Physics Help Forum Gravitational Time Dilation question

 Special and General Relativity Special and General Relativity Physics Help Forum

 Oct 4th 2012, 10:19 AM #1 Junior Member   Join Date: Oct 2012 Posts: 3 Gravitational Time Dilation question I was just curious if, for example, a rocket ship passed equidistant between two equally massive black holes, would it be slowed twice as much relative to an outside observer as it would if it passed by just one? Or would it not be slowed at all? My guess is that it would not be slowed at all, due to being pulled in opposite directions at equal strengths cancelling out directional energy from its velocity. But I'm not a physicist, so I need some professional input.
Oct 5th 2012, 08:04 AM   #2
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 Originally Posted by LamoyRL My guess is that it would not be slowed at all, due to being pulled in opposite directions at equal strengths cancelling out directional energy from its velocity. But I'm not a physicist, so I need some professional input.
I believe you are correct, sort of. Yes the effects of the two masses cancel each other out, but it's not due to some "directional energy." Time dilation under general relativity can be caused by the curvature of space-time by a massive object. At the midpoint between two black holes the curvature caused by one effectively cancels the other out. Consequently the magnitude of the gravitational field at that point is zero, so there's no time dilation due to gravity. However, if the rocket is moving at relativistic speeds through that point the outside observer would still prerceive that the clock on the rocket ship runs slow, per the effects described by special relativity.

 Oct 11th 2012, 11:14 AM #3 Junior Member   Join Date: Oct 2012 Posts: 3 And I guess due to quantum mechanical effects, the uncertainty in the location of both the masses and the passerby would cause an unequal pull on the passerby, yanking him inevitably towards one of the gravitating bodies. Right? However, if this could be done on a controllable scale, probably with miniature black holes, this would allow a much closer look at what happens near an event horizon. Why hasn't this been used to explore quantum mechanical properties of gravity yet? Last edited by LamoyRL; Oct 11th 2012 at 11:16 AM.
 Oct 11th 2012, 01:06 PM #4 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,352 The midpoint between two massive bodies is indeed an unstable point, in that its impossible to stay precisely in the center without some form of propulsion and control. More precisely what you would do is position the ship at the Lagrange 1 point, where the pull of gravity from the two masses and the orbital cenrtripedal force all cancel out. Unfortunately it's an unstable orbit point, and sort of like balancing a pencil on its point you need some form of "station keeping" propulsion and control to stay there, but it's not too difficult. Of course this assums that the black holes are far enough away that tidal forces don't rip the space ship apart! This has nothing to do with quantum effects however - I don't understand why you mention that. I'm amused by your question as to why people aren't experimenting with "minature black holes." Obviously no one has ever seen one up close - you can't exactly pick one up at your local hardware store! Last edited by ChipB; Oct 12th 2012 at 07:02 AM.
 Oct 16th 2012, 08:04 PM #5 Junior Member   Join Date: Oct 2012 Posts: 3 So we can make antimatter but we can't squeeze stuff together really tight? I really wanna make a joke about black holes right now...
 Dec 20th 2012, 03:34 AM #6 Junior Member   Join Date: Dec 2012 Location: France Posts: 2 Wrong. Time is slowed twice as much relative to an outside observer. The amplitude of the general relativistic effect of time slowing, is given by the gravitational potential. This remains true in any case, whatever the distribution of masses, including in a local experiment near the equator where the centrifugal force of the Earth's rotation is included (substracted) in the value of "gravity" as it appears, so that the special relativistic effect of time dilation by the difference of speed between altitudes is reinterpreted as (substracted from) the general relativistic effect. The gravitational potential in the presence of 2 masses, is the sum of potentials given by each mass. So the time slowing effects ad up.
Dec 24th 2012, 08:01 PM   #7
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 Originally Posted by LamoyRL I was just curious if, for example, a rocket ship passed equidistant between two equally massive black holes, would it be slowed twice as much relative to an outside observer as it would if it passed by just one? Or would it not be slowed at all?
It depends on where in its path the rocket ship was. As it starts to pass from a great distance the effects of gravity is small. Then the closer the rocket gets to the mid point the faster its moving. Then it slows down as it leaves from between the black wholes. The difference in rate can't be reaily compared since without the other black hole present the path the object takes also changes.

 Originally Posted by LamoyRL oles were I'd guess that it'd be pulled with twice the speed [/quopte] I don't know where this came from and thus I can't figure out what it means.
 Originally Posted by LamoyRL My guess is that it would not be slowed at all, due to being pulled in opposite directions at equal strengths cancelling out directional energy from its velocity. But I'm not a physicist, so I need some professional input.
You're only considering the tangential component of the velocity, not the vertical component. Since the vertical components face in the same direction the velocities add
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Last edited by Pmb; Dec 24th 2012 at 08:24 PM.

 Jan 13th 2013, 11:32 PM #8 Junior Member   Join Date: Jan 2013 Posts: 1 No, the fastest light will get to us will still always be light speed. Gravitational lensing will give you a better image of the supernova, but in that case it will actually take light longer to reach us because it will travel through a gravity well, which is where the gravitational time dilation will occur.
Jan 14th 2013, 06:28 AM   #9
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 Originally Posted by Gerry22 No, the fastest light will get to us will still always be light speed. Gravitational lensing will give you a better image of the supernova, but in that case it will actually take light longer to reach us because it will travel through a gravity well, which is where the gravitational time dilation will occur.
Gerry - What are you talking about? From your comments above it looks as if you're posting in the wrong thread. What post/comment were/are you responding to?

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