proton

Yes.

I'm not suggesting that the term matter cannot be defined. It's easy to define terms. What is not so easy is to define a term for which the definition is not problematic. For example; suppose we define the term "matter" as "that which has non-zero proper mass". What then of a two photon system for which the proper mass of the system is non-zero? That would mean that one photon is not matter, a system of two photons moving in the same direction is not matter but a system of two photons moving in opposite directions is matter. What of systems for which all we have is the stress-energy-momentum tensor. In that case all we know is what the energy density, stress and momentum density is. We don't know what the system is comprised of. Should we be able to quantize the quantity of matter in that case? What is we say that anything composed of particles which has zero proper mass is not matter. Would you object to the consequence of non-matter being able to generate a gravitational field? Recall that the physicist John Wheeler said "Matter tells space how to curve, and space tells matter how to move." This would no longer be true if that was the case. No. They have kinetic energy. One does not say that they "are" kinetic energy. Kinetic energy, like mass and charge, is a property. Yep. The energy of anything is frame dependant. Special relativity dictates that such a thing is impossible. I.e. it is impossible for a photon to be a rest. Do you have access to a library? If so then you might like to read the following article Does nature convert mass to energy?, Ralph Baierlein, Am. J. Phys. 75(4), April 2007 Merged post follows: Consecutive posts mergedI just found and interesting quite in Physics for Engineers and Scientists - Third Ed. Vol. II by Ohanian and Markert, page 722 Similarly in Classical Electrodynamics - Second Edition, Hans C. Ohanian, page 50 Note: Others refer to the Bose-Einstein condensate as the fifth state of matter.

What picture. Are you suggesting that you took a camera and took a picture of this forum? If so then exactly how did you accomplish that? How exactly do you expect people to react to someone who refers to themselves as the Seventh Angel of the apocalypse? Most people would correctly consider such a person to not be in their right mind. You are aware of that fact, aren't you?

I'm not sure what you're seeking? If you're looking for a definition of the term matter then I don't think I can be of any help for the reasons I mentioned above. Is it really that important? I'd say that it's more important to understand what people mean by it when they use it. Thus if you see someone use the term then you'd be better off asking them what they mean by it. For example; when you heard the term "convert matter to energy" it referred to changing proper mass to kinetic energy. In quantum mechanics there is a wavelength associated with every particle so I don't see how you can't resolve it in that way.

Current is defined, of course, as the flow of charge. The term is defined because it appears as the source term in the defining equations for the magentic field. In relativity the Electric and Magnetic fields are unified into one field, the Electromagnetic field. When Maxwell's equations are expressed in tensor form the source term in the equations is called the 4-current and is defined in terms of charge density and current density (current per unity area). No. It's possible to have more than one charge density in the presense of a given current density and vice-versa. That's one of the reasons why the source is given in terms of charge density and current density.

As I said, it depends on how you define the term "matter". When you asked that question you already had a definition in mind as far as what it means for something to be matter. When fission occurs there is a change from rest mass energy to kinetic energy (where radiation having all kinetic energy). As far as a "modern" definition goes, there really is none. Different physicists mean different things by it. It's really a vauge term which is either ill-defined or highly avoided. Sure, you can find a text here and there which attempts define the term but its always a bit vauge. Not really solid enough to determine qauntitatively and even then different texts define it diffferently or avoid the term altogether. Merged post follows: Consecutive posts merged I seriosuly doubt that in that most, if not all, physicists prefer to avoid that term when at all possible. but if you believe differently then p\Please post a reference to a modern textbook which provides such a definition. Preferably a few so that we can see if they all agree. Wikipedia attempts to define it here - http://en.wikipedia.org/wiki/Matter Typically the definition alludes to mass, which of course differs from one person to another. The definition says that matter is what has mass and occupies space. Does a point particle occupy space? Notice the comment in the link Dark energy is characterized as having negative pressure which results in a negative active gravitational mass. Thus pressure is in a sense a form of matter. Have you looked at Taylor and Wheeler's SR text? They address the term "amount of matter" commenting that history has struck down every attempt to define such a term. They mentioned this when they addressed the question as far as whether mass is a measure of "amount of matter". It's for these reasons that I choose not to accept any definition as being universally accepted. But once you choose a definition and explain how to quantify it we can talk about it if you'd like.

In that case the texts mentions so far too advanced for you at this time. The best book I can think of for you is Gravity from the Ground Up by Bernard F. Schutz. Later in your education, after you have the calculus sequence and basic physics sequence behind you, you should read this author's text A first course in general relativity. It's a great book to learn GR from and is used at many universities, including MIT, for their GR course.

It depebds on what you call matter. In an article written by Einstein where he published a review article on his general theory of relativity he referred to matter as anything for which the stress-energy-momentum tensor is not zero. In layman's terms one can say that Einstein defined matter as anything which has energy. Thus Einstein would have considered an electric field to be a form of matter. Typically one is describing changes in the form of matter from one from to another. Thus if you have a particle annihilating its antiparticle then energy in the form of proper energy + kinetic energy is changed to electromagnetic energy. Always remember that no matter what, the energy before a reaction equals the energy after a reaction. When you asked does it always form in pairs only you must have swansont's comment in mind. I don't know why he said that. Perhaps he was thinking only of pair production in which case that's true. But the idea of conversion has to do with a change in the form of energy taking place and can happen in other circumstances such as nuclear fission in which case the comment about pairs being produced does not apply. Only on TV and in the movies. When people use the term "pure energy" they tend to be referring to matter for which there is no proper mass (aka rest mass).

The only ones I know of are very advanced. E.g. Principles of Physical Cosmology by Peebles Cosmological Physics by Peacock What level of math/physics do you have in mind? Do you know tensor analysis and general relativity?

What's your point? I was merely pointing out that it is a term which is well defined in physics and as such it's incorrect to say technically there is no such thing as deceleration. It's merely a term which isn't used much. I myself have never found cause to use it in fact.

The term decelleration is a well defined term in physics. It refers to an acceleration which results in a decrease in speed. You can find it defined in Fundamentals of Physics - Third Edition by Halliday and Resnick, page 18 - footnote

(never mind)

Perhaps you're right. In any case focusing IR radiation won't violate the second law of thermodynamics. For example; heat a body such that it becomes white hot. Now use a lens to focus some of the optical light comming off the body, like focusing the light from the blackbody we call the sun. How is this violating the second law of thermodynamics? We can't focus the light from the sun to heat a body to a temperature higher than the sun.

Heat is defined as the energy transferred between a system and its environment as a consequence of a temperature difference between them. I don't think that's what you're referring to here. Perhaps you're wondering how to focus the thermal energy from the radiator, correct? If I recall correctly, most of the thermal energy comming from a radiator is blackbody radiation, most of which is IR radiation in this case. So if you know of a way to focus IR radiation then you've accomplished your task. In the present case there is no problem with the second law of thermodynamics because the energy flow is not in the form of heat since the flow is not due to a difference in temperature.

Thanks. I don't see where in that page it mentions radio telescopes. Perhaps you did make an error after all.

What new element?

The first thing you need to tell your students is that it is impossible to explain the existance of stable atoms in terms of classical mechanics. This is one of the facts that motivated the creation of quantum mechanics. Classically an electron orbiting a positive nucleus should continuously radiate electromagnetic energy and spiral into the nucleus thus collapsing. Classically atoms can't exist. That's one of the reasons quantum mechanics was created. What you now need to do is to find the simplest way to explain this to highschool students in a way that they'll be able to grasp without first learning quantum mechanics in its entirety. Since the terminology of quantum mechanics uses the term "orbital" you will need to explain that this term is not what it means in classical mechanics. As far as the description goes I suppose you can simply relate the following postulate made by Bohr (as phrased by Hans C. Ohanian) I hope that helps.

I agree. I mentioned it only because if one were to classify antiparticles under quantum mechanics or particle physics then it would be apopropriately listed under particle physics. If someone was curious about antiparticles and wanted to search the forum on previous posts then they should start searching under the Modern and Theoretical Physics section to begin with. However, speaking for myself, I frequent those sections in which have some exertise more than I frequent other sections. For example; I look at the relativity section quite often because that's where my education and experience lies and for that reason that is where I can be of most use. I look at the Modern and Theoretical Physics section much less often for similar reasons. I assumed that others are the same way to at least some extent. Of course this may be quite wrong, I don't know. I'm merely going by my personal experience. For that reason it was why I said it was merely a suggestion. After all, what is the purpose of having different sections if not for similar reasons? It would be a waste of time and energy to put any thought into that suggestion beyond what I said. It's not as if it was very important. In fact it has more to do with where people should look in the physics literature for the subject than anything else. Merged post follows: Consecutive posts merged Because photons don't annihilate themselves.

Yes. That is quite correct. My point was that the conversion pertains to a conversion of the form of radiation from one form to another. I used radiation as an example and neglected to elaborate. Thanks for correcting me. Laymen have the tendancy to think of light as being "pure energy" and that was something I was trying to steer readers away from.

You're most welcome. May I make a suggestion? When you want to ask another question about antiparticles/antimatter I recommend that you post the question under the Modern and Theoretical Physics section. Typically textbooks on quantum mechanics don't cover antiparticles. That's usually covered in texts on Modern Physics and Particle Physics. Particle physicists who frequent this forum may monitor those sections more frequently than this one and may more readily get to your question than otherwise.

No. Can you show me where? That would not only be news to me but it also seems quite impossible. A digital camera takes images using CCDs which are designed to record optical images. A radio telescope is, by definition, an instrument that receives radio signals. A CCD cannot read radio signals. There are two modes in which a radio telescope works; active and passive. In active mode an electromagnetic wave, in the radio wavelengths, is sent out in the direction of the object which one wishes to observer. The wave is the reflected by the object and subsequently detected by the radio telescope. In this more they can observe asteroids, satellites, etc. In passive mode the radio telescope only receives signals. In this more they can observe things like stars, pulsars, quasars, galaxies, the CMBR, etc. Off hand I would imagine that a radio telescope, in active mode, would only be useful in observing an NEO after its existance was detected. An array of radio telescopes work by the principles of interferometry. This requires the distance between the dishes to remain fixed. If the distance changed by even a centimeter (over thousands of miles) then it would become useless. Can distances like this be maintained? What benefit do you see of using radio telescopes over optical ones in detecting Near Earth Objects (NEO)?

To be precise, one doesn't convert energy into matter. One converts radiation into matter. What happens is that the form of energy changes. An example would be pair production. In nuclear physics this occurs when a photon is in the vicinity of a nucleus, otherwise momentum wouldn't be conserved. The photon goes into the reaction and an electron/positron pair come out of the reaction. The energy remains the same. Only the form of the energy changes. oops! I see Severian beat me to it. Sorry!

By definition - the anti-particle of a particle is the particle which as the same mass but opposite charge and magnetic moment. In those situations when the charge and magnetic moment is conjugated (made opposite) and it results in the same value then the resulting particle is the same as the original particle. The mass, charge and magnetic moment of the photon is (0, 0, 0). Therefore its anti-paricle has the properties (0, 0, 0). This is just a photon. However when a particle is identical to its anti-particle they do not annihilate each other.

That is widely held misconception among both physicists and laymen alike. For this reason, Don Koks, the physicist who maintains the Relativity FAQ, updated the FAQ on relativistic mass to reflect the actual usage. The (updated) FAQ is at http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html Another physicist, namely Gary Oas of the “Education Program for Gifted Youth” at Stanford University, wrote an article on the use of relativistic mass. The article is called On the abuse and use of relativistic mass. It is online at http://arxiv.org/abs/physics/0504110 If you feel the need to post an objection to each instance of the use of relativistic mass as it arises then may I suggest that it would be more prudent to simply list those two URLs and let the reader decide for themselves rather than create a new post claiming that it is the "prefered" term? By the way. People have preferences, not discussion forums. What you said might be valid if everyone that posts here both now and in the future, prefers rest mass in all instances. But that is not the case. I'm pretty sure that nobody wants a debate to be sparked every time someone uses or mentions relativistic mass. Not only would it derail a thread but it would be irritating to the person who uses it. All that merely because we have a different opinion. That would be unfortunate, wouldn't you agree? Also, suggesting that people only use rest mass is not a good idea. They may never have the occasion to hear about or learn about things like the inertia of stress or the fact pressure is a source of gravity or that these two phenomena are related and plays an important role in modern cosmology. You argue that It is more common to consider just one mass.. It's certainly more common for the particle physicists to use it but that is not the case for cosmologists. A perfect example is the concept of Dark Energy and its role in both inflation and the accelerating expansion of the universe. The term Dark Energy is defined in the glossary of Schutz's text Gravity from the Ground Up (a textg which relies heavily on the concept of relativistic mass). Peebles also uses the term active gravitational mass as well as passive gravitational mass in his (as does Schutz) his cosmology text. Schutz also uses the term passive gravitational mass. Cameron - I mentioned what I did because the concept is both still used and very important. Since I don't know you I am unfamiliar with your level of understanding of physics and don't know what you may or may not be interested in. I assume that you'd find it interesting that inertia has stress, right? I guess that because I myself find it fascinating as do most people when they first learn about it. Relativity textbooks and journal articles, some of the texts being dvanced, by some well-known and respected physicists, have been published within the last ten years that utilize relativsitc mass. I myself find it to be the most logical, if not the only logical way, to define mass. In recent years there have been some very interesting papers in the American Journal of Physics on the more complex issues regarding the concept of mass. I found the following articles quite interesting The mass of a gas of massless photons, H. Kolbenstvedt, Am. J. Phys. 63(1), January 1995 Let me ask you this - Before I explained that a gas of photons has rest mass would you have thought it to be true? 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 Simply saying that every occurance of the term "mass", that you will encounter in your studies in physics, means "rest mass" is quite misleading, wouldn't you agree? Also note that what I said also pertains to both active gravitational mass and passive gravitational mass.

If this were in the context of Newtonian gravity you'd be correct. But this is in the context of general relativity. In GR the inertial force contribution due to the rotation of the Earth is, by definition, part of the total gravitational force acting on the body. All that means is that the value of the gravitational force is observer dependant. Just because something is observer dependant it doesn't mean that it doesn't exist. It only means that its existance depends on the observer. E.g. the electric field 4-vector is a vector which is defined in terms of the Faraday tensor and the observer's 4-velocity. For some observers this 4-vector is zero. For other observers its non-zero. In some instances radiation can have an observer dependant existance. For example; Suppose there is a charged particle at rest in a gravitational field. An observer who is at rest with respect to the charge will not detect any radiation. However if there is an observer who is in free-fall in the field then that observer will detect the radiation. That means that the existance of the radiation can be measured by some observers and for other observers it can't be detected. In any case, I'd rather not discuss it in this thread. I'd rather not hijack the thread by going off on a tangent merely to discuss different opinions on metaphysics.