# Energy to mass

• I
member 342489

## Main Question or Discussion Point

As I understand E=m*c^2 you can create mass from energy

In my imagination that means, that if you create a high enough energy density, then you will create mass equvilant to that energy. (with some loss i imagine)

That understanding have made me think, could we, in theory, create enough energy to create a universe?

Related Special and General Relativity News on Phys.org
LURCH
Well, we could not technically do that because of the first law of thermodynamics, energy cannot be created (or destroyed). Whatever energy we use to “create “ matter, we must borrow from the universe.

But, if we borrow the total amount of energy in the universe, it would indeed be exactly enough to form the universe.

member 342489
Well, we could not technically do that because of the first law of thermodynamics, energy cannot be created (or destroyed). Whatever energy we use to “create “ matter, we must borrow from the universe.

But, if we borrow the total amount of energy in the universe, it would indeed be exactly enough to form the universe.

It does.I did realize, that it would be problematic to create our own universe from energy within this universe. I was only curiousity. Thnak you :-)

Orodruin
Staff Emeritus
Homework Helper
Gold Member
There are several threads in this forum already discussing the misconception of $E = mc^2$ that you somehow "create" mass from energy or vice versa.

Energy and mass are properties of a system and for an isolated system they are both conserved. The mass of the system is a particular combination of its energy and momentum and it would be appropriate to identify the mass with the energy in the rest frame of the system (up to the conversion factor $c^2$). Neither mass or energy is any sort of substance. They are properties of a system and rather than saying that mass is "converted" into energy or vice versa, it would be more appropriate to consider mass a form of energy.

Ibix
I'm not sure that your question is really answerable. Mass and energy are different things; simply having energy doesn't give you mass. There are a number of ways to explain that - possibly the easiest is to note that light has energy but no mass. $E=mc^2$ is a special case of a more general equation $E^2=m^2c^4+p^2c^2$, where $p$ is momentum.

More fundamentally, we don't understand where this universe came from (or even if that question makes sense). So we have absolutely no idea how to go about creating another one.

member 342489
There are several threads in this forum already discussing the misconception of $E = mc^2$ that you somehow "create" mass from energy or vice versa.

Energy and mass are properties of a system and for an isolated system they are both conserved. The mass of the system is a particular combination of its energy and momentum and it would be appropriate to identify the mass with the energy in the rest frame of the system (up to the conversion factor $c^2$). Neither mass or energy is any sort of substance. They are properties of a system and rather than saying that mass is "converted" into energy or vice versa, it would be more appropriate to consider mass a form of energy.
I am sorry for creating an extra post with an already debated question, I should have performed a search first.

I do not doubt your answer, but cannot claim to fully understand either.

I have seen several articles claimeing, experiments transforming photons into electrons and positrons were underway. So that made me think about the creation of the universe, and how the particles formed to begin with.

Thank you for your time :-)

member 342489

Ibix
Despite a lot of sloppy pop sci, describing light as energy is wrong. As Orodruin says, energy is a property of things, not a thing. It's like velocity - your car may have a certain velocity, but it is not velocity. Nor is anything "pure velocity" outside of Flash fan fiction.

Yes, we can turn things with a low mass and a lot of energy into things with high mass. That kind of thing is bread and butter for nuclear and particle physicists. As I say, we've no idea how the universe came to be, so whether or not this is relevant to that isn't known.

Mister T
Gold Member
As I understand E=m*c^2 you can create mass from energy
No. In a reaction a system changes. Part (or even all in some cases) of the system's mass can become rest energy. All that's changed is that what you used to call mass afterwards you now call rest energy. Mass and rest energy are equivalent, so which name you decide to use is based entirely on tradition; these names were created before this equivalence was known.

In my imagination that means, that if you create a high enough energy density, then you will create mass equvilant to that energy.
What makes you think density has anything to do with it? That energy is a property of something, what is the something?

I have seen several articles claimeing, experiments transforming photons into electrons and positrons were underway.
There was a Nobel prize awarded for that in 1948. It's called pair production. The photon-nucleus system that existed before the reaction is transformed into a system consisting of the nucleus and a matter-antimatter pair. The total amount of mass before the reaction equals the total amount of mass after the reaction. The total amount of rest energy before the reaction equals the total amount of rest energy after the reaction. Before the reaction the photon's energy was one of the things that made a contribution to the system's mass, afterwards that same contribution is now made by the nucleus and the pair. The same can be said of the rest energy.