To know why time is not absolute, we must date back in time and have to see the evolution of theories regarding space and time. At first, Aristotle proposed that the natural state of a body is to be at rest and starts moving only when it gets acted upon by some external force. It followed that a heavy body should fall faster than a lighter one on to the Earth. Later it was proved wrong by Galileo,and Newton proposed his three laws on the basis of Galileo's experiments. He proposed that, the natural state of a body is not to be at rest but to be in uniform motion and only when it gets acted upon by some external force, It starts accelerating. It follows from Newton's laws that there is no unique standard of rest. Lets for instance, a train was moving past an electric pole standing tall along the embankment, at a speed of 70 mph, one could equally say that the pole was at rest and the train was in motion or the train was at rest and the pole was moving past at the rate 70 mph; we can't really say which one is at rest preferably. If for someone inside a train which is in motion, an object appears at rest, and it appears in motion for someone outside the train. This implies, we can't assign any event an absolute position in space. hence space is not absolute.

They came up with non absolute space but still it was believed that 'time' was completely independent from 'space' and that one can unambiguously measure the interval between any two events that happen in space. In 1676, Christensen Roemer came up with a significant discovery that light travels at a finite speed and he calculates it to be around 140,000 miles per second, However later Clerk Maxwell precisely measured it to be 186,000 miles per second. Whatever it is, It adds a significant implication that light travels at a finite and more importantly fixed speed.

Here comes the most interesting part, if light has to travel at some fixed speed, It must travel relative to something at rest and what could be that 'something'?

They came up with a shockingly interesting and imaginary space, 'Ether', supposedly presents everywhere and obviously, at rest.

Later scientists tried to measure the time taken for the light (from a fixed source) to reach an object that is moving towards it, in one instance and moving away from it in another instance. Shockingly, they measured the same 'time' in both instances.

At that stage Albert Einstein came up with his famous Theory of relativity, which implies that time is not absolute, obviously no need of ether anymore. A remarkable consequence of relativity is the way it has revolutionized our ideas of space and time. In Newton's theory, if a pulse of light is sent from one place to another, different observers would agree on the time that the journey took (since time is absolute), but will not always agree on how far the light traveled (since space is not absolute), Which implies that different observers would measure different speeds of light. In relativity, on the other hand, all observers must agree on how fast light travels. They still, however, do not agree on the distance the light has traveled (since no absolute space), so they must now also disagree over the time it has taken. In other words, the theory of relativity put an end to the idea of absolute time, which means that time measures between any two events in space by two identical clocks would not necessarily agree, even though they show the same.

We must accept that time is not completely independent from space, but is combined with it to form an object called space-time

## 5 comments

commentsI watched this very good video on SpaceTime here: https://www.youtube.com/watch?v=DdzWuPh581I

ReplyBetween 15:16 and 17:10 on the runtime (in video link above) the narrator talks about space and time constantly "adjusting" to keep the speed of light fixed at 671,000,000 miles per hour.

Question 1

What exactly does "adjusting" refer to? In the runtime mentioned, as the car approaches the speed of light, spacetime shrinks the car to a few inches wide and it moves very slowly past the viewer watching on the corner.

Question 2

What is "space and time" actually doing to keep the speed of light at a fixed speed? How and what makes the car shrink and his watch to tick slowly to the listener "outside" on the corner who is watching? I do have a concept of how time slows down as it reaches the speed of light but there seems to be a piece missing that ties it all together in that segment of the video.

Thank you kindly

Jenny,

ReplyIf you can answer your Q2 exactly you will probably be awarded a Nobel prize. :D

You can look at it this way. We are constantly traveling through space-time at the speed of light. No speed through speed means all the speed through time. As the speed through space increases the speed through time decreases.

I don't understand. The narrator in the video specifically mentions that "if you were watch my car approach the speed of light, it would be only inches long and you would hear my watch ticking very slowly". To make a statement like that leads me to believe he knows the reason why (see 16:00 thru 17:10 on the runtime in the video https://www.youtube.com/watch?v=DdzWuPh581I ).

ReplyWe look at it like this.

ReplyLight is the fastest influence we can find. To make something move we have to push it. Push is an interaction of EM (light). In your frame of referance you are motionless. Everything moving is reletive to you. So all motion is stated in terms as if it has been pushed (rE) by you. Since any push is limited by the speed of the pusher reletive to you, and the push can't be faster than light that's the limit of how fast things can go reletive to you. However there is a concept called the cosmological principle. That is that everything else sees the same thing you do from its own frame of reference. To it you are the one moving. To it light moves at the speed of light. But if you add its speed reletive to you and the speed of light reletive to it you get a speed of light faster than light reletive to you. That violates the first rule; nothing faster than light. To reconcile these two (you vs not you) divergent frames we can only model it in certain ways. Time and length are what gives. If your lengths and time measures are the same as his there's a problem. So consider the measure of length of a set of waves of light. Say ten waves per foot. That foot passes you at the speed of light so you get the number of waves per foot times the number of feet per second to get waves per second. So now understand you have a length and time measure. Something moving past you also has a foot and 10 waves per foot. To maintain your measure of the speed of light you have to measure his feet as shorter and his time as longer than yours.

Jenny, I don't have the time to watch the movie, but anyway :D

ReplyLet's look at these analogies.

1)

Imagine huge field like a fotball ball field with a long side and a short side.

Imagine a car driving at a constant speed in the field.

Imagine that the the long side is time and the short side is space. (space-time do indeed have an orthogonal relation)

Now imagine the driving car is driving parallel to the long side. This is equivalent to "all speed through time". Now the car turns and also starts to travel parallel to the short side. This is equivalent to increasing the speed in space and as a result the speed parallel to the long side (time) decreases. When the car has turned so that all the speed is parallel to the shrt side it is equivalent to "all speed through time".

2)

In space-time there is one constant, the speed of light and this is what we measure with.

Imagine you, a friend, two flash lights, two one meter rods and a flatbed truck.

The truck is standing still with with your friend on the bed facing you.

Both of you flahing your light at exactly once per second measured by your own clocks. As long as the truck is at a stand still you both have the same beat.

Now the truck starts to move forward, then since each subsequent flash have longer to travel the time between your friends flashes measrured by you will be longer than one second. By the same token your will also measure the time between your flashes to be longer than one second. The same is true for all events.

You can see we have two vantage point from where the world can be observed, your's and uyour friends. We call those "local frames". Normally we would choose one of them as a reference and call it "inertial frame". You can look up "Lorenz transformation" to learn more about this.

Imagine that somehow you could take your "stationary rod" and in the direction of travel lay it down beside your friends "moving rod". This is impossible but we can pretend it can be done. That would be meauring your friends rod in "your local frame". You would then see that your friends rod appears to be shorter. This is because since the speed of light is constant in relation to "all frames" and time measured by you runs slower in your "friends local frame", it can only cover a shorter distance. This will make your friends rod appear shorter.

Now the truck turns around an comes towards you, then the opposite will be true, you both measure intervals shorter than one second, and your friends rod will appear longer.