Why time is a separate physical quantity

[Vanadium 50]

A voltmeter doesn't directly measure voltage either. The voltage is converted to a current, which produces a magnetic field, which causes the meter mechanism to turn and the arrow to move.

As far as a measuring device for coordinates, what's wrong with a ruler?

 

[ehpc]

Vanadium 50, I don't remember saying anything about voltmeter directly measuring voltage, but whatever. The wrong with the ruler is the same as with watches. There is no special space-force that helps to move the ruler. The ruler moves through space. For comparison, magnetic field does help arrow to change it's position. If there is a field, there is movement. Field causes movement. But does space causes you to take a ruler and make a measurement? Or you think there is no difference between space and voltage measurement? Then please explain.

 

[adaptation]

Vanadium 50, I don't remember saying anything about voltmeter directly measuring voltage, but whatever. The wrong with the ruler is the same as with watches. There is no special space-force that helps to move the ruler. The ruler moves through space. For comparison, magnetic field does help arrow to change it's position. If there is a field, there is movement. Field causes movement. But does space causes you to take a ruler and make a measurement? Or you think there is no difference between space and voltage measurement? Then please explain.

I believe Vanadium 50 was just clarifying a point. (A point I didn't get the first time around, so thanks.)

I see what you're saying about space also. It's kind of the same as time. You can measure space with feet, light years, meters, the wingspan of a canary. It doesn't really matter. Generally we choose to use meters because meters are easy to communicate to others.

A ruler should not move when you use it to make a measurement. The ruler needs to be motionless (relative to something) for you to be able to use it in a meaningful way.

Both time and space are relative to the observer. We use common units of space (usually meters) and time (usually seconds). The units we use are less important than the things we are measuring. The units we have created to measure these things are only tools for us to use to describe the world.

Where would things happen if they didn't happen in space? How would things happen if there were no time?

And as a suggestion, maybe you should stop thinking of space and time as separate things. Try to think about space-time as the "arena" of physics. Everything happens in space-time.

 

[ehpc]

Thanx, adaptation. I've just read wikipedia article about space and it says: "Space is one of the few fundamental quantities in physics, meaning that it cannot be defined via other quantities because nothing more fundamental is known at the present.".
Does this mean that there is no actual definition/explanation of what space is, so kind of the space is the space?

 

[adaptation]

Thanx, adaptation. I've just read wikipedia article about space and it says: "Space is one of the few fundamental quantities in physics, meaning that it cannot be defined via other quantities because nothing more fundamental is known at the present.".
Does this mean that there is no actual definition/explanation of what space is, so kind of the space is the space?

You're certainly welcome!

There are many models that describe what space is. The popular idea at the moment is that space-time is a continuum composed of three dimensions of space and one of time. "en.wikipedia.org/wiki/Spacetime"[/URL]

On that page you can read how a space-time interval is defined. Maybe that will also help answer some of your questions.

 

[Studiot]

This is not really an answer to your question, which floats on the boundaries between science, mathematics and philosophy, it is some things for you to think about.

When we talk about a physical property or quantity we look for a mathematical model. We want the mathematical rules to reflect the physical ones (and vice versa) as far as practicable. I don't say as far a possible because there is rarely if ever a total match.

Often we use the real numbers.

So what interesting properties do the real numbers come with?

Well, apart from the obvious arithmetic ones they are well ordered. This is a mathematical statement of the fact that we can place them one after another in order. This affords us the concept of greater than or less than. It also implies that every number has its place and cannot be placed somewhere else on the number line.

Next comes the property of completedness. This is a mathematical way of saying that there are no gaps or numbers not included between the numbers on the number line.

Is this always a good correspondence to physics theory in the light of quantum mechanics? Is time quantised?

Then the reals posess a distance function. This guarantees us that the difference (distance) between say 5 and 7 is the same as the distance between say 5000 and 5002.

This feature can be very useful but does it fit with more complicated (relativistic) theories of space-time? It does however provide the ruler you guys were talking about and suggests that 1 metre or 1 second on Mars the the same as 1 metre or 1 second on Alpha Centauri.

Some physical properties obey rules not reflected in the reals so we introduce imaginary numbers - at the cost of the well ordering principle as complex numbers are not well ordered and cannot be put in greater than less than order.

 

[ErikD]

You might want to watch the BBC documentary http://www.bbc.co.uk/programmes/b00fyl5z" I found it very useful in simply describing the history and our current understanding of time.

 

[ehpc]

Studiot, I think I see your point. So it is not very proper to discuss the essence of spacetime using incomplete language.

ErikD, thank you I'll definitely find this film.

 

[Dale]

Ok, I have read through this forum posts about 'time'. And have another stupid question, this time about clock.

Apparently we measure time with clock. The clock has arrows in it just like in any other measuring device. So we can talk about time changing by looking at indications of that arrows. If they move, we know the time has changed. But I don't get what exactly does this clock measure..

For comparison if we use voltmeter. It is a measuring device too. And it measures electrical potential difference through magnet. So there is an electrical force iteracting with magnet and that's why the arrow moves. So there is an observable participation of some force.
But what with the clock? What drives arrows in the clock? The tension of the spring for example. Or quartz crystal. Or whatever. But not the thing we are measuring. So the time does not actually move the arrows? Or if the time is interconnected with space, than any motion carries a time within itself? So that's why we are talking about time on clock, because when arrow moves, that means there is a movement in time? But then I have a question. Does voltmeter shows time too? It's arrows are moving too, so it should show time shouldn't it?

All measuring devices are essentially the same in principle. We have some unknown physical quantity and a device which operates according to some physical theory to produce a human-readable result which depends on the physical quantity of interest. Usually the device must be calibrated to some reference standard.

In the case of the voltmeter the unknown physical quantity is the voltage and the device operates according to the theory of Maxwell's equations to deflect a needle to some position along a dial which has been properly calibrated to correspond to the number of volts. In the case of an atomic clock the unknown physical quantity is the time and the device operates according to the theory of quantum mechanics to produce a repeating EM wave which can be counted and calibrated to correspond to the number of seconds.