# What Happens to Mass as it Approaches the Speed of Light?

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1. Feb 1, 2014

### teetar

Pretty straight-forward, I'm just curious. My current level and understanding of physics has lead me to believe that as an object accelerates it's mass becomes higher and if it were to be able to reach the speed of light it would get infinite mass. I believe this currently, however, two of my friends have denied this statement saying that, rather, an object's mass will become zero when it reaches the speed of light. I'm just wondering if I'm correct or if they are. If they're correct, is it just a simple misunderstanding, or mistake on my part, or is it that I don't understand physics to the correct level to attempt to understand this yet?

Thanks for anyone who can respond to this, and please have a nice day!

Last edited: Feb 1, 2014
2. Feb 1, 2014

### Staff: Mentor

It is a very common misconception that an object's mass changes at its speed increases. The term "mass" refers to "rest mass", which is the mass the object has in its own inertial frame. This never changes. That, along with the fact that an object with mass cannot reach the speed of light, means that your question is a nonsensical one. Any object with mass cannot reach the speed of light so we cannot say what will happen "if" it did.

3. Feb 1, 2014

### ZapperZ

Staff Emeritus
In addition to what Drakkith has stated:

1. The "mass" that you know is what we term as the rest or invariant mass. This mass doesn't change. So both you and your friends are mistaken. Do I get to win the bet, if any?

2. If you are invoking this thing called "relativistic mass" then read this first

Zz.

4. Feb 1, 2014

### teetar

Thanks guys! I'm sorry I posted this here, I should've looked into it more to find an answer, I obviously need to learn more :P

5. Feb 1, 2014

### Staff: Mentor

Here's an answer from another perspective. Suppose that instead of you being "stationary" and the object zooming past you at a speed approaching warp 1, it is the object that is "stationary," and it is you who are zooming past the object at a speed approaching warp 1 (in the opposite direction). I think you will agree that your motion as a passive observer cannot affect the mass of the object in any way (after all, you are not even coming close to contacting it). Yet from your perspective, it is the object that is zooming past you. So, either way, its mass can't change.

6. Feb 1, 2014

### vanceEE

Please explain. Light as a wave carries energy, and energy is equivalent to mass, right?

7. Feb 1, 2014

### ZapperZ

Staff Emeritus
No it is not. Read the Relativity FAQ!

That question is also off topic to what the OP is asking. Either ask this elsewhere, or better yet, read the numerous other threads and our FAQ that have addressed this.

Zz.

8. Feb 1, 2014

9. Feb 1, 2014

### ZapperZ

Staff Emeritus

Zz.

10. Feb 1, 2014

### Staff: Mentor

Nonsense, it's a perfectly valid question to ask. Why else would this forum exist if not to help people?