h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Four Momentum - A72640767
Author: EnergeticRobb978 - U14573969

Welcome to my guide entry and I hope you find it interesting and informative.

Sounds interesting, but it is not very informative. I know quite a bit of physics, but I have no idea what this is all about. Could you explain it in simpler terms?

I know that momentum is the product of velocity and mass. Velocity is a vector, having both magnitude and direction, while mass is a scalar, having magnitude but no direction.

What is 'four momentum'? Is it a different sort of momentum, or just a way of thinking about traditional momentum in 4-space?

Thank You very much for your post.
Four Momentum is the name allocated to a new vector which applies to Special Relativity. It was basically a way of describing momentum in 4 dimensional space. It is as you said a new type of momentum, which is compatible with Einstein's theory of relativity.
It was based upon the fact that normal momentum described the 3 dimensions of direction just fine, so they made a new type for a larger scale. In order to base it on a larger scale, they had to include the temporal dimension (dimension of time), because on a scale of metres or kilometers, you don't exactly go back in time, but on a spatial scale, it is quite different, the further you go out, the more you see back in time.
So yes it is a new type of momentum.
Thank you for your post again and I wish you a pleasant day.

Thank you for your post.
The four momentum is as you predicted a way of allocating regular momentum to the 4 dimensional fabric of space-time. So in all essence yes it is a new type of momentum, just an altered version designed to cope with relativity.
Thank you again and I wish you a pleasant day.

Sorry I did that twice. I didn't see my first one.

Doesn't seem to inform. I'm left Wiki-ing which also leaves me none the wiser.

I'm rather confused by this - it doesn't take a genius to state that in four dimensional space-time, you need four coordinates to define a position (though as Hawking points out, these can be a mixture of space and time in the same way that you could express a 2D position as coordinates NW and NE of [0,0]). The logical conclusion is that to describe a translational movement in four dimensions, you need a vector with four parts to it. This in itself is not novel or interesting enough to justify its own Guide Entry.

Also, I don't understand what this has to do with momentum. Momentum is equal to the product of mass and velocity, both of which are scalars. Where is the need for a space-time vector?

Alex

Oops, my bad, just occured to me that velocity is a vector. Still confused as to the need for a four-part vector, though - doesn't velocity include time anyway?

Minkowski diagrams manage to express spacetime events in two dimensions...

I notice that energetic rob has 2 pieces in Peer Review, but from his personal space it may appear that he has left us I do hope that this is not the case.


Lanzababy

He has indeed, apparently, left. He appeared to fancy himself quite the physicist, but didn't know the difference between energy and charge, which is a bit like fancying yourself quite the footballer, but not knowing the difference between the offside rule and Manchester United.

Both articles currently in PR seem like cut-n-paste jobs, done by someone who amply demonstrated they don't really understand the subject. How long is it appropriate to leave something like this in PR before removing it?

Oh, longer than two weeks - I just hope that he returns

Looks like the author has elvised afterall.
I recommend either Flea Market or return to entry.

BMT Scout

I'd suggest a return to entry, since I'm still not sure where this was going!

Agreed