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time travel

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Applying to Uni? Let Universities come to you. Click here to get your perfect place 20-10-2014
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    (Original post by Superstar6318)
    I am in deep regret over one day, trivial as it sounds, I need to go back

    Will time travel ever exist?
    Man up, and shrug it off.
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    (Original post by dknt)
    It changes by quite a bit. If you travel at 0.9c, time passes ~2 times faster relative to you. At 0.99c its ~7 times, 0.999c is ~22 and 0.9999c is ~70.


    And yes, sort of. That's length contraction. If something is moving, relative to you, then the object appears to be smaller. Relative to that object however, it remains the same size and its you that's smaller.

    When he said more massive, I think he was referring to its energy, not physical size.
    Ah, thank you for the explanation! Point taken & rep given

    About the shrinking of objects when they move faster - If i remember rightly: Einstein gave an example of a car which is 4m wide, and said for it to fit into a garage which is 3.9m wide, it would have to travel just over 22% of c. If the object does not change size relative to itself, how does it work? Does it still shrink relative to the garage?
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    (Original post by ThatPerson)
    Relatively speaking, you could travel forward in time faster. Time Dilation means that as you approach the speed of light time moves slower relatively to you.

    So if you you have person A and B; person B remains on earth, person A goes around and flies in his spaceship at the speed of light. Relative to person B, time moves slower for person A. This means that when Person A lands back in earth, he would have perceived less time than person B perceived so technically he would've traveled to the future. It's impossible to travel back in time I think, apart from something random I read a while ago about wormholes.
    not being a dick... but... well trying not be...


    this is technically wrong as I hope you will see:
    (lets take out at speed of light for 'close to' as I think you meant that anyway)

    you are right about the person on earth seeing time going slower in the spaceship so they would be younger when they get back

    however

    the person in the spaceship would also see time moving slowly for the person on the earth so would think they were young too when they get back!

    it's symmetrical see.

    this is called the twin paradox because of this symmetry problem.


    The resolution to it (i.e why should one twin be the older one in a symmetrical situation) is that it actually violates the premises of special relativity which is derived for inertial frames. When the spaceship accelerates away from earth it is not an inertial frame anymore but an accelerated frame. So general relativity must come into play to explain the effect.
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    Travelling forward in time is theoretically possible, but backward would require an object faster than the speed of light. That's not impossible, but would violate so many principles of physics that it's incredibly unlikely.
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    (Original post by dknt)
    It changes by quite a bit. If you travel at 0.9c, time passes ~2 times faster relative to you. At 0.99c its ~7 times, 0.999c is ~22 and 0.9999c is ~70.
    I was wondering why I was only getting factor increase of only 22.37 on my calculator when travelling at 0.999c! It clearly didn't fit in with my original statement of 59'000 years passing on earth!
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    (Original post by CharlieBoardman)
    Ah, thank you for the explanation! Point taken & rep given

    About the shrinking of objects when they move faster - If i remember rightly: Einstein gave an example of a car which is 4m wide, and said for it to fit into a garage which is 3.9m wide, it would have to travel just over 22% of c. If the object does not change size relative to itself, how does it work? Does it still shrink relative to the garage?
    I don't know if you have a source for that but to me, if it doesn't fit in at rest then it won't fit in at any velocity. However much the car is length contracted in the garage frame to fit through, the garage is length contracted by the same amount in the cars frame, stopping it fitting
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    (Original post by a.partridge)
    I don't know if you have a source for that but to me, if it doesn't fit in at rest then it won't fit in at any velocity. However much the car is length contracted in the garage frame to fit through, the garage is length contracted by the same amount in the cars frame, stopping it fitting
    Well, I read it in Brian Cox's book - 'Why does E=mc^2? And why should we care?'. Surely if the car shrinks relative to us, it would shrink relative to the garage too? - as the garage is at rest like us.
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    (Original post by CharlieBoardman)
    Well, I read it in Brian Cox's book - 'Why does E=mc^2? And why should we care?'. Surely if the car shrinks relative to us, it would shrink relative to the garage too? - as the garage is at rest like us.
    It might be at rest to us but it's not at rest relative to the car
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    Future yes, past no
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    (Original post by CharlieBoardman)
    Ah, thank you for the explanation! Point taken & rep given

    About the shrinking of objects when they move faster - If i remember rightly: Einstein gave an example of a car which is 4m wide, and said for it to fit into a garage which is 3.9m wide, it would have to travel just over 22% of c. If the object does not change size relative to itself, how does it work? Does it still shrink relative to the garage?
    This is actually a very good question. Let me use my own example if you will (its actually a common example used)

    Consider a barn of length 15m and very far away is a pole of length 20m. Now a friend stands at the front of the barn and you pick up the pole a long way away. Now as it stands, you both agree that the pole can not fit in the barn.

    You now run towards the barn at 0.8c. Now let's look at the viewpoint of your friend stood at the barn. He sees you coming towards the barn at 0.8c. Due to length contraction (going through the calculation), your friend sees that the pole is now 12m in length. So from his viewpoint the pole can now fit in the barn, with 3m to spare. You then reach the barn and he closes the door once you're all inside.

    Now look at the viewpoint of you running. You're running with the pole. So you see that still at 20m. However, you see the barn approach you at 0.8c, so by length contraction the barn seems even shorter, now at 9m long. So according to your viewpoint you cannot possibly fit in the barn. However, your friend has closed the door on the barn and you're inside. You both MUST agree that you're inside. So what has happened?

    I'll leave that for you to ponder on
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    (Original post by a.partridge)
    It might be at rest to us but it's not at rest relative to the car
    Ah I see what you mean now. So if the garage isn't at rest relative to the car, the garage would actually be shrinking relative to the car, right?
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    (Original post by dknt)
    This is actually a very good question. Let me use my own example if you will (its actually a common example used)

    Consider a barn of length 15m and very far away is a pole of length 20m. Now a friend stands at the front of the barn and you pick up the pole a long way away. Now as it stands, you both agree that the pole can not fit in the barn. (This is a very important point, all (inertial) observers must agree on the laws of physics, or in this case that the pole cannot fit in the barn.

    You now run towards the barn at 0.8c. Now let's look at the viewpoint of your friend stood at the barn. He sees you coming towards the barn at 0.8c. Due to length contraction (going through the calculation), your friend sees that the pole is now 12m in length. So from his viewpoint the pole can now fit in the barn, with 3m to spare. You then reach the barn and he closes the door once you're all inside.

    Now look at the viewpoint of you running. You're running with the pole. So you see that still at 20m. However, you see the barn approach you at 0.8c, so by length contraction the barn seems even shorter, now at 9m long. So according to your viewpoint you cannot possibly fit in the barn. However, your friend has closed the door on the barn and you're inside. You both MUST agree that you're inside. So what has happened?

    I'll leave that for you to ponder on
    A good example! They definitely contradict each other! Are you saying that he definitely got inside the barn?
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    (Original post by CharlieBoardman)
    A good example! They definitely contradict each other! Are you saying that he definitely got inside the barn?
    heres the answer in better terms than I can give (its a very long vid but watch only from 1 hour 3 minute 10 seconds.

    http://www.youtube.com/watch?feature...2fU9qIVK4&NR=1

    this guy is a pro at STR - learnt all his lectures before I started uni and have clearly been drinking too much
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    (Original post by CharlieBoardman)
    A good example! They definitely contradict each other! Are you saying that he definitely got inside the barn?
    Yes there certainly seems to be a contradiction! But there is a way to resolve this and explain how you and your friend both see that you're inside the barn.

    Spoiler:
    Show
    So according to you, the pole is 11m "too long" when you're rnning towards it. The key here is the transfer of information, which can only happen at the speed of light. If you view the pole is 20m long, then the time taken for information to travel from the front of the pole to the end is just time=distance/speed, t=20m/c = ~ 7x10^(-8) s, i.e. it takes 7x10^(-8) seconds for the front of the pole to "tell" the back of the pole that it has hit the front of the barn. However, travelling at a speed of 0.8c, you can travel a distance of 16m in 7x10^(-8) seconds. So in the period where the information that the pole has hit the front tells the back of the pole, you can travel 16m, when you only needed 11m, so actually you can fit easily in the barn, because you can still move forward until the information that you have ran into the back of the barn reaches the rest of the pole.
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    (Original post by dknt)
    Yes there certainly seems to be a contradiction! But there is a way to resolve this and explain how you and your friend both see that you're inside the barn.

    Spoiler:
    Show
    So according to you, the pole is 11m "too long" when you're rnning towards it. The key here is the transfer of information, which can only happen at the speed of light. If you view the pole is 20m long, then the time taken for information to travel from the front of the pole to the end is just time=distance/speed, t=20m/c = ~ 7x10^(-8) s, i.e. it takes 7x10^(-8) seconds for the front of the pole to "tell" the back of the pole that it has hit the front of the barn. However, travelling at a speed of 0.8c, you can travel a distance of 16m in 7x10^(-8) seconds. So in the period where the information that the pole has hit the front tells the back of the pole, you can travel 16m, when you only needed 11m, so actually you can fit easily in the barn, because you can still move forward until the information that you have ran into the back of the barn reaches the rest of the pole.
    Hmm. I'll have a good think about it before I click your spoiler! will it require knowledge above GCSE level to figure out?
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    (Original post by CharlieBoardman)
    Hmm. I'll have a good think about it before I click your spoiler! will it require knowledge above GCSE level to figure out?
    Should be fine for GCSE! It's conceptually hard, as opposed to calculations. You'll need to know that "information" travels at the speed of light. As for the calculations, some simple speed equals distance over time.
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    (Original post by dknt)
    Should be fine for GCSE! You'll need to know that "information" travels at the speed of light. As for the calculations, some simple speed equals distance over time.
    Rightio then! I shall get some sleep, and then ponder on this in the morning - when my brain can function more efficiently! Thank you for giving me something to think about! I love puzzles
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    The time traveller - Ronald Mallett , give it a read.
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    (Original post by Superstar6318)
    I am in deep regret over one day, trivial as it sounds, I need to go back

    Will time travel ever exist?
    Maybe, but, definitely not in our lifetime.

    I also wish life had a reset button, life would be more fun, easier to improve on certain skills, and undo big mistakes; but that's what life is, what's done is done. Best thing to do is to learn from the mistake, stop feeling bad about it, and just keep moving forward - do things you enjoy, always grab opportunities as they come by, and live life to the fullest - YOLO!

    But, then again, some of my friends said they travelled back in time after using a "special drug."
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    Isn't it a final and complete proof that time travel backwards in time will always be impossible that we do not have amongst us time travellers from the future? I'm sure they would take an interest.

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