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Old Aug 1st 2009, 06:49 PM   #1
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Common Misconceptions in Physics

The post that follow this one was made by the member Pmb. He deserves all the credit.
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Old Aug 1st 2009, 06:49 PM   #2
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Common Misconceptions in Physics

Misconception – Light has no mass.

Correct Physics – Whenever you see someone use the term mass you need to make sure you understand exactly what he or she means by it since the term mass when used unqualified, can refer to several different things. It can refer to proper mass (aka rest mass), inertial mass (aka relativistic mass/mass-energy), passive gravitational mass or active gravitational mass. Only when one is referring to proper mass can it legitimately be said that light has no mass.

Misconception – Einstein proved that gravity is a curvature in spacetime.

Correct Physics – Einstein argued that gravity is the result of viewing nature from a non-inertial frame of reference. He showed that when that doesn’t hold as in the case where tidal forces are present the result is due to spacetime curvature. Other physicists objected to the notion that a gravitational field has a relative existence so they chose to interpret tidal forces (i.e. spacetime curvature) as determining the presence of a gravitational field. Einstein actually objected to this interpretation.

Misconception – You cannot simultaneously measure both the position and the momentum of a particle.

Correct Physics – This is the wrong phrasing of Heisenberg’ s Uncertainty Principle (HUP), which actually states that the position and momentum are not simultaneously determined. This has a very different meaning than what is stated above. Uncertainty is a property of the quantum state and not determined by how measurements are done. No matter how a measurement is taken one cannot change the inherent uncertainty determined by the quantum state of the system.

Misconception – Mass does not depend on velocity.

Correct physics – This is a statement made by people who choose to use the term mass only to refer to what is known as proper mass aka rest mass. All the statement really means is that the proper mass of a particle does not depend on the particle’s speed.

Misconception – Physicists no longer use relativistic mass.

Correct physics – This is an incorrect statement. For example; Gary Oas, the author of the article On the Use and Abuse of Relativistic Mass, did a survey of literature on relativity and concludes that the concept of relativistic mass is still prevalent in the literature on relativity. The survey done by the author included 637 works. Of those 477 relied on the concept. This is 74% of the literature examined! Hardly more than the claim of zero! For a complete analysis one should read the above article and see exactly what the numbers mean. For example Figures 1 and 2 are diagrams illustrating the use over the years. It’s interesting that those textbooks, which are about special and general relativity, tend to use the concept more than books on modern physics that only contain a few chapters on relativity. As figure 1 shows, a greater percentage of relativity texts published in the years 2000-2005 used relativistic mass! I.e. 4 didn’t use it whereas 8 did! The claim “Physicists no longer use relativistic mass.” is a very serious mistake.

The following articles are from the American Journal of Physics. They are examples of the use of relativistic mass in a highly respected physics journal. This particular journal is a teaching journal.

The mass of a gas of massless photons
, H. Kolbenstvedt, Am. J. Phys. 63(1), January 1995

The inertia of stress
, Rodrigo Medina, Am. J. Phys. 74(11), November 2006

Apparatus to measure relativistic mass increase
, John W. Luetzelschwab, Am. J. Phys. 71(9), September 2003

When someone tells you that mass does not depend on speed your caution antenna should go up!

Misconception – Since a light has no mass it isn’t affected by gravity

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

(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, $\displaystyle \bold p =$) $\displaystyle \mbox m \bold v$ 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 the 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 know its energy. For a photon $\displaystyle |\bold v|= \mbox c$. Since the energy of a photon is related to its momentum by $\displaystyle \mbox E = \bold p \mbox c$ we obtain $\displaystyle \bold p = \frac{\mbox E}{\mbox c}$. Setting this equal to $\displaystyle \mbox{mc}$ we obtain $\displaystyle \frac{\mbox E}{ \mbox c} = \mbox {mc}$. Solving for $\displaystyle \mbox m$ gives $\displaystyle \mbox m = \frac{\mbox E}{ \mbox c}$

Richard Feynman points all of 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.

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

: Suppose a particle is moving at constant velocity in the inertial frame S’. The momentum of the particle will be constant in S. Now consider another frame S which is accelerating relative to S’. The particle i When the motion of the reference system causes the momentum, as measured in the reference system, to be a function of time, i.e. $\displaystyle \bold p = \bold p(t)$, we say that there is an inertial force acting on the particle, the value being $\displaystyle \bold F \equiv \frac{d \bold p}{dt}$. The value of the inertial force will always have the form $\displaystyle \bold F = m\bold g$

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-force or 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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Last edited by arbolis; Aug 2nd 2009 at 05:05 PM.
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Old Nov 14th 2016, 02:56 AM   #3
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Great explanation. I haven't heard about it before. Thanks for the wonderful information.

Last edited by topsquark; Nov 14th 2016 at 05:03 AM.
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