What is time? Does gravity really slow down time?

Image source: [1]

Time, in physics, is but a magnitude to measure the change of state of an object.

The most common conception of time is that of classical mechanics, where this is a scalar magnitude (a constant) for all observers. It receives the name of absolute time. The time elapsed between two events is the same for all observers.

Then there is the more precise conception, that of relativistic mechanics, introduced by Einstein. It conceives time as a magnitude whose value depends on the frame of reference of the observer. That is, the time between two events will not be the same if it is measured by someone on Earth or someone on a ship at 0.5c (half the speed of light).

Time would also be a dimension, similar to the three spatial dimensions, but radically different. Time, as far as we know, is one-dimensional (there is only one timeline). Its spatial equivalent is a road: you can only go in two directions. Well, in the temporal realm this is not so: you can only go in one direction, forward; You can not stop, and your speed depends on the type of particle you are (in general, massless particles like photons and gravitons will move at velocity c and the rest of particles with mass will move at a slower speed than the light).

There is something called temporal dilation, predicted by the Theory of Relativity. Basically, it consists of a change in the rhythm of the passage of time due to a deformation of the space-time tissue.

It means that if you accelerate to speeds close to that of light, the tissue deforms and time expands in strange ways: you, at speeds close to that of light, will see the others, outside, in a fast chamber; And the outsiders see you in slow motion. Therefore, for all, the light would continue traveling at 300,000km / s (approx).

If you want more details here I leave a link: [2]

Here an explanatory video:

Video credits: The Science Channel