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05 Oct 13
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Originally posted by KazetNagorraRJHinds: Yes you apply a force to the football, then it falls "down". This was what I meant with "hidden joke". My first post; "...not down wich you normally expect, yes there is a joke hidden in this". Note this as well. The earch falls towards the football, but the fall is so small that it is not notable.
Einstein's notation is now no longer conventionally used. In modern notation, we generally write:
E = gamma*mc², where gamma now takes care of the change in energy when going to a different reference frame. In Einstein's equation, the "mass" changes and is called relativistic mass. In the newer equation, it does not change and is known as rest mass. Pa ...[text shortened]... in which case they are known as "massless."
Read more: http://en.wikipedia.org/wiki/Rest_mass
KazetNagorra: The course I took on said subject was to me very hard. Have for sure forgotten most of what I learned back then. I don't know this subject at all and would be the first to admitt it.
I really shoud have read KazetNagorras link but I did not.
Read more: http://en.wikipedia.org/wiki/Rest_massShame on me.
I could be very wrong in this. If so, could someone correct me? I will try to answer what my logic was to wether photons have mass or not. twistedhead wrote: "... did you conclude that things must have mass, or not? Your post wasn't at all clear on the matter.
I believe it is unknown for sure whether photons have mass ..."
A: E = m*c^2 , here E is a number wich changes but is always positive. E cannot be 0 and can not be a negative number.
B: Photons is a particle which have Energy.
C: Therefore; [A] + [B] means that. In the case o photons, E is a positive number. c is a constant, do not change. Then m must be positive. One photon means that E is a constant number which makes m a constant number. We can call it m[photon] and E[photon], substitution gives, E[photon] = m[photon]*c. of course c is very big therefore m[photon] is very small.
D: m[photon] is so small that it is not able to be measured. Any measurement, an experiment, would mean introduction of errors in measurement. These errors would be much larger than the mass of a photon, therefore. We can not measure the mass of the photon. No one can! (Yet). It has not been done yet. (Because errors is large).
E: My assumption (C) E[photon] = m[photon]*c is correct. Hence the photon has a mass. But we have not been able to measure this mass, m[photon]. (m can't be zero, since photons have energy. I have not heard of a photon standing still, I assume (wild guess) that such a photon would cease to exist, become energy only, not a photon. Such a imobalized photon could perhaps have zero energy but not a moving one).
This was my logic for concluding that photons have mass.
I do not know about this. But I'll give it anyways, although it could embarasse me. But if you found E = m*c^2 to not be correct then you and this experiment would be famous. Famous enough that I would have noticed it.
Sorry for not using the more modern variation of the equation. It is because I am unsure how to use it. (Since I assume that gamma is not a constant but a function, and therefore could vary, according to circumstanses). If I am be wrong in this one (my a,b,c,d,e logic). I would like to know. Am I wrong?
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05 Oct 13
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Taking on the moderator hat. What is Vaccum - Two users disagree - This is leaving the main subject.
KazetNagorra, lemonlime: If either of you intend to continue talking about vaccum. DO SO ! But please, then START A NEW THREAD about that. If you think it is important.
I know of two uses of the word vaccum.
1. Vaccum. As in how laymen use it;[/b] the mose common use; as in Space with atoms in it. These atoms have quite some distance between them. Like in a glass bown, in wich air have been pumped out. Vaccum! Still lot and lots of atoms in it.
2. Vaccum. A more scientific approatch to the word. Nothingness. No atoms at all. Perhaps a cube containing space (call it A) which lies inside another cube (call it B). (A) do not contain any atoms at all. (B) do contain atoms. Now we call (A) for Vaccum.
Stop here and pretend that you were having fun in playing with the Vaccum word. As in trying to be funny. No one than you are having fun. If either of you continue in discuss Vaccum. Then - start a NEW thread. Pretend that you were playing with words and stop there.
05 Oct 13
The energy of a photon is given by E = hf, where h is Planck's constant, and f is the frequency of the photon.
E = mc² can be written in a different form using momentum, in which case having massless particles gives no issues (in fact, this predicts that massless particles must travel at the speed of light).
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05 Oct 13
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Originally posted by KazetNagorraVery long, i know. Lots of, perhaps unclear, things as well. Sorry for that.
The energy of a photon is given by E = hf, where h is Planck's constant, and f is the frequency of the photon.
E = mc² can be written in a different form using momentum, in which case having massless particles gives no issues (in fact, this predicts that massless particles must travel at the speed of light).
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Ok, am I understanding you correctly? Theoretically we can have massless particles. Wich travels at the speed of light.
hf = gamma*mc²
I assume then that if m = 0 (a massless particle) then goes that m*c^2 = 0.
"Changes frame" - What is a frame? - can you give an example?
And, c is the speed of light in vaccum.
If not vaccum then the speed of light is slower.
When the speed of light slows in - then photons gets mass? - Since they start oscillating, hence have a frequency.
And there is also something called 'virtual photons' - dunno what that is - is that a photon with no frequency? A massless photon? Can that virtual photon be converted to a photon or do we have 'virtual photons and photons?
Then comes the question. Can something travelling at the speed of light leave the speed, slow down? Sounds unprobable to me but really, I don't know. Does physics have the answer to that?
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In this article I am reading that ... http://en.wikipedia.org/wiki/Mass-energy_equivalence
"This energy is often released in the form of light and heat, which is too quickly and widely dispersed to be easily weighed, though it does carry mass."
"...and the energy of a very long-wavelength photon approaches zero. This is why a photon is massless; this means that the rest mass of a photon is zero."
" In this frame the two photons, as a system, have a mass equal to their total energy divided by c2."
" Thus, by calculating the invariant mass of pairs of photons in a particle detector, pairs can be identified that were probably produced by pion disintegration."
"when massive particles (particles with rest mass) within the system are converted to massless particles (such as photons)"................"In such cases, the photons contribute invariant mass to the system, even though they individually have no invariant mass or rest mass."
"by comparing the mass-lost to the energy of the emitted gamma ray associated with the neutron capture. The binding mass-loss agreed with the gamma ray energy to a precision of ±0.00004 %, the most accurate test of E=mc2 to date."
"Although mass cannot be converted to energy, in some reactions matter particles (which contain a form of rest energy) can be destroyed and converted to other types of energy which are more usable and obvious as forms of energy, such as light and energy of motion (heat, etc.). However, the total amount of energy and mass does not change in such a transformation. Even when particles are not destroyed, a certain fraction of the ill-defined "matter" in ordinary objects can be destroyed, and its associated energy liberated and made available as the more dramatic energies of light and heat, even though no identifiable real particles are destroyed, and even though (again) the total energy is unchanged (as also the total mass). Such conversions between types of energy (resting to active energy) happen in nuclear weapons, in which the protons and neutrons in atomic nuclei lose a small fraction of their average mass, but this mass-loss is not due to the destruction of any protons or neutrons (or even, in general, lighter particles like electrons). Also the mass is not destroyed, but simply removed from the system. in the form of heat and light from the reaction."
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In this article I find a formula for "two massless particles" - http://en.wikipedia.org/wiki/Invariant_mass
"The invariant mass of a system made of two massless particles whose momenta form an angle [\theta] has a convenient expression".......Picture of formula.
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Then we also have this wikipedia-article - http://en.wikipedia.org/wiki/Frames_of_reference
And perhaps - unsure how much these two due to clearify things.
( * http://en.wikipedia.org/wiki/Inertial_frame_of_reference * )
( * http://en.wikipedia.org/wiki/Frame_fields_in_general_relativity * )
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This article have nothing to do with the others, I simply include it because I found it to be interesting.
http://en.wikipedia.org/wiki/Laser
05 Oct 13
Originally posted by bikingvikingWhat happens in a non-vacuum, is the photons get absorbed and re-emitted, which results in an overall slowdown. However they still travel at c between emissions and absorptions. They also interfere with each other and themselves resulting in phenomena like reflection, refraction etc.
If not vaccum then the speed of light is slower.
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05 Oct 13
Originally posted by twhiteheadI am probably sounding quite dumb but twhitehead, can you be clearer in your explanation. It is incompatible with my intuition (perhaps wrong) which say this:
... which results in an overall slowdown. However they still travel at c ...
OK. They slow down. But they still travel at c (assuming c a constant, speed of light in vaccum, not speed of light travelling in materia, slower).
Is that making photons then thavel faster than the light in materia, i.e. photons travelling faster than the actuall light? Does not sound right.
Perhaps we are in another reference frame than the photons is. I don't know, wild guess.
If so, then the photons start ocillate, having a frequense. Then they automatically get's energy. Then they get some kind of 'matter' automatically, or is it something else they get instead?
06 Oct 13
Originally posted by twhiteheadThere is no refraction of light in a vacuum. By definition, the index of refraction for a vacuum is 1.
What happens in a non-vacuum, is the photons get absorbed and re-emitted, which results in an overall slowdown. However they still travel at c between emissions and absorptions. They also interfere with each other and themselves resulting in phenomena like reflection, refraction etc.
http://capone.mtsu.edu/phys2020/Lectures/Part_1__L1-L5/Lecture_1/Index_of_Refraction/index_of_refraction.html
The Instructor
06 Oct 13
Originally posted by bikingvikingA true vacuum is space containing nothing. Man can only cause a partial vacuum, because we do not have the ability to evacuate all matter from a space. Therefore, man can only approximate the speed of light.
[b]Taking on the moderator hat. What is Vaccum - Two users disagree - This is leaving the main subject.
KazetNagorra, lemonlime: If either of you intend to continue talking about vaccum. DO SO ! But please, then START A NEW THREAD about that. If you think it is important.
I know of two uses of the word vaccum.
1. Vaccum.[/ ...[text shortened]... cuss Vaccum. Then - start a NEW thread. Pretend that you were playing with words and stop there.
The Instructor
06 Oct 13
Originally posted by bikingvikingPhotons always travel at c. When a photon encounters an electron it is absorbed then re-emitted a short while later. This makes it appear to have slowed down. However it is emitted in a random direction.
I am probably sounding quite dumb but [b]twhitehead, can you be clearer in your explanation.[/b]
It then interferes with itself. Interference is a quantum effect but it can be calculated using wave mechanics.
When a photon passes through a substance like glass, it takes every possible route. It is absorbed and re-emitted by every single electron in the glass. It also interferes with itself. This results in an overall slow down effect. The photon then 'appears' or 'reaches' some point with a probability based on all possible routes, electron interactions and the interference.
It also results in some photons getting reflected at the two surfaces of the glass, or passing through the glass but being refracted.
I am not an expert at quantum mechanics and only learnt the above recently.
06 Oct 13
Quantum mechanics rules the universe. We can visualise and calculate a photons path through empty space using newtonian mechanics and treating the photon as a particle.
We can visualise and calculate the path of light through substances like glass using wave mechanics and treating light as a wave.
However, photons are neither particles nor waves and we should not forget that. They are quantum particles and follow the rules of quantum mechanics.
06 Oct 13
Originally posted by RJHindsWe have already gotten within a hair's breath of c inside the big atom smashers like the one at Cern. Vacuum levels are not what stops us from getting to c, what stops us is adequate propulsion. Chemical propulsion is barely able to get us to mars and Jupiter, you can see for instance, the voyager, it has been in space for almost 40 years and it is just barely into interstellar space.
A true vacuum is space containing nothing. Man can only cause a partial vacuum, because we do not have the ability to evacuate all matter from a space. Therefore, man can only approximate the speed of light.
The Instructor
We could get to at least 0.5c with some kind of nuclear propulsion, fusion or antimatter, the latter being preferable if we can manage it since it would be hundreds of times more efficient than fusion. 0.999c will take some doing I don't think even the big boys have plans for.
1/2c gets us to Alpha Centauri in maybe 10 years so its not impossible. There are a couple dozen stars within 20 light years so it may be we could send people that far especially if we can master some kind of hibernation technique.
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06 Oct 13
Originally posted by twhiteheadClearification. Perhaps someone missed this, therefore: This is what I meant with my "fotballs that fall upwards", i.e. a new kind of particle. Sometimes people assume they are f.e. "waves" but no, they are a new kind of particle, we and we sometimes call this particle a wave. Now I have said what I wanted to say, you can stop reading here if you want to.
However, photons are neither particles nor waves and we should not forget that. They are quantum particles and follow the rules of quantum mechanics.
Here comes me ranting,
Meaning: not part of our discussion, i.e. ranting.
Same goes for electrones (E) by the way, (correct me if wrong), they are a new kind of particle. They are not golfballs circulating a center of mass, meaning, they do not behave like golfballs do. Since they live in the quantum mechanics world and is a new kind of particle (not golfballs). By this I mean that the classical picture of rings of E in "round" shells is vrong. (It is). However it is a convenient model, a useful model, which can be used to teach persons about electrons. But in this model E very close to some kind of golfballs. When they in fact are quantum particles, following quantum mechanics.
Meaning, the E in fact is spread in a probabuility "cloud" (PC), probably not technically called a cloud btw. Where they, in such a PC are spread everywhere in a "zone", a PC. Determined by probabuility. Electrons are all at onece but there are some places where the E is more likely to be.
06 Oct 13
Originally posted by RJHindsLight follows the path of space but that path can bend which was proven by Eddington in 1919 by seeing the position of stars that grazed the surface of the sun and seeing them in a total eclipse of the sun and measuring where they appear vs where they would have appeared if the sun had not been there.
There is no refraction of light in a vacuum. By definition, the index of refraction for a vacuum is 1.
http://capone.mtsu.edu/phys2020/Lectures/Part_1__L1-L5/Lecture_1/Index_of_Refraction/index_of_refraction.html
The Instructor
A black hole can refract light all the way in, forcing light to go first into a tight orbit and if the mass is a bit higher, the photons are led down a rabbit hole and will disappear so in that sense space CAN refract light. The path light takes nearby a massive object will change depending on how close the photons get to the surface and the mass and density of the object.
For light skimming by the sun, the deflection is 1.75 arcseconds, like around one part in 740,000, not much but measurable.
I did some work with that and found if two straight beams just graze the surface of the sun, they come together, will meet, at around 55 billion miles on the other side of the sun following the light path. I have a lot more to say about that but won't tie up the thread.