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Time going in reverse

  1. Feb 8, 2005 #1
    If time were to go in reverse, how exactly would the universe, and humans act? For instance, humans can think and be conscious while time is pointing in a forward array, but can humans be conscious while time is running backwards? That is what one would think, right? Would mathematics have to change? It seems like everything is normal while time is going forward, so why would there be a great difference if time were to run backwards? Why can't we be conscious, thinking, humans? I'm not asking for an argument on the validity of my question, but just a general discussion, if possible.
  2. jcsd
  3. Feb 8, 2005 #2


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    1] I think you may have hit on a question that gets at the very nature of time. If everything *were* going in reverse, who is to say? We wouldn't know the difference, since our consciousness and everything we see and experience is going the same way. This sort of suggests that there is no such thing as forward *or* reverse.

    2] The problem with events happening in reverse is that they violate caasality. A puddle of milk on the kitchen floor could plausibly be suddenly repelled from the Earth, fly up into the air, gathering in one spot, where coincidentally, a pile of glass shards would have done exactly the same thing; they are thrown together with exactly enough force and direction to assemble themselves in a single object down the molecular level. The force of these two masses converging at the same spot in the kitchen would fling them four feet into the air, where they would miraculously have just enough energy to touch the edge of the fridge shelf with none left over, and in doing so, suck the fridge door closed behind them.
  4. Feb 8, 2005 #3
    Very nice response. I think I can give some more insite on it. With out current laws of physics, everything happends for a reason, things fall towed the ground because of the gravitational attraction of the earth. This is my theory, I am just throwing it out there because it makes the most sense.

    When time goes in reverse, the laws of physics, in a sense, are reversed.

    Perhaps, multiplication becomes division, addition becomes subtraction, and so forth. :bugeye: I have not given this much though, so I guess I will think about it for a while, then maybe I can come to a logical conclusion. Ok, let me try to incorporate relativity into this. If, according to relativity, you were to back in time, the events that would go in reverse already happened. I can't travel back in time from the year 2020 to 2010, if it is the year 2005. First I must pass up the year 2020, then I can travel back in time. Now, I am not saying there is no other way of doing this, for instance, a looped univserese, you travel back in time to the point where you end up in the future, but lets just assume that the events have had to already happened for you to reverse them. So this means, that going forward in time would be default, or, regular. And going back in time would be something special. If indeed, the ONLY way to reverse time is for v > c, then it would be default for time to go forward, if v is always restricted to v < c If the notion of forward time is when v > c So, when the time reverses, the laws of physics must change, obviously, there is no gravity, but anti-gravity, but there is still positive motion. I think I sound a little confusing, oh well.
    Last edited: Feb 8, 2005
  5. Feb 10, 2005 #4
    If you ask me what time is I do not think I'm able to provide an answer. I do not think I'm able to define space too. The only think I'm able to say is the difference in space-time between time and space is that time flow in just 1 direction.

  6. Feb 10, 2005 #5


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    Most laws of physics actually have a property called "time-symmetry" which means they look exactly the same in reverse. Suppose I am running a simulation involving some billiard balls bouncing around on a frictionless table, bouncing off each other according to Newtonian laws (with 'elastic collisions', meaning no energy is dispersed as heat or soundwaves). If I pick some initial conditions, let the simulation run for a minute, then freeze the simulation at the end of that minute and note each ball's position and instantaneous momentum vector at that moment, then if I reverse the direction of each ball's momentum vector and keep their positions the same, and use this configuration as a new set of initial conditions which I run forward for a minute using exactly the same laws, then this second simulation will look like a backwards movie of the first simulation. The most fundamental laws of quantum physics are not time-symmetric, but they are CPT-symmetric, which means that if you took the final configuration of a system and reversed all the momenta and reversed the charge of each particle and reversed the directions of your coordinate axes (so that left would become right, up would become down, etc.), then if you played this forward it would look like a reverse version of the original system's evolution.

    There are plenty of physical situations where a backwards movie of a given process would seem impossible, like an egg falling to the floor and breaking. But physicists think that this is not because the laws of physics rule out the backwards version at a fundamental, but because the backwards version involves a decrease in entropy rather than an increase, making it statistically unlikely. But physicists think the only reason increases in entropy are more likely than decreases in entropy is because of the low-entropy starting conditions of the universe at the big bang, which has never been fully understood.
    Gravity is an example of a law of physics that is time-symmetric, so if time reversed it would work exactly the same way. When an object falls to the ground, some of its kinetic energy is dispersed as heat and soundwaves--this is an increase in entropy. If none of the energy was dispersed in this way, so that there was no change in entropy, then the object would bounce back upwards to exactly the same height it was dropped from, and continue to fall and bounce over and over again, so that a backwards movie of this process would look just like the forward version. In general, if you are shown a movie of any gravitational interaction that doesn't involve a change in entropy (a planet orbiting a star, for example), there's no way for you to know whether you're watching a forward version or a backwards version.
  7. Feb 11, 2005 #6
    Good point there JesseM, but as far as the entropy thing :Confused: This universe has so little entropy, and earth has so much organization. The Big Bang theory, nor the theory of Evolution, nor an explination of time has successfully explained it, in fact, I would go far as to say its the only logical way of scientifically, explaining it. But thats a whole topic all together. I personally and not convinced that time could go in reverse. Although, I could be dead wrong. Worm holes were mathematically derived, but it does not mean they exist. So let me ask this, is is possible for a being to be conscious while time is going in reverse, after all, we are conscious when time is going forward. That is assuming that consciousness is just made out of energy (which I cannot confirm, nor anybody).
  8. Feb 11, 2005 #7


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    The earth 'gets' it's entropy 'from' the sun. The sun is giving off masive amounts of energy which means ++order for us.

    Also, it may seem unlikely for a glass to 'jump' up. But remember that the energy that it lost/gained during the fall is all going back into the glass! (IE: the sound is propagating backwards into the glass, the earth is passing the enegy the glass gave to it back to the glass, etc)
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