# Thread: Object gaining relativistic mass as it accelerates

1. I'm somewhat confused about this aspect of Special Relativity. I'm trying to think of this in terms of what an observer would see if they were a particle in a particle accelerator, and then what the person operating the accelerator (stationary with respect to the accelerator, but not the particle) would see.

Case 1: Observer who is an accelerating particle.

Since, from your perspective, the particle accelerator is moving and you yourself are stationary, the particle accelerator appears to be time dilated. The amount of energy you experience its electromagnets to be generating must also appear to be less, then. As you approach C, the amount of energy would begin to seem negligibly small. That seems to explain why it takes greater and greater amounts of effort to accelerate particles as they approach the speed of light.

Case 2: Observer who is operating the accelerator

I guess you perceive the mass of the particle to be increasing, because it's harder and harder to increase its speed by using your electromagnets to accelerate it?

2.

3. From what I read in textbook (undergraduate physic text) there's 2 side of opinion: one side said that the mass increase and that explain why it take more energy to accelerate, but another side said that the energy requirement increase and it appears as increase in 'mass' in the equations. I don't know more than that but it seems that beyond that point there's still debate/opinion. Also in youtube and everywhere else (+ here) people tend to say the mass itself increase and therefore it explain why energy requirement increase.

4. Originally Posted by kojax
I'm somewhat confused about this aspect of Special Relativity. I'm trying to think of this in terms of what an observer would see if they were a particle in a particle accelerator, and then what the person operating the accelerator (stationary with respect to the accelerator, but not the particle) would see.

Case 1: Observer who is an accelerating particle.

Since, from your perspective, the particle accelerator is moving and you yourself are stationary, the particle accelerator appears to be time dilated. The amount of energy you experience its electromagnets to be generating must also appear to be less, then. As you approach C, the amount of energy would begin to seem negligibly small. That seems to explain why it takes greater and greater amounts of effort to accelerate particles as they approach the speed of light.

Case 2: Observer who is operating the accelerator

I guess you perceive the mass of the particle to be increasing, because it's harder and harder to increase its speed by using your electromagnets to accelerate it?
Yes.

5. Originally Posted by msafwan
From what I read in textbook (undergraduate physic text) there's 2 side of opinion: one side said that the mass increase and that explain why it take more energy to accelerate, but another side said that the energy requirement increase and it appears as increase in 'mass' in the equations. I don't know more than that but it seems that beyond that point there's still debate/opinion. Also in youtube and everywhere else (+ here) people tend to say the mass itself increase and therefore it explain why energy requirement increase.
I've been wondering if it's kind of like the role Lorentz contraction plays in analyzing the different perspectives observers might have about time in Special Relativity. If a third object is moving between two objects in the same frame, like say a rocket traveling between planet Earth and Proxima Centauri, then the way we rectify the fact the rocket doesn't perceive itself to be time dilated, is by noting that it does perceive the distance to be contracted. So an observer on Earth sees the rocket traveling for the normal expected length of time and sees that the people on board the rocket appear to be aging slowly. But the people on board the rocket simply think the trip took less time (because the distance was perceived to be shorter).

So, maybe mass increase plays a similar role in the energy exchange question in an acceleration? If a (very very long range) tractor beam on Proxima Centauri were pulling on a space ship to help it accelerate from Earth toward Proxima Centauri, the tractor beam operator would note that it takes more and more power to achieve less and less acceleration as the space ship approaches the speed of light. From the space ship's perspective, the tractor beam is getting time dilated, and therefore transfers less and less power. However from the tractor beam's perspective, the space ship appears to be getting more and more massive.

Is that analysis more or less correct? Only one of the two observers perceives the mass increase?

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