# why is time and space relative [duplicate]

I was reading the theory of relativity and though I think I understood the effects of it, I am not sure why it happens in the first place. I will summarise below

1. Time and space are relative:

Why so? I read how clocks move at different speeds when in motion but what is the underlying phenomenom that makes it happen ?

1. You can never reach the speed of light because As you move faster mass increases and more force is needed for acceleration.

Cool, but then how does light move at the speed of light? What property does light have that no other matter does that enables it to move at this speed?

1. Space and time are relative but space time is not

What is space time. Not theoretically but from an understanding point of view. Any practical example ?

• Sorry, but there is nothing we can tell you here that isn't already described in a great detail in different sources. Take a look at wikipedia, there is lots of references and information there. – Darkseid Oct 5 '17 at 22:48
• At the very bottom of every chain of scientific reasoning, once you've plumbed the deepest theories currently backed up you get to the statement 'because that is how the universe is observed to behave'. – dmckee Oct 5 '17 at 23:14
• Welcome to Physics! Note that while we typically prefer 1 question per post, your questions are fairly broad and are likely to be addressed in other posts on this site. – Kyle Kanos Oct 6 '17 at 1:00

Take a sheet of paper and draw two axes, time and distance. Then draw a line for a static object. This line would be parallel to the time axis, because for this object time changes, but distance does not. Now draw a line for an object moving with a constant speed. It would be a straight line on an angle, as both time and distance change.

Finally look at your drawing as a whole. It is static. Any motion that happened is already drawn as lines (called "world lines"), so now you just have a static picture of these events. This sheet of paper irepresents spacetime. It is static ("not relative" as you called it). No matter how you turn it, the picture on it doesn't change.

Now let's measure spacetime "distance" (called "inerval") on this sheet. Imagine some object moved by $x$ in space and by $ct$ in time (were $c$ is the speed of light used as a constant to make sure we don't add meters to seconds). In the "normal" (Euclidian) geometry, the interval $s$ would simply follow the Pythagoras theorem:

$$s^2=x^2+(ct)^2$$

There is a problem though. If this were true, we could draw a line on such an angle that time would go backwards. This is not an option, because, unlike in space, we can't turn around back in time. To reflect this difference betwen space and time, we put a minus sign between them and write the spacetime interval this way instead:

$$s^2=x^2-(ct)^2$$

This fixes the problem, but also makes this spacetime (called Minkowski space) counterintuitive. For $x=ct$ the interval is... zero. What does it mean? It means that time stops at the speed of light. Space and time cancel each other out.

Can we exceed the speed of light? No, but not because of the mass increasing (it does increase, but this is more of a result than the cause). We cannot exceed the speed of light, because time cannot move slower than not moving at all. It is like traveling to the North. Once you've reached the North Pole, can you move any "farther" to the North? No, your distance to the North Pole is already zero and cannot be smallr than that.

Can we move with the sped of light? No, but light does. What special property does light have? It has no mass. We have mass, so we can stay and move with different speeds, but we can't reach the speed of light. Light has no mass (called "invariant mass", formerly known as "rest mas"), so it can't stay or move slower without disappearing. In vacum, light can move only with the speed of light.

Why are time and space relative? What is the underlying reason? It is the fact that the speed of light is finite. If the geometry of spacetime allowed any speed, no mater how high (like in the classical Galilean theory), then we couldn't relate space and time to each other like above. There would be no spacetime intervals, only spatial distances and temporal periods separately. It is the speed of light that makes time and space relative and links them together.

Of course this explanation is simplified, but I hope it helps :)

• Thanks .. a little clarification .. for x = ct. It basically means the distance travelled is equal to the time elapsed. So the interval is 0.How did you deduce time stops at the speed of light? And light does not have mass. Yes it is a wave and it does not have mass . But so do the emtire spectrum of waves. Does this hold true for all of them ? – john Oct 7 '17 at 6:25
• In your own frame you don't move in space, only in time. Essentially, your spacetime trajectory (world line) is the time axis in your frame of reference. So the distance (interval) measured along your world line is simply time measured by your wristwatch. It is called your proper time $\tau$. Now take $x=ct$ and put it in the formula for your interval $\tau=\sqrt{x^2−(ct)^2}$. You will get zero. So the time measured by your wristwatch is zero, if you move with the speed of light – safesphere Oct 7 '17 at 7:01
• It would take an infinite energy for anything with a rest mass to accelerate to the speed of light. So anything that moves with the speed of light doesn't have rest mass and for the same reason cannot stop or slow down. All forms of electromagnetic radiation move in vacuum with the speed of light: light, UV, x-rays, gamma-rays, IR, microwave, radio waves. All of them, the entire spectrum (but only electromagnetic waves). Gravitational ways should too, but these are hard to detect. – safesphere Oct 7 '17 at 7:01