
#19
Nov1607, 12:36 AM

Sci Advisor
P: 2,341

Right, a massless particle can't be at rest, so it cannot have and does not have a "rest mass", but it can and does have energy and momentum.




#20
Nov1607, 01:51 AM

P: 72





#21
Nov1607, 02:32 AM

Sci Advisor
PF Gold
P: 3,273

i.e. 'a massless particle can't be at rest, so it cannot have and does not have a "rest mass", but it can and does have energy and momentum when it is moving at c'. Garth 



#22
Nov1707, 04:55 PM

P: 26

instead to say does time cease to exist if it cannot be measured or observed because surely all clocks require to be made of mass and energy




#23
Nov2107, 08:25 AM

P: 72





#24
Nov2307, 06:52 AM

P: 72

Is it going to rest in peace? Is there nobody out there to reply?




#25
Nov2307, 07:28 AM

Sci Advisor
PF Gold
P: 3,273

You may appreciate the quote from the fourth century: In that same chapter he also said , as a prayer to God, amongst other sayings: and You may find the discussion of this old thread useful to read: The Nature of Time?. What other questions do you want to ask? We will do our best to answer them. Garth 



#26
Nov2307, 03:53 PM

P: 8

The existence of time is the sole requirement of the assumption that spacetime is really a curved matter. Gauss studied curved surface, very realistic. Based on Gauss forms of curved surface, Riemann proposed curved `surface` of whatever dimension. These are math!! GR, a physical theory, considers the math to be reality. So the universe based on BB (big bang) must be the curved entity in the meaning of Gauss but 4dimensional. This is the standard interpretation of GR. Another interpretation is based on real matter and is consistent to Quantum Mechanics: Physical measurements give an impression of curved spacetime. Based on this interpretion, cosmic redshift is Doppler redshift. The physical universe is infinite but physical constants may change and the corresponding physical measurements give an impression of `Big Bang`. And time does not exist as you suggest! Please do not ban me!! Jin He 



#27
Dec307, 05:45 PM

P: 53

According to me "no mass and not motion" , so no observers as well, means nothing, so why the time should exist?




#28
Dec407, 07:46 AM

P: 72

And y do u think so? I think the answer is c.




#29
Dec407, 02:55 PM

P: 53





#30
Dec1107, 07:07 AM

P: 72





#31
Dec1107, 02:50 PM

P: 53

a "no place" ? (no space, no time , no everything) 



#32
Dec1207, 08:22 AM

P: 72





#33
Dec1207, 10:40 PM

P: 2

I believe the line of reasoning that a photon has zero rest mass is this:
According to Special Relativity as an object with mass approaches the speed of light its mass increases. The limit equation is that as an object with mass approaches the speed of light its mass approaches infinity. So if an object is moving at the speed of light it cannot have any rest mass. One must understand the purpose of an equation and from what I have seen many explanations are not very good at getting into explaining the full essence of such equations. That is because typically a large amount of background is assumed. There are nonmathematical books that explain physics, like Brian Green books. These are good for gaining some insights into physics. There are textbooks with strong mathematical books. These usually assume you took all the prerequisites, i.e. they assume a strong background. There are technical papers that almost always assume a strong background. There are articles like those in wiki that run the gambit. The in between books and especially articles which show the math and do a detailed explanation of each term with simple explanations are more harder to find. Let me explain the equation . This is a composite equation. The energy contained in “rest mass of matter” is the value m2c4. The motional energy and photonic energy is the p2c2. The following web page has some good information about energy. http://en.wikipedia.org/wiki/Energy An except from this is: 2. On the other hand, in the key equation m2c4 = E2 − p2c2, the contribution mc2 is called the rest energy, and all other contributions to the energy are called kinetic energy. For a particle that has mass, this implies that the kinetic energy is 0.5p2 / m at speeds much smaller than c, as can be proved by writing E = mc2 √(1 + p2m − 2c − 2) and expanding the square root to lowest order. By this line of reasoning, the energy of a photon is entirely kinetic, because the photon is massless and has no rest energy. This expression is useful, for example, when the energyversusmomentum relationship is of primary interest. Technically speaking, this paragraph does not explain why a photon has zero rest mass. It simply states it. So I do not like the wording in this document about why the photon rest mass is zero. Always remember there is a big difference between photons and particles. When you look at equations you must ask two questions: does this apply to particles, if so then how; does this apply to photons, if so then how. You will likely get different perspectives depending on whether it is a particle or a photon. One other thing to note is to always be careful about rest mass energy verses relativistic energy of a particle due to motion. Let me explain my point by discussing the wave equation. http://en.wikipedia.org/wiki/De_Broglie_hypothesis http://en.wikipedia.org/wiki/Waveparticle_duality When one uses the wave equation especially for matter realize that it deals with the interaction of objects. The is explained as follows: From http://en.wikipedia.org/wiki/De_Broglie_hypothesis “the greater the energy, the larger the frequency and the shorter (smaller) the wavelength” So now ask yourself, what energy are they talking about? Another question, are we talking about particles or photons? By the earlier principle, differentiate particles and photons. It does apply to both. So let me simply discuss the application to particles. Is the energy dealing with the rest mass, motion, or both? When you split apart the statement into more detailed questions like this you begin to better understand the nature of the statement and the equations. This energy is the energy of motion not in rest mass because the equation uses the mass and velocity of a particle. Higher velocity is higher energy is a shorter wavelength. Principle: motion has no meaning for a single isolated particle. It must be with respect to two or more. A simple statement but remembering it can help clarify. The energy of motion is in relative velocity. Something moving toward you, which increases its velocity, will increase its energy with respect to you. This will increase this frequency factor. However, you can also increase the energy by accelerating towards this object. If you accelerate towards the object you increase the relative velocity. This increases the relative motional energy and increases this frequency factor also. Your increase in speed effect your view of this objects “frequency”. This frequency factor is what is used on calculating how any interactions between you and this other object will happen. It is an equation used to help compute outcomes for interactions. So I hope this helps. I tried to add some principles for understanding what you are reading on physics and how to explore thoughts on physics. If you might guess these days I do like wiki articles on Physics. They are a nice resource for reference and look up. 



#34
Dec2607, 05:07 PM

P: 53





#35
Dec2707, 07:05 AM

P: 72





#36
Dec2707, 12:49 PM

P: 67

To EssentialNature. That was a great article! I have always been curious to know where did E=M_0C2 came from. I know the derivation of E=MC2 but it uses on its way M_0C2 as a term that represents the rest mass energy of an object. How do we know this? Is there a concrete derivation of this or did this come to Einstein as a dream? 


Register to reply 
Related Discussions  
Does matter exist?  General Discussion  13  
Does matter exist?  General Physics  8  
Matter Forms Space & Time is Relative Motion  Special & General Relativity  1 