What Is Time: Answers to Your Questions

[Stratosphere]

I understand that time has effects and that time can be slowed by gravity and such but I don't understand what time really 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 spatial coordinates, and the time coordinate for the time that the ball was at that point in space. The time coordinate is different from the spatial 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 don't 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

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

 

[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
http://blogs.discovermagazine.com/cosmicvariance/2009/03/08/the-envelope-please/

 

[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]

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]

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.

 

[UncertainOne]

"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."

saying this though is going by the arbitrary value we give time here, namely the ticks of a clock! Think of how different the answers to these questions would be for me, a mere land dweller as opposed to someone in a weaker gravitational field/travelling super fast. This is a very profound question which isn't very easily answered, however i think there were a few good answers given hier, and i think the one about the reference frames was the best one given.

 

[cragar]

ya it is a very complex question , Einstein thought he figured out what time was in
1915 but then changed his mind in 1916

 

[rosie]

Is time related to distance? I think what I'm asking is this. If I were to travel into space with a clock on board my ship and I were to look back at Earth - at differing distances I would see Earth moving in and out of daylight hours at differing frequencies. So the frequency of that transition in and out of daylight would depend on the distance I am away from earth. If the regularity of that flicker could be seen as say, every 10 minutes on my clock, but each flicker relates to an advance of 24 hours time on Earth then my distance from Earth could be calculated. The further the distance, then the faster the flicker into and out of daylight hours. It's as if everything has it's own time frame.

It makes we wonder what it would be like to ride an electron around the orbit of an atom. Would that time frame have any correspondence to daylight hours?

And on and on. When we look at galaxies spinning in the distance - it's a given that we are looking at the galaxies past. But if we can see it describe an entire orbit in, say, 24 hours, then we are not only looking back into time but at contracted time. Like a fast forward. Nothing whatsoever to do with real time on that object.

 

[cragar]

space and time are one thing , if i move through space i am moving through time so i guess distance and time are related.

 

[tiny-tim]

Welcome to PF!

Hi rosie! Welcome to PF!

Is time related to distance? I think what I'm asking is this. If I were to travel into space with a clock on board my ship and I were to look back at Earth - at differing distances I would see Earth moving in and out of daylight hours at differing frequencies. So the frequency of that transition in and out of daylight would depend on the distance I am away from earth. If the regularity of that flicker could be seen as say, every 10 minutes on my clock, but each flicker relates to an advance of 24 hours time on Earth then my distance from Earth could be calculated. The further the distance, then the faster the flicker into and out of daylight hours

Time is related to speed, but not to distance.

The "regularity of the flicker" that you would see would be the same, no matter how far away you were.

It changes when your speed (relative to the Earth) changes, because of Special Relativity, but so long as your spaceship stays at the same speed (doesn't even have to be in the same direction), the flicker rate will be the same.

It's as if everything has it's own time frame.

It makes we wonder what it would be like to ride an electron around the orbit of an atom. Would that time frame have any correspondence to daylight hours?

Every velocity has its own time frame, but every thing (in every position) doesn't.

For example, an electron (assuming it has a "classical" circular orbit) has constant speed relative to the Earth, and so has a different "time frame" … not to be confused with a Special Relativity space-time frame of reference, which of course only applies to constant speed in a fixed direction.

 

[rosie]

thanks tiny tim for your patience. But I'm still confused. Here's my scale. I'm in a busy street. Hectic. Cars everywhere - people rushing around. Then I go to the top of a really tall building and look down. Cars seem to be crawling - no sense of rush. Distance has widened my perspective of the ground and the relative to that perspective the traffic appears to be slower. Coincidentally my time frame is in fact also slower - marginally.

Now I go up in a spaceship and orbit the Earth at a distance, say of 5000 miles. Now provided I can adjust my speed to the necessary I could position myself at precisely the point where the sun forever rises - early morning and stay in that orbit. Then my time frame is consistent with Earth clocks. And I'm traveling at a constant speed.

Now I decrease that speed and I'm now falling behind Earth's time frame. Now I increase my speed and I'm exceeding Earth's time frame. At each position I am traveling at a constant velocity.

Then - I position my orbit at 5 million miles away. And I repeat the exercise. Velocity and distance can still coincide with Earth's time frame - obviously assuming I could travel that fast. And when I'm out of synch with Earth's time would I lose time and gain time exponentially and respectively? And surely - depending on that distance and that velocity I could see Earth flicker in and out of daylight.

 

[rosie]

I'm trying to get my head around the fact that if I traveled ever outwards, so to speak - at a constant velocity - then I'd share Earth's time frame. I just can't get there - but will keep trying.

 

[Jack21222]

Here's a link to a documentary by Brian Cox entitled "What is time?"

http://www.youtube.com/watch?v=vYmdgHyCF_Q&feature=PlayList&p=ACC16BC09031C4EE&index=0&playnext=1

Seems appropriate, plus I've got a little bit of a man-crush on Brian Cox.

 

[rosie]

jack - just watched that youtube link. V interesting but not much help. He's also looking for answers. Thanks for this. I see there may be other links. I'll try and get more.

 

[Andrew Mason]

Is time related to distance? I think what I'm asking is this. If I were to travel into space with a clock on board my ship and I were to look back at Earth - at differing distances I would see Earth moving in and out of daylight hours at differing frequencies. So the frequency of that transition in and out of daylight would depend on the distance I am away from earth. If the regularity of that flicker could be seen as say, every 10 minutes on my clock, but each flicker relates to an advance of 24 hours time on Earth then my distance from Earth could be calculated. The further the distance, then the faster the flicker into and out of daylight hours. It's as if everything has it's own time frame.

It makes we wonder what it would be like to ride an electron around the orbit of an atom. Would that time frame have any correspondence to daylight hours?

And on and on. When we look at galaxies spinning in the distance - it's a given that we are looking at the galaxies past. But if we can see it describe an entire orbit in, say, 24 hours, then we are not only looking back into time but at contracted time. Like a fast forward. Nothing whatsoever to do with real time on that object.

The exact relationship between time and distance is:

d = ct

where d is the distance that light travels in time t. The ratio between d:t (which is, of course, c) is the same in all inertial frames of reference. So you can definitely relate t to a distance. But this does not define it. Time is a measure of separation of events. It is fundamental. So it cannot be defined in terms of simpler concepts.

AM