The existence of Black Holes is based on the idea that gravity can influence (bend) light rays in some manner. It is a simple premise – can a strong source of gravity effect light rays as they pass by, even though photons have no mass?? This has been a favorite enigma of comologists and other scientists for many years.
In forming his General Theory of Relativity, Albert Einstein used his famous thought experiments to try to prove that gravity can effect light rays. He claimed that acceleration creates the same effect on a beam of light as gravity and are therefore equivalent. “The Principle of Equivalence”. Although their effects are similar (but not exactly the same) they are not equivalent. Inertial acceleration is an inertia requirement that states a certain force must be exerted on a body for it to accelerate at a certain rate”. Gravity is just one of the primary energy/forces of nature that fulfills this requirement.
One of the ideas that misled Einstein to his conclusion was his thought experiment, that states: a body dropped in an accelerating elevator behaved in exactly the same manner as it would if was dropped on the surface of Earth. If someone was inside the elevator there would be no way of knowing whether the elevator was “resting” on the ground or accelerating out in space.
Actually, the effects in both cases, are similar but not exactly the same. The truth is, Gravity causes bodies, dropped from various heights in the room “resting” on the ground, to fall at different rates of acceleration (due to their different distances in the room, to the center of gravity of the Earth) where as, all bodies in the accelerating elevator would seem to “fall” at exactly the same rate. This is true whether it takes place in a small room or dropped from an airplane at different altitudes in the sky.
Another factor that adds doubt to Einstein’s thought experiment is – the room was not actually “resting” on the ground – it was actually in a constant state of “restrained acceleration” as is everything else that is “resting” on the surface of the earth. Its acceleration was constantly being restrained by the obstructional force of the earth’s surface, (rather then just resting on the Earth’s surface with no force being exerted on it). If this obstructional force is removed the body will immediately resume falling.
In both cases, the elevator is in a state of acceleration one caused by the force of gravity the other caused by the electromotive force of the electric motors propelling the elevator upward.
Actually the displacemnet of the star’s light passing by the sun was an optical (passing through the corona) rather then a gravitational phenomena.
Donald L. Hamilton author: “The MIND of Mankind”: Human Imagination, the source of Mankind’s tremendous power!
http://novan.com/mind.htm
http://novan.com/cosmol.htm (Cosmology for the 21st Century)
Now i have not done any further study of physics whatsoever but jarnold’s example of a spacecraft simulating an orbit and then actually being in an orbit is flawed as the force of gravity is acting on both the observer and the spacecraft whereas the thrust was only acting on the spacecraft and therefore on the observer as is pressed up against the walls. However if the spacecraft itself was considered to be the observer and it couldn’t observe the fact thrusters were functioning or not it could not determine whether is was the thrusters or the gravity that was acting on it.
Back to the original question of how gravity interacts with light i understand by disregarding the statement that objects have energy proportional to their mass according to E=mc^2 and gravity acts weakly on the small energy of the photon just as it would on the “mass energy” of any other object. The difference being that since a photon isnt an object it doesnt have a gravity of its own (the same way waves and particles have some similarities and some differences but interact with one another) to interact back with the black hole thus it loses energy instead
When the photon is losing energy (i suppose an interpretation of this could be that the photon is moving into a more compressed region of space where the speed of light is “faster” relative to the rest of space) the photon isnt traveling at the speed of light anymore and thus can be accelerated back up to ‘c’ curving it around the black hole. The event horizon represents the point where gravitational interactions with photons cause them to lose all of their “straight line” energy and instead take on a path of orbit, whether sustained or not, around the black hole.
What i am trying to say is that my theory is gravity is a force interacting with energy rather than with mass even the energy which is expanding the universe and thus why space is “curved” towards objects of significant mass.
I might not be thinking multidimensionally enough about the whole subject but that comes from no formal teaching.
With regards to the original post. Gravity doesn’t “pull” light. But it does warp space. Light travels unwaveringly in one direction in empty space. If the space fabric is distorded by a mass, the light will follow in suit. This I beleive is the cause of gravitational lensing.
I agree with Jarnold as well. We cannot feel gravity itself because it is not a force as much as a simple distortion of space-time. You feel pressure under your feet pinning you to the ground. This is what I beleive Jarnold means when he says obstructed gravity. Like it or not, you are always falling: only there is something solid continuously stopping you.
No physical body in space is at ever at rest, it is moving forward through the dimension of time. Along with gravity this constitutes the cosmological constant of the 20th century. It’s the most amazing part about general relativity, yet few manage to understand it fully. Knowing the laws of general relativity top to bottom is nothing compared to actually grasping it’s significance.
Ridiculed. Opposed. Accepted as self evident!
I enjoyed Brian Greene’s explanaition of this in “The elegant universe”.
Here is a link I found of interest.
http://www.outersecrets.com/real/2_motion.htm
Peace~
It happens every time. Challenge the ancient association of gravitation with force long enough, and you’ll end up talking to yourself, as those few willing to indulge you at all will, if finally cornered by specifics, abandon their indulgence with the self-assurance that you just don’t understand.
In my previous comment I showed that the association of inertial acceleration with rest or uniform motion, and of gravitation with inertial acceleration, can be invalidated with concrete experimental observations. But so powerful is the pull, the adherence to an established paradigm, that the concrete carries no weight against the abstract.
Evidently, the state of rest/uniform motion can’t be considered absolutely distinct from inertial acceleration, never mind the observable, absolute distinction, because an established abstract theory considers them relative. Inertial acceleration can’t be considered absolutely distinct from gravitation, never mind the observable distinction, because they coincide in our most familiar experience on the earth’s surface, and an established abstract theory considers them equivalent. Distinctions can’t be distinctions because they’re irrelevant. Distinctions can’t be distinctions because they’re not.
I’ll abandon my annoying objections with one last illustration which may manifest the distinction between gravitation and inertial acceleration most clearly, for the benefit of anyone genuinely curious – or maybe just for my own sense of closure. (Warning: The following may be disturbing to conservative physicists and young children.)
Let a spacecraft in a region far from significant mass simulate an orbit around an imaginary star with some specified mass and location. Navigation of the simulated orbit is plotted by means of the “fixed stars” and accomplished by the continuous firing of a thruster. Let the inertial effects on the craft due to the simulation be detectable. (A “static gravitational field” of some intensity will be observed at the inside surface of the craft that is farthest away from the imaginary star as it rotates around the imaginary position — unsecured objects will press against that surface.) Now introduce an actual star of the specified mass at the location posited in the simulation, and let the engine of the craft be simultaneously switched off. The craft will continue on the accelerated path, but now without inertial effects — the “static field” immediately disappears and the occupants of the craft float weightlessly as the craft follows an actual geodesic around the star. Thus, without altering the craft’s trajectory relative to the “fixed stars”, their supposed influence has been entirely canceled by the introduction of a local mass. A fundamentally different sort of acceleration has replaced the earlier acceleration, even though its trajectory as it would be described by a distant observer remains consistent. The former “gravitation†that is supposed to be equivalent to inertial acceleration has been replaced by an actual gravitation, with manifestly different characteristics, and without a trace of inertial involvement.
No doubt, gravitation can be found on occasion to be involved with inertial acceleration, as it is when a body presses against the surface of another of sufficient mass, as it is for residents on the earth’s surface, but instead of identifying gravitation and inertial acceleration by their coincidence, and ignoring their distinction, surely their manifest independence must be taken as fundamental. Surely a coincidence implies a distinction. Unless, of course, other – non-physical – considerations are more pressing.
A state of rest is indistinguishable from a state of relative uniform motion. That’s relativity. A state of inertial acceleration is distinguishable from a state of rest and/or uniform motion by, to use Fred’s example, the introduction of a spring between a body and a force. A gravitational acceleration is distinguishable from an inertial acceleration by, for example, observing an electrically neutral test body in a container; it will distinctly express a situation as exclusively gravitational or inertial by either floating freely or tending toward one wall of the container. An exclusively inertial acceleration is distinguishable from an interrupted gravitational acceleration by “dropping†two objects; if gravitation is involved they will, if the measuring apparatus is sensitive enough, be observed to converge. All these latter cases are manifestly, self-evidently NOT relativistic. Please dispense with the condescension and either refute the concrete principles involved (without resorting to abstract, un-provable principles like “equivalenceâ€) or accept them, and begin considering the implications.
Jim-
A final clarification, though I am concerned you still won’t understand.
You write: “Otherwise, when you step on the gas at a green light, the rest of the universe could just as well be said to be accelerating in the opposite direction.”
That is precisely what can be said of what you observe when you are in an accelerated (therefore non-inertial) frame of reference.
You are always at rest in your own frame of reference, inertial or otherwise. As anon says, that is the definition of the term. In this example, you can tell it is a non-inertial frame by putting a spring between you and your seat-back.
Leaving out the knowledge of the gas pedal, that observation can be interpreted in either of two equivalent ways. You are in a gravitational field pointing the backwards direction, or you and your frame of reference are accelerating forward.
Either interpretation explains why the rest of the Universe is accelerating backwards with respect to you.
And now I am taking anon’s advice and getting back to my book writing. If you don’t “get it” now, then I have to consider the situation hopeless.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Anonymous,
You wrote that I “remain mystified and make incomprehensible statements like: ‘confusion of obstructed gravitation with the inertial obstruction’”
Thank you for kindly pointing out that I should have said “…with the inertial acceleration.â€
“And nonsense like: “a body moving freely in a gravitational field is always moving uniformly from its own frame of reference” — by definition, a body is at rest in its own frame of reference, though it may be subject to a balanced set of forces, such as gravity and the normal force from a surface on which it sits.â€
Actually, a body that is undergoing an inertial acceleration is not at rest in its own frame of reference. Considering a body in a lab on the earth’s surface to be at rest is a practical convenience, as per Dicke’s “weak principle of equivalence.†Otherwise, when you step on the gas at a green light, the rest of the universe could just as well be said to be accelerating in the opposite direction.
But you seem to be someone interested and able to bring clarity to the world, and to shame nonsense into enlightenment or silence. So maybe you can explain how gravitation, which you will presumably agree is a geometric distortion of spacetime, and which is force-free from the reference frame of a body moving unobstructed (or “moving freely”, or “at rest” in space, or falling in an elevator), suddenly becomes a force from that same reference frame when it intersects with the surface of a massive body – if meeting the inertial resistance of the surface is not an “obstructed gravitation”, if the resulting surface acceleration is not the sole instance of force. I’d be very grateful for your assistance.
Jim,
I find Einstein’s discussion clear, and more importantly, supported by a century of observation.
The “proof” you offer makes an assumption different from the thought experiment, which concerns, in Einstein’s words, “a large portion of empty space, so far removed from stars and other appreciable masses that we have before us approximately the conditions required by the fundamental law of Galilei.”
In such conditions, the convergence you describe would not happen.
Consider this: A person in free fall with no ability to observe the outside world, whether in orbit or in an elevator with a broken cable in an evacuated shaft, is “weightless.” From the perspective of someone on the outside, that person is in a gravitational field and is accelerating. From the perspective of the person falling, the local “g” is 0.
Going further: We don’t feel the Sun’s gravity because we are falling toward/around the Sun with our planet. Nor do we feel the net gravitational attraction of the galaxy, even though the Sun and Solar System are moving in an orbit around the galactic center of mass and are bobbing up and down through the galactic plane.
We only know of such accelerations by making observations of the rest of the Solar System and the galaxy.
We don’t even feel Earth’s gravity when we are in free fall, but it is there.
As far as I can tell, your difficulty in understanding this leads to phrases that are muddled at best and incorrect at worst, like: “Gravitation is only involved with inertial acceleration when it is resisted.” That seems based on the perception of gravity, not gravity itself.
And now I must close my part of this. I have a book to write based on my “Our Next Planet” school visit program.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Fred,
I’m so glad you came back! If only temporarily –
The issue seems quite simple, and yet, astoundingly, I’ve yet to find a physicist who will step out of the paradigm to address it, much less critique in directly. You see like a reasonable person, I have an abiding faith in reason, and so I continue to hope that you’ll address my point:
Not only is there a way to distinguish between an inertial acceleration and gravitation, the two are entirely distinct. Gravitation is only involved with inertial acceleration when it is resisted.
“ an accelerated frame of reference or the presence of gravity yield the same result, and there is no way to distinguish between them.â€
First, I showed that there is a way to distinguish them: Objects dropped in the situation where gravitation is involved will converge (see Eotvas). Second, unobstructed gravitation manifests no inertial acceleration whatever. I welcome an example that violates this NON-EQUIVALENCE principle. The “equivalence principle†only shows that the inertial acceleration that occurs when gravitation is obstructed resembles (in some ways – again, see Eotvas) the inertial acceleration that occurs when force is applied.
If the non-equivalence principle (gravitation is only coincident with inertial acceleration when gravitation is resisted) can’t be refuted, if the equivalence principle confounds inertial acceleration and gravitation when they coincide, doesn’t that give pause?
Don, I’ve read Einstein’s various statements of the equivalence principle. He’s a worthy authority, but authority isn’t supposed to carry weight in physics. The equivalence principle only demonstrates a limited equivalence of inertial accelerations, with and without the involvement of gravitation. To the extent that there’s an equivalence, there’s an equivalence of inertial acceleration with inertial acceleration.
Fred, per your last comment, the equivalence of gravitational mass and inertial mass is to be expected, as it’s the same mass that is whether accelerated by force or prevented from gravitating by force.
The chapter you cite explains the principle of equivalence quite well. I now realize I have been forgetting the most important aspect of the principle, namely that the inertial mass is equivalent to the gravitational mass.
In other words, Newton’s second law, which Einstein would write F=dp/dt (force equals rate of change of momentum), has a term, momentum, p, which depends on inertial mass, m, and velocity, v. (p=mv is a very good approximation at speeds much less than c, the speed of light.)
The gravitational force on an object also depends on its mass — in this case the gravitational mass. If you think about it, the two masses are not necessarily the same. There could be a gravitational equivalent of electric charge that is unrelated to the inertial mass.
The principle of equivalence states that these two masses are indeed the same, and Einstein’s thought experiment, so clearly described in that chapter, uses that to develop the equivalence of gravity and an accelerated frame of reference.
By the way, getting back to the original point of this thread, the curvature of light in a gravitational field is explained with equal clarity in Chapter 22. (http://www.bartleby.com/173/22.html) For some reason, Don has a hard time accepting that as valid and proposes a different mechanism that doesn’t fit with observations, such as the Solar eclipse data cited by Einstein and the additional evidence, such as gravitational lensing, which is available to us today.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Fred has left with a mystifying response. I don’t know what he means by a “convergence” which he believes I’m describing that “would not happen.”
“From the perspective of someone on the outside, that person is in a gravitational field and is accelerating. From the perspective of the person falling, the local ‘g’ is 0.”
I have no problem with this description, or any description confirmed by observation. The problem is the standard interpretation that confuses gravitational acceleration with inertial acceleration. “The local ‘g'” is ALWAYS 0 when gravitation isn’t obstructed, because “g” is the measure of inertial resistence to gravitation. Fred seems to think I’ve been arguing that there is no gravitation if we don’t feel it. We NEVER feel gravity itself – gravity is a geometric distortion of spacetime, and a body moving freely in a gravitational field is always moving uniformly from its own frame of reference. The experience of ‘g’ only occurs when gravitation is resisted. There is no observation, either here or far out in space that conflicts with this distinction, whereas the confusion of obstructed gravitation with the inertial obstruction is a fundamental error that pervades theoretical physics – at least here on earth.
Is the problem a difficulty in understanding the difference between a purely relative gravitational acceleration (where “the local ‘g’ is 0”) and an inertial acceleration (where the local ‘g’ is proportional to the force involved)? Is the problem just the inertia of an established paradigm? In either case, it’s astounding to me that the distinction can’t even be heard without imposing one’s own implicit association of gravitation and inertia, the very association that’s at issue.
The reasoning seems to be “Gravitation and inertial acceleration can’t be distinct because gravitation and inertial acceleration are two aspects of the same principle.” It’s called a tautology. And this is the state of physics today?
Did any of you actually read Einstein’s own words on this thought experiment??
Albert Einstein (1879–1955). Relativity: The Special and General Theory. 1920.
Chapter XX. The Equality of Inertial and Gravitational Mass as an Argument for the General Postulate of Relativity
WE imagine a large portion of empty space, so far removed from stars and other appreciable masses that we have before us approximately the conditions required by the fundamental law of Galilei. It is then possible to choose a Galileian reference-body for this part of space (world), relative to which points at rest remain at rest and points in motion continue permanently in uniform rectilinear motion. As reference-body let us imagine a spacious chest resembling a room with an observer inside who is equipped with apparatus. Gravitation naturally does not exist for this observer. He must fasten himself with strings to the floor, otherwise the slightest impact against the floor will cause him to rise slowly towards the ceiling of the room.
To the middle of the lid of the chest is fixed externally a hook with rope attached, and now a “being†(what kind of a being is immaterial to us) begins pulling at this with a constant force. The chest together with the observer then begin to move “upwards†with a uniformly accelerated motion. In course of time their velocity will reach unheard-of values—provided that we are viewing all this from another reference-body which is not being pulled with a rope. Etc. Etc.
(Here are Einstein’s own words on the subject)
http://www.bartleby.com/173/20.html
Don Hamilton
Jim, I feel Fred’s pain.
He explained why your argument of converging masses was not relevant under the conditions that Einstein postulated, but you dismiss that.
He explained why inertial (F=ma) and gravitational (F=GMm/R^2) mass are not necessarily the same.
He points out how Einstein’s thought experiment connects the two masses.
And you remain mystified and make incomprehensible statements like: “confusion of obstructed gravitation with the inertial obstruction”
And nonsense like: “a body moving freely in a gravitational field is always moving uniformly from its own frame of reference” — by definition, a body is at rest in its own frame of reference, though it may be subject to a balanced set of forces, such as gravity and the normal force from a surface on which it sits.
Fred, please write your books. Your time is too valuable to waste trying to straighten out Jim’s misconceptions.
The principle of equivalence deals with one frame of reference. The equivalence is between two interpretations of a measurement: an accelerated frame of reference or the presence of gravity yield the same result, and there is no way to distinguish between them.
My suggesting that we add scales sent us off in the wrong direction, because that permits us to explore the non-uniformities in the acceleration. The premise of the original discussion was that all the two observers could see was each other. From such a measurement, neither could distinguish whether they were accelerated or experiencing gravity.
By doing a series of experiments, the changing accelerations (or the equivalent changing gravitational fields) suggest the presence of a large mass.
Somehow, I think we need to get into the mathematics of general relativity to sort this out. So I’m signing off because I am about to fall over the edge of my knowlege :)
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
“The principle of equivalence deals with one object’s accelerated frame of reference. It applies to each object individually, but the convergence implies multiple objects and multiple frames of reference.”
I’m sorry to see you sign off with just a questionable quote from scripture. Obviously, the principle of equivalence deals with two frames of reference, else the putative equivalence would be a self-equivalence. In any case, the principle is what’s in question, and in any case, I’ve cited a well-known, accepted, and confirmed phenomenon (see the Eotvas Experiment) which argues for centers of mass.
Jim, struggling but not quite succeeding in giving up the idea of a center, writes:
“In fact they are distinguishable, as objects dropped in (1) will converge, however slightly – because there’s a convergence toward the center of mass(!)”
The principle of equivalence deals with one object’s accelerated frame of reference. It applies to each object individually, but the convergence implies multiple objects and multiple frames of reference.
Jim, you might like a book that I reviewed less than favorably: The View from the Center of the Universe.
Time for me to sign off on this topic.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)