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?

  • $\begingroup$ positronium (massive) decays into pure light (massless). $\endgroup$ Nov 12, 2012 at 18:27

2 Answers 2


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.

  • $\begingroup$ In the second sentence, it might be helpful to specify that the rest mass can be zero. $\endgroup$
    – user12029
    Nov 12, 2012 at 11:16
  • $\begingroup$ I would disagree. All known particles with zero mass (i.e. photons) travel at the speed of light and so there's no frame in which they are at rest. $\endgroup$
    – David H
    Nov 12, 2012 at 11:22
  • 1
    $\begingroup$ Can you please elucidate for me what you mean by " So in this sense, "the answer to your question is "no, you accelerate an object to light-speed by expelling mass."". How can an elementary particle expel mass? The relativistic mass increases with velocity so I am really puzzled by this statement. $\endgroup$
    – anna v
    Nov 12, 2012 at 15:00
  • $\begingroup$ You can't That was indeed a typo, which made the sentence say the exact opposite of what it was meant to say. Sorry about that, and thanks for helping me catch that. $\endgroup$
    – David H
    Nov 12, 2012 at 15:07
  • $\begingroup$ to err is human. the answer is ok now. +1 $\endgroup$
    – anna v
    Nov 12, 2012 at 16:00

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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.