# What is time? From a relativistic perspective.

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I know about time dilation, for instance if I have two hour glasses. and I place one on a spaceship and leave the other on earth, then I accelerate the spaceship to near light speeds.

The hour glass on the spaceship will have far more grains of sand left in the top half of the hour glass, than the hour glass on Earth.

What mechanism was responsible for this? How does time effect the grains of sand in the hour glass? And one of the things I'm having trouble understanding is; how does the clock measure time against the universe.

If a clock is just small mechanical or electrical parts moving about, why does the clock slow down?

What I'm asking is, Is time purely mechanical or is it some kind of substance and the clocks measure this substance like a volt meter measures volts in a current?

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The grains are just used for measurement. They drop down because of the gravity. Now that mechanism will only work for the earth as it is measured that the gravity takes an hour to pull down the sand grains in the hourglass, in the moon it will take more time for the gravity to pull down the sand in the hourglass this is because the gravity is less there,but time passes in the same speed in both places it isn't slow in the moon nor fast on earth . the time doesn't have anything to do with the hourglass, the hourglass is just an apparatus used for the measurement. The time just keeps on going but the hourglass is just made by seeing that it will always take an hour for the sand to drop in the hourglass if it is placed on earth.

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Time dilation has nothing to do with which apparatus you use to measure "time". Time is relative to space. Think of measuring "time" as distance traveled by a photon between two differenct spatial boundries. The closer you can get to the speed of that photon the slower time will seem to you because of the distance being traveled is being observed from much closer distance.

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Ranjha is saying the same thing Time dilation has nothing to do with which apparatus you use to measure "time". hourglass is just an apparatus it has no connection with time it is just used to measure time, time itself passes on in the same speed it is no substance and the hourglass cannot stop time nor slow it down, time keeps on traveling in its own speed it is when we use an hourglass or any type of clock that we see to it as a measurement

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Time is what is measured by a clock. If that isn't satisfactory then I suspect that "What is time?" is really more of a philosophical question than a physics question.

Time isn't measured relative to the universe, per se; time is measured in your local frame, and is relative to the rest of the universe. The relative rates of time occur because the speed of light is a constant in all inertial frames. For that to happen, there cannot be an absolute frame of reference, and transforming from one frame to another requires a Lorentz transformation rather than a Galilean one. (i.e. speeds do not add linearly and time does not pass at a constant rate)

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So what your saying is that their is no "real" time. It isn't a particle or substance? It's just the movement of objects and distance?

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The grains are just used for measurement. They drop down because of the gravity. Now that mechanism will only work for the earth as it is measured that the gravity takes an hour to pull down the sand grains in the hourglass, in the moon it will take more time for the gravity to pull down the sand in the hourglass this is because the gravity is less there,but time passes in the same speed in both places it isn't slow in the moon nor fast on earth . the time doesn't have anything to do with the hourglass, the hourglass is just an apparatus used for the measurement. The time just keeps on going but the hourglass is just made by seeing that it will always take an hour for the sand to drop in the hourglass if it is placed on earth.

Yes I understand but why is then that people talk about time travel and time going by slower here on earth than when I'm in a spaceship flying near the speed of light? If time dilation is purely mechanical then why should I not age, and my friend on earth ages. If time passes at the same speed and the clocks are just dilating?

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So what your saying is that their is no "real" time. It isn't a particle or substance? It's just the movement of objects and distance?

No, I was saying that nailing down what time is is an exercise in philosophy/metaphysics. As such, I was not offering any clue about what time is. Science is interested in how time behaves.

Yes I understand but why is then that people talk about time travel and time going by slower here on earth than when I'm in a spaceship flying near the speed of light? If time dilation is purely mechanical then why should I not age, and my friend on earth ages. If time passes at the same speed and the clocks are just dilating?

Time dilates, not clocks. Time does not pass at the same rate in different frames.

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But why does it do that? Why does time dilate?

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But why does it do that? Why does time dilate?

It might be of benefit to offer up what you think time is, then it can be contrasted to what time is in terms of general relativity. Time and space in terms of general relativity become I think somewhat intertwined, like spacetime.

It sort of goes back to that example of people on a train, vs people not on the train all viewing a lighting strike and how and when each observer sees it, which then ties into the concept of frames and the speed of light with distance and so on, not sure though.

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Just as distance is the separation between objects, duration is the separation between states of a system. Time and space are more similar than most people think. They are both relative and their magnitude is dependent on the energy in a reference frame.

In [acr=Special Relativity]SR[/acr], certain vectors are now four dimensional(called 4-vectors). It's easy to see the above by showing the position 4-vector.

X=(ct, x, y, z) Now, we use ct, rather than t for the time part due to units. In this vector, t is the time for your chosen reference frame.

Velocity is also a 4-vector. $V=(c \frac{dt}{d \tau}, \frac{dx}{d \tau}, \frac{dy}{d \tau}, \frac{dz}{d \tau})$ We take the derivative with respect to Tau(time as experienced by the particle under examination) because it is invariant(everyone will agree on it). Since $\tau=t\gamma$, we can say that $V=(\frac{c}{\gamma}, \frac{dx}{d\tau}, \frac{dy}{d\tau}, \frac{dz}{d\tau})$

Another invariant quantity is the dot products of 4-vectors. These are a bit different from normal dot products, however.

$X \cdot X=(ct)^2-(x^2+y^2+z^2)$

That will be the same for all observers. This is very useful, because you can set the dot product of your 4-vectors equal to the dot product of another frame's 4-vectors in order to solve for an unknown.

Edited by ydoaPs
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But why does it do that? Why does time dilate?

Because the speed of light is constant for all inertial observers. This is what makes length contract, too.

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You mean the object contracts not the length. Length doesn't exist.

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You mean the object contracts not the length. Length doesn't exist.

o.O

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You mean the object contracts not the length. Length doesn't exist.

The object's length. Length is the name of what we call the side that goes along the same direction as the movement. That thing is contracting.

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Length is the measure of how long a object is. So when you say length contraction you mean the object contracts thus the length contracts.

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The same way a clock doesn't measure the time in the universe, the ruler doesn't measure the length.

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Length is the measure of how long a object is. So when you say length contraction you mean the object contracts thus the length contracts.

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The same way a clock doesn't measure the time in the universe, the ruler doesn't measure the length.

The best way to say what you are saying is that there is no absolute time, nor absolute length, for any object or event. However, there is a rest length for the object, and as it begins to move, its length contracts.

=Uncool-

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It's a definition, so there's really not something to argue about.

We defined length in a specific way, such way that includes the definition that a ruler measures such length. Period. It's a definition created by man to measure a natural property.

That property (called length) is changing in accordance to Relativity..

Same goes with time.

Other than resurrecting some ancient fruitless issues, I don't quite see the point you're making...

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Well, I am having problem understanding things. For instance we measure time as movement of a object. The sun moving, a shadow moving, water droplets falling, etc.

If it takes the water droplet 1 second to fall one foot, we say one second has gone by and if a water droplet fell six feet, we would say six seconds had gone by. The water clock is not measuring anything in the universe. The water clock is just interpreted by humans, to know how long it takes a person to run five miles, or how long I have to wait at the bus stop.

Now, lets say I take the water clock to the moon, where their is less gravity. The clock will appear to run slower.

Time didn't slow down because the water clock wasn't measuring time in the first place. It was just our interpretation of the water droplets falling.

Even Einstein's photon clock, where a light beam bounces between two mirrors. The stationary light clock runs faster than the one that is moving. And again the moving clock's light beam just traces out a longer path, until it hits the mirror.

So why do we say that time slowed down when it was just the clocks and the environment of the clocks changing.

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If the clocks slow down, and everything around the clocks slow down, including the human brain, light, any and all things that move in the same reference frame slow down (in comparison to another reference frame), then we define it as time slowed down.

It's not just the clocks that slow, Peron. The environment slows.

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But the people inside don't think it slowed down, because the universe is relative?

I think I'm starting to get it.

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That's why it's called relativity... it's relative. If you're moving quickly, then I will see YOU moving slower... relative to *my* frame of reference, your time slowed.

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But does anyone know why this happens? Why the environment slows down?

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As far as I understand it, it seems to be a basic property of the universe. Just like the existence of gravity. Why does gravity exist? It's a basic property of the universe.. the why part is a bit irrelevant for physics, as it's dealing more with the "what" and "how", the implications, mechanism and consequences.

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Let me restate the question, How does the environment slow down?

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Whats the mechanism that makes time slow down?

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Whats the mechanism that makes time slow down?

It's not a physical mechanism. It's a consequence of the speed of light being constant.

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You mean the object contracts not the length. Length doesn't exist.

It doesn't?

I can perceive and measure three spatial dimensions and a time dimension. As far as I'm concerned, they exist. (I'm not going to get into a discussion of what "exist" means.)

I mean the length contracts, just as I mean time dilates. Items used to measure those dimensions will behave accordingly. The distance between two coordinate points (be it time or distance) will depend on the reference frame you are in.

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Well, I am having problem understanding things. For instance we measure time as movement of a object. The sun moving, a shadow moving, water droplets falling, etc.

If it takes the water droplet 1 second to fall one foot, we say one second has gone by and if a water droplet fell six feet, we would say six seconds had gone by. The water clock is not measuring anything in the universe. The water clock is just interpreted by humans, to know how long it takes a person to run five miles, or how long I have to wait at the bus stop.

Now, lets say I take the water clock to the moon, where their is less gravity. The clock will appear to run slower.

Time didn't slow down because the water clock wasn't measuring time in the first place. It was just our interpretation of the water droplets falling.

Even Einstein's photon clock, where a light beam bounces between two mirrors. The stationary light clock runs faster than the one that is moving. And again the moving clock's light beam just traces out a longer path, until it hits the mirror.

So why do we say that time slowed down when it was just the clocks and the environment of the clocks changing.

Any clock, regardless of construction, will experience the same change. Not all clocks are based on classical movement, though movement is a convenient way of building a clock.

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