[knarf]

There goes the first Law of Thermodynamics.

[Bishop_Malachi]

There goes the first Law of Thermodynamics.

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Yeah...I thought matter to energy was a one-way street...no converting it back.

[AndyJackson]

There goes the first Law of Thermodynamics.

No. Frist law is about conservation of energy. Einstein's mass energy equivalence states that there is no issue here. Now, there are other conservation laws as well - charge conservation [so for every electron you also make an anti-electron, a positron] angular momentum / spin conservation.

So, for instance to make an electron - positron pair out of vacuum requires at least the rest mass energy of two electrons which is 2 x 511 keV [measuring mass as energy per Einstein's E=mc2 formula and measuring energy in electron volts, which is usual in high energy physics].