Conservation

Today's post is a short one on the conservation laws of Physics.

There is no doubt that I will return to each of these in turn in later posts, so consider this as a bit of an introduction. There are four that will get a mention here and they are, in no particular order;

Conservation of mass-energy

Conservation of linear momentum

Conservation of angular momentum

Conservation of electric charge

Before we start in on these, just what is a Physical law? simple question right? Nope, it is actually really difficult both philosophically and physically to state what a law is!

The sun converts mass into energy

So, for the length of this blog, a law is something that is obeyed! sorted that one eh? In terms of the four lines above this means that in ALL the valid experiments that have ever been carried out have found them to be true. 

So we have four laws, big deal I hear you say, and I can hear your voice, but it is a big deal, because these four laws help define the universe. This universe, so large that it genuinely is beyond the imagination of man, can be partly described by these four laws that can be tested in a lab at a school or university.

Just think about that for one minute. These are four laws that appear to be obeyed everywhere, through out the entire length and breadth of this mind boggling vast universe

So let's take a quick look;

Conservation of mass-energy. Energy can neither be created or destroyed ever - although it can be converted into mass according to Einsteins wonderful equation E = mc². So what this means is that the energy in the universe today is exactly the same as at the very start of the universe at the time of the Big Bang.No more, no less.

Conservation of linear momentum. If no external force acts on a closed system of objects, the momentum of that closed system will remain constant. If we think of the universe as the closed system, does this mean that the total momentum of the universe is constant? Yep, reckon it does. So what does that mean? 

Well if the Big Bang really happened then surely it implies that the total linear momentum now is the same as it was at the moment of the Big Bang.

If the total linear momentum of all the objects in the universe is not zero then this implies that there must have been a total none zero linear momentum at the Big Bang. So the singularity itself had linear momentum, but what does that mean? after all it did not have any size!

Conservation of angular momentum. In a closed system angular momentum is constant. Once again, if we take the universe as our closed system, is the total angular momentum constant? I reckon it could be. 

Following the same argument we just used for linear momentum, if the total amount of angular momentum is not zero then it surely it implies that the singularity of the Big Bang had angular momentum, in other words, it was spinning. If it was spinning, where did it get its spin from? 

Further what does it actually mean to have spin in something so small that it has no physical size?

Conservation of electric charge. Electric charge has to be conserved. The total amount of positive change minus the amount of negative charge in the entire universe is always the same. 

For this one I think that the actual total charge of the universe is zero. If it is not then we have the same problem we had with the momentum laws. In this case the singularity at the point of the Big Bang must have had a total net charge, if so, where did it come from? 

So based on the above I have come to a couple of conclusions;

1) the total linear momentum of the universe is zero

2) the total angular momentum of the universe is zero

3) the total electric charge of the universe is zero

4) the total energy of the universe is zero

Now, I can see that while it may be possible to argue that the first three may be zero, especially when you think about things like Newton's 3rd law of motion; 

every action has an equal and opposite reation. 

you push something it pushes back, so you both end up with the same momentum, but in opposite directions. For conservation of charge you get a stack of neutrons, they decay into protons and electrons, balancing each other out.

How can we possibly say that the total energy of the universe is zero? After all, there are literally billions and billions of stars that have enormous masses. There is thermal radiation all over the place. If we add all this up it is most certainly not zero. 

So if there is all this energy, what is the equivalent negative energy to cancel it out?... I'm still working on that bit! might be wise not to hold your breath.

Update: Was reading some stuff on Mach's principle for another post and it turns out that three of the four conclusions mentioned above are given a Mach number by Bondi & Samuel.

Mach5: The total energy, angular and linear momentum of the universe are zero.

Image is probably from NASA. Drop me a line if you want me to pull it.