Question Regarding Relativity

[SCOETZEE]

Hi I am new to this forum and am currently in year 12 at high school. We are in the middle of learning about relativity in Physics class and i came across a question my teacher couldn't answer.

 

I understand that when an object starts to reach a velocity near that of light, it reaches a point where its kinetic energy cannot increase any further so instead the energy is converted into mass. And that to the person in the craft going light speed there wouldn't be any noticeable change in mass however to an outside observer there would be a mass change.

 

My question is, if the person in the craft going light speed can't see a change in mass, and they obviously can't go any faster, where does the energy go?

 

~ Seth

Edited January 30, 2015 by SCOETZEE

[xyzt]

Hi I am new to this forum and am currently in year 12 at high school. We are in the middle of learning about relativity in Physics class and i came across a question my teacher couldn't answer.

 

I understand that when an object starts to reach a velocity near that of light, it reaches a point where its kinetic energy cannot increase any further so instead the energy is converted into mass. And that to the person in the craft going light speed there wouldn't be any noticeable change in mass however to an outside observer there would be a mass change.

 

My question is, if the person in the craft going light speed can't see a change in mass, and they obviously can't go any faster, where does the energy go?

 

~ Seth

...into increasing the person's speed , i.e. into increasing the person total energy.

[Strange]

I understand that when an object starts to reach a velocity near that of light, it reaches a point where its kinetic energy cannot increase any further so instead the energy is converted into mass.

 

Just to be clear, the velocity doesn't need to be near the speed of light, this happens at all speeds. It is just that it doesn't become noticeable at "everyday" speeds.

 

Also, describing this as "increasing mass" is not very accurate (even though it is often used) and can lead to misunderstandings. It is really the total energy that has increased - but as we know, mass and energy are equivalent so we can talk about a "relativistic mass" made up of the rest mass (the real mass) plus the kinetic energy.

 

And that to the person in the craft going light speed there wouldn't be any noticeable change in mass however to an outside observer there would be a mass change.

 

From the point of view of the person in the craft, there is no change in (relative) speed either. They are still stationary wrt the craft. So there is no increase in kinetic energy.

 

My question is, if the person in the craft going light speed can't see a change in mass, and they obviously can't go any faster, where does the energy go?

 

Energy is not an absolute thing, it depends on the observer. The stationary observer sees the spacecraft increasing in kinetic energy; the observer on the craft doesn't.

[swansont]

 

I understand that when an object starts to reach a velocity near that of light, it reaches a point where its kinetic energy cannot increase any further so instead the energy is converted into mass.

 

It actually doesn't work that way. The energy to mass idea is one thing (it comes from a mis-application of E=mc²) but there's no point where the kinetic energy can't increase.

 

The kinetic energy of an object is [latex](\gamma-1)mc^2[/latex] where [latex]\gamma = 1/\sqrt(1-{v^2/c^2})[/latex] and m is the rest mass. This reduces to being the familiar 1/2 mv² when speeds are small, but as v approaches c, a small change in v corresponds to a large change in kinetic energy. There is no limit for KE, though — there's no point where it stops increasing. The limit is on how fast something can go — that limit is c.

[SCOETZEE]

Thanks for replying! It makes much more sense now.

We don't seem to learn Physics in much depth at school, so it is helpful to hear the 'why' and 'how' behind the facts we are told.

[Devin Of Byzantium]

 

From the point of view of the person in the craft, there is no change in (relative) speed either. They are still stationary wrt the craft. So there is no increase in kinetic energy.

 

 

A question from a rookie: If from the perspective of the vehicle occupant speed does not increase nor decrease, then how would one be aware of any changes in speed? Does this mean that speed can be infinitely increased and the occupant would be unable to track how fast they are going?

[Strange]

 

A question from a rookie: If from the perspective of the vehicle occupant speed does not increase nor decrease, then how would one be aware of any changes in speed? Does this mean that speed can be infinitely increased and the occupant would be unable to track how fast they are going?

 

You would be aware of changes in speed (acceleration) because you would feel the force of acceleration (like when you are pushed back in your car seat). But you would have no way of knowing your speed without something to measure it realive to.

[Devin Of Byzantium]

 

You would be aware of changes in speed (acceleration) because you would feel the force of acceleration (like when you are pushed back in your car seat). But you would have no way of knowing your speed without something to measure it realive to.

 

So, let's use space travel as a setting. At a certain speed you will be unable to know how fast you are going with out something to measure relativity with. Does this mean that you would have to compare your speed to how quickly stars are changing positions from distance to your craft? If not, what exactly is it that you'd measure your relativity to.

[Strange]

 

So, let's use space travel as a setting. At a certain speed you will be unable to know how fast you are going with out something to measure relativity with. Does this mean that you would have to compare your speed to how quickly stars are changing positions from distance to your craft? If not, what exactly is it that you'd measure your relativity to.

 

It doesn't really matter what you measure it relative to. Initially, you would probably measure it relative to Earth (or whatever your starting point was). And then you would measure it relative to Mars (or wherever you go).

 

The point is, speed is purely relative. You can always consider your spaceship stationary and other stars and planets whizzing past. Or you can choose one or a series of references points to measure you speed against.

[StringJunky]

 

So, let's use space travel as a setting. At a certain speed you will be unable to know how fast you are going with out something to measure relativity with. Does this mean that you would have to compare your speed to how quickly stars are changing positions from distance to your craft? If not, what exactly is it that you'd measure your relativity to.

It can be more fruitful sometimes to consider that there is no thing in the universe that is absolutely stationary that one can measure everything else against; it must always be relative to some arbitrary reference.

 

If you are sitting on a small rock in space with nothing else around you, how fast are you going? If you are moving in a straight line with a constant speed you will consider yourself stationary.

[Devin Of Byzantium]

It can be more fruitful sometimes to consider that there is no thing in the universe that is absolutely stationary that one can measure everything else against; it must always be relative to some arbitrary reference.

 

If you are sitting on a small rock in space with nothing else around you, how fast are you going? If you are moving in a straight line with a constant speed you will consider yourself stationary.

 

Very helpful, thank you.