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Why is there 0 gravity in space

  1. Sep 21, 2004 #1
    Why is there 0 gravity in space?

    From what I know, in fact I do not know much about SR and GR, so I'm hoping to learn more. In GR, it states that anything which has mass causes a curvature in spacetime, which causes gravity. Why is it that even though we have mass, astronauts travelling in space do not experience gravity at all?

    Also, why is it that massless photons do feel the gravitational force.
  2. jcsd
  3. Sep 21, 2004 #2
    An astronaut orbiting the earth is most certainly affected by gravity. The gravity field in low earth orbit is only a little bit weaker than that on the ground. It is in fact the gravitational force that keeps the astronaut and his vehicle in orbit. Imagine swinging a ball on a string around your head. You must hold the string tightly to keep the ball moving in a circle. In the same way, gravity acts as the "tension in the string" that keeps a spacecraft moving in a circle around the earth.

    As you can see, gravity is still very much present in orbital space. The reason for the astronaut's weightlessness feeling is because he or she is in continual freefall around the earth. The feeling is sort of what it's like being in a falling elevator -- if you and your surroundings are falling in a certain way, it feels like you're floating.
  4. Sep 21, 2004 #3
    This is a very excellent question !! In fact, it is similar to the question Einstein once asked himself, which led to the General Theory of Relativity.

    The story goes that a painter once fell off the roof of a tall building which he was painting. Fortunately, he survived the fall and latter told reporters how strange he felt while falling. He said that he did not FEEL gravity pulling him down, that, in fact, he felt weightless while falling.

    When Einstein read the story he asked the same question you are asking. How could this be? :confused: The detailed answer he developed is called GR.

    The modern view of gravity is this. A large mass does not directly pull other masses to itself. Thus, astronauts, or a photon, or a falling painter do not experience a force pulling them towards the Earth. Rather, a mass (and energy also) curve the spacetime surrounding it. Anything travelling through that area of spacetime therefore does not go in a "straight" line, but only the "straightest" line it can. In the case of the falling painter, that line led to the ground. In the case of orbiting bodies, that line is a 4-d spiral. In the case of the photon, the line is usually only slightly bent.

  5. Sep 22, 2004 #4
    to TAlewis: I guess that the weightlessness is more than a feeling for the astronaut, right? So, if I didn't understand wrongly. Everything around the astronaut is being affected by the gravity field, and so this explains the weightlessness of the astronaut? But what happens if the astronaut is not near a planet or a gravitational field. Why is it he still does not feel gravity even though he has mass?

    to mijoon: I'm sorry, but can you explain the the straight line thing again?

    Thanks alot....

  6. Sep 22, 2004 #5


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    Gravitational force is an interaction between two objects with mass - if he's not near any massive objects, he'll feel no force.
  7. Sep 22, 2004 #6

    Sure !

    This will not be an exact analogy, so experts, please dont nit-pick. The following is ONLY intended to give a general idea of things.

    Suppose that you take a VERY pliable rubber sheet and lay it flat on a table top. Then you take a little toy car and send it across the sheet. It will travel in a straight line, right ?
    Now imagine that you clamp one side of the sheet tightly to the table top. Send the car across the sheet for another trip. This time, as the car moves across the sheet, you pull on the unattached side of the sheet making it stretch more and more. The path of the car would now be curved. How curved would depend on how fast and how much you stretch the sheet.

    4-D spacetime is like a 4-d version of the rubber sheet. Far from gravitating objects, the spacetime is like the unstretched rubber sheet but near a large mass , it is more like the sheet when you are stretching it.

    BTW, The latin word for a pliable object such as the rubber sheet is "Tensor" . That is the name we now use for the math object which describes the stetching of spacetime.
  8. Sep 22, 2004 #7
    This is the Newtonian version of gravity. In GR, a massless object, such as a photon, will also be affected (indirectly) by proximity to a large mass, as the spacetime through which it travels will be curved by that mass.
    Weither an astronaut's ship is floating in inter-galactic space or plunging towards the surface of a planet, the physics inside the capsule are the same.
    In neither case would he feel "gravity" pulling on him.
  9. Sep 22, 2004 #8


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    That's true, but the astronaut is not massless.
    Granted, but he can't feel his own gravitational field. He's not pulling himself in any direction.
  10. Sep 22, 2004 #9
    We seem to be agreed on these things. I would add, for the sake of completeness, that the photon also generates a very tiny field.
  11. Sep 22, 2004 #10
    Gravity is a body force. If we consider the local gravitational field to be uniform, then Earth's gravity pulls on each atom of my body evenly. Because of this, I cannot feel gravity directly. What makes me feel so heavy is the ground pushing up on my feet. Or the chair pushing back on my rump. Or whatever surface supports my weight.

    However, that support force doesn't have to be solid ground. If I get in my spaceship and fire the engines, accelerating at 1 G, I will feel my weight as my chair pushes against me. If the rockets are steady, I can get up and walk around the cabin wall if I want to. It's the ship's acceleration that provides me with a support force here.

    If there is no support force pushing against the soles of my feet, I don't feel my own weight. If I jump out of a tree, for the brief moment that I'm falling I feel weightless just like an astronaut. If I'm in an elevator, and the cable snaps and the car begins to fall, I will float around just like an astronaut. Astronauts can also experience weightlessness in the "vomit comet," a plane that flies on "parabolas" following free-fall trajectories. The feeling of weightlessness is purely the absence of a supporting force to push against your body.

    So it doesn't matter what gravity is doing. If you're not immediately being pushed against anything, then you're weightless. You can be weightless on a trampoline, or in orbit, or far away from any other bodies. It's a condition of your motion, not directly due to gravity.

    Edit: Maybe this Wikipedia article on weightlessness will be helpful:

    Last edited: Sep 22, 2004
  12. Sep 23, 2004 #11


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    My high school physics teacher would scowl whenever a kid used the terms "zero gee" or "weightless." He wanted us to say "free fall."

    By the way, a sentient being whose dimensions were sufficiently large would feel a gravitational gradient.
  13. Sep 23, 2004 #12
    I agree. To me, "zero gravity" and "microgravity" both imply that gravity somehow disappears in orbit, which is not the case. "Weightlessness" is also kind of a misnomer, since the astronaut orbiting the Earth is still acted on by his weight of mg, he just doesn't "feel" it. Free-fall is probably the best term, though it's not what most people think of when they imagine the cause of an astronaut's floating in space.
    Last edited: Sep 23, 2004
  14. Sep 23, 2004 #13


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    Just a quick word on your ability to detect the 'bending' of spacetime that results from your own gravity ... even in Newtonian physics you could, in principle, detect your own gravitational field - place a grain of sand 1 m (say) from you, and watch it. If there's just you (no radiation pressure, no interplanetary plasma, no magnetic fields, ...), and if you put the grain there with zero net motion wrt you, then it will accelerate towards you ... ssslllllooooooowwwwwwwlllllllllyyyyyyyy.

    In GR, you could also (again, in principle) measure your own gravity, by detecting the gravitational redshift, for example (using a variation of the famous Pound/Rebka experiment perhaps).
  15. Oct 2, 2004 #14

    Andrew Mason

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    There can be one point at which the astronaut is in zero gravity, weightless and free fall at the same time. It would be the point at which the forces of solar. lunar and earth gravity sum to zero. If you ignore solar gravity, the point at which lunar and earth gravity are equal in opposite directions it is at a point that is:
    [tex] \frac {r_{moon}} {r_{earth}} = \sqrt{\frac {M_{moon}} {M_{earth}}}[/tex]
    or about 171,000 miles from earth.

    Quite correct. Falling into a black hole, for example, is something you would 'feel' because the gravitational gradient would rip you apart.

    Andrew Mason
  16. Oct 2, 2004 #15


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    This may be nitpicking, but this point is not a 'zero gravity' point. :eek:

    While the net gravitational force from the Sun, Earth, and Moon may be zero, there are plenty of other massive bodies in the universe, indeed, even in the solar system. Is it possible, even in principle, to find a point where the gravitational forces from all bodies in the universe net to zero? If so, where would this point be? And how fast would it be moving, wrt the solar system barycentre?
    Last edited: Oct 2, 2004
  17. Oct 2, 2004 #16


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    Hmmm, remeinds me of the topological theorem that there is no nonvanishing vector field on a two-sphere. The "hairy eight-ball always has a cowlick" theorem. I don't know if the same thing is true of a three-sphere (assuming our universe is like S3 X R1).
  18. Oct 25, 2004 #17

    Andrew Mason

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    Universe's zero gravity point depends on Observer?

    Yes. I should have said: "if you ignore the sun's gravity and the gravity of all the other objects in the universe".

    The N-body problem where [itex]n\rightarrow\infty[/itex] is infinitely complicated. The classical 'Three body' problem is complicated enough!

    But, theoretically, it would occur but in only one place in the universe at any given instant in time. It would be a function of the mass distribution in the universe (ie. the various masses and their relative distances from each other and from the observer).

    The question you raise is an interesting one, though. According to Relativity Theory, one's measurement of distances and masses depend on one's motion. I think that this means that that the zero gravity point in the universe depends on the observer's frame of reference.

  19. Oct 25, 2004 #18
    oil tankers in the ocean have a tendency that if they get to close to each other they bump into each other do to their gravities. if we keep getting more and more people on earth and we keep building more and more will this make earths gravity increase?
  20. Oct 26, 2004 #19
    No. Conservation of matter, the material those people are composed of was already here.
  21. Oct 27, 2004 #20
    ahhhhhh, i guess i missed this very thread i started, thanks for the help guys. But there's still the question on why massless photons do feel the force of gravity.......

    : )
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