Flatland, emc2, Einstein, and Dimensions

The recent review of Flatland, by Edwin A. Abbott, suggests some questions. All us folks are 3rd dimension beings, but we can experience the 4th dimension of "Time". Abbott's thoughts for mid 19th century are really amazing. 

We exist in the past, present and future. Perhaps we can find a way to take the 3 dimensions and move into the 4th, but what about the 5th and are there more? 

Einstein's theory stated that an entity’s velocity, or its momentum, is only measurable in relation to something else, and secondly that the speed of light is a constant in a vacuum, regardless of the person measuring it and the speed at which the person travels. The third part of the equation is that nothing goes faster than light in contrast to Newton's gravitational laws. To make it work, Einstein needed the fourth dimension called space-time. He expressed his theory using the famous mathematical equation E= MC2

Today, scientists use 10 dimensions and string theory to explain where gravity and light from the electromagnetic spectrum meet.