Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I wonder what is time?

  1. Jul 27, 2003 #1
    What is time? I've heard this questions asked many times on other forums, but since they weren't physics forums nobody seemed to be able to offer an answer. I know time is another dimension, but is it merely that? Is it just hard for people to grasp because they can't see it?
  2. jcsd
  3. Jul 27, 2003 #2
    Time can be considered a dimension in the sense that we define a positional coordinate in our world based on latitude, longitude, and altitude. Well take that a step further and define that coordinate by latitude, longitude, altitude, and moment of existence in time. We observe the effects of time every day; cellular aging, nuclear decay, oxidation of metals. Time is also just a unit of measurement.

    Anyone have some other definitions?


    This thing all things devours:
    Birds, beasts, trees, flowers;
    Gnaws iron, bites steel;
    Grinds hard stones to meal;
    Slays king, ruins town,
    And beats high mountain down.
  4. Jul 29, 2003 #3

    I believe time is a universal inverience, whereby time doesnt stop, you cant force time to stop. We can use time in mathmatics e.g. measure time to reach a certain distance. Time can be precise if properly calculated. As far as Time travel, well its theorectical and that we cannot go back because time is erased, its history. But someday we could go forward in time, depending on how the theory is coming along by that time.
  5. Jul 29, 2003 #4
    Re: Time

    But isn't time relative, i.e it changes if you travel at great speeds?
  6. Jul 29, 2003 #5


    User Avatar
    Science Advisor

    Re: Re: Time

    Yes. In fact, there is a very useful analogy which Brian Green uses near the opening of his book "The Elegant Universe". He proposes that it might be useful to think of all material objects as having a total velocity of c. The analogy he uses is that of a car driving across a dry lake bed. On this lake bed there are lines drawn north-to-South. These lines are one kilometer apart. If you drive your car straight eastward, at a speed of 60 kilometers per hour, you will cross one line every minute. But if you turn to the Northeast, at a 45o angle, it will take you two minutes to get from one line to the next. Although your total velocity is still 60 kph, only half of that total velocity is assisting you in your eastward progress.

    In the same way (says Green), whenever we move through one of the three "spatial" dimensions, that velocity must be subtracted from our progress through time, so that our total velocity still equals c. If you were to turn your car straight north, you would never reach the next line on the lake bed because 100% of your total velocity would be devoted to NorthWard progress. Likewise, if you accelerate to c in any of the three spatial directions, your progress through time will stop. The next "moment" of your life will never arrive, because you have brought yourself parallel to it.

    So, when you are stationary relative to some spatial frame of reference, time passes at lightspeed. When you travel at lightspeed through space, you become stationary relative to time.
  7. Jul 29, 2003 #6


    User Avatar
    Science Advisor
    Gold Member

    Yes Lurch, Brian Greene's The Elegant Universe (I picked it up in a second-hand shop for £3 and though I have already had a formal education in most of the areas he talks of, he really brings SR and GR down to an intutive level)is by and away the best pop-sci book on the subject of physics, it beats the pant's off A Brief History of Time.

    The overriding fact about (special) relativity is that you should always remember that the speed of light in a vacuum will appear the same whatever your frame of reference, everything else follows on from this.
  8. Aug 11, 2003 #7
    Time is the movement of a particle Q from point a to point b, and because it typically happens in 1 second at 78 degrees under a certain gravitation and electromagnetic force we call it one second, all matter influences all matter and so time is variable. In order for one to go back in time all matter would have to take on it's exact opposite characteristics, - would be +, it is more human to want to correct the mistake of the past and make good science fiction.
    OMV, as a clock approaches the speed of light the atom in it's spring winding must slow it's internal motion to compensate for increased motion outside and maintain constancy of energy, remember energy can't be created or destroyed and energy has been transfered in motion, as a result the spring metal's atoms interact and move slower and the clock hand moves slower and time seems to slow down except if you are travelling with the clock then your atoms are also moving slowly and things appear normal.
  9. Aug 14, 2003 #8


    User Avatar

    Re: Re: Re: Time

    Very interesting...

    Perhaps you could compare the situation with the speed of sound, for example. If you're travelling at sonic speed, you will not hear sound waves travelling "behind" you as they are trying to "catch up" with you. They have to "wash over" you in order for you to be able to hear them, so-to-speak.

    Perhaps due to the consequences of luminal speed in vacuo, it is impossible to feel the effects of time because you're travelling "as fast as" or "faster" than time itself.
  10. Aug 15, 2003 #9


    User Avatar

    No. You still feel the effects of time because you are in an inertial frame relative to yourself. However, the people around you will to you, experience no time, while to the people around you, you will also seem to be experiencing no time.
  11. Aug 15, 2003 #10
    Time is both a relative and quantized phenomena from a human point of view. I'm not a physicist, so if anyone sees any mistakes with my arguments feel free to point them out.

    For a photon traveling at the speed of light the trip from even the most distant galaxy happens instantaneously. As far as the photon is concerned, no time has passed whatsoever and it has magically been teleported across the universe and into the future. From our point of view, the photon has been traveling for billions of years.

    The same can be said for a photon falling into a black hole. For the sake of argument, lets assume this black hole forms a wormhole connected to the other side of the universe. From the photon's point of view, it then emerges instantaneously on the other side of the universe in the distant future. From our vantage point of watching the photon fall into the black hole, it never enters the black hole, but steadily slows down and becomes virtually frozen in both space and time.

    Again, whether we perceive time as relative or quantized, just depends upon our point of view. Having mass, we cannot accelerate to the speed of light without first shedding our mass. If we could ask Scotty from the star ship enterprise to beam us up, to convert us to pure energy and transport us, again time would appear quantized. If we could then take off on impulse power and accelerate the ship towards the speed of light, time would appear relative.

    Thus, exactly what time is or isn't, is impossible to say. After all the observations and analyses are done, all we can say is it is apparently just a question of your personal point of view, but time, space, and existence cannot be distinguished one from another. Time without space, and spacetime without events between "things" that exist is evidently impossible. Or, at least, paradoxical or ineffable.
    Last edited: Aug 15, 2003
  12. Aug 15, 2003 #11
    Forgive me if I am wrong, but, as stated above, what if time is mearly a point of view? We have no clear way to prove the existance of it, and the only evidance we have is it's effects. Couldn't time be rather, a measurement made up by man to explain why things move? A measurement of an object moving from point A to point B over a period of x? Now, I must agree with the definition provided by Aeonic333 at the begining of this thread. Time is only a measurement used to define more accurately where something happened.
    It is thought that powers the world
  13. Aug 16, 2003 #12
    You could say the same thing about size or shape or existence itself! Maybe we really don't exist! To me such thoughts are a moot point. We have this word we call "time" with an assortment of definitions and whatnot, but it's apparently a useful concept and that does seem to matter to me. If you can prove everything is just thought, it still won't make any difference in my life unless you can also prove it's apparently useful. For all I know time is in reality a fuzzy bunny!
  14. Aug 16, 2003 #13

    Les Sleeth

    User Avatar
    Gold Member

    My little explanation of time might be too tame for some who want to time travel or think of it as a spatial dimension.

    I believe time can be reasoned out from the way the universe has worked and continues to work. Time is said to have begun with the big bang -- that is why the universe is believed to be upwards of 13 billion years old -- and the big bang, for now, is considered the source of physical existence. Since the big bang, the universe has expanded, radiated away its light, and surrendered its energy.

    So what is time? The atomic organization of the universe is essentially what makes it endure, and so its dis-organization, or entropy, is what is ending it. Well, one might ask, how many entropic events are left in the universe before its organization is gone (i.e., when all its light and energy have escaped organizational structure)? That’s important because if there is no more structure, then the universe will be done for, and so “time” will come to an end (I am predicting the end strictly based on how the universe has worked and is working now).

    Then there is a second factor and that is, what is the rate of occurrence of those entropic events? Well, that depends on where you are and what is going on. Since time is determined by cycles, and the ultimate cycle for the universe is atomic and light oscillation rates, anything which affects those rates also affects time. If one travels, for instance, the cycles are lengthened and so time slows relative to a non-moving frame of reference. If one accelerates, it has a constrictive effect on the cycles and so also slows time relative to a non-accelerating frame of reference.

    Time therefore is the rate of entropy. There is that time, or rate of entropy, for any relative situation; and there is that time, or rate of entropy, for the universe as a whole. How much time is left for the universe is how many entropic events there are before all organization has disappeared. As far as time travel goes . . . with this interpretation of time, time travel is nonsense, as is the notion of it being an actual dimension (beyond a measurement tool).
    Last edited: Aug 16, 2003
  15. Aug 17, 2003 #14
    It seems to me that time could be the result of distortions in the dimensional plane. Of course, I'm just an amateur at this.
  16. Aug 17, 2003 #15
    Here is some interesting excerpts I saw on the inet when just browsing around. It touchs on this topic of time and light. All bolding emphasis is mine. The first is a statement that agrees with what Wuli was saying. The second is the conclusion of what time is.

    "For a long time it was assumed that space and time were fundamental to the underlying reality. Einstein's Theory of Special Relativity came as a great shock to this assumption. He showed that what we observe as space and what we observe as time are but two aspects of a more fundamental spacetime continuum. What this continuum is like we never know -- in this respect it sounds very much like Kant's noumena. All we ever know of it are the ways in which it manifests as the two very different qualities of space and time. Moreover, how much of the continuum manifests as space and how much manifests as time varies according to the relative motion of the observer.

    Space and time have fallen from their absolute status. They are both created through the act of perception, and so belong to the relative world of experience. This is not to imply that they are not fundamental to our experience; they are the dimensional framework within which we structure our mental image of the world. But we deceive ourselves when we assume that they are also fundamental to the underlying reality."

    ...snip snip...

    "According to Einstein's equations, as an observer's speed increases, time slows down, and space (in the direction of motion) contracts. At the speed of light, time has slowed to a standstill and space contracted to a point. Although no object with mass can ever attain the speed of light (Einstein's equations predict that it would then have an infinite mass), light itself does (by definition) travel at the speed of light. From light's point of view it has traveled no distance, and has taken no time to do so.

    This reflects a unique property of light. In the spacetime continuum there is no separation between the emission of a light ray and its absorption. What Einstein called the "spacetime interval" bewtween the two ends of a light ray is always zero.

    How should we understand this? The answer is that we probably should not even try to understand it. Any attempt to do so would once again fall into the mistake of applying concepts derived from our image of reality to the underlying reality. All we need to recognize is that from light's perspective it traverses no spacetime interval.

    However, when we perceive the world from our human frame of reference we do indeed observe a separation between the two ends of the light beam -- the exact amount of separation depending upon our speed. We could say the act of perception "stretches out" the zero interval, and divides it into a certain amount of space and a certain amount of time. Since the total interval remains zero, the amount of space created exactly balances the amount of time created. For every 186,000 miles of space, we create 1 second of time.

    What we conceive of as the speed of light is actually something completely different. From light's point of view -- and this after all must be the most appropriate perspective from which to consider the nature of light, not our matter-bound mode of experience -- light travels no distance in no time, and therefore has no need of speed. What we take to be the speed of light is actually the ratio in which space and time are created in our image of reality. It is this ratio that is fixed -- and this is why in the phenomenal world the apparent "speed" of light is fixed."
  17. Aug 17, 2003 #16
    I had a thought earlier today that if the 4th dimension is supposedly time, and gravity is the same as a constant acceleration in the 4th dimension, then perhaps time is the constant acceleration that is gravity
  18. Aug 18, 2003 #17
    Cytokinesis, Change and motion have no meaning without the concept of time. To speak of acceleration outside of the context of time is an oxymoron, a paradox.
  19. Aug 18, 2003 #18
  20. Aug 18, 2003 #19
    Thanks Captain Obvious!

    Measurement usually implies a dimension of one form or another, and it's a well known fact that time is a dimension, or measurement.
  21. Aug 19, 2003 #20

    Les Sleeth

    User Avatar
    Gold Member

    No . . . I think he meant time is ONLY a measurement . . . it has no actual reality beyond that. Calling time a "dimension" was simply to indicate the three spatial dimensions are invariably linked to change, and that change is, overall, the universe's steady march (as of now) toward total entropy.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook