# If mass can be converted to energy than how is it said that energy can't be created?

From the mass-energy equivalence E=m(c*c), it can be seen that energy can be created and it is not converted from one form of energy.

Or am I wrong ? Can you explain?

• Mass is a form of energy. Jan 29, 2019 at 14:44
• Possible duplicate of physics.stackexchange.com/questions/339282/… Jan 29, 2019 at 14:49
• Welcome to physics SE. The assumption that energy can be created is not seen (at least by me) from the formula. I would argue, that it is rather seen that e.g. total energy of the moving system is proportional to its relativistic mass. $E^2=m^2 c^4=p^2c^2+m_0^2 c^4$. That you have a tool to compare the kinetic energy and total energy of a system in rest. Yes, you can use the formula in your scenarios of creating and destructing particles while maintaining the total energy (in a form of kinetic E or mass), but it needs a bit more assumptions about the world and the laws of conservation. Jan 29, 2019 at 15:31
• That equation in actual mean that mass and energy are equivalent
– user212727
Jan 29, 2019 at 16:31
• stanford.library.sydney.edu.au/archives/spr2004/entries/equivME Jan 31, 2019 at 8:58

I think the best way to understand this equation $$E=mc^{2}$$ is just reinterpret the mass a another kind of energy. If you do this, you are still able to say that energy can't be created. Actually in general relativity to talk about a conserved quantity one normally form a tensor first and then identify a conserved quantity.

In the case of the energy, the tensor is the stress–energy tensor which has a close relationship with the mass of a body or field.

No, because mass can't be created in a way that it violates the law of energy-conservation. Take as an example the color confinement. It says, that you can't observe one single quark, because the energy necessary to do so would be large enough that another hadron is created.

• Congratulation to the first answer. Just try to pay more attention. What is the answer "no"? No, I cannot explain? And, please, the question shows that the author is a bit puzzled by a basic concept. Bringing in the subtle theme of quark observation doesnt help much, I think. Jan 29, 2019 at 15:21

When it is said that "Energy cannot be created" what is actually meant is, "Energy cannot be created from nothing.". The reason that energy cannot be created is that in your example the energy already existed locked in the mass. You started with something.

In your example E=mc^2 the energy in principle can be converted back into mass. This can be done as many times as you please. Now, if you did this conversion back and forth ten times, would you say that you had created ten times the energy of the original mass?

From the mass-energy equivalence E=m(c*c), it can be seen that energy can be created and it is not converted from one form of energy.

Have you checked what this m is in the famous equation? m is the relativistic mass, and is not an invariant.

It depends on velocity, and kinetic energy depends on velocity too,it is what happens to the inertial mass, the mass in F=ma, at relativistic velocities but a useful formula only for space ships going at relativistic velocity.

In particle physics the the concept of relativistic mass is not used because of the confusion in terms it introduces. One works with four vectors of energy and momentum, where energy and momentum are conserved quantities and $$m_0$$ is the invariant mass of the particle or system of particles described by the four vector, which does not change.

The length of this 4-vector is the rest energy of the particle. The invariance is associated with the fact that the rest mass is the same in any inertial frame of reference

Mass and energy are equivalent terms. From the $$E=mc^2$$ equation,the change in mass corresponds to change in the mass. For better understanding, consider a collision between two bodies A and B of equal mass $$m_0$$ and moving with opposite velocities to each other in a inertial frame of reference. Let us consider a collision to be inelastic (kinetic energy is not conserved), the two bodies form a third body C which is at rest (by law of conservation of momentum). Now the resulting body doesn't have mass $$2m_0$$ as one may expect. But the resulting mass is the sum of $$2m_0$$ and the conversion of kinetic energy of two bodies into mass. In this way, both mass and energy are equivalent.

For further understanding, refer to Special Relativity by Robert Resnick.

• I really don't think this is a convincing example: there're so many other ways in which the energy could be dissipated.
– user191954
Jan 29, 2019 at 16:27
• "the change in mass corresponds to change in the mass" Is that a typo? Jan 29, 2019 at 16:34
• From the equation,it is clear that dE=(dM)(c^2) Jan 30, 2019 at 2:47