# Doubt on time dilation

1. Dec 2, 2014

### Ashes Panigrahi

Why must time slow down when a body moves with respect to a reference frame at rest ?
Why should its mass and length increase and decrease respectively when a body travels faster and faster ?

2. Dec 2, 2014

### ShayanJ

Its light's fault. She wants to have the same speed in any frame. She's very obstinate!
(Although its preferred not to use relativistic mass so people don't usually talk about mass increase as a prediction of SR because its almost useless and troublesome.)

3. Dec 2, 2014

### Jonathan Scott

When you look at an object from a different angle, it gets wider, shorter or whatever from your point of view. The object doesn't change, but your view of it changes.

In special relativity, those geometric transformations are extended to cover what happens when you look at it with a different velocity. The more general transformations are called Lorentz Transformations. Still nothing actually different happens to the object from its own point of view, but when you try to describe it using your own coordinates, there are some odd effects which are somewhat similar to a sort of rotation between space and time. The maths is very closely related to ordinary rotations (but uses cosh and sinh instead of cos and sin, as if the angle were imaginary rather than real).

For speeds which are small compared with the speed of light, velocities add up in the usual way and time hardly changes at all, but for larger speeds, it is necessary to adjust the rules of mechanics and electromagnetism to give consistent results from all points of view at all speeds.

Experiments have confirmed that known physical laws are consistent with this simple mathematical model, and it is now accepted as being totally accurate, to the extent that the speed of light is now fixed by definition, providing a fixed relationship between time and space units.

4. Dec 2, 2014

### Staff: Mentor

It's not her fault... No one has been able to find her a nice comfortable ether (that awkwardly contrived Lorentzian ether just isn't good enough for a lady of her delicate sensitivities) to move around in.:)

5. Dec 2, 2014

### HallsofIvy

Staff Emeritus
The answers given may seem facetious to you but you should understand that you are essentially asking for an explanation of special relativity! That is much too complicated to be given here.

It might help you to review the thread "The experimental basis for relativity" at the top of this sub-forum.