In this the "length" of the four momentum becomes the $\sqrt$ with one timelike coordinate and $n-1$ spacelike coordinates, for an $n$-dimensional space. The momentum vector above is modified and called a four momentum ( $c=1$ in this): Relativistic mechanics which hold for high energies and momenta, and which are necessary in order to make sense of the plethora of particle data, requires four vectors, Which is invariant under rotations and translation of the reference system. Momentum for example $(p_x,p_y,p_z)$ are the three vector components whose length is $p$ and is given by In classical mechanics a vector in space is a three dimensional row or column with values from the field of real numbers that follow eucledian tranformation properties under translations and rotation.
![space in time that connot be explained space in time that connot be explained](https://cdn.mos.cms.futurecdn.net/4ugoTjVhaT7FQiDgHFriYd-320-80.jpg)
In particle physics the data show that one is not dealing with a three dimensional space that has as a parameter time, but that space and time are united in a specific mathematical way in what is called a "four vector space". In that view spacetime is far from static, it is more like a sort of fluid. But since all space and all time are intrinsically part of it, it actually cannot be conceived from an external point of view, and indeed Einstein's equations are strictly local and relational: they describe how the distribution of energy defines its own playground and how time and space can be seen in the way we are used to only instant by instant for specific observers, whose mutual perspectives are always shifting and transforming. The price to pay to see spacetime as a background is that this scene is completely static, sometimes called the block universe. By forming a space (in the mathematical sense) combining traditional space and traditional time in an intricate way allowing space to rotate into time and the other way round, we can still get by with the idea that there is some smooth universal scene where everything happens. Spacetime is then the notion we use to still have a background after all. So there is no universal spatial background, and no universal time flow. This traditional space/time framework somewhat holds but only relatively to an object it is not the same for all objects.
![space in time that connot be explained space in time that connot be explained](https://static.wikia.nocookie.net/antimattered-space/images/1/15/PandoraReboot.png)
Physicists discovered that there is no such box, and there is no such flow of time.
![space in time that connot be explained space in time that connot be explained](http://www.theimagineershome.com/blog/wp-content/uploads/2022/07/3.jpg)
The intuitive and traditional idea of space and time is that objects live in an infinite three-dimensional box, space, and that their motion in space happen in time in such a way that at each definite moment in time all objects have a position, and we can compare those positions because time flows the same for all objects.