Excerpted from: How Things Work.Virginia.edu
Louis A. Bloomfield fields questions about physics:
January 19, 1999
"If you were at the back of a bus going the speed of light, and you were to run toward the front, would you be moving faster than the speed of light or turn into energy?" -- TM, Ft. Bragg, NC
"First, your bus can't be going at the speed of light because massive objects are strictly forbidden from traveling at that speed. Even to being traveling near the speed of light would require a fantastic expenditure of energy.
But suppose that the bus were traveling at 99.999999% of the speed of light and you were to run toward its front at 0.000002% of the speed of light (about 13 mph or just under a 5 minute mile). Now what would happen?
First, the bus speed I quoted is in reference to some outside observer because the seated passengers on the bus can't determine its speed. After all, if the shades are pulled down on the bus and it's moving at a steady velocity, no one can tell that it's moving at all. So let's assume that the bus speed I gave is according to a stationary friend who is watching the bus zoom by from outside.
While you are running toward the front of the bus at 0.000002% of the speed of light, your speed is in reference to the other passengers in the bus, who see you moving forward. The big question is what does you stationary friend see? Actually, your friend sees you running toward the front of the bus, but determines that your personal speed is only barely over 99.999999%. The two speeds haven't added the way you'd expect. Even though you and the bus passengers determine that you are moving quickly toward the front of the bus, your stationary friend determines that you are moving just the tiniest bit faster than the bus. How can that be?
The answer lies in the details of special relativity, but here is a simple, albeit bizarre picture. Your stationary friend sees a deformed bus pass by. Ignoring some peculiar optical effects due to the fact that it takes time for light to travel from the bus to your friend's eyes so that your friend can see the bus, your friend sees a foreshortened bus--a bus that is smashed almost into a pancake as it travels by. While you are in that pancake, running toward the front of the bus, the front is so close to the rear that your speed within the bus is miniscule. Why the bus becomes so short is another issue of special relativity. "
Louis A. Bloomfield
Marilyn W. Lathrop, author of the Over the Edge science fiction series:
"Mr. Bloomfield's explanation applies to vessels powered by the usual means. But what if the vessel is powered by gravity?
"The concept behind the physics in Over the Edge is that the gravitational drive is able to use the increasing mass of the vessel to correspondingly increase the power it generates. Of course, you can't achieve 100% light speed, but I factor in achieving only 90%. If the vessel gains mass as it accelerates and that gain in mass, which essentially is an increase in gravity, can be transformed into more speed--what would happen then? In the Over the Edge series, "deep space" ships are powered by gravitational drives that transfer the force of gravity into power and power into speed.
"Will this work? Don't know, but it's certainly more plausible than Gene Roddenberry's warp drive or Scotty's transporters! Gravity is one of the last frontiers of normal physical forces we've yet to understand. Will we eventually understand gravity? Wait and see."
October 25, 2005
Marilyn W. Lathrop