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If I drop a key

  1. Dec 21, 2003 #1
    If I drop a key....

    Dear friends!

    I'm new here but have read a lot of inforamtion and books about GR. However I struggle as much as all others to get a picture of the specetime.

    I will challenge you - not with a question but with an invitation - who tell the best story that explains why my key drops to the floor when released from my hand! You are allowed to use entities as bent space, equvalence principle and so forth but not any equations or math. I.e to do it the eisteinian way!

    I do want to understand gravity from the bent spacetime perspective. No rubber sheets - I want the truth....

  2. jcsd
  3. Dec 21, 2003 #2


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    so do we all, it is impossible to give you a true answer
    gravity is the most vexing question for science, you are
    free to scan all the evidence, at this time ST seems out
    of favour, LQG is your best bet, but you must study and
    form your own oppinions. good luck:smile:
  4. Dec 21, 2003 #3
    Consider an object accelerating in a region of flat space-time. As the object's velocity increases wrt an arbitrary rest frame, it's orientation increasingly becomes more space like. If you plot it on a space-time diagram, it will trace out a curve as it accelerates. I would say it's reasonable to conclude that an object which accelerates curves through space-time. Now if gravity, whatever it may be physically, causes an object to accelerate, it's likewise curving through space-time. How is gravity causing this to happen? I think it was a fantastic, conceptual leap to assume that space-time is curved. Who can prove that space-time is physically curved? I can't. But GR is based on this premise and has passed experimental testing of it's predictions. Does that prove that space is curved? I suppose not, but until a better idea comes along, I'll stick with
    it for now.

    An object which accelerates curves through space-time. An object which moves through curved space-time accelerates. That's a visual description of the equivalence principle.

    Why does curved space-time cause an object to accelerate? The analogy of the rubber sheet with the heavy sphere deforming it does have its problems. If you put the rubber sheet in free-fall, then nothing is going to happen. It needs gravity in the first place for the sphere to deform the sheet. It needs the very thing which it is trying to describe.

    Consider that all matter in the universe is in motion. When an object moves through a region of curved space-time it follows the curvature and its orientation is changed such that it will move toward the center of the curvature. Of course, depending on the objects velocity, it will either fall to the planet's surface, orbit the planet, or slingshot away. The point is, the object's orientation in space-time is changed just like an object accelerating in a region of flat space-time.

    Considering the key you mentioned originally, if you were just floating in space (moving at a constant velocity) and you release the key, it will just float with you. You can consider yourself and the key at rest. From a four-dimensional perspective, however, you still have a motion in time. When you drop the key on the surface of the planet, it follows the curvature of space-time and it's motion is oriented toward the center of gravity (curvature) and it will move in that direction until the surface of the earth stops it. It will still try to move toward the center but it can't move through the surface obviously and this is felt as weight.

    Have I suceeded in confusing you?
    Last edited: Dec 22, 2003
  5. Dec 22, 2003 #4
    Thank you very much Jimmy, I'm gonna print this out and read it over and over during the holidays :)

    On my way throght spacetime I always gett happy to be close to others like you, I'm far away, and read this when wooking up, i.e. another time, but still, I'm greatful to you for sharing!

  6. Dec 22, 2003 #5
    Your welcome. I would like to mention that I am not a physicist but I have studied quite a bit on my own. This is how I visualize space-time and how objects move. I'd watch for responses from experienced and more knowledgeable users. I believe what I have said is essentially correct but it is difficult trying to explain it visually. Someone else may point out an error of which I am unaware. Remember, GR and SR are mathematical models and a simplified explanation is limited. Still, I hope it increases your understanding.

    I recommend a book by Lewis Carroll Epstein: Relativity Visualized
  7. Dec 29, 2003 #6


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    Re: If I drop a key....

    Gravity is what we see as the behavior of spacetime in the presence of matter/energy. Through GR, we can visualize gravity as "curved space"...and the math works out well...but according to GR, there is no 4th dimension of space into which our 3D "curve" (so, it's not actually curved in a sense we are familiar with).

    The key drops from your hand because the opposing force from your hand was the only thing keeping the key away from the floor in the first place. Spacetime was constantly accelerating the key toward the floor (gravitational "force" resulting from the Earth's mass).

    Technically speaking, science does not offer absolute truth. It offers explanations and descriptions, as best can be figured out from the current evidence. So far, the phenomenon of gravity is best explained by General Relativity. Gravity is essentially equivalent to spacetime, and as such is at the very foundation of the universe. But what is that made of? Gravitons? Strings? It's unknown.
  8. Dec 29, 2003 #7
    Re: Re: If I drop a key....

    So what's going on with my posts on the other thread?
  9. Dec 29, 2003 #8
    Re: Re: If I drop a key....

    He knows that already. He wants to know why.
  10. Dec 30, 2003 #9


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    Re: Re: Re: If I drop a key....

    Yeah, and I answered that. He wants to know what gravity IS (the whole, unadulterated Truth). That is unknown, but GR best explains it so far.

    Send me a PM with a link to the topic you are referring to.
  11. Mar 25, 2011 #10
    Re: If I drop a key....

    Dear Friends,

    You will not believe how relieved I was when I saw this thread yesterday. I know very well how teachers explain GR. All those bookish explanations including stretched rubber sheet and all. Tensor calculus is also not that difficult to me. But still, like Per, I want to know what exactly make the key move towards earth.

    I don’t have that much difficulty in imagining a deflection in a moving body’s path when it comes near a massive body which curves the space-time.

    But for a body at rest, like the key in Per’s hand, what makes it move when it is released is not yet within the limits of my visualization power.

    After reading this thread yesterday, I ordered the book “Relativity Explained” from Amazone. (Thanks Jimmy).

    I would like to thank Per for this thread and all others who replied. As this thread is very old, could you all great minds please let me know if you guys are still active in the forum?

    I would also like to ask Per if he has received a better explanation for his question in the past decade.

    Thanking you all,

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