# Total Momentum of the Universe

Can the total linear momentum of the universe be zero? Momentum is defined as the product of mass and velocity. But the universe as a whole is not moving (an assumption) although it is expanding therefore velocity=zero. To really see whether the universe is moving or not, a vantage point outside of spacetime is needed. But this is impossible.

## Answers and Replies

Antonio Lao said:
Can the total linear momentum of the universe be zero? Momentum is defined as the product of mass and velocity. But the universe as a whole is not moving (an assumption) although it is expanding therefore velocity=zero. To really see whether the universe is moving or not, a vantage point outside of spacetime is needed. But this is impossible.

Could not also the total velocities cancel out? I.e. for every galaxy heading in one direction at x speed, there is an equal and opposite galaxy traveling at -x speed. I'm not an expert on physics though.

mee said:
Could not also the total velocities cancel out? I.e. for every galaxy heading in one direction at x speed, there is an equal and opposite galaxy traveling at -x speed. I'm not an expert on physics though.

In reality, it's a lot more complex than that. There are also angular momenta and energy transformation (potential to kinetic and vice versa). With these other parameters, it is next to impossible to track all the movements of stars and galaxies, clusters and superclusters. The movements seem to conform to some fractal geometries (existence of self-similarities and great attractors, bifurcations, etc) inherent in nature.

Njorl
Obviously, you can choose an inertial reference frame for which the universe has 0 linear momentum. The more difficult question is; can you choose an inertial reference frame for which the universe does not have 0 linear momentum?

Njorl

Njorl said:
...choose an inertial reference frame...

Do we really need any reference frame (inertial or otherwise) in order to formulate a new physical theory. To keep it frugal and simple, any reference frame is a luxury that any poor theorist cannot afford. Frame needs math and more frames need more complicated math. One can stuck one's whole lifetime just by learning everything in math and still would be farther from the truth sought originally in the theory. Math should never be the master of the search for truth but as a mean, not the end of the search. Math is best if it is the mean to an end.

Last edited:
krab
Antonio Lao said:
Do we really need any reference frame (inertial or otherwise) in order to formulate a new physical theory.
You seem to have the idea that a reference frame is a complicating feature and we should try to live without it. On the contrary, reference frames are simplifying constructs. They are simply a fixed set of conditions from which measurements are made, so that others can set up the same conditions and verify the results. No general statements can be made in physics unless such conditions are controlled. To take a simple example, I can tell my neighbour that the sun always rises over the tree in my back yard. If he verifies this from his house (frame), he will see it to be false.
The question Njorl asks is fundamental. A second one is "Is the universe spinning?"

If the universe is spinning this could be why galaxies are moving apart.
Is there an inertial reference frame for which the universe does not have zero linear momentum?
This can only be the case if there are any reference frames that do not accelerate or rotate? There could be briefly but since the masses in the universe are dynamic
such a reference frame could only exist for a fraction of a second.If there was more than one inertial frame at any one time then it would be interesting to know how these frames are spatially distributed-it would imply an order in the universe we don't currently know about.

Well assumming the Big bang model is correct in in some form (namely that the universe is expanding from an intial explosion) the the total linear momentum would have to be zero (measured of course from the fram of reference of the whole universe) because the initial linear momentum previous to the expansion would have been zero (because there can be no velocity if all matter is compacted into one object, that one object can never have a velocity because there is nothing for it to be moving relative to.) So the total linear momentum must be zero.

krab said:
You seem to have the idea that a reference frame is a complicating feature...

If I was going to formulate a theory of the vacuum, do I need a reference frame? In this case the vacuum is referencing itself. If I was going to formulate a quantum theory of spacetime, do I need another spacetime frame? Again, spacetime is referencing itself. What is the meaning when we say that a distance is zero? What are we referencing it to? What is the meaning when we say that a force is zero? If the force is not zero then we can introduce a frame to indicate that the force varies from point to point and forming a force field.