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Old Jul 20th 2009, 11:59 AM   #1
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"Common Misconceptions in Physics" page

In Suggestion for Improvements we discussed a few things that might add to the quality of this website. One suggestion I made was a page called Misconceptions in Physics. I started working on this last night. I created two of them so far. I'm posting them here for discussion.

Common Misconceptions in Physics


Misconception – Since a light has no mass it isn’t affected by gravity[/FONT]

Correct Physics – Physicists recognize three types of mass according to threes aspects of the concept of mass. They are as follows

(1) Inertial mass – The ratio of a particle’s momentum to its speed
(2) Passive gravitational mass – The mass on which gravity acts
(3) Active gravitational mass – The source of gravity

Inertial mass is properly defined as the quantity m such that the quantity (momentum, p = ) mv is conserved. To be precise we say that inertial mass is defined so that momentum is conserved. The equivalence principle, from Einstein’s general theory of relativity, postulates equality of inertial mass and passive gravitational mass. Since photons have momentum they have inertial mass; since they have inertial mass they have passive gravitational mass; since they have passive gravitational mass they are acted upon by gravity. We can also find the mass of a photon if we have its energy. For a photon |v|= c. Since the energy of a photon is related to its momentum by E = pc we obtain p = E/c. Setting this equal to mc we obtain E/c = mc. Solving for m gives m = E / c[sup]2[/sup].

Richard Feynman points this out in The Feynman Lectures on Physics – Volume I, by Feynman, Leighton and Sands. From page 7-11
In the Einstein relativity theory, anything which has energy has mass – mass in the sense that it is attracted gravitationally. Even light, which has an energy, has a “mass.” When a light beam, which has energy in it, comes past the sun there is an attraction on it by the sun. The light does not go straight, but is deflected.
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Misconception – Gravity is not a force, it’s a curvature in spacetime.

Correction – Einstein’s general theory of relativity treats the gravitational force on the same footing as inertial forces. First consider the definition of inertial force

Definition: When the motion of the reference system causes the momentum, as measured in the reference system, to be a function of time, i.e. p = p(t), we say that there is an inertial force acting on the particle. The value of the inertial force is F = dp/dt.

An example of an inertial force is the centrifugal force which is the force felt by an object moving in a curved path that acts outwardly away from the center of rotation. Prior to general relativity inertial forces were viewed as being a result of viewing nature from the wrong frame of reference. It was for this reason that they are more often referred to by the fictitious force, pseudo-forceor apparent force. Non-inertial forces can be expressed with a non-zero 4-force. It should be noted that Einstein viewed inertial forces as being “real.” Einstein also never interpreted gravity to be a curvature in spacetime either. Laymen often confuse spacetime curvature with the curved path of a particle being deflected in a gravitational field. A more common name for spacetime curvature is gravitational tidal gradient, i.e. tidal force. Loosely speaking, gravitational tidal force is the difference in gravitational force in a gravitational field. For an object to experience a tidal force it must have a finite spatial extension. The gravitational force can act on a single point particle. Point particles are not subject to tidal forces but are affected by inertial forces. It is therefore misleading to say that gravity is not a force but merely a curvature in spacetime.

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Old Jan 3rd 2014, 07:23 PM   #2
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I recently ran into a another common misconception in physics. Quite often many people assume that Einstein's General Theory of Relativity is a theory that explains gravity. This misconception was addressed by Sir Aurthur Eddington. From Gravitation and the Principle of Relativity by A.S. Eddington, Nature, March 14, 1918, page 36
The purpose of Einsteinís new theory has often been misunderstood, and it has been criticized as an attempt to explain gravitation. The theory does not offer any explanation of gravitation; that lies outside its scope, and it does not even hint at a possible mechanism. It is true that we have introduced a definite hypothesis as to the relation between gravitation and a distortion of space; but if that explains anything, it explains not gravitation, but space, i.e. the scaffolding constructed for our measures.
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Old Jan 8th 2014, 08:53 PM   #3
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hi there

hat do you think abput this ideas, the validy of the theory flaws, etc. im not o phisics student so its based only on assumptions and things ive learnd on my own, thanks any comentary apreciated
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Old Jan 8th 2014, 09:25 PM   #4
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Originally Posted by vrgdaniel View Post
hat do you think abput this ideas, the validy of the theory flaws, etc. im not o phisics student so its based only on assumptions and things ive learnd on my own, thanks any comentary apreciated
Sorry but those are merely statements which don't make a great deal of sense. E.g.

From “Basic Principles”

1-matter exists in parallel with dark matter

I assume that you mean that matter coexists with dark matter. I’ll accept that as an axiom.

2-they are made of different stuff and they obey different laws

It’s not clear what that means. I.e. what constitutes something as being “different stuff”? However there’s nothing to support the axiom that they obey different laws.

3- forces that govern matter are weaker related to the ones that govern antimatter

Wrong. Contradicts observation.

4-both matter and antimatter can “compress” (beyond its equilibrium point)

The rest of it reads like that

Last edited by Pmb; Jan 8th 2014 at 09:37 PM.
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Old Jan 10th 2014, 12:37 AM   #5
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sorry for the langauge mistakes englshi is not my language im actually spanish :P, so yes i meant it coexists in the first statement

By different sutff i mean matter atoms, or at a smaller scale elementary particles (quarks, leptons.etc) and antimatter well have no idea about it elementary partciles but being different well they should obey different laws.

i thought its interesting explaining gravity like a "bend" in the stuff that forms the antimatter

its just an idea i had the other day that made sence to me but like i said im an architecture student so my understanding of physics is not that high :P

thanks for the answer.
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Old Jan 10th 2014, 03:59 AM   #6
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Originally Posted by vrgdaniel View Post
By different sutff i mean matter atoms, or at a smaller scale elementary particles (quarks, leptons.etc) and antimatter well have no idea about it elementary partciles but being different well they should obey different laws.
That's incorrect. The laws that govern matter are identical to the laws that govern antimatter. Antimatter is simply matter with the opposite charge and spin. And the laws are the same for both.

Originally Posted by vrgdaniel View Post
i thought its interesting explaining gravity like a "bend" in the stuff that forms the antimatter
I don't know where you got ideas like this but they're all quite wrong, this one included.

Originally Posted by vrgdaniel View Post
its just an idea i had the other day that made sence to me but like i said im an architecture student so my understanding of physics is not that high :P
Then you're in the right place. Keep asking questions and you'll keep learning.

Originally Posted by vrgdaniel View Post
thanks for the answer.
You're very welcome.

Last edited by Pmb; Jan 10th 2014 at 04:08 AM.
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Old Jan 11th 2014, 01:47 AM   #7
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so then one final question :)

ok thanks again for your answers, so i have one more question, which is actually at the basis of what i asked before, its very basic fisics, but i think the answer its quite complicated,

I never understodd in school What is Gravity? but like really understanding the concept , and by that i dont mean like the fact thats it the force betwen two objects, , relaccion between their masses and the disctane betwent them, whit the a constant g (the matematical formulas), i dont mean either at the other explinacion that Einstein gived that gravity is a bend in space and time, which starts to make more sence, i mean what and why makes an object attract another whit a diferent mass? why this force appearce? and why is it only noticalbe only past a certain mass, size objects, planets, etc, and not mathematical explanacion, numbers but the concept that stand behind it, hope i it a clear question thanks again
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Old Jan 11th 2014, 01:56 AM   #8
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and to explain in more detail waht do i mean by gravity a "bend" in the antimatter

regarding the latest post, like gravity a bend in the stuff that makes the antimatter, well camed from the fact that ensiteins saw gravity like a bend in space and time, so like i said it makes much more sence for me understandig gravity like that then like the previous version of an invisible force between two objects, so starting from that idea, what is this space and time that ensiten refers that are bend by an object, by it mass? Waht is space and time? and how come mass bend both of them?

if you see space as empty, which latley from what i know scienties start to realize that the space, particulary the spcae we thoguht its empty , vacum, the scpae between plantes it not actually empty, in fact it could contain more energy than what we can find anywere else, so

if energy=mass*speed of light squared, and there is more energy in the vacum it could be that it has a huge amount of "mass" only a differnt kind of mass, mass more like a concpet, a way to store energy, not like the actual object that you can touch, the matter we can see around, which could be antimatter

so from that i conclued that gravity being as einsteyn explained it a bend in space, if you see space , the vacum not like an empty space, but like a very high energy , mass "stuff" , then it makes much more sence , for me at least, that this could bend, and thus for creat what we call gravity

so thats what stands behind the concepts that i wrote before in the image, that i imagine what we see as empty space, vacum, full of antimatter, and metaforicaly speaking you could comapre this to a huge rubber cube in which if you insert something big enough (and whit big i mean related to its mass) it will generate a force whic tends to fill back the space it displaced so like a "bend" but in 3D..... but like i said before im not a phisicy student so its only from books i ve read raleted to the subject , and then apllynt to that knologe common sence some conclusions that again to me make sence, so only looking for the onpinion of people whit more practical knolowege of the subject

Thanks again

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Old Jan 11th 2014, 03:53 AM   #9
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....

and to continue on this ideas in the image ive uploaded, the black holes coud be clustters of matter that are compressed under this pressure forces, gravity, and end up sucking everthing around it, all matter from planets to stars, even light it self, exanding the amount of matter they contain ,but being coinstantly compresed by this outised pressure that animatter generates over it, the example of the rubber, till this pressure, gravity compresess tha mattert to such an extente that it blows up, this procces happening at a hole range of diferent scales, from smal black holes in systmes, to biggwer ones at the center og galaxys like our one, to one huge final balck hole that will encompass all the matter in the univers, whic is the first end last slides in the image i uploaded, recahing a point were this pressure is so high that in explodes, the big bang, and this process of compresion ,explosion expansion , and recompression big bang al over again, is like a cylce in the uniers, if big bang happned once? why shuold we think it was a one time process? it makes more sence to see it as a cicle that repeates it sefl over very very long periods of time ... so hope now the image i first uploaded is more clear jejejeje. any opinions on this last post?
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Old Jan 11th 2014, 04:21 AM   #10
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Originally Posted by vrgdaniel View Post
... i mean what and why makes an object attract another whit a diferent mass? why this force appearce? and why is it only noticalbe only past a certain mass, size objects, planets, etc, and not mathematical explanacion, numbers but the concept that stand behind it, hope i it a clear question thanks again


Nobody knows that. You're looking for the mechanism of how gravity does what it does. Unfortunately all that physics knows so far is how to describe it. Nothing more. E.g.
The purpose of Einsteinís new theory has often been misunderstood, and it has been criticized as an attempt to explain gravitation. The theory does not offer any explanation of gravitation; that lies outside its scope, and it does not even hint at a possible mechanism. It is true that we have introduced a definite hypothesis as to the relation between gravitation and a distortion of space; but if that explains anything, it explains not gravitation, but space, i.e. the scaffolding constructed for our measures. - Gravitation and the Principle of Relativity by A.S. Eddington, Nature, March 14, 1918, page 36
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