## What does "time" really mean?

 Quote by DaleSpam No. There are several cases where it introduces confusion in beginning students, but not self-contradiction. That is the whole point of establishing a unified mathematical framework for a theory.
Ok.

"The term 4-D means that it takes 3 spacial coordinates and 1 temporal coordinate to specify the position of a point or event.

"An object is said to have as many dimensions as there are axes required to locate its position in space"

Are both definitions above correct?

 Making accurate predictions about the results of experiments is the only scientific criteria. Other criteria amount philosophical or personal preference.
Ok. ur right.

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 Quote by TheAlkemist u realize that implicit in ur question is the notion of measuring time as a distance?
That's why I put "distance" in quotes, so that you would not take exception to my terminology because I can't tell what terminology you prefer.
 Quote by TheAlkemist besides, spacetime interval addresses an issue created by the conception of space and time as a single entity. it's like a custom designed solution.
Spacetime does not create the issue, nature does, and spacetime interval is a solution.
 Quote by TheAlkemist My short answer to ur question is that it's irrational and conceptually impossible to measure space (emptiness) between objects. U measure the physical object in space. But i'll leave it here. i'm not changing any minds in here anyway. and i'm not trying to offend anyone. just commenting.
But you said you could solve the same problem that "spacetime interval" solves except by another method. Now are you telling me that you don't believe there is any solution?

Let's take, for example, the first half of the twin paradox. The twins (or two identical clocks) start out at the same age (or set to the same time) at the same location. One of them accelerates away and travels at a high speed for awhile and then decelerates and comes to rest with respect to the first twin (or clock) some distance away. This defines two events: the first is when the traveler starts out and the second is when the traveler stops. When this situation is analyzed from different frames of reference, different answers will be determined for the actual physical distance the traveler traversed and for the actual physical time that it took the traveler to make the trip. Do you agree? If yes, then how do you reconcile the different measurements of distance and time? If no, then please explain why.

 Quote by TheAlkemist OK, then why u can't have just tag on, say, temperature, and static charge yo x,y,z,t and call it 5D?
You can as long as it is mathematically useful. Physical "spacial dimension" only has 3 dimensions x,y,z. Time is a dimension but not a spacial dimension. Because Special Relativity uses both spacial dimension and time dimension then we combine them so that it is mathematically useful and we call it "space-time dimension".

I am just saying that you were not defining dimension abstractly enough but you are rather defining spacial dimension in specific instead.

 Quote by TheAlkemist Ur right, temperature, pressure and attitude have nothing to do with shape. As for spacetime, this interval u speak of is simply a number-line that's been added as an extra "time dimension". A metric for duration so to speak. The purpose of adding this is to endow the model with Lorentz symmetry right? Is this in anyway related to the concept of T-symmetry? If so, isn't the the physical universe we observe time asymmetric (because of 2nd Law of thermodynamics?). Hope i'm not way off here...
As in space alone, there is no preferred direction in spacetime that you can call the time direction. In this regard they are inseparable, unlike space+temperature, say. A Lorentz symmetry is more of a rotational symmetry, similar to rotating spacial vector.

 Quote by Phrak As in space alone, there is no preferred direction in spacetime that you can call the time direction. In this regard they are inseparable, unlike space+temperature, say. A Lorentz symmetry is more of a rotational symmetry, similar to rotating spacial vector.
I could be misunderstanding you but the time dimension is different from the other spatial dimensions; it is "timelike", whereas the others are "spacelike". At least one defining distinction between the two is that timelike dimension(s) only permit movement in one direction.

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 Quote by TheAlkemist "The term 4-D means that it takes 3 spacial coordinates and 1 temporal coordinate to specify the position of a point or event. "An object is said to have as many dimensions as there are axes required to locate its position in space" Are both definitions above correct?
The first is basically correct, although I would have been more specific (e.g. "The term 4-D spacetime"). The second is not correct, it seems to be describing the dimensionality of a space and not the dimensionality of an object as it says.

 Quote by DaveC426913 I could be misunderstanding you but the time dimension is different from the other spatial dimensions; it is "timelike", whereas the others are "spacelike".
It was in my opinion that to throw in that sort of detail would cloud the issue at the level of understanding of the question. However I didn't make my case very well, did I?

To try again: <We cannot pick-out any one direction in spacetime and say "this is the time direction". Observers in relative motion will not agree. In this regard space and time are inseparable.> How's that?

 At least one defining distinction between the two is that timelike dimension(s) only permit movement in one direction.
I don't know what the meaning of movement in time is. However, world lines of classical and incoherent particles are confined to the interior of the light cone.

To be really abstract, the difference is that rotations in space have a real valued parameter of rotation, whereas rotations between space and time have an equivalent imaginary parameter. But this doesn't really tell us what class of objects must have their world lines confined to the interior of the light cone... hmm...

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 Quote by TheAlkemist My short answer to ur question is that it's irrational and conceptually impossible to measure space (emptiness) between objects. U measure the physical object in space.
I hope you realize that the issue we are talking about isn't confined to distances between objects in empty space, the same issue exists between the shape of a single physical solid object on the surface of good old earth. The Michelson-Morley experiment is what started this whole thing. It was a single very large, very solid, very massive object that seemed to be changing its dimensions simply by being rotated. How do you understand Lorentz contraction when applied to MMX?

 Quote by ghwellsjr The Michelson-Morley experiment is what started this whole thing. It was a single very large, very solid, very massive object that seemed to be changing its dimensions simply by being rotated. How do you understand Lorentz contraction when applied to MMX?
I'm not sure what you are attempting to infer, but the Michelson Morley experiment yielded null results. They measured no difference in dimensions, or anything else.
 Recognitions: Gold Member MMX was the inspiration for Lorentz to explain the null result by saying that the physical dimension of the apparatus was contracted along the direction of the aether wind. Michelson, on the other hand, believed that he could not measure the aether wind because he thought the earth was dragging the aether along with it.
 I don't think even the best physicst can explain exactly why time is a dimension. The best explanation I have read in any book written by one is that if they where to point out on a map the exact coordinates where you will meet them you would never be able to meet them there without knowing when they will be there. Then the time coordinate allows your meeting.

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 Quote by John232 I don't think even the best physicst can explain exactly why time is a dimension. The best explanation I have read in any book written by one is that if they where to point out on a map the exact coordinates where you will meet them you would never be able to meet them there without knowing when they will be there. Then the time coordinate allows your meeting.
Seems like even amateur physicists can explain exactly why time is a dimension.

 Quote by ghwellsjr That's why I put "distance" in quotes, so that you would not take exception to my terminology because I can't tell what terminology you prefer. Spacetime does not create the issue, nature does, and spacetime interval is a solution. But you said you could solve the same problem that "spacetime interval" solves except by another method. Now are you telling me that you don't believe there is any solution? Let's take, for example, the first half of the twin paradox. The twins (or two identical clocks) start out at the same age (or set to the same time) at the same location. One of them accelerates away and travels at a high speed for awhile and then decelerates and comes to rest with respect to the first twin (or clock) some distance away. This defines two events: the first is when the traveler starts out and the second is when the traveler stops. When this situation is analyzed from different frames of reference,different answers will be determined for the actual physical distance the traveler traversed and for the actual physical time that it took the traveler to make the trip. Do you agree? If yes, then how do you reconcile the different measurements of distance and time? If no, then please explain why.
No. I believe theres a fundamental error in the relativist's notion of distance. What's actually being measured as "distance" is actually "distance traveled". hence why "distance" is defined in terms of c. Relativity alludes to the qualitative static distance (between two objects or "events") but explains the theory with respect to dynamic distance traveled. Case in point is the phenomenon of "length contraction".
If i'm standing 20 yards away from a tree (where there's a measuring device) and a muon zips past me at near the speed of light, relativity theory says that the distance between the muon an the tree contracts. Now did the static distance between me and the tree shrink or was it the distance between the muon and the tree? And if the answer is the later, then say there's a rock between me and the tree, did the distance between the rock and tree also shrink?

 Quote by cshum00 You can as long as it is mathematically useful. Physical "spacial dimension" only has 3 dimensions x,y,z. Time is a dimension but not a spacial dimension. Because Special Relativity uses both spacial dimension and time dimension then we combine them so that it is mathematically useful and we call it "space-time dimension". I am just saying that you were not defining dimension abstractly enough but you are rather defining spacial dimension in specific instead.
As far as i'm concerned, time is not a dimension. it's simply a number line. it's then endowed with orthogonality and combined with the 3 "spatial" dimensions. as such mathematicians have simply just created, lie u said, a useful framework for modeling the physics of objects in space.

 Quote by Phrak As in space alone, there is no preferred direction in spacetime that you can call the time direction. In this regard they are inseparable, unlike space+temperature, say. A Lorentz symmetry is more of a rotational symmetry, similar to rotating spacial vector.
So what's the preferred direction of temperature?

 Quote by ghwellsjr I hope you realize that the issue we are talking about isn't confined to distances between objects in empty space, the same issue exists between the shape of a single physical solid object on the surface of good old earth. The Michelson-Morley experiment is what started this whole thing. It was a single very large, very solid, very massive object that seemed to be changing its dimensions simply by being rotated. How do you understand Lorentz contraction when applied to MMX?
I understand it as an indirect inference from a null-experiment.

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 Quote by TheAlkemist No. I believe theres a fundamental error in the relativist's notion of distance. What's actually being measured as "distance" is actually "distance traveled". hence why "distance" is defined in terms of c. Relativity alludes to the qualitative static distance (between two objects or "events") but explains the theory with respect to dynamic distance traveled.
In flat spacetime distance can be defined as coordinate separation in a consistent way. What is a ruler but a stick with coordinates marked off ? In SR each inertial observer has a set of coordinates and a definition of distance. The Lorentz transformation allows us to transform the coordinates between frames.

 Case in point is the phenomenon of "length contraction".
Which is what happens when a distance in one frame is expressed in the coordinates of another frame.

It is completely consistent and your assertion
 there's a fundamental error in the relativist's notion of distance
is incorrect.

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 Quote by TheAlkemist Case in point is the phenomenon of "length contraction". If i'm standing 20 yards away from a tree (where there's a measuring device) and a muon zips past me at near the speed of light, relativity theory says that the distance between the muon an the tree contracts.
SR says that in your rest frame, the muon itself is contracted but the distance the muon has to travel is not contracted. In the rest frame of the muon, the distance between you and the tree is contracted but not the muon itself. That's why it can survive long enough to make the trip.
 Quote by TheAlkemist Now did the static distance between me and the tree shrink or was it the distance between the muon and the tree? And if the answer is the later, then say there's a rock between me and the tree, did the distance between the rock and tree also shrink?
There are two answers depending on whether you are using your rest frame or the rest frame of the muon. In your rest frame the distance between the rock and the tree does not shrink. In the rest frame of the muon, the distance between the rock and the tree is shrunk.

You can use either rest frame (or any other frame) to analyze the situation and they will all get the same answer, which is even though the half-life of the muon is too small for it to survive traveling "long" distances", from your rest frame, it survives because it's clocks are running slow and from its rest frame, it survives because it doesn't have very far to travel.

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Quote by TheAlkemist
 Quote by ghwellsjr I hope you realize that the issue we are talking about isn't confined to distances between objects in empty space, the same issue exists between the shape of a single physical solid object on the surface of good old earth. The Michelson-Morley experiment is what started this whole thing. It was a single very large, very solid, very massive object that seemed to be changing its dimensions simply by being rotated. How do you understand Lorentz contraction when applied to MMX?
I understand it as an indirect inference from a null-experiment.
Yes, that is very true. I'm glad you agree with me and everyone else on this point.
 Wow, what a question. Well, we all know that time is the duration it takes actions to happen. Time is the fourth dimension, as Einstein viewed it. People before Einstein, like Newton, viewed time as a definite quantity. They viewed it as a definite measurement that is the same for everybody. Then came Einstein, and said that time is in fact relative, it is not an equivalent quantity for everyone. First, he said that the ultimate speed of the universe is the speed of light (you can't go faster than the speed of light). He also said that time is a relative measurement, it depends on your speed; the closer you travel to the speed of light, the slower time beats. Also, time beats faster if you are away from gravitational pull (that's why our GPS works the way it does. It has to take General Theory of Relativity into consideration).That is our basic understanding of time.Time travel to the future is very possible, you just have to go on fast speeds and you age less than your twin, you are in some sense a traveler to the future, However, we don't really know for time travel to the past, because you can't change your past. There are also some new theories on time such as wormholes, and string theory's tiny curled up extra dimensions... The subject of time is really a huge subject, and physicists are still investigating on time.

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