A claim that 'something cannot come from nothing' is actually a type of conservation law: that the amount of some type of thing can't be zero at one time and non-zero at another time.
So, for example, there is no conservation law for entropy. In fact, quite the opposite: in any spontaneous reaction, the overall entropy will increase.
On the other hand momentum is conserved: in any reaction the total momentum before and the total momentum after will be the same. In a certain sense, we cannot get momentum from no momentum.
There are many conservation laws in science: momentum, angular momentum, charge, baryon number, etc.
But ALL of the conservation laws have ways that different aspects can cancel each other. So for example, two equal masses going in exactly opposite directions at the same speed will each have a non-zero momentum, but *together* the total momentum will be zero. So it is possible to start with a situation with zero momentum and end with a situation with two things lie this *because* their momenta cancel each other to give zero,
Now, when people say you can't get something from nothing, they usually are intuiting some for of conservation of matter. But, we *know* that matter is no conserved. The main way to measure the amount of matter is through the concept of mass and nuclear reactions do change the amount of mass in a system. They do so by converting mass to energy or vice versa.
Now, you can ask for conservation of mass-energy and that *is* a valid conservation law, at least locally. If you include mass in your energy calculation (using E=mc&2), then the total energy is conserved. Fair enough.
But, and this is crucial, gravity has energy. And the energy of gravity is opposite that of mass and energy. The two cancel! Gravitational energy is *negative* energy.
The upshot is that you *can* get 'something from nothing' as long as whenever you made mass and energy, there is enough negative gravitational energy to cancel them in the conservation law.That requires a very high degree of curvature and is *exactly* what is produced in certain types of quantum 'bubbles'.
So, the intuition that something can't come from nothing, while one that works in everyday life with low gravitational curvatures, isn't something that is universal. And, in fact, at least theoretically, it is possile our whole universe can out of such a bubble and the total energy of our universe *exactly* balances to zero! In other words, we still have 'nothing'.
So, for example, there is no conservation law for entropy. In fact, quite the opposite: in any spontaneous reaction, the overall entropy will increase.
On the other hand momentum is conserved: in any reaction the total momentum before and the total momentum after will be the same. In a certain sense, we cannot get momentum from no momentum.
There are many conservation laws in science: momentum, angular momentum, charge, baryon number, etc.
But ALL of the conservation laws have ways that different aspects can cancel each other. So for example, two equal masses going in exactly opposite directions at the same speed will each have a non-zero momentum, but *together* the total momentum will be zero. So it is possible to start with a situation with zero momentum and end with a situation with two things lie this *because* their momenta cancel each other to give zero,
Now, when people say you can't get something from nothing, they usually are intuiting some for of conservation of matter. But, we *know* that matter is no conserved. The main way to measure the amount of matter is through the concept of mass and nuclear reactions do change the amount of mass in a system. They do so by converting mass to energy or vice versa.
Now, you can ask for conservation of mass-energy and that *is* a valid conservation law, at least locally. If you include mass in your energy calculation (using E=mc&2), then the total energy is conserved. Fair enough.
But, and this is crucial, gravity has energy. And the energy of gravity is opposite that of mass and energy. The two cancel! Gravitational energy is *negative* energy.
The upshot is that you *can* get 'something from nothing' as long as whenever you made mass and energy, there is enough negative gravitational energy to cancel them in the conservation law.That requires a very high degree of curvature and is *exactly* what is produced in certain types of quantum 'bubbles'.
So, the intuition that something can't come from nothing, while one that works in everyday life with low gravitational curvatures, isn't something that is universal. And, in fact, at least theoretically, it is possile our whole universe can out of such a bubble and the total energy of our universe *exactly* balances to zero! In other words, we still have 'nothing'.