There are many forms of energy. Moving objects such as cars and planes have kinetic energy. A boulder sitting on the edge of a cliff has gravitational potential energy because it has the potential to fall if the cliff below it crumbles or is somehow removed. According to the Law of Conservation of Energy, the total amount of energy in the universe never changes. Energy can only be converted from one form to another. However, Einstein discovered another type of energy which is sometimes referred to as "rest" energy. We already know a moving object will have kinetic energy. What Einstein pointed out was that the same object will have energy even when it is perfectly motionless. The amazing thing is that this "rest" energy is far greater than any other type of energy the object may possess. You may have seen this equation at one time or another (it is, after all, kind of famous):

E = mc²

• E is the amount of energy
• M is the mass of the object
• C is the speed of light

For the most part, "rest" energy stays in its present form because it is rather difficult to convert to other forms of energy. However, small amounts are converted in nuclear power plants, nuclear weapons, and the sun. In reality, when rest energy is released, some of the mass of the object is converted into some type of energy (usually heat and sound, in the case of the three examples). By the way, as an interesting side note, scientists have recently been able to convert energy into mass. All they had to do was manipulate the equation a bit, so m = E / c². In order to utilize this, incredibly powerful lasers were focused on one tiny point in space. The resulting amount of energy was enough to be converted into a single subatomic particle. At this point, you are probably thinking to yourself, "Big deal. One little particle of mass from all that energy?" Well, as you can see from the equation, a great amount of energy can be obtained from a very small amount of mass. However, it would take this same amount of energy (or even more, depending on the conditions) to produce that same very small amount of mass. So, converting tremendous amounts of energy into normal-sized objects does not seem to be feasible or even practical anytime in the near future.

Getting back to what we were discussing earlier, the sum of the kinetic and rest energy of an object can also be determined fairly easily with the following equation:

 E = mc² γ

Notice, that at everyday speeds, gamma is approximately 1, so the sum of the rest energy and kinetic energy is approximately the same as the rest energy alone. In other words, at everyday speeds the rest energy of an object is much greater than its kinetic energy. However, at speeds very close to the speed of light, gamma can become much larger than 1, and the kinetic energy of an object can become much larger than its rest energy. This equation also explains why an object with mass cannot physically reach the speed of light. As we have seen already, if an object reaches the speed of light the gamma factor will be infinity. Since gamma and energy are directly proportional in the equation, E = mc² γ, if the gamma factor is infinity the amount of energy is infinity also. This means that an object with any mass at all would require an infinite amount of energy to reach the speed of light. Particles that don't have mass, such as photons of light, must travel exactly at the speed of light, for reasons that are unnecessary (and too difficult) for us to explain (We believe it has something to do with zero mass and infinite gamma canceling out in the equation).

There are other more logical reasons why an object cannot exceed the speed of light. One of these involves "causality", the relation between cause and effect. Let's go back to Bob and Tom. Suppose Bob gets very furious with Tom for some unknown reason and decides to throw the baseball at him at a speed faster than the speed of light. In Tom's frame of reference, he may actually get knocked out with the ball before he even sees it coming to hit him.

For now we will rule out the possibility of superluminal (faster than light) velocities. However, this is not a reason to lose hope in reaching other distant places in the universe. If mankind is able to build vehicles that can travel at speeds close to that of light in the future, the effects of time dilation and length contraction will cause the distance and amount of time to reach these places to be much shorter than expected. However, even if a relatively short time passes for the ship, a much longer time will pass on the clocks back on Earth.

• Simultaneity and the Space-Time Continuum

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