# Increased mass at light-speed -- then you stop and go back to Earth? Problem?

So you hop in the ol’ spaceship, and throw in a couple of special fuel cells which catapults you to let’s say almost the speed of light–then according to special relativity, as you try to reach the speed of light your mass increases as you pour more energy (vainly) into accelleration .

Giving up, you turn around, and slowly deaccelerate back to regular speed, and reenter earth orbit. You land and say hi to your friends (who may have aged rapidly during your travel). But, what about YOU? Does your mass now make you weigh on earth 60000 tons or so?

Or did the mass increase from your ill-fated attempt to exceed the speed of light no longer a problem? Are you now some kind of freak/superhero, that is, beyond Jenny Craig’s help when it comes to the weight department?

Your mass decreases when you slow down, fatty.

Proof that running and Middle School Gym classes are causing obesity!

Also, your mass only increases with respect to the (relatively) stationary Earth. If you weighed yourself on the ships’s scale, you would weight the same no matter if you were moving or stationary.

Where does it go?

Regards,
Shodan

It’s converted into the energy you need to decelerate back to zero from light speed.

That makes sense - thanks!

Regards,
Shodan

Because of misunderstandings such as this physicists have discarded the entire concept of relativistic mass.

Rest mass, real mass, or just mass is equal to the energy of a system that cannot be transformed away. RM can always be transformed away.

If RM were real mass all kinds of weird things would be true:

A single particle traveling close to c could collapse all stellar objects into black holes.
An object would have different masses in different directions.
Mass would wind up being some kind of a matrix function.
The gravity of a starship approaching c would crush all its occupants.

But your dimension in the direction of travel would shrink, so you would be really skinny.

Wouldn’t it actually suck all the occupants against the walls?

My phyics is pretty rusty but I’m wondering about this. If you decelerate (which is really just an acceleration in the direction opposite that of velocity), the decrease in mass is unrelated to any energy used to change velocity. After all, you had to expend energy to reach your speed to begin with but your mass increased. Just because the force is now being applied in the opposite direction, it doesn’t cause your mass to decrease. It decreases because your velocity decreases. It is unrelated to energy expenditure.

Or am I misunderstanding something?

It depends. Transverse RM is significantly less than longitudinal RM mass.

Ring, the abbreviation “RM” for “relativistic mass” might be a bit confusing, given that you’re contrasting it against “rest mass”, or “real mass”.

Not only that but I just realized I wrote relativistic mass mass. (RM mass) Is there an accepted abbreviation?

Also, how about giving your take on the subject. Other than physicists who understand what they’re saying, do any (many) scientists still use the term?

I had nothing to add to your post beyond pointing out the confusing abbreviation. There’s no standard abbreviation for relativistic mass, because nobody talks about it any more.

Very short, very sweet, and very to the point.

Actually…where does the increase in mass come from?

Also, why do people always use the impossible example of firing a gun on a spaceship moving at the speed of light instead of the infinitely more practical example of turning on your headlights in a moving automobile?