Are photons energy? What is energy, anyway?

Discussion in 'Physics & Math' started by origin, Aug 19, 2019.

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  1. foghorn Valued Senior Member

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    If a gravitational wave is a disturbance in a gravitational field, would it be right to say a photon is the same to a electromagnetic field?
     
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  3. paddoboy Valued Senior Member

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    Here are some interesting answers which again validates the main point I have been making since the start of this...Pedant/Pedantic/Pedantry

    http://scienceline.ucsb.edu/getkey.php?key=3971
    That's a really great question. In fact this question caused a great debate among physicists during the time of Isaac Newton. Newton thought that light was a stream of little particles and other physicists such as Christian Huygens thought that the universe was filled with tiny particles called aether particles and that light was just a wave moving through the aether.

    To make a long story short, it turns out they're both kind of wrong... and they're both kind of right. In some situations light sort of acts like a stream of particles and in others it acts sort of like a wave. Today many physicists (myself included) like to say that light is neither a particle or a wave... it's just light.

    So, some might say that because light can carry energy from the sun to the earth it must be matter. Others might say that because you can't hold a lump of light in your hand light isn't matter. It all depends on what you mean by the word 'matter.' But the important thing to remember is that light doesn't care what you call it, it will still act the same way no matter what. That's why in physics we try not to worry too much about what things are called and focus on understanding how things actually behave.


    Answer 2:


    "Both your class, and the entirety of science is having this debate. First, some precision. Photons are not in light, they are light. Light is made up of photons, so one photon is like one unit of light. Additionally, photons aren't really matter, although this depends on how you define things. Traditionally people say that for something to be called matter, it has to have mass and photons don't really have mass... although this also depends on how you define it.

    I'll cut to then end, and then explain the controversy. Light is a form of electromagnetic radiation, which is a form of energy. This energy always comes in distinct units, which we call photons. In this way, light is like a particle, but it still doesn't have mass, so it's not really matter. The way these particles behave though, is very strange. Sometimes they behave more like a wave, like a ripple moving across a pond. Sometimes they behave more like actual solid objects, like bullets being shot from a gun. And the craziest thing is that whether they behave one way or the other depends largely on whether or not anyone is watching them at the time. I cannot explain this behavior... actually no one can. All we can do as scientists is observe that it is the case and describe it using mathematical laws. If you want to know more about how we know this to be true, there is a famous physicist by the name of Richard Feynman who explains it quite nicely....


    Now the video is a little old, but it's a good one and the laws of physics, as we know them, have not changed since those days.
    Now I said that whether light has a mass or not depends on your definition, let me explain that. As I said, photons, and thus light, is energy. But there is a famous equation E=mc2 that tells us that the mass and energy are perfectly equivalent. E is energy, m is mass and c is the speed of light in a vacuum.

    You can convert mass into energy and energy into mass. As a matter of fact, that's what the sun does every day, it converts some of its mass into energy, which we see as light. And so there is a perfectly reasonable way to talk about the "mass" of a particle of light. All you do is convert its energy to mass, although conventionally we people try not to do that these days. And so... light is energy, but energy and mass are equivalent. And if you're asking whether light is a particle or a wave... no one in the universe, that we know of, has a good answer to that one.

    photons are entirely made of energy,"
     
    Last edited: Sep 6, 2019
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  5. paddoboy Valued Senior Member

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    As my time is short and precious, let's add some more while I have the time.....
    http://curious.astro.cornell.edu/ab...ow-are-photons-created-and-destroyed-advanced
    QUESTION:
    I'm reading an excellent book by Kauffmann, Discovering The Universe, in which he describes when electron jump to a higher level or a lower level. If they go to a higher level they emit a photon. If they go to a lower level they absorb a photon.

    It causes me to ask if photons exist as a seperate entity within all atoms or are they created at certain energy levels for the purpose of absorption, or emission, or do they exist omnipresently in the fabric of the universe? Anyway, where does the photon for absorbtion come from?

    Hope this isn't too stupid a question.
    ANSWER:
    It certainly isn't a stupid question, and really cuts to the heart of the question of interpreting quantum mechanics.

    The simplest answer is that when a photon is absorbed by an electron, it is completely destroyed. All its energy is imparted to the electron, which instantly jumps to a new energy level. The photon itself ceases to be. In the equations which govern this interaction, one side of the equation (for the initial state) has terms for both the electron and the photon, while the other side (representing the final state) has only one term: for the electron.

    The opposite happens when an electron emits a photon. The photon is not selected from a "well" of photons living in the atom; it is created instantaneously out of the vacuum. The electron in the high energy level is instantly converted into a lower energy-level electron and a photon. There is no in-between state where the photon is being constructed. It instantly pops into existance.

    So the question is: where does the photon come from?

    Strangely, it doesn't seem to come from anywhere. The universe must put the extra energy somewhere, and because electrons in atoms are electromagnetic phenomena, a photon is born with the required energy. In a weak-force interaction, say the decay of a neutron, that energy goes into a neutrino particle which is also instantaneously created. Each force has its own carrier particles, and knows how to make them.

    That's really all we can say about it. There are many interpretations of what this and other phenomena in quantum mechanics mean on a deeper level, and whole libraries worth of books which argue points of view on the matter. But my personal philosophy is that of the famous physicist Richard Feynman, who said: "Shut up and calculate."
    <<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>

    Dave Kornreich
    Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.


     
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  7. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

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    This needs to go in the quotes thread

    Cheers

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  8. paddoboy Valued Senior Member

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    A photon is simply a " quanta" of the EMF...or the smallest part thereof. While as you say gravitational waves are just disturbances in a gravitational field/spacetime.
     
  9. arfa brane call me arf Valued Senior Member

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    Can you see energy?

    If you throw something like . . . a cricket ball ... so it follows some trajectory through space, what you see is its motion.

    Physics tells you that you accelerated the object to an initial velocity (which it has at the "instant" you release it). It therefore has a momentum.
    Can you see its momentum?

    Well, maybe when it lands, eventually, it will make a thud and you will hear some sound (sound energy interacts with you), the level or amplitude of this sound is exactly like a physical measurement--an indicator of the object's mass and velocity at the point of impact.

    Mass is something you can measure too. In fact, if you go to the extent of making a video recording, with sound, you can analyse the path of the object, get some approximations etc.

    So analysing the path. It has a highest point; when the object "moves through" this highest point it has the greatest potential energy. You can see where this is, but there isn't anything else to see, or hear. So it seems the potential is indeed a theoretical quantity.

    However the mass and the velocity at each point of the continuous path are things you can estimate, and yes, these are things that you can see (although with a lot of approximation, you need more instrumentation--a video camera and a microphone, some weighing scales or a beam balance, to get closer to the physics).

    Also, a recording is exactly a store of information about the physics of the object's motion; it therefore constitutes a memory; applying equations of motion etc, to this store of data is therefore exactly algorithmic.

    So, remembering that the physics is in a frame with a constant gravitational field, now you can talk about energy being "in" this field. You can say the object gains energy as it falls "spontaneously" from the highest point.

    Where does the energy come from? The object gets it from the field (!). So although you never see (or hear) energy, you can say you see an object gaining energy from a field (of . . . gravitational energy) . . .

    Is energy physical? What constitutes physicality is a whole other debate--Einstein and Bohr had a few about this. I think it's ok to say energy is physical because it's "constructed" out of physical units --kilograms, meters and seconds. If time is physical, and space is too, also kilograms, that's a reasonable conclusion.

    If time doesn't really exist, and if mass is also as theoretical as energy, things get a bit less reasonable.
     
    Last edited: Sep 6, 2019
  10. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

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    Not really

    Mass can be seen and measured and even the shape described

    You can shock horror, add or subtract stuff to other stuff which, shock horror, would increase or decrease the mass

    More shock horror at the same time mass is being added or subtracted its (the stuff) is gaining or loosing its POTENTIAL energy

    How does it gain said POTENTIAL energy? Well take a example off stuff called a cannon ball, (name - because of other aspects it has). Place said cannon ball in a cannon and fire it thus transferring energy from the combustion of material to the cannon ball

    How is it known energy is real? Calculations can be made about how much energy is in the combustible material and how much it can transfer to any particular cannon ball

    As for TIME, what can I say which I have not posted elsewhere

    No evidence of its existence

    Anyone have any?

    Please provide

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  11. arfa brane call me arf Valued Senior Member

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    No it can't. What you see are solid objects, with mass. That means you can say a cricket ball, or a rock, or a pile of sand are carriers of mass.
    Or more pedantically, are carriers of mass-energy.

    You can weigh rocks and piles of sand, and you could use a cricket ball as a standard weight.
    Newton "framed no hypothesis" about the ultimate source of gravity, and he also, therefore acknowledged that mass is, well, entirely theoretical; it does explain inertia really well though.
     
  12. arfa brane call me arf Valued Senior Member

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    Sort of. The photon is emitted by an electron--a "matter field"--because electrons lose energy by "conserving angular momentum".

    So "a disturbance in the EM field" is generated by a particle which generates the field--electrons have an electric field around them (remember?) the magnetic part of the field depends on (relative) electron motion. Photons can be considered a quantum of electron (more generally fermion) angular momentum, which is conserved by "exchanging momentum" with another fermion. Locally at least, energy is conserved.
     
    Last edited: Sep 8, 2019
  13. arfa brane call me arf Valued Senior Member

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    What "something else"? . An electron spontaneously emits radiation (in discrete amounts), when it accelerates. Accelerating an electron means it has to interact with an electric field (or a magnetic field). In the first case the electron accelerates linearly; in the second case it follows a curve.

    Electrons accelerating in a strong magnetic field generate synchrotron radiation.
    LEDs emit radiation which is monochromatic, because electrons in the LED accelerate (not very far) in an applied (constant) electric field--the "something else" . . .
     
  14. arfa brane call me arf Valued Senior Member

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    Well, look, I'm sorry, but that completely contradicts Einstein's 1905 paper.

    If a cricket ball "contains" mass, what is it? If I carry the cricket ball as I walk around a field, am I carrying its mass around?
     
  15. arfa brane call me arf Valued Senior Member

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    And from my earlier quote from "that" textbook. First of all I'll say that the authors have also published more advanced textbooks and universities still recommend them, or have in the past.

    These same authors say that energy or momentum are both a kind of "frame of reference" and either leads to the same kind of result--the laws of physics are preserved.
    I suppose one could also ask, if energy is such a key component in analysis (of the physical kind), and if I'm right that Joules are entirely theoretical, which implies that mass at least could be, how much of physics is really not theoretical, but "real"?

    Reality is something we like to think we have an adequate philosophy of. I think that's entirely reasonable. I can't though, take the notion of even "quantum information" quite seriously, although it seems what's meant is the probability of a dot appearing somewhere, and not somewhere else, or something. Energy and/or momentum are involved--there is no dot unless that's true--so information and an exchange of momentum seem to be closely related.

    So when particles exchange momentum and neither is a classical device (or memory), that's related to quantum information, one supposes.
    What this exchange of momentum amounts to, is a discrete amount of energy, but not in the particle "frame", the energy propagates once "free" of the matter-wave of the electron; I think that all kind of falls out of a Feynman diagram.
     
  16. origin Heading towards oblivion Valued Senior Member

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    Paddo, if you are so sure that a photon is energy, why don't you go to that site where you have hundreds of likes and start a thread called, "Are photons energy?" . I bet you won't, because I suspect you know you are wrong and you are just arguing to be a pain. It would be funny to have your favorite forum point out your error though.
     
  17. origin Heading towards oblivion Valued Senior Member

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    I noticed that I am apparently the author of this thread! Since the questions have been satisfactorily answered, to all but the most bull headed, I request that my thread be closed.
     
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  18. arfa brane call me arf Valued Senior Member

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    A photon is energy, but when you see light you don't see energy. Seeing light means you interact with photons and they exchange momentum with electrons.

    OK? That's a fairly standard explanation; energy can't be measured but it can be calculated.

    But keep telling yourself that most modern textbooks are wrong, that a university education in physics won't be helpful. No point in consulting any literature at all, really.
    Considering Einstein, Schrodinger, probably Maxwell too, were all confused about it and so managed to mislead a lot of people . . . some of them went to the extent of authoring physics textbooks which were enthusiastically recommended by professors, who were just as confused as Einstein clearly was.
     
  19. paddoboy Valued Senior Member

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    Perhaps you need consider your own silly bull headiness and self gratuitous attitude, since all I am saying is that I see this as nothing more then pedantry...one may certainly be more factual then the other, but the amount of contrary reputable links I have given, would support what I say with regard to pedantry.
     
  20. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

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    You don't see light. You see objects light has illuminated

    Light is invisible. The rods and cones of the eye are activated when hit by photons of light. The energy, from the impact, is then converted into chemical/electrical signals, which travel to the brain

    Brain translates signals into image

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  21. paddoboy Valued Senior Member

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    What colour is an Orange in the dark?

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    While this is fun, it's all pedantry.

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  22. origin Heading towards oblivion Valued Senior Member

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    Not going to make that post I suggested at the other site, are you?

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  23. paddoboy Valued Senior Member

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    Are you in some way disabled? Why not do it yourself?

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    Like I said, pedantry, as could be said about many other areas of physics and science.
    But perhaps you need to gather the intestinal fortitude to question James on his seemingly objection to the earlier statement I made re "mass is a form of energy and all mass is energy, while not all energy is mass"
     
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