Question

What happens to the mass of an object as it is accelerated? Why can’t objects travel faster than the speed of light?

Answer 1

It is commonly known that, if you accelerate an object, its mass will increase; however, to understand why this phenomenon occurs, we mustn’t think of the object’s mass increasing. Instead, we should think of its energy.

In physics, mass is just simply locked up energy. We call this type of mass, ‘inertial mass.’ Essentially, inertial mass is the amount of resistance that a physical object has to any change in its motion (this includes the resistance that a body has to acceleration or to directional changes). According to the theory of relativity, gravitational mass is always the same as inertial mass. However, when we speak of an objects mass increasing due to acceleration, we are really talking about its inertial mass increasing.

So when we think of mass as energy, we can begin to understand why an object will increase its ‘mass’ as it speeds up. As an object increases in speed, so does the amount of energy that it has, this energy is what we refer to as ‘the increase in mass’ (just remember, this is inertial mass).

This understanding of mass and energy makes it a little easier to understand one reason why we can never reach the speed of light. The energy of a particle diverges to infinity as it approaches the speed of light. Since we can never have infinite energy for a particle, the speed of light cannot be reached. Since an object has infinite kinetic energy when it approaches the speed of light, it therefore has infinite mass as well.

According to Einstein’s equation E = mc^2, mass and energy are the same physical entity and can be changed into each other. Because of this equivalence, the energy an object has due to its motion will increase its mass. In other words, the faster an object moves, the greater its mass. This only becomes noticeable when an object moves really quickly. If it moves at 10 percent the speed of light, for example, its mass will only be 0.5 percent more than normal. But if it moves at 90 percent the speed of light, its mass will double.

As an object approaches the speed of light, its mass rises precipitously. If an object tries to travel 186,000 miles per second, its mass becomes infinite, and so does the energy required to move it. For this reason, no normal object can travel as fast or faster than the speed of light.