an interesting question from Y!Answers about Einstein's relativity and the constant nature of time. this was a nice conceptual question to answer, which is refreshing from time to time.
i also very much enjoy helping people (especially those who have not had much exposure to the topics) about these complicated subjects in ways that are easy for anyone to understand. i find this is often difficult, but it is an extremely worthwhile endeavor. and nothing beats a strong conceptual understanding base from which to build your knowledge of a subject - it is immensely useful. so that is what i tried to give when answering this question...
Q:
How do we know that time is what is what we believe it to be?
Basically I would like to know whether time is what class it as i.e. that how do we know that a second or an hour lasts the duration that we say a second or an hour lasts. Also will the strength of gravity change the speed at which time travels (and if so is time therefore not a constant and as Einstein said only "Relative") I hope that makes sense I'm not really fully sure what I mean myself, i know that the theory of relativity says that space and time are relative but relative to what?
A:
a second or an hour lasts as long as it does because that is the way that we defined it. for example, a second is defined (in modern terms) as the time it takes light to travel 299,792,458 meters. other modern measurements include the time it takes cesium-133 to radiate a certain number of times.
in other words, the universe did not invent the duration of the second, we did. we are using universal constant to define it now, which will never vary so that we can accurately measure time in any circumstance!
regarding your gravity question, no the speed of light will never vary! but it is interesting that you brought this up! let's analyze this situation. if we have light traveling against a gravitational field, the light will need to expend or lose energy. however, the energy it loses will not come out of its speed, but rather, its wavelength. there is a property of waves which relate their wavelengths (or frequencies) to their energy. so, light will expend energy from its wavelength rather than its speed to travel in a gravitational field. this will result in the wavelength of light getting longer or shorter (depending on which way it is traveling) and also changing its apparent color! (this property is partially responsible for the phenomenon known as red-shifting).
yes, einstein's theory of relativity states that everything is relative, well almost everything (the exception being light). the actual title of his paper on what we now call 'special relativity' was actually called something along the lines of "On the Invariance of the Speed of Light", which tells us what einstein really had in mind when he was developing his theory.
in any case, in order to illustrate this 'invariance', we'll look at two similar examples (which i'm sure you may have seen before):
example 1:
you are at one end a train which is moving at a constant velocity, let's say 30 km/hr. and your friend is standing at the other end (let's say he's at the front and you're at the back of the train). now, you have a frisbee and you throw it to your friend at 5 km/hr. both you and your friend see the frisbee traveling at 5 km/hr. however, an observer who is outside the train (on the ground) will observe the frisbee traveling forward at 35 km/hr! (and the train is still moving at 30 km/hr to him as well).
so how is it possible that we have 2 different measured speeds for an object? is the object actually traveling at 2 different speeds? is there an objective way to measure the absolute speed of an object, such that it has one speed all the time? this is where einstein comes in! einstein said that there is no way to objectively measure the speed of the frisbee. he says that it's speed is always measured relative to some other stationary object. for you and your friend, you measured the speed relative to the train (which you considered stationary because you were in it). for the outside observer, he measured the speed relative to the ground (which was stationary to him). (also, imagine the apparent speeds and path that the frisbee would seem to travel to someone on mars or on the moon! they would be very complicated indeed!)
now, example 2:
you are again on a train moving at a constant velocity, 30 km/hr again. but this time, you have a flashlight instead. so, from inside the train, you turn on your flashlight and you observe the light coming out the end of it. this light is traveling at a velocity c (about 300,000 km/sec). and this light will hit the front wall of the train in some time t (based on the length of the train). now, there is also an observer who is outside of the train. at what speed does he see the light travel? does the light travel at 300,000 km/s + 30 km/hr? no! again, einstein comes in and says that the light will travel at the same apparent speed regardless of the observer! so, the light will travel at c, but how long will it take to strike the end of the train? will it be the same time t? well this is a complicated answer, and this gives rise to the phenomenon known as time-dilation. this states that in order for the speed of light to be kept constant (to any observer), time will actually shorten to allow light to make the distance.
so, in short, what einstein is saying is that space and time (and velocities) are relative to their observers. however, the speed of light (the universal constant) is the only thing which will never vary (due to complex phenomena like time dilation and many others).
i know that i have introduced a lot of complicated topics here, and i hope that i haven't confused you (relativity is a very confusing topic anyway). but i hope that you've gained something from this!
please ask questions if i wasn't clear about something (which is likely the case haha)!
