No, I don't believe that motion is absolute.
CONCEPT OF RELATIVE MOTION- How Can We Say That Planets revolve around Sun?
- Thread starter ash64449
- Start date
- Status
No, I don't believe that motion is absolute.
That's because we are confined to a single reference frame and our adaption to this condition.
I am Absolutely sure that the Planets should move around the Sun and Not Sun moving around a planet Earth is just the impression of our's. Don't you think that's wrong?
You can absolutely understand that different objects have different velocities and accelerations in different frames. So why should the concept of absolute be introduced?
If you don't believe motion is absolute,How can you believe All planets revolving around sun to be absolute?
I don't.
After looking through the discussion so far, it seems that the conditions established within special and general relativity are being confused. Special relativity is where you find the statement that all inertial systems are equivalent, meaning that for inertial systems, one cannot say with certainty, which of two objects is moving relative to the other. Keep in mind this is limited to inertial systems, not accelerating systems. Where the word "inertial" referrs to systems or objects which are moving with a constant velocity. It is confined to conditions consistent with special relativity and in practice is only applicable in a locally defined frame of reference. The laboratory or motions defined relative to some assumed fixed reference.., the motion of two objects relative to the earth or one's own unique frame of reference.
(From a naive perspective one might also assume the same of accelerating Objects in that same locally defined frame of reference. This however, assumes that acceleration cannot be measured directly. The inertial resistence to the constant change in motion can both be felt and measured and so the object which is accelerating can be determined. An exception to this is an object whose motion is accelerating as it falls into a gravity well.)
The world, on the other hand, where one is speaking of the kinetic relationship between objects within the solar system or the universe, do not lie within the confines of special relativity. They lie within the scope of general relativity. Nothing within that context can be thought of as inertial, as described within the context of special relativity. Everything is accelerating in some fashion, weather that is accelleration in the basic sense or accelleration(s) defined by the varying influence of gravity. It is within this difference that the answer to your question(s) lie.
Yes, we could construct a model that describes the motion of the planets, (a historical fact).., relative to ourselves here on the earth, that assumes that we are at rest and everything else is in motion. The test and disproof of that perspective lies in the success of both manned and unmanned explorations of the moon and planets. Had we continued to believe that the earth was at rest, we could never have predicted the true locations of the moon and planets, relative to the spaceships and satellites launched to explore them.
For any two objects moving at constant velocities (inertial objects) neither can say without some external reference which is moving, relative to the other or some common frame of reference.
Where acceleration, other than as defined by the influence of gravity.., is involved, the accelerating object can measure or feel the inertial resistance to its changing state of motion. In this case both frames can know which is accelerating.
Both of the above are valid within the context of both special and general relativity. In practice, within the context of general relativity and gravitation, the motions of all object's must be defined based on the involved center of gravitational mass. When we look at the solar system we define that as the sun, or at times very near the sun. As we expand our observations to galaxies and beyond, so must our frame of reference expand for our observations and conclussions to have any meaning, within the larger context.
Last edited:
Well, the Statement "All Inertial Systems are equivalent" means that even if we conduct experiments in any of the inertial reference frames, All experiments would yield only the same results and as a result says that all laws of physics are valid in inertial systems. It doesn't say anything more than that.
You used the word "certainty", "one cannot say with certainty, which of the two objects are moving".
Well, you used the word "certainty" because in your mind still the concept of absolute motion prevails.
No. Both frames can't understand which one is accelerating.
OK. "If both frames can understand which frame is accelerating", say i take earth and sun as the reference frame. Earth obviously moves around the Sun. And the Earth is accelerating.( Kepler's Second Law helps you to understand that). If Earth is accelerating,then we should know that we are accelerating. But do we feel accelerating? NO. Because Motion along with acceleration is relative.
Obviously, The Concept of Relative Motion is hard to Grasp. Even if someone understands or believes Motion is Relative, That person would still think in his inner mind that Motion is Absolute.
So Tackling a point by just saying wouldn't help. Provide relevant mathematical equations or examples to show that Acceleration is not relative. That would work.
But through examples,it is easy to show that acceleration is relative. Please see my post #7 which shows with simple mathematics that Acceleration is relative.
So I stress again. Geocentric Theory And Heliocentric Theory are equally valid, Only in their own given frame of reference due to the fact that motion is relative. Motion is a combined effect of the observer and another object.
So the word Absolute motion or the introducing the word "certainty" to explain the motion of objects doesn't make any sense.
So Tackling a point by just saying wouldn't help. Provide relevant mathematical equations or examples to show that Acceleration is not relative. That would work.
But through examples,it is easy to show that acceleration is relative. Please see my post #7 which shows with simple mathematics that Acceleration is relative.
So I stress again. Geocentric Theory And Heliocentric Theory are equally valid, Only in their own given frame of reference due to the fact that motion is relative. Motion is a combined effect of the observer and another object.
So the word Absolute motion or the introducing the word "certainty" to explain the motion of objects doesn't make any sense.
Math is only useful insofar as it is representative of reality. You can use math to demonstrate all sorts of things that aren't true.
The reality is that if you have two objects stationary next to each other, one has to DO SOMETHING to accelerate away from the other. That's the one really accelerating.
Even if you weren't wrong about acceleration being relative, the two models would not be equally valid. One model is simple and powerfully predictive while the other is extremely complicated and ad hoc, with no predictive power unless constructed in a way that reduces to newtons laws.
And as far as I know, no one has ever successfully created even a limited geocentric model of the solar system (just a few objects).
I don't believe you have sufficient information to support the portion in bold above.
That said you are wrong. Given only two objects or observers both moving with a constant velocity and no other external data, neither can say which is moving relative to the other or whether both are moving. All they can say is that they are moving relative to one another. IOW they can both say with certainty that they are moving relative to one another, but neither can say with "certainty" who is doing the moving or if both are.
I must have missed something of the earlier discussion because I have no idea how you arrived at absolute motion.
Since accelleration can both be felt and measured, as a resistance to the constant change in motion, both observes can tell who is accelerating and through a bit of math who is not! Any observer who is not accelerating would not feel or measure any acceleration. Any observer accelerating will feel and measure that acceleration. If both are accelerating, they can esily determine that the distance between them is changing at a rate differnt than suggested by their own rate of acceleration.
So if only one observer is accelerating the inertial observer will not feel or measure any acceleration and conclude that the other observer is accelerating.
In your example of earth orbiting the sun, you obviously forgot the qualification excluding acceleration due the the influence of gravity, from the general statement. Yes the earth is accelerating, in a free fall state that takes it around the sun. And no we do not feel it fall around the sun, just as we do not feel our own acceleration, toward the fround, as we jump off a cliff. (Don't try it to test it, the landing is ...)
As soon as you add gravity to an example it is necessisary that the source of the acceleration be considered and things become more complex.
This is why I began by saying it seems as if the conditions of special and general relativity were being confused. One accounts for the affect of gravitation and the other does not.
I agree. In my textbook, to derive the equation of relative velocity(constant velocity),it used a method. I used the same method to prove that acceleration is relative. I don't think there is no problem with that derivation.
Yes. you are right. Mathematics can be used to derive whatever you want. Mathematics being language of physics. we can actually use any sort of assumptions and mathematics helps us to make predictions from our assumptions. Mathematics does only that. Help us to make predictions. Nothing more.
Yeah. You are right. Geocentric Theory is not qualitative. Of course Heliocentric Theory is more qualitative.
Does that mean you believe that Motion of object executed in our reference frame is an illusion ? I got this sort of feeling when you said that Neither can we know with "certainty" who is actually doing the "moving".
And also that statement doesn't make any sense to me. The word "moving" is relative.
OnlyMe, The above paragraph is in direct contradiction with the below paragraph.
In the 1st paragraph, you said acceleration is something that can be felt.
In the 2nd paragraph, you said that earth is accelerating but it is in free fall,so we do not FEEL the acceleration. i.e acceleration that cannot be felt.
I totally agree with the 2nd paragraph.
From Equivalence principle, we can understand that acceleration is indistinguishable from gravity.
Now An observer is standing on the earth. He has a ball in his hand. Then he lets go of the ball, the ball falls down the earth with an acceleration.
Equivalence principle helps us to state that we can think the above event like this too: earth along with observer moves upwards. when the observer drops the ball,acceleration is not imparted to the ball. so Ball move with constant velocity upwards. Since the earth is accelerating, According to the observer's reference frame,ball move downwards with an acceleration.
You can see over here that ball moved with constant velocity in some other reference frame. But not in the reference frame of the observer.
So how can you think acceleration is not relative?
Note that I said with no other external reference. It is a pure special relativity situation.
When you qualify the question with "our reference frame", you suggest some additional information. Perhaps both observers stand on the earth and assume the earth as a common frame of reference for their motion. They now measure their motion relative to oneanother, as defined within the context of their relationship to the earth.
The discussion has seemed to intermingle conclussions that fit with the pure special relativistic conditions where there are two inertial objects, with nothing else to compare their motion, apart from each other, and conditions that involve gravity and general relativity. Two objects alone in space moving with constant velocities cannot tell who is moving relative to who or if they are both moving. All they can say is that they are moving relative to each other.
Yes the word moving is relative, but it is relative to the frame of reference one assumes as a standard. When we set that standard vaguely as "our frame of reference".., by common experience we imply reference to the earth as if the earth were a fixed frame of reference. Special relativity, as far as this issue is concerned, deals only with the two objects/observers and no other external references.., and it does not include the presence or affects of gravitation on either or both observers.
So yes motion is relative, but it is important to clearly define what it is relative to.
The two paragraphs are describing different conditions/environments. The first describes acceleration not due to the affect of gravity. The second involves the affect of gravity.
Be careful here. While superficially the above is true, the way you seem to be interpreting or applying it is not consistent with what it actually says or represents.
The equivalence principle says that acceleration, without any external reference will feel and be measured to be equivalent to standing still on the surface of a gravitating body. Standing on the surface of the earth feels just like standing in a spaceship that is accelerating. It is assumed that the spaceship is far distant from all sources of gravity and accelerating. (Assume the acceleration is the result of the spaceship's rockets.) In effect it assumes conditions that exclude the affect of gravitation and are consistent with special relativity.., and compares that with how gravity feells and is measured standing still on the surface of.., say the earth.
The above sounds confused. When you drop an object here on earth it does not fall at a constant velocity. It accelerates toward the ground at 32ft/sec/sec. That acceleration would not be felt as equal to standing still on the earth and is not what is described by the equivalence principle. The example defines the earth as the frame of reference and the grvitational field of the earth as the source of the ball's acceleration.
Acceleration which is the result of gravity, is an exception to the comparrison of acceleration and gravity, within the context of the equivalence principle.
There is no easy way to explain why, because it involves the relationship between inertia and both acceleration and gravitation. At present inertia is an a priori. It is a generally accepted intrinsic quality of mass. There is no completely consistent model that describes or defines the origin of inertia and its relationship to both acceleration and gravitation. There are continuing efforts to do so, but they lie in the domain of quantum mechanics and as yet have not been completely successful.
Free fall acceleration is not what is being comparred to gravity, by the equivalence principle.
I didn't use the word qualitative and don't know why you think it applies.
The problem is that it is just a transformation equation. It doesn't analyze the interactions causing the acceleration.
The modern concept is the same as Galileo's (motion is relative) plus Einstein's (length contracts and time dilates at high relative speeds).
The Geocentric view was that all of the objects in the sky orbited Earth. This is feasible for most objects but not all. In particular the planets pose a problem, because they move quickly enough out of their expected orbit to appear in the wrong position. To explain this a complex system of epicycles was invented to try to account for their erratic trajectory. It was never quite satisfactory, no matter how many improvements were made to it.
Enter Galileo. His principal discovery was that Jupiter was orbited by its own moons. If those moons did not orbit Earth, then perhaps Jupiter did not orbit Earth. If Jupiter did not orbit Earth, then perhaps the rest of the known planets did not orbit Earth. If they did not orbit Earth then perhaps they orbited the Sun. If that were true, then perhaps the Earth orbited the Sun also. This set the stage for the Heliocentric view.
Enter Copernicus, Kepler and Newton. Copernicus proposes that the planets orbit the Sun. Kepler shows that the area swept out along the orbit accumulates at a fixed rate. Newton confirms both men's ideas through the invention of calculus and the discovery of the universal law of gravitation.
No. The planets obviously do not orbit the Earth. The Geocentric view is therefore wrong. The Heliocentric view is Copernicus's correction to this. It eliminates epicycles and accounts for Galileo's discovery of the moons of Jupiter.
It's not a theory. It's just a correction to the older incorrect beliefs that prevailed in medieval Europe.
No, it's fully correct once Kepler demonstrated the absolute rate of change of the area swept by an arc of an orbit. This is more fully proven and explained by Newton's calculus and his discovery of gravity.
- Status