Light goes at the fantastic speed of 670 million miles per hour, which is all the way to the moon and back in less than a second. Can we achieve this speed? What if a rocket going at half the speed of light shot a bullet forward at half the speed of light? Unfortunately, doing this changes time and space, so the bullet only goes at four-fifths the speed of light!
What if we gradually accelerate a rocket up to the speed of light? Nature again intervenes by increasing the rocket mass until at near the speed of light its near infinite mass needs a near infinite force to move it, so this doesn’t work either. In theory, in a rocket going 5mph slower than the speed of light, one could throw a ball at 5mph per hour to reach the speed of light but in practice, we can’t produce the force needed to throw the ball. And if the rocket had headlights, one might expect light to leave it at almost twice the speed of light, but again Nature plays with space and time to keep the speed of light the same.
This denies the conservation of mass and the thermodynamic law that energy in a closed system can’t be lost, but Einstein noted that energy and mass convert by E=mc2, so nothing is really lost. He didn’t say whether mass was a form of energy, energy a form of mass, or both were aspects of something else.
Relativity contradicts all our intuitions about movement. For example, if two rockets left the earth at half the speed of light, one to the sun and one to Pluto (Figure 5.6), relativity requires light from the sun to pass both rockets at the same speed! How can the same photon pass both rockets, one going to the sun and one away from it, at the same speed? This makes no sense in classical terms.
In relativity, as in quantum theory, the equations work but they don’t make any sense. How can space that is the measure of movement, itself move? How can time that is the measure of change, itself change? It cannot be, yet the evidence is that it is so.
Einstein deduced that space and time had to change for our world to be as it is, but didn’t say why. Perhaps he expected physics to explain later but a century on, we are no wiser. To unravel this mystery, of how matter changes space and time, it must be a product painted not a substance.