# The Naked Scientists Forum

### Author Topic: general relativity question  (Read 5840 times)

#### thebrain13

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##### general relativity question
« on: 24/10/2007 06:29:10 »
Last time I asked a similar question it went unanswered, I figured nobody knew the answer, so im guessing this one will go unanswered as well, but its worth a shot.

Relativistic objects fall faster, in the presence of gravity, than "stationary ones". And at the speed of light an object falls twice as fast.

I have a few questions about that statement I just said.

Relative to what does a "moving" object fall faster than a "stationary" one?

Also consider this experiment, what if you had two parallel mirrors, placed in a free falling elevator on the surface of the earth. You then fire a photon so it would bounce back and forth from mirror to mirror. would you witness the light being affected by gravity more than you, or would the beam stay stationary relative to you(or at least bouncing in the same place over and over relative to you)?

Allright, im going to think out loud, somebody correct me if I make a mistake.
I figure that due to the equivalence principle, the person and the photon would fall at the same rate. Being that my experiment is pretty much local, all observers would have to agree that the person in the elevator and the photon would fall at the same rate as well.

So somehow, bouncing light back and forth causes the light to fall, at least in the long run, at the rate newton would of expected. So what does the statement, relativistic objects fall faster than stationary ones mean, and how does acceleration (ie. bouncing light back and forth) affect that statement?

#### another_someone

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##### general relativity question
« Reply #1 on: 24/10/2007 06:48:53 »
So what does the statement, relativistic objects fall faster than stationary ones mean?

You tell me - I have never come across such an assertion - you want to tell us where you saw this said.

#### DoctorBeaver

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##### general relativity question
« Reply #2 on: 24/10/2007 09:52:17 »
I've never heard that either; and I can't imagine what it could mean.

#### Mr Andrew

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##### general relativity question
« Reply #3 on: 24/10/2007 17:31:05 »
Einstein showed that light was bent by gravity twice as much as it was predicted to bend with classical mechanics.  This is not to say that light is affected by gravity more than other matter but that classical mechanics inaccurately describes how light is affected by gravity.

#### sleuth in kilt

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##### general relativity question
« Reply #4 on: 24/10/2007 18:17:03 »
That elevator is like anything in freefall: as regards the behavior of light, it's as if the vehicle weren't moving at all. So the light reflects back and forth without aberrations. Earth's gravity is too weak to bend light significantly anyway, but it's not a factor here.

As far as the assertion, "Relativistic objects fall faster, in the presence of gravity, than 'stationary ones'; and at the speed of light an object falls twice as fast", there is zero truth to it. What possible definition could there be for "relativistic object"?? and certainly, no object can ever move AT light speed.

Einstein showed that light was bent by gravity twice as much as it was predicted to bend with classical mechanics.
That sounds intriguing but I don't know of it. I don't know that classical mechanics predicts light being diverted by gravitational pulls. Of course, I could've missed something. (Naah)

#### thebrain13

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##### general relativity question
« Reply #5 on: 24/10/2007 19:02:24 »
http://en.wikipedia.org/wiki/Arthur_Stanley_Eddington

Heres an excerpt from a wikipedia article. Ive seen statements like this in other places, although ive never seen its explanation. maybe its because only three people understand it, and theyre all dead by now

After the war, Eddington travelled to the island of Príncipe near Africa to watch the solar eclipse of May 29, 1919. During the eclipse, he took pictures of the stars in the region around the Sun. According to the theory of general relativity, stars near the Sun would appear to have been slightly shifted because their light had been curved by its gravitational field. This effect is noticeable only during an eclipse, since otherwise the Sun's brightness obscures the stars. Newtonian gravitation predicted half the shift of general relativity.

Eddington's observations published next year (Dyson, F.W., Eddington, A.S., & Davidson, C.R. 1920 A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919 Mem. R. Astron. Soc., 220, 291-333) confirmed Einstein's theory, and were hailed at the time as a conclusive proof of general relativity over the Newtonian model; the news was reported in newspapers all over the world as a major story. It is also the source of the urban legend that only three people understand relativity; when asked by a reporter who suggested this, Eddington jokingly replied "Oh, who's the third?"

sleuth, I was asking what relativistic object meant, hence my question, relative to what does a moving object fall faster than a stationary one. And I know objects(other than light) cant travel at c, but an object traveling at .999 percent the speed of light will fall at 1.999x the normal gravitational strength, according to an object resting on the surface of the earth. So stop knitpicking.

Lastly might I suggest an explanation, maybe einstein means objects traveling towards you at the speed of light(or near) fall twice as fast(or near twice as fast) as an object with no relative radial velocity with you. And an object traveling away from you at the speed of light doesnt experience gravitation, relative to you.(traveling in a curved path, relative to you balances out the normal pull of gravity)  That way we could view parallax (like in 1919) while maintaining the equivalence principle.

#### lyner

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##### general relativity question
« Reply #6 on: 24/10/2007 19:19:03 »
And as for "What is a relativistic object?" question.
Well, electrons, flowing through a wire are moving fast enough for relativistic effects to occur. That's a few mm per second, so it 'relativistic' includes just about everything.

#### Soul Surfer

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##### general relativity question
« Reply #7 on: 24/10/2007 19:22:03 »
The term "falling" gives the wrong image of what is going on.  What Eddington was observing was the minute deflection (a few seconds of arc)of rays of light as they pass through the gravitational field of the sun on ther way to the earth.  The deflection is seen as a change of momentum.  The same would apply to a relatavistic object  The object is moving very fast so the extra change of momentum does not have a big effect on the direction of travel  but it is greater than one would expect if there was simply gravity acting.  The anwer I think may be the "gravitiomagnetic force" which acts in a direction perpendicular to the direction of travel.

#### sleuth in kilt

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##### general relativity question
« Reply #8 on: 24/10/2007 21:02:00 »
I looked at the cited wikipedia article about Eddington, wherein this Newtonian shift of light is mentioned, and under the "discussion" tab at that article, under the heading "eclipse question", a person does question the validity of there being any defined deflection of light due to gravity under classical Physics. There was suggestion that that part of Newton's hypothesis was later obsoleted and was actually a reference to some kooky kind of diffraction.

Um, as for your (thebrain13's) continued insistence that an object moving at 1.999c experiences about double the gravitational pull, I'm not following that clearly nor am I familiar with any such formulation.

As for what an object's speed is relative to, I can assure all that it is never relative to "space" ie. some master cosmic framework comprising a 3D coordinate system, for there is no such thing in Nature. Material speed is always defined as being relative to another cited material object. So no object is a "relativistic object" per se.

And as for electrons moving at a few mm per second in a wire, that's a snail's pace of course (yet the voltage signature traverses at lightspeed), and I can't think of any particular "relativistic effects" one could ascribe to those slogging electrons.

"Knitpicking"?? Guilty as charged, I guess. I've got a lot of excess time, truthfully.

#### jartza

• Sr. Member
• Posts: 230
##### general relativity question
« Reply #9 on: 17/11/2010 02:14:03 »
Last time I asked a similar question it went unanswered, I figured nobody knew the answer, so im guessing this one will go unanswered as well, but its worth a shot.

Relativistic objects fall faster, in the presence of gravity, than "stationary ones". And at the speed of light an object falls twice as fast.

I have a few questions about that statement I just said.

Relative to what does a "moving" object fall faster than a "stationary" one?

Also consider this experiment, what if you had two parallel mirrors, placed in a free falling elevator on the surface of the earth. You then fire a photon so it would bounce back and forth from mirror to mirror. would you witness the light being affected by gravity more than you, or would the beam stay stationary relative to you(or at least bouncing in the same place over and over relative to you)?

Allright, im going to think out loud, somebody correct me if I make a mistake.
I figure that due to the equivalence principle, the person and the photon would fall at the same rate. Being that my experiment is pretty much local, all observers would have to agree that the person in the elevator and the photon would fall at the same rate as well.

So somehow, bouncing light back and forth causes the light to fall, at least in the long run, at the rate newton would of expected. So what does the statement, relativistic objects fall faster than stationary ones mean, and how does acceleration (ie. bouncing light back and forth) affect that statement?

Ball bouncing up and down has the largest speed closer to the ground.
Light bouncing up and down has the slowest speed closer to the ground.

That's why bouncing light is pulled with larger force than bouncing ball.
(Gravity must be stronger closer to the ground for this to work.)

If ball and light have same mass, light weighs more, vertically bouncing light, that is. Horizontally bouncing light weighs the normal amount.

« Last Edit: 17/11/2010 11:58:35 by jartza »

#### JP

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##### general relativity question
« Reply #10 on: 17/11/2010 10:33:55 »
Ball bouncing up and down has the largest speed closer to the ground.
Light bouncing up and down has the slowest speed closer to the ground.

I can't understand most of what you're trying to say here, but that is wrong.  Light moves at a constant speed.  It isn't slower closer to the ground.

#### jartza

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##### general relativity question
« Reply #11 on: 17/11/2010 12:27:49 »
I can't understand most of what you're trying to say here, but that is wrong.  Light moves at a constant speed.  It isn't slower closer to the ground.

Well that's offending. Leave the "trying" out.

Light spends some extra time traveling if there is a gravity well along the way. Newton would say light saves some travel time if there is gravity well along the way.

#### JP

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##### general relativity question
« Reply #12 on: 17/11/2010 13:06:07 »
I can't understand most of what you're trying to say here, but that is wrong.  Light moves at a constant speed.  It isn't slower closer to the ground.

Well that's offending. Leave the "trying" out.

Light spends some extra time traveling if there is a gravity well along the way. Newton would say light saves some travel time if there is gravity well along the way.

Fair enough.  I didn't mean to offend.  :)  Now that you explained it again, it makes sense.  I agree with you, but physicists are generally very careful about how they use speed, and light speed is constant under normal use of the term.

#### Soul Surfer

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##### general relativity question
« Reply #13 on: 17/11/2010 23:02:09 »
To go back to the original question I think that what you are talking about could be the effect of Gravitomagnetism.  Like moving electrical charges produce electromagnetism (which can be described as a relativistic process) moving gravitating mass produces gravitomagnetism.  This can enhance or neutralise gravitational effects dependant on the particular setup  it can even produce the thing that lots of of wilder type would love to have, a repulsive gravitational effect.  This is particularly seen in the vicinity of a Kerr or rotating black hole.

I am currently reading up on this in "Gravitation from the ground up"  by Bernard Schutz (already mentioned elsewhere on these pages) and hope to be able to explain things a little better soon.

#### jartza

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• Posts: 230
##### general relativity question
« Reply #14 on: 18/11/2010 06:37:17 »
See the second animation here:
http://www.astro.ucla.edu/~wright/deflection-delay.html

The lower part of light ray moves slower. And therefore light ray bends downwards.

Would lower part of a potato move faster? And would potato therefore be deflected upwards?

« Last Edit: 18/11/2010 06:38:48 by jartza »

#### yor_on

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##### general relativity question
« Reply #15 on: 09/12/2010 08:57:15 »
Just want to make some remarks, as usual :) Interesting idea but slightly weird as light always 'propagate' at the speed of light. Light is also always 'uniformly moving', at the same speed, in, and from all 'frames of reference' (assuming a vacuum). That creates some remarkable effects when it meets proper mass. To get the idea on have to understand that SpaceTime is bumpy, like an uneven road due to the way planets and suns 'bend' the space 'geodesics' that makes the lights road.

That light 'deflects outwards' may seem counter intuitive. Why doesn't it just pull inwards when it meets the sun? Why 'outward'? Well, the light that never reach us do get pulled inwards. But, for the light escaping, as it meets the sun for example, assuming that it comes from behind, hidden to us (I'm exaggerating here).  Then, as it meet the gravitational 'field' or 'room time geometry' surrounding the sun, you could think of it as it starting to prepare for an orbit bending slightly outward to follow the gravitational 'geodesic'. And the light entrapped could be seen as (exaggerating a lot!!) 'spiraling' around as it falls down into the sun.

Think of a ordinary orbit and the way it will decay as Earth drags the debris in. It doesn't go in a straight line down to some center, instead it will spiral down following an ever decreasing orbit around Earth. The same will happen with any matter passing that gets caught by Earths gravity. And if that matter came from the back side of our Earth you will still see it pass you on the other side, before it will hit.

It's all about 'frames of reference' and 'room time geometry'. For us standing at Earth the light will take a longer path in space as it gets deflected, and so seem 'delayed', as compared to following a 'straight line'. But according to the light itself it took the 'straightest line' it could, just following 'SpaceTimes geodesic road'. And if you don't like that way of thinking you can always think of light as trying to avoid 'expending energy' always choosing the 'cheapest path' through SpaceTime.

But 'deflecting' do sound weird when you 'know' that light bends towards gravity, not away from it. Thank (God of your choice) it's explainable in words :) And, that effect is also known as 'gravitational lensing'. ( Never liked that word, although I think I can see how they think. But every time I read it I think of a magnifying glass and the way it concentrates light, not 'deflecting' it?)

"The gravity from the massive object will “pull” on the photons as they pass, shifting their paths, and thereby affecting the image that we see in our telescopes. In the simple case of a distant point source of light (e.g., a far away star), and a compact spherically symmetric lens (e.g., a black hole), the bending angle is given by



In this equation M is the mass of the lens, r is the minimum distance between the (unperturbed) line-of-sight to the source and the lens, G is the gravitational constant, and c is the speed of light. This was a crucial prediction of Einstein’s new theory, and one way to test it was to see if the stars on the sky “jump” as the Sun (which is quite massive, and traverses the sky quite briskly) comes nearby on the sky. total solar eclipse (July 22, 2009)If you plug in the appropriate numbers above ((G/c^2)*M_sun = 1.5 km [geometric units], R_sun = 700,000 km), you find that a star should shift on the sky by 1.75 arcseconds (8.57e-6 radians) as the Sun approaches."

?==

Ive seen this kind of statement though too "In a very real sense, the delay experienced by light passing a massive object is responsible for the deflection of the light... The rays are always perpendicular to the wavefronts which mark the set of points with constant travel time from the star. In order to bend the light toward the star one needs to delay the wavefront near the star." Which surprises me?

How does one part of 'space' differ from another other than in its geometry? Snell's law about refraction is about light moving from one medium to another, e.g. from air into glass and then out on the other side, but 'from space to space'??
« Last Edit: 09/12/2010 11:55:47 by yor_on »

#### The Naked Scientists Forum

##### general relativity question
« Reply #15 on: 09/12/2010 08:57:15 »