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I Trying to understand gravity

  1. Nov 19, 2017 #1
    How much gravity is needed to dim a light bulb?
    Could this exsperiment ever actuly be created?

    Also is the shape of a spiral galaxy like ours due to the effect of the supper massive black hole at its center has on all the other stuff in the galaxy ? Or is it because of the effect the black hole has on spacetime all the other stuff is just falling inwardly?
    From an observatinal stand point since we are in a spiral gallixy looking outwardly under the abouves inplyed effect wouldn't from our perspective the univurs seem to be expanding ?? Woulnt we need to be in a place in the univurs that is as far away from any mass as possible in order to make an observatinal messurment of exspantion and time dielation?
    Last edited by a moderator: Nov 19, 2017
  2. jcsd
  3. Nov 19, 2017 #2

    stefan r

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    Gold Member

    By "dim" do you mean red shift? Any amount of mass (including the light bulb) causes some gravitational redshift. The redshift from the mass of a lightbulb would not be detectable with current tech.

    If the bulb is inside the event horizon of a black hole then none of the photons escape. If a very tiny black hole existed the hawking radiation could be brighter than the bulb. P= 3.562×1032 W kg2/M2 = 100W so M = 1.9 x 1015 kg. Any black hole smaller than 1.9 x 1015 kg will be brighter than the light bulb.

    The mass of the galaxy effects its structure much more than the black hole. The Milky Way has a relatively small black hole. There is 1012 solar mass in the galaxy and at most 4.5 x 106 in the black hole. 1/200,000 is not enough to dominate the structure.
  4. Nov 19, 2017 #3
    I was thanking if I built a device that could create artificial mass/ gravity as strong or as weak as I needed .. Put a bright light bulb on top. And from some distance away how much gravity would it take for from my prospective I could no longer see the lightbuld light??
    And any thoughts on how such an exsperiment could be pulled off??
  5. Nov 19, 2017 #4

    stefan r

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    Does shifting the visible light to infra-red count. You would not be able to see infra-red with your eyes. It can be detected.

    The type of light bulb makes a difference. The light from a standard incandescent (Edison type) bulb is not the same as a compact fluorescent or LED bulb. Typical street lights in the united states are high pressure sodium bulbs. Halogen bulbs and mercury vapor are also common. Wikipedia has a list of light sources.

    Incandescent bulbs can be tungsten, carbon or others. The power of a bulb effects the filament temperature and therefore it's light output. HPS bulbs give off a lot of 590 nm radiation which makes it easier for this question[assuming I understood the question]. Most human eyes cannot see 750 nm red.
    z = (750-590)/590 = 0.27
    So maybe you want to know how to "cause a red-shift of 0.27"?

    "How much gravity?" is more complicated than it sounds. Earth has a fairly constant mass but force of gravity changes with altitude. But you can plug that into the equation:
    2GM/(rc2) = (1+z)-2-1 = 0.38
    GM/r = 1.7 x 1016
    M/r= 2.5 x 1026
    r = 2.5 x 1026/M

    G is gravitational constant
    M is mass of object creating gravity. (probably what was meant by "amount of gravity")
    c is speed of light
    r is radius from center of mass of bulb

    So, for example, at 1 meter radius from an object with 2.5 x 1026 kg mass the light from sodium vapor bulb will to redshift to infra red. Not likely to find an object with that mass and a radius less than 1 meter. If the Sagitarius A* black hole has mass 8.6 x 1036 kg then a 400 watt HPS bulb at 3.4 x 1010 m or 34 million km will shine in infra-red and would not be visible to human eyes.

    [check arithmetic, is late and i am tired]
  6. Nov 19, 2017 #5
    Sure it counts becouse you just made my random thought interesting thank you..
  7. Nov 19, 2017 #6
    Besides gravitational lensing is there any other proof that gravity can effect light to the exstreem that it can not escape? Is there any other process that can be directly observed .. Is there any possible way to prove that a black hole can't be seen becouse gravity? And not another process that dousnt allow light escape?
  8. Nov 20, 2017 #7


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    Staff: Mentor

    There's the Pound-Rebka experiment confirming gravitational redshift.
  9. Dec 2, 2017 #8
    No such machine exists, as far as I know, to create artificial gravity.
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