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Posts posted by pmb

  1. m_0 = 0 for a photon.


    The way light has mass is to introduce a new definition for mass, e.g. m=E/c^2

    The definition which nefver needed changing is the way mass was defined by newton and how his second law was defined. Newton defined mass so as to be consistent with the m in p = mv. Plug in c for a photon and you get p = mc. Or if we follow French we'd get


    From Special Relativity, A. P. French, MIT Press, page 20

    Let us now try to put together some of the results we have discussed. For photons we have


    E = cp




    m = E/c2


    (the first experimental, the second based on Einstein's box). Combining these, we have


    m = p/c

    The nice thing is that m = p/v is a definition whereas m = E/c2 is an equality. One that doesn't hold true in all concievable cases. E.g. the inertial energy density and the inertial mass density can be different. E.g. consider a rod which is under stress. In such a case the inertial mass density is not the same as the inertial energy density.


    If you have Schutz's GR text please see page 110


    Thanks for clearing the doubt...

    But as we know that photons are the elementary particles of light.. they have no mass, but how come light has mass???

    Einstein proved that an object travelling at the speed of light has infinite mass..

    I.E. m = m0 /sqrt(1 - v2/c2)

    We will get to know that the mass is infinite (of an object travelling at the speed of light)

    If photons are the elementary particles of light (with no mass), then how is it possible for light to have mass?????

    They have zero proper mass but the dop have inertial mass m = p/c. This comes fom the definition of mass which is implicitly defined through the relation p = mv. Such a definition says "mass m is defined such, for a system of particles, mv is a conserved quantity." This might be of use - http://home.comcast.net/~peter.m.brown/sr/inertial_mass.htm


    The expressioin m = m0 /sqrt(1 - v2/c2)[/size] is not one I choose to use to define the mass of a luxon (a particle whose speed is c)

  2. This could go on forever. I don't see a day that where everyone would decide on when they've arrived at a solutio. I'm not sure when to explain the solution.


    Perhaps I could point to a relativity text which would give you a hint to the solution if you'd like. Here is a list of the ones I have.




    I could also point to an article in the American Journal of physcs which has a similar problem with the electric field. They won't give the answer but it should head you off to the right direction.


    Do any of you have the text Gravity From The Ground Up by Bernard F. Schutz?


    When would you like to see the answer, if at all?

  3. [math]\rho \equiv T^{00}/c^2=\frac{1}{2\mu_0 c^2}B^2=\frac{1}{2}\epsilon_0 B^2[/math]


    which is what I had in my original post. Of course, should one choose to boost to another frame then things would get a lot messier and other components of T (from the original frame) would contribute.

    Quite interesting! I've never see it written that way and didn't recognize it. Thanks!

  4. It's moot, because you can't determine the exact energy, from [math]\Delta{E}\Delta{t}>\hbar[/math]

    That relation is not a strict Heisenberg relationship. It doesn't mean that energy can't be measured precisely.


    Recall the derivation of that expression and the meaning of E in it.



    See also http://arxiv.org/abs/quant-ph/9807058


    uncertainty in energy x uncertainty in time? > reduced planck's constant?

    There is no such thing as uncertainty in time. A real Heisenberg expression requires two observables, Time is not an observable and as such there is no time operator. Time is a paramenter.


    That relation is not a strict Heisenberg relationship. It doesn't mean that energy can't be measured precisely.


    Recall the derivation of that expression and the meaning of E in it.



    See also http://arxiv.org/abs/quant-ph/9807058



    There is no such thing as uncertainty in time. A real Heisenberg expression requires two observables, Time is not an observable and as such there is no time operator. Time is a paramenter.

    juanrga - Excellant!! Way to go! You"ve got it precisely right in my opinion.


    Some people were making those kinds of arguements to me a long time ago. I found that a lot of people misuse that relationship that swansont. I also found that those people didn't understand time. They thought it was an observable.


    Griffiths has a nice insight into this. From Introduction to Elementary Particlesby DAvid Griffiths, 2004). See page 51-52, regarding that relation

    Nevertheless, it is a useful device for "back-of-the-envelope" calculations, and it does very well for the pi meson. Unfortunately, many books present it as though it were a rigorous derivation, which it certainly is not. The uncertainly principle does not license violation of conservation of energy (nor does any such violation occur in this process.... In general, when you hear a physicist invoke the uncertainty principle, keep your hand on your wallet.

    Oh how I love that quote! :)

  5. In modern physics one would tend to make the distinction between the fundamental particles that "build-up" matter and the fundamental particles that "transmit" forces.


    Photons are the particles that are responsible for the electromagnetic interaction and thus are not usually considered as matter.

    The term matter is a term which I tend to stear clear of. If I use it I mean more or lass "stuff". I'd never use it in a scientific sense myself. But if people ask me for a definition in a relativity context I quote Einstein.

  6. .... Faith represents more than just "belief."

    I agree. Where did you get the idea that what I said implied differently?


    It represents claiming to know something you do not know, and is not equivalent to either belief or hope.

    I dsagree. This time let's simply refer to the dictionary and be done with it.


    From http://dictionary.reference.com/browse/faith

    confidence or trust in a person or thing: faith in another's ability.

    2. belief that is not based on proof: He had faith that the hypothesis would be substantiated by fact.

    3. belief in God or in the doctrines or teachings of religion: the firm faith of the Pilgrims.

    4. belief in anything, as a code of ethics, standards of merit, etc.: to be of the same faith with someone concerning honesty.

    5. a system of religious belief: the Christian faith; the Jewish faith.



    Faith - Belief that does not rest on logical proof or material evidence.



    belief - 1: To accept as true or real ...1: To have firm faith, especially religious faith.


    I sent you the URL http://plato.stanford.edu/entries/faith/. Did you take a brief look at it?


    •the ‘belief’ model: faith as belief that God exists

    •the ‘trust’ model: faith as belief in (trust in) God

    •the ‘doxastic venture’ model: faith as practical commitment beyond the evidence to one's belief that God exists

    •the ‘sub-doxastic venture’ model: faith as practical commitment without belief


    Faith as belief



    I can't quote them all since the word belief appears in that page 80 times.



  7. I am new at quantum physics, so dont be harsh..

    my question is that "Photons"(quanta of light) they are present in the forms of electromagnetic radiations. they have no mass..

    but still they are said to be a Matter.. WHY????


    Einstein defined matter as follows. From The Foundations of the General Theory of Relativity, by Albert Einstein, Annalen der Physik, 49 (1916). Reprinted in The Principle of Relativity, Dover Pub. See bottom of page 143.

    We make a distinction hereafter between “gravitational field” and “matter” in this way, that we denote everything but the gravitational field as “matter.” Our use of the word therefore includes not only matter in the ordinary sense, but the electromagnetic field itself.


    Regarding whether photon's have mass please see


    which explains that Photons have zero proper mass and non-zero inertial mass.


    Particle physicists have a strong tendancy to use the term "mass" to mean "proper mass". They don't have applications otherwise. It's different when working with continuous media. Cosmologists use the term differently such as when it comes to gravitational mass.

  8. I said, "Faith is little more than claiming to know things you do not know, and it is not equivalent to the concept of hope."

    I my humble oinion, faith is beleving something that you don't know to be true.


    For exmaple; a man may have faith that his wifeisn't cheating on him when she's not in his presence. Yes. Sometimes this proves false.


    A particulr physicist may have faith that a particular element was discovered after readingaout it i s science journal. And yes again. Sometimes this has been proven false too.


    Nope. Don't feel like doing your work for you. Try a dictionary.

    The question is what you believe it means. Not what we find that we'd think you mean. For that reason its up to you to do the work.

  9. I wish the 1% of people who are convinced that they understand physics would come out of the woodwork and supply us with a theory of everything!

    Have you ever looked up the meaning of the word physics in a dictionary? If not then please see



    a science that deals with matter and energy and their interactions


    You'e confusing the defintion that readily comse to mind when you're talking to physicists to the physics properties of the universe. When you do this intentionaly withot stating so then you are intentionally being misleadingwith you make the above clam.

  10. It's tricky to know what exactly you're asking for. Of course, you could define a "mass" density with units of kg/m3 by dividing its energy density by c2. In that case it would just be:


    [math]\rho =\frac{1}{2}\epsilon_0|\bold{B}|^2[/math]

    Hint: What is the energy density of a magnetic field?


    Question: What units are you using?

  11. It's tricky to know what exactly you're asking for. Of course, you could define a "mass" density with units of kg/m3 by dividing its energy density by c2. In that case it would just be:


    [math]\rho =\frac{1}{2}\epsilon_0|\bold{B}|^2[/math]

    Thank's for responding. re - It's tricky to know what exactly you're asking for. - I'm asking that people take a crack at solving this problem. It's the same question asked in the SR text I mentioned, i.e. from Special Relativity: A Modern Introduction by Hans C. Ohanian, Physics Curriculum & Instruction, 2001, page 149, problem 14.


    The strongest magnetic fields produced in Laboratories, by explosive compression of magnetic field lines, is 103. In such a magnetic field, what is the energy density and what is the mass density.


    So basically the question I posed is to solve this problem. Let's wait until we get multiple responses before I comment on the answers.


    Of course, there is a good reason why I chose this problem and why I called it challenging.




    ps - If you don't want to wait then let me know and I'll PM you my comments.

  12. People often ask me about when proper mass can't work as good as relativistic mass or even better. I wrote an entire paper to answer this question but people don't read it carefully enough. Probably because they believe that no matter what the paper says they've already made up their mind because the already thought about it carefully alread a long time ago. And that is a good reason. I'd probably do the same thing - Too much reason with no real expectations of changing what they think in any sape or form


    So to take a shot at perhaps clarifying why physicists hold on to the notion of relativist mass. So I've decided to create a challenge for everyone. I have a SR text by Hans C. Ohanian. One of the homework problems is to find the mass density of a magnetic field. That's my challenge to you all. Solve this introductory level SR problem. Let's make it as simple as possible and assume that the magnetic field be uniform. Find the mass density of a the magnetic field. Use whatever definition of mass that you see fit.


    Good luck. :)

  13. I hear so many religious leaders claim that God exists out of time. Here is an example of their belief. No logical arguement has ever been given in m experience.


    See Does God exist outside of time and sees all of time at once. at http://lavistachurchofchrist.org/LVanswers/2008/11-09.html


    This particular author seems inteligent enough to know better. E.g. her quotes the following

    Author - But it doesn't match what the Bible says. Consider Abraham's sacrifice of Isaac. God stop the sacrifice and said - Genesis 22:12 12 And He said, “Do not lay your hand on the lad, or do anything to him; for now I know that you fear God, since you have not withheld your son, your only son, from Me.”


    Author - Or think about the grief God expressed over men's sins. In the days leading up to the flood, the Bible says - Genesis 6:6 6 And the Lord was sorry that He had made man on the earth, and He was grieved in His heart.


    Author - Another event to consider is Jesus' discussion with the centurion. The man assured Jesus that all Jesus need to do was speak and it would happen. - Matthew 8:10 - 10 When Jesus heard it, He marveled, and said to those who followed, “Assuredly, I say to you, I have not found such great faith, not even in Israel!

    It warms the heart to hear someone in religion is intelligent. Others just can't get their mind on what it means to exist outsie of time. I'd betcha that none of them has ever spoken to a physicist or a philosopher of science. lol!

  14. I am a bit of a technoklutz I am afraid. Are you using a new browser? I am using Firefox 12 On Fedora 16 - the emoticons show up fine for me. I only get them on the full editor (not the quick reply) and I can toggle the selection on and off by using the single smiley face about a third of the way along the icon-bar.

    I'm using the same browser that I always have for the last 12 years, Internet Explorer. They used to how up with this browser but not now. :(

  15. To be honest Peter - I like to bask in the reflected glory; it's like a degrees of separation game: I have discussed physics with you, you have worked with Edwin Taylor, who co-authored one of the most important texts in physics with John Archibald Wheeler, who collaborated with Einstein

    Thank you. I am truly honored.


    Do you think that you can help me?. I can't get the Emoticons on the right panel back. Clicking on Show All does nothing. If I had an Emoticon I'd click on the one that makes me look sad. LOL!!

  16. I copied and pasted the following from Unexplained-mysteries.com:


    "Imagine an electromagnet, hooked up to some energy source. Attached to it by a frame is a piece of iron, at some distance from the magnet. The electromagnet is switched on for a fleeting instant, and then switched off again. Its magnetic field travels out in all directions as a wave, propagating through space at light speed, about 300,000 kilometers per second. When it reaches the piece of iron, the iron is pulled toward the electromagnet, and its own magnetic wave travels back to the electromagnet. But by the time the wave reaches it, the electromagnet has been switched off…so it is not attracted toward the iron. The forces within the system are unbalanced, and there is an “impossible” net momentum in one direction. The entire apparatus, including the piece of iron, is accelerated in that direction."

    I don't see any reason to assume that the momentum is unbalanced. When the EM wave hits the piece of iron the iron radiates an EM wave in all directions, not just back to the source. The stress wave inside the frame does not move at c so it hits the magnet later. When the wave hits the magnet it will stimulate another EM wave and it goes on and on. The momentum of any system is always conserved.


    See http://home.comcast.net/~peter.m.brown/em/momentum_of_radiation.htm

  17. I was wondering something. Some of the physics I learned came from well-known physicists. When I say something like says something that Guth I know personally ... I fell like I'm unnecessarily name-dropping, which I hate. Is this an unwise way to argue opinion in physics?

    I just looked up the term "name-dropping". From http://www.thefreedictionary.com/name-dropping

    To mention casually the names of illustrious or famous people in order to imply that one is on familiar terms with them, intended as a means of self-promotion.

    Let me state this now and perhaps I'll come to terms with it. The purpose of my posting a quote from Guth had nothing to do with implying anything about my familiarity wih Dr. Guth. The purpose was physics related and not fame related. As if I'd ever become famous by name-dropping here. LOL!!


    Although now that I think back ... I did do a bit of shameful name-dropping here. It was more along the way of being proud to know and work with them. After all that part is in print so ...

  18. Can a particle have an irrational amount of energy?

    The limit of E is the total energy of the universe (if such a thing exists) so an irrational amount of energy would be more energy that exists. There's always the scenario where a tardyon (which always moves at the speed of light) moves faster than light. The energy of the particall would be conceptually irrational as well as mathematically a complex number, both of which are nonsense and therefore impossible.

  19. Jesus never said he was the son of God


    Luke 22:66-70 [66] At daybreak the council of the elders of the people, both the chief priests and teachers of the law, met together, and Jesus was led before them. [67] "If you are the Christ, " they said, "tell us." Jesus answered, "If I tell you, you will not believe me, [68] and if I asked you, you would not answer. [69] But from now on, the Son of Man will be seated at the right hand of the mighty God." [70] They all asked, "Are you then the Son of God?" He replied, "You are right in saying I am."


    Good enough for a logical man. Jesus would never have told someone he was the son of God if it wan't true. Jesus never lied! To disagree with that is to be a kind of Christian that Jesus wouldn't want me debating with. Jesus said to bring his news to the town and if a town rejcts you then kick the dirt of the town off your shoes and move on to the next town.


    Kick, kick, kick!! :)

  20. <br /><font face="Times New Roman"> </font><font face="Times New Roman"> </font><font size="3">Assume that</font><font size="3"> the adopted</font> <font size="3">standard</font> <font size="3">time in our</font> <font size="3">region of the universe</font> <font size="3">is  </font><font size="3">a selected</font> <font size="3">pulsar</font> <font size="3">with a rotation cycle of</font> <font size="3">1 second</font><font size="3">,</font><font size="3">and that the</font> <font size="3">clocks</font> <font size="3">on Earth and</font> <font size="3">on all other</font> <font size="3">space</font> <font size="3">objects</font><font size="3">, including </font><font size="3">those with very</font><font size="3"> big</font><font size="3"> mass</font><font size="3">, </font><font size="3">are synchronized</font> <font size="3">by the</font> <font size="3">pulses</font> <font size="3">of the</font> <font size="3">pulsar.</font> <font size="3">Thus,</font> <font size="3">these</font> <font size="3">clocks</font> <font size="3">will</font> <font size="3">always indicate</font> <font size="3">the same</font> <font size="3">time</font><font size="3">, everywhere </font><font size="3">in space</font><font size="3">, regardless of the </font><font size="3">mass of  </font><font size="3">the object  </font><font size="3">upon which</font> <font size="3">they are located.</font> <font size="3">This example</font><font size="3">, therefore, contradicts </font><font size="3">the truth</font> <font size="3">of the gravitational</font> <font size="3">time dilation</font><font size="3">. </font><font size="3">Am I right</font><font size="3">?<br /><br /></font><br /><br /><font face="Times New Roman"> </font><font face="Times New Roman"> </font><br />
    <br /><br /><br />

    No. This doesn't account for gravitational time dilation. Clocks near gravitating bodies which generate a spacetime for which there is a spatial variation in [math]g_00[/math] will cause the local clock to run at a different rate as the distance source. If you locallysync that clock to the distance source then according to distant clocks they will be out of sync.

  21. <br />As you mention black holes I have a question.<br /><br />Please keep in mind my knowledge of maths is very limited, also, this question might be utter nonsense for reasons I don't understand.<br /><br />However;<br /><br />The centre of a black hole is a singularity and at this singularity our understanding of physics breaks down because things tend to infinity. But why is the centre a singularity, why isn't this singularity also governed by the uncertainty principle? Is it therefore not a singularity?<br /><br />**http://www.youtube.com/watch?v=hydDhUNvva8** in the first minute of this vid<br /><br />i.e. does r not = 0<br /><br />can you not just throw some uncertainty into the equation<br />
    <br /><br /><br />


    The text is on black holes and that is my extent of knowledge. What you're asking about is outside my area of expertise. You want someone versed in quantum gravity. Hopefully someone will chime in.


    I have a luncheon on friday where I'll be around a lot of experts. I'll raise the question if I remember it.

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