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Homework Help: Time Traveling

  1. Apr 2, 2008 #1
    I think I know why we cant time travel backwards. I have been researching like mad at my skool and I think i have the answer. If I'm wrong, PLEASE tell me. This is for a research paper, and I am trying to navigate my way out of the math and stuff:

    We are in the present, while physical and psychological (mental) time are in effect. The future has not happened yet, so it will happen. That means that Physical time will be is in effect and mental time is not in effect yet. The past has neither working on it, since it already happened. Also, if you did go into the past amd meet yourself, would there be one person or two?

    your responses have to be a little bit more.....simple, since I'm only in Middle School. Thanx
  2. jcsd
  3. Apr 2, 2008 #2
    Welcome grimlove527.

    I'm not sure what the difference in physical and psychological time is you describe here. In science physical and psychological time are the same thing. Adrenaline (fear) can make time seem like it slows down for some people but it's just a perception. We can time travel into the future faster (not past) but it has nothing to do with psychological time. So physical and psychological will be exactly the same thing in my explanation.

    Everybody is traveling into the future. The only thing you can change is how fast you travel into the future. There are two ways to do this. One is in a strong gravitation field time slows down. When time slows down you will not notice because psychological time is always the same rate as physical time. A better way to travel into the future is to travel very fast relative to where you want to see the future of, such as Earth. If you leave in a spaceship travel at 86% the speed of light and come back in 5 years then everybody else on Earth will be 10 years older. If you could accelerate your spaceship at 1 G for 12 years then the Earth would be over 100,000 years older since you left. All the people you knew would be gone.

    If I misunderstood your question any let me know and I'll do what I can. If interested I can show you how to calculate time dilation from just a cheap little desk calculator, or windows calc in your start menu. Middle school is old enough to learn it well enough to do time dilation calculations for uniform velocities.
    Last edited: Apr 2, 2008
  4. Apr 2, 2008 #3
    ok, thanx. your reply hepled me a lot.

    But you didn't answer the secong question:If,hypothetically, you could travel back into the past and me yourself, would there be one person or two? and also ,do you have an answer to the grandfather paradox?
  5. Apr 2, 2008 #4
    Oh yes I forgot. That one is a bit more difficult because it starts with, "if the impossible was possible what would happen". Suppose the time machine worked but that meant that the person who built it wasn't born yet therefore can't meet the younger self. You couldn't know if yesterday was the year 2324 and the person who reversed time is not here to know it either. If the time traveler somehow separates himself to go back with the time how does time reverse? They're in the time machine so they're not there to reverse that accident they got into last month.

    So if you ask if they will meet themself you must assume the past you go to is different from the past you remember, just very close to it. It also means the future can be much different than the one you remember while another you thinks the time machine never worked.

    You also have the problem of how do you reverse time for the whole Universe. It is not even possible for all the parts of the Universe to know when the reversal began so parts of it wouldn't go back in time at the same time.

    This is why these kind of "if" questions really have no answers.
  6. Apr 2, 2008 #5
    right. I forgot that you would have a different memories than you did before you went back in time. I have another question: Do you know anything about wormholes? I already know that they are shortcuts to get to a distance far away. I also know that thet apply to the theory of realitivity. I just can't find out how we came upon them. I heard there is no evidence that they exist.
    Last edited: Apr 2, 2008
  7. Apr 2, 2008 #6
    Time travel- simply impossible and always will be.

    When you wrote "we are in the present" you were absolutely correct. That simple statement of fact is the key to understanding why we cannot nor will not- ever- physically travel, either forward or backward, in time. We, the computer keyboard I am using, my house, the planet, all the planets in our solar system not to mention all the galaxies; in fact the universe and everything in it; only PHYSICALLY exists in the present- in the NOW. There is no future- physically now. We and the whole universe does not PHYSICALLY exist in ten minutes AT PRESENT. We can be confident that it will because it DID ten minutes ago.
    And thats the point. Thats the simple truth of it all. Because we remember the past we therefore anticipate the future.
    We (and everything) physically exist only in the NOW. The PRESENT. We and everything (and I mean everything) do not physically still exist ten minutes- one nano second ago. Nor do we physically exist at present, one minute from NOW.
    Simple eh?
  8. Apr 2, 2008 #7
    yeah, thanks for clearing that up. The way you put it, it does sound simple. I know the future isn't real yet, and the past has already happened. So how can we go back to something that has already happened, right? This topic of "time" started on a busride to skool, and now its blossoming into new discoveries . Before I started researching, I had this theory that if the earth stopped moving or revolved backwards that time would stop or we would go back in time. Now that sounds rediculous. That was only a month ago.

    I think we should eliminate the future tense all together
  9. Apr 2, 2008 #8
    No probs. Thats just my theory anyway. Forget all those "grandfather" paradoxes etc, to explain the complexities and consequencies of travelling back in time- its never going to happen. Its PHYSICALLY impossible- and, like I said, always will be.
  10. Apr 2, 2008 #9
    Well, what about the theory of realitivity? I mean, would going forward in time faster be considered "time travel"?
  11. Apr 2, 2008 #10
    No, not really, although I know what you mean. Try to get away from the idea of forward travel in time. The concept of time exists only, I believe, in us humans. We remember past, we anticipate future and we call that imagine space- time.

    Regarding moving or travelling, either slow or fast (even very very fast) does not, in my opinion alter either the travellers now or anyone or anything else's now. They, the two nows remain the same- they exist as one together.
    All that speed does is manupulate distance. Years ago the idea of being able to get to Australia from uk in a day would have been thought of as impossible. If I, back then, telephoned my Aunty Sheila in Australia drom London and said "I am just leaving the UK to come and visit you" and then arrived less than twenty four hours later on her doorstep; she would be amazed to say the least and perhaps thought I had used a "time machine". Thats the point. I, of course, didnt. But I did manipulate distance by speed. However both mine and my Aunty Sheila's now remained the same throughout my travel and her waiting.
    My belief in all this is to break all the silly stuff down and simplyfy it, Only then does the truth become clear.
  12. Apr 2, 2008 #11


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    This is the best thing ever written on http://xkcd.com/209/" [Broken]. To me the moral of the story is that your have to be careful in defining what you mean by the term. This is why I don't think time dilation in relativity counts as time travel.
    Last edited by a moderator: May 3, 2017
  13. Apr 2, 2008 #12
    I agree. Its simple when you break it all down eh?
    Last edited by a moderator: May 3, 2017
  14. Apr 2, 2008 #13
    It does become clear. Exept for the two "nows" part. And i agree that speed does manipulate distance.
  15. Apr 2, 2008 #14
    What I mean is there is only ONE NOW. For everone and everything. When the concept of this is grasped then the impossiblity of physically travelling in time (forward or back) becomes obvious. The past, the future does not physically exist right now. And, I'm sure you will agree that, if it is physically impossible to travel to a place that does not exist then it is, therefore, equally physically imposible to travel back or forward in time. Because the past and future do not exist in the now.
  16. Apr 2, 2008 #15
    I can't be very exact with this description but I think you will be happy with it.

    I'll describe the effect then describe the cause. A good way to imagine what a worm hole does is to imagine a hallway in your front yard. You can look through this hallway and see out the other side. There is something funny about it though because the hallway is 50 feet long on the outside but inside it is only 5 feet long. Now a wormhole is a bit more complicated (and dangerous) but the effect is about the same. Except it happens in space with no hallway around it. So how can this be with known laws of physics?

    Remember above where I said gravity slows time down? Well to the person in the gravity field time is not slow, it's looks to them like time outside the gravity is fast. The person in the gravity field, because they measure time as normal will measure the distances between things as shorter. This means that if your friend is upstairs in a two story house you are closer to your friend than your friend is to you by a very very tiny amount.

    Now for a wormhole to make two places a lot closer, if you go through the right invisible hallway, it needs to be a very very powerful gravity field. So powerful that if you got too close the gravity would rip you apart. So how do we get through without being ripped apart?

    We'll the first step is to spread the gravity field out over a big area of space. This works because space and time dilation does not depend on how strong the gravity is around you. It depends on how deep in the gravity field you are. For instance, if you were to go to the center of the Earth you would feel weightless as if there is no gravity. However, time will be slowed just as much, and space shortened just as much. So we can have these effects without being ripped apart by gravity. The gravity must still be around you just like in the center of the Earth.

    Now to make this hole more like a worm we need to change the shape. The round shape of the Earth doesn't work because you must climb back out the same amount of gravity as you went through going in. So we stretch this gravity out into a long tube such that you are at the center all the way down the center of the tube. Then you can travel a great distance before climbing back out of the gravity on the other end. This distance through this hole will be much shorter than the distance outside the hole just the the hallway in the front yard.

    No in the description I give it is normal gravity with normal mass. This means that you may think you went through a short hole in a few minutes but to the people outside you spent many years going through a very long hole. To change this theorist use what they call exotic matter. This is basically matter that has has negative energy density, where normal matter has a positive energy density. This allows for people to get through these wormholes in a few minutes or hours to the people both inside and outside the wormhole. This exotic matter is also thought to help prevent the wormhole from collapsing on itself.

    There are theoretical problems.
    General Relativity (gravity) does allow these things to exist if you can arrange enough matter in the right way. It does not say whether you can actually arrange matter like this in a stable manner, only that if you did it would work.

    This exotic matter is another problem. Exotic matter is thought to have negative mass so the gravity repels instead of attracts. This is how it helps stabilize the wormhole by balancing attraction and repulsion. This is a purely theoretical substance which has never been observed. Though it is unknown whether exotic matter is absolutely required for wormholes.

    It is unknown if these solutions will remain valid in a theory that include both General Relativity and Quantum Mechanics.

    You are correct that there is no evidence that they exist in a meaningful way. The best we can say is that they are allowed if matter can be configured certain ways. However, in a very limited sense you travel through a hole in space every day. To a person in orbit around the Earth the distance is longer from your house to your school than it is to you on the ground, by a tiny amount. You are going through a tiny wormhole of sorts every time you go to school.
  17. Apr 2, 2008 #16
    Relative speed does manipulate distance. Nobody can say who is really moving and who is not except relative to something. When I said speed slows down time it doesn't feel slow to the person it slowed down for. The person that you think their time slowed down for says no, it's normal, but the distance is shorter than what your saying it is. So if your time is slow you'll not think that the bus ride to school is slower than normal, you will think the distance to school is shorter than normal.

    The distance to school is how long it takes to get there at a certain speed. If time rate changes it means it changes how you measure that distance. Distance divided by time.

    It can get very complicated to think about until you work through it and get used to it.

    ETA: Oops I misread that. Good job, got that right.
    Last edited: Apr 2, 2008
  18. Apr 3, 2008 #17
    huh. that makes sense. I wonder how we came upon them?
  19. Apr 3, 2008 #18
    Can you please tell me where you found all this info? I need to give citations in my report and I know I can't cite a forum.
  20. Apr 3, 2008 #19
    I would not put it that way. I would say that acceleration, and by equivalence gravitation, influences an observer's measure of distance. It is not the speed but the change in speed that matters.
    Last edited: Apr 3, 2008
  21. Apr 3, 2008 #20
    I'm not sure what to do about citations. The description I posted I created to make it easier to visualize both the effects and what physics was involved in making it possible. It's not perfect but is basically correct.

    Perhaps if you could quote sources for all the physical effects then you wouldn't have to have quotes for the analogy.

    Good place to start. It describes even more than I did;
    It was written by this guy: http://en.wikipedia.org/wiki/John_Gribbin

    The quote for 1 g acceleration in my post #2 can be found here;
    http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

    This Georgia State University site talks about gravitational time dilation and includes some experiments that have been done;

    The length contract is mentioned here and is the inverse of time dilation. I can show you how to solve this equation with any cheap calculator. If you can calculate 9 times 9 on a calculator you can solve these equations.

    Here's a good one for General Relativity;

    Another one that talks about the problems and includes exotic matter.

    Wiki in often not a good source for references but;

    The distance shortening effect of time dilation can be thought of as the reason space is curved in General Relativity.

    Perhaps others can point to some more that are not overly technical.
    Last edited by a moderator: May 3, 2017
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