistently in cases where the theory also contained higher dimensional objects. An M-theory that contained only 1-dimensional strings would be an inconsistent M-theory.
Polchinski defined what types of D-branes string theory allows and some of their properties. Polchinski’s D-branes carried charge, which meant that they interacted with each other through something similar to the electromagnetic force.
A second property of D-branes is tension. The tension in the D-brane indicates how easily an interaction influences the D-brane, like ripples moving across a pool of water. A low tension means a slight disturbance results in large effects on the D-brane. A high tension means that it’s harder to influence (or change the shape of) the D-brane.
If a D-brane had a tension of zero, then a minor interaction would have a major result — like someone blowing on the surface of the ocean and parting it like the Red Sea in The Ten Commandments. An infinite tension would mean the exact opposite: No amount of work would cause changes to the D-brane.
If you picture a D-brane as the surface of a trampoline, you can more easily visualize the situation. When the weight of your body lands on a trampoline, the tension in the trampoline is weak enough that it gives a bit, but strong enough that it does eventually bounce back, hurling you into the air.
If the tension in the trampoline surface were significantly weaker or stronger, a trampoline would be no fun whatsoever; you’d either sink until you hit the ground, or you’d hit a flat, immovable trampoline that doesn’t sink (or bounce) at all.
Together, these two features of the D-branes — charge and tension — meant that they aren’t just mathematical constructs, but are tangible objects in their own right. If M-theory is true, D-branes have the capacity to interact with other objects and move from place to place.