The resulting electroweak theory fueled many speculations about a Grand Unified Theory that would also incorporate the strong and gravitational forces. Einstein attempted to construct a theory that would encompass both gravity and electromagnetism with no success. But mathematical and theoretical advances at the end of the 20th century revived Einstein's dream. Superstring theory may hold the key to constructing a theory of quantum gravity, the essential step in linking gravity to the other fundamental forces. Future experiments will provide guidance as physicists compare theoretical predictions with real-world data.
Nobody knows the actual size and shape of elementary particles such as quarks.
Experiments have yielded upper limits on their size, but nobody knows how tiny they really are. Particle physicists usually think of quarks and other fundamental particles as point-like objects that have no volume at all. String theories, however, envision particles as little loops that can vibrate like the strings of a violin. Though we perceive our world as consisting of one time dimension and three spatial dimensions, each point in the universe could have tiny extra dimensions attached to it. Only certain types of particles and interactions would feel their presence.