# Is Feynman right? Does mass change with speed? [duplicate]

In chapter one of Feynman's famous Lectures on Physics, he writes the following.

Mass is found to increase with velocity, but appreciable increases require velocities near that of light. A true law is: if an object moves with a speed of less than one hundred miles a second the mass is constant to within one part in a million.

Because speeds are relative to an observer, if I as an observer move quickly away from an object, is there some sense in which I change its mass?

This seems like it can't be right. Is the above even a well-posed statement? Namely, can an object even have an objective speed? If so, does a change in speed really change something's mass? If so, to which observers?

• The mass increases to the observers who see the speed increasing, of course. To other observers, who see the speed decreasing, the mass decreases. Jun 21, 2022 at 18:57
• maybe the links in physics.stackexchange.com/q/525047 this answer of mine might help , also the answers to that question Jun 21, 2022 at 19:01
• Does this answer your question? Why is there a controversy on whether mass increases with speed? Jun 21, 2022 at 19:18

Because speeds are relative to an observer, if I as an observer move quickly away from an object, is there some sense in which I change its mass?

This is one reason that modern physicists have largely abandoned the concept of relativistic mass. When modern physicists say the word “mass” they are usually referring to the invariant mass. The invariant mass is an intrinsic property of the object itself, while the relativistic mass is a relationship between an object and a reference frame.

• Comments are not for extended discussion; this conversation has been moved to chat.
– Buzz
Jun 22, 2022 at 1:59

I as an observer move quickly away from an object, is there some sense in which I change its mass?

No. You do not change the object in any way by moving away from it. But, if you observe somebody performing an experiment on the object as you whizz past, the mathematically simplest way to explain your observations will be to apply the Lorentz transformation to the length, mass, and rate of ticking clocks for everything in the experiment.

I personally do not understand what this fact tells us about the reality in which we live.