# What Exactly is Time?

#### Stratosphere

I understand that time has effects and that time can be slowed by gravity and such but I dont understand what time realy is. Gravity is the bending of space and time but what is time? What causes time?

#### flatmaster

Before trying to understand the GR definition of time, you must understand the classical description. Time is a coordinate. That is, it is simply a number that describes either when, or where something is. Immagine a ball bouncing around a room. You need exactly four numbers to describe the motion of the ball. You need three spacial coordinates, and the time coordinate for the time that the ball was at that point in space. The time coordinate is different from the spacial coordinates in that it only moves in the positive direction.

#### Stratosphere

I thought the General Reletivity says that an observer traveling near the speed of light would not agree with the time though. I also thought that time CAN travel backwoards if you have enough energy. The laws of Physics dont prevent time travel.

#### flatmaster

If you want to really understand this, you need to learn classical first. You're want to run and you can't walk yet. Unfortunately, that's the way it is.

#### FredericGos

Still, this is general physics and I think the question is still a good one. Otherwise, people wouldn't write stuff like:

http://www.chronos.msu.ru/EREPORTS/rovelli_time.pdf [Broken]

Maybe the OP thought that there is this a priori concept of time that lacks explanation? And
that it might be the root of many problems? What do you guys think about the above paper?

/Frederic

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#### v2kkim

My simple thought is that we human feel time as things happen sequentially and we experience that certain physical reaction takes a certain time. But how about photons point of view ? A photon travel at speed 'c', so its clock (if they carry one) looks frozen, meaning that how far it travels no time elapse from photon point of view. So photon does not have time and no aging -- which is quite different from us.

#### harvellt

This is a shameless plug for my favorite blog, pretty interesting little over my head but interesting

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#### nuby

Time perception is an interesting subject. Not sure if it ties into physics though... Quantum physics maybe?

#### Gear300

Time perception is an interesting subject. Not sure if it ties into physics though... Quantum physics maybe?
quantum physics is physics

#### HMS

Good quote on Time

Time is the quality of nature that keeps events from happening all at once.

#### Fredrik

Staff Emeritus
Gold Member
I'm just going to quote myself, and say that the Rovelli paper is interesting, but I didn't really understand it (when I read it a few months ago). I'm going to have to have another look some time.

Regarding the definition of time...

We can define a coordinate system in Newtonian mechanics, SR and GR as a function $x:M\rightarrow\mathbb R^4$, where M is spacetime, and then define "coordinate time" as a component of that function. In SR and GR it's also necessary to define "proper time", which is the integral of $\sqrt{-g_{\mu\nu}dx^\mu dx^\nu}$ along a curve.

That takes care of the definitions in the mathematical models used in these three theories, but the theories must still include postulates that tell us how these things are related to what clock's measure. In Newtonian mechanics, clocks measure coordinate time. In SR and GR, a clock measures the proper time of the curve that represents its motion.
Time is certainly more than that mathematical expression, but any answer to the question of what time "is", will always be in the form of a mathematical model and a set of instructions about how to use that model to make predictions about the real world. The best answer we have so far is the one provided by general relativity. The relevant "instruction about how to use the model to make predictions" says that what a clock measures is the proper time of the curve that represents the clock's motion. I don't think anyone has a better answer than that at this time.

Note that two definitions of time are needed. First we have to define time in a mathematical model (in this case as a certain integral), and then we have to define it operationally (as "what a clock measures"). Then we postulate how the two are related.

#### cragar

no one will ever truly understand time.

#### NoobixCube

The direction of Time is shown in thermodynamics by the increase in entropy..
that's my two cents

#### harvellt

The direction of Time is shown in thermodynamics by the increase in entropy..
that's my two cents
I feel the thermodynamic definition of time is to narrow, yes entropy increases and we can't un-crack an egg but I think the fact that processes that are exactly the same will increase in entropy at different rates according to the same observer. Take an atomic clock on earth take one exactly the same and send it spinning around the earth as fast as you can. Few years later grab the one spinning around the earth and put it next to the one sitting in your lab and ...they are different. Same processes same "time" from the observers point of view different change in entropy.

#### Gear300

Time could simply be our perception of the 4th dimension. It is sort of like saying that the there are no spatial, time, etc... types of dimensions; there are simply dimensions and time might not be different from the 3 spatial dimensions...simply that we are unable to perceive 4 spatial dimensions, so any dimension after 3 is a different experience.

#### Bob S

We believe that the speed of light is c in every inertial refrence frame. If I had a cesium atomic clock at rest in my reference frame, and I counted x million-billion wavelengths of a certain atomic transition, wouldn't that qualify as measuring time, and indeed quantify what a second was in my reference frame?

#### confinement

"What is time?" is not a proper question according to the use of the word "time." Examples of proper uses are:

"What time is it?"
"How much time do we have remaining?"
"At what time will it arrive?"
"How much time is this going to take?"
"I don't have time."
"This is a waste of time."

#### Klockan3

Time as specified in SR is what governs all processes, while proper time is what we can actually use as a coordinate system, they are not the same.

As an example the twin paradox is perfect to show the effects, by moving away and then back one twin becomes older than the other yet they they both have exactly the same coordinates. Therefore you must conclude that the time we measure and the proper time used as a coordinate system are not the same things.

#### Andrew Mason

Homework Helper
Here is my attempt: Time is a measure of the separation between events that occur at the same location (in a spatial frame of reference).

#### cragar

What is time is prolly one of the most complex questions that could be asked ,
i was wondering does time have a field. And if time is the fourth dimension
the could we say using newtons laws for every action there is an equal but opposite
reaction , if we can travel forward in time then we must be able to travel backwards in time.

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