QR5.2.5 The Universal Speed Limit

Light goes at the fantastic speed of 670 million miles per hour, about the distance to the moon in a second. Can we achieve this speed? What about a leap-frog method, like a rocket going at half the speed of light that 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!

How about gradually accelerating a rocket to reach the speed of light? Nature again intervenes by increasing the rocket’s mass, until at near the speed of light it’s near infinite mass needs a near infinite force to move it. This seems to contradict the conservation of mass and the law of thermodynamics, that energy in a closed system can’t be lost. Einstein’s answer was 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.

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 one can’t produce the force needed to throw the ball. One might expect light in the rocket to move at almost twice the speed of light but Nature plays with space and time to keep every reference frame the same.

Figure 5.4. Light always travels at the speed of light!

Again, the implications are quite counter intuitive. For example, if the earth sent off two rockets at half the speed of light, one to the sun and one to Pluto (Figure 5.4), relativity says that light from the sun passes both rockets and the earth at the same speed! One might ask how can the same photon pass both rockets, one going to the sun and the other away from it, at the same speed? This makes no sense in classical or indeed any other terms.

The problem with relativity, as with quantum theory, is that the equations work but they don’t make any sense. How can the space that is the measure of movement itself move? How can time that is the measure of change itself change? Einstein deduced that space-time had to change for our world to be as it is, but he didn’t explain how it is so. Perhaps he expected us to unravel the mystery later but 100 years on, we are no wiser. The next section suggests why our world behaves the way Einstein describes.