# The Naked Scientists Forum

### Author Topic: Are relativity equations compliant with the energy/mass conservation principle?  (Read 1571 times)

#### zordim

• Jr. Member
• Posts: 46
##### Are relativity equations compliant with the energy/mass conservation principle?
« on: 27/11/2012 17:52:14 »
?

#### yor_on

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##### Re: Are relativity equations compliant with the energy/mass conservation principle?
« Reply #1 on: 29/11/2012 16:24:04 »

#### a_dark_knight

• First timers
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##### Re: Are relativity equations compliant with the energy/mass conservation principle?
« Reply #2 on: 05/12/2012 06:41:12 »
I'm not an expert but yes, I'm pretty sure it all works out perfectly. For example, let's say a spaceship is moving "near" the speed of light. Then you could measure its kinetic energy, using its apparant mass (or inertia) instead of its rest mass. This will tell you exactly how much work/energy has been put into moving the spaceship. If it weren't for relativity, the spaceship would be moving faster instead of "weighing" extra, but either way, the energy's the same. That's also exactly how much work/energy it would take to slow it down. So basically, none of the work is wasted. Please correct me if I'm wrong about this!

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Are relativity equations compliant with the energy/mass conservation principle?
« Reply #3 on: 05/12/2012 06:51:45 »
I'm not an expert but yes, I'm pretty sure it all works out perfectly. For example, let's say a spaceship is moving "near" the speed of light. Then you could measure its kinetic energy, using its apparant mass (or inertia) instead of its rest mass. This will tell you exactly how much work/energy has been put into moving the spaceship. If it weren't for relativity, the spaceship would be moving faster instead of "weighing" extra, but either way, the energy's the same. That's also exactly how much work/energy it would take to slow it down. So basically, none of the work is wasted. Please correct me if I'm wrong about this!
Yes. Not only are alll the equations SR consistent with the conservation of energy and mass but they are so because we use conservation of mass-energy in deriving their relativistic form.

#### zordim

• Jr. Member
• Posts: 46
##### Re: Are relativity equations compliant with the energy/mass conservation principle?
« Reply #4 on: 11/12/2012 10:00:03 »
Dear Pmb,

Gravitational time dilation equation is:

We have a body with a mass . We observe some point which is at a distance    from that body. The time equation at that point is

If another body with a mass comes (from very far away) at some distance from the observed point, the time equation at that point becomes

So, bodies will produce the following time behavior at the observed point:

is the distance of the i-th body from the observed point

Hence, the superimposed time-behavior at some point which is equally distant from two or more objects will not be the same as the time-behavior produced at the same distance from one object with the mass equal to the mass-sum of those two or more objects:

How does this fit with the continuity principle, that is, with the mass (energy) conservation principle?
« Last Edit: 11/12/2012 10:03:15 by zordim »

#### The Naked Scientists Forum

##### Re: Are relativity equations compliant with the energy/mass conservation principle?
« Reply #4 on: 11/12/2012 10:00:03 »