Is there any law that prevents an object with mass to become massless?

Question

I got into a discussion with my physics teacher about the speed of light and I asked

What if an object with mass was to lose mass as it gained speed-- would that allow for an object to eventually reach the speed of light or surpass it if possible through that thinking?

My teacher didn't really have an answer to that and I was wondering if there was any law of science that prevented an object with mass to become mass-less. Because I know this may be simple minded in thinking but if an object with mass must have infinite energy to reach the speed of light and at that point will have infinite mass, what if an object was to reduce its mass until it had none? Would it then not need infinite energy and be able to move at or past the speed of light in theory?

Answer 1

In order to answer this question, you should first ask yourself what you mean by "object". From an elementary particle perspective, every particle has a characteristic constant rest mass. These masses aren't thought to change, just like the charge of an electron doesn't ever change. So in this sense, the answer to your question is "no, you cannot accelerate an object to light-speed by expelling mass." It's not so much based on any particular law of physics, but rather on a body of empirical facts about elementary particles.

There is a process in particle physics called pair-production/annihilation where, for example, a photon decays into an electron positron pair, or vice versa. Do not mistake the process of two massive particles (like the electron and positron) annihilating each other to produce a pair of a photons (massless, and traveling at the speed of light) for being representative of the idea you had in mind, because you don't have the same set of objects at the end of the process that you did at the beginning.

If by object you mean some macroscopic aggregate of matter, the answer is pretty much the same. Yes, an object can radiate away all of its mass, but after that you no longer have an object.

Answer 2

The question is how your object could lose mass. Energy has to be conserved so you can have the mass just disappear. The best you can do is have the mass turn into massless particles like photons. This isn't as hard as it sounds and I can think of two ways to do this:

1. Suppose your object is made from positronium. At some point the object is going to decay into photons, and photons travel at the speed of light.

2. If you have some system of particles and your raise the energy to above the electroweak transition energy the particles will all become massless and they will then travel at the speed of light.

You ask if the particles would be able to move past the speed of light, but this isn't possible even for a massless particle. A massive particle always moves at less than the speed of light, and a massless particle always moves at exactly the speed of light.