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Where does a photon get it's infinite energy from?

  1. Feb 1, 2012 #1
    I know frequency and energy vary in photon. I also know that mass is the same as energy, which relates to Einsteins famous equation. What I want to know first is, can energy exist without mass? Some energy being transported somewhere for e.g. without no mass.

    So, the real question is, if you send a photon to travel somewhere doesn't matter which color it will travel all until it reaches some kind of surface, then it can be absorbed or bounced. Still, that photon wouldn't be lost if it traveled either 5000km or 1000000000000Gm's. Perhaps it could reduce it's energy, and acquire another color, but it wouldn't stop moving.

    How is this possible? Doesn't it constantly have to spend energy to move with its velocity? I know distance is contracted when you reach the speed of light so PERHAPS it relates to something of this. But I can't find a way to link up the 2. If you do, please reply :D

    P.S. I simply love thinking about this stuff and getting answers for it.
  2. jcsd
  3. Feb 1, 2012 #2


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    If by "mass" you mean "rest mass", then yes, energy can exist without mass; photons have zero rest mass but nonzero energy.

    Einstein's equation E = mc^2 is really a simplified equation that only applies to objects with nonzero rest mass and zero momentum (in a particular frame). The full equation is:

    [tex]E^{2} = p^{2} c^{2} + m^{2} c^{4}[/tex]

    If the momentum p = 0 and m > 0, this reduces to the "famous equation". But if, instead, m = 0 (as for a photon), it reduces to E = pc, the energy-momentum relation for a photon.

    No. No object has to "spend energy" to move at constant velocity in free space. In our everyday experience this fact is obscured because objects are not in "free space"; there is friction and air resistance. But space probes, for example, don't need to "spend energy" to keep moving at a constant velocity.

    There isn't really a useful link that I can see between length contraction/time dilation and the ability of photons to transport energy/momentum despite having zero rest mass. For one thing, the Lorentz transformation with v = c is singular, so you can't really consistently define "length contraction" or "time dilation" for a photon.
  4. Feb 2, 2012 #3
    On top of that, I have one more question...
    Why doesn't a photon move faster than the speed of light? Why can't it actually move faster? I know the explanation regarding for e.g. some object humanly made or any other why it can't move faster. But why is the photon's limit that particular speed? If it has no mass, it will not gain mass with accelerating. That is, what defines the actual speed of light?
  5. Feb 2, 2012 #4


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    1) Photons don't have infinite energy. From your other remarks, I expect that you've probably already seen that a photon's energy is finite and proportional to it's frequency.

    If you haven't run across this yet, look up the photoelectric effect, E = h * frequency

    Rather than taking a guess as to why you posted that they have infinite energy when you probably know better, you should probably think about where you came up with the incorrect idea that they do.

    Hopefully you'll spot the flaw yourself - especially if you do some reading.

    2) The energy of a photon depends on the observer, so it's waht we call "frame dependent". If one person standing still sees a green photon, a person moving relative to the first will see the photon as having a different color and being a different frequency due to the doppler shift.
  6. Feb 2, 2012 #5
    emirhasa "On top of that, I have one more question...
    Why doesn't a photon move faster than the speed of light? Why can't it actually move faster? I know the explanation regarding for e.g. some object humanly made or any other why it can't move faster. But why is the photon's limit that particular speed? If it has no mass, it will not gain mass with accelerating. That is, what defines the actual speed of light?"

    Im not sure if this is the "correct" way to think of it.

    Perhaps ignore the fact that the photon has a speed (or that the speed has "meaning" relative to you), and consider it to be in a certain "state" that the photon is in.

    I don't know much of anything regarding four-momentum (or four-velocity), but imagine that simply because it has (specificaly is) energy but no rest mass it must have a speed of c.

    Given that "state" of having energy but no rest mass equates to c. Pretty sure that's why a photon has a speed of c.
  7. Feb 2, 2012 #6
    I know that given the state that it has no rest mass, and a speed, it will go with the speed of c. What I ask is, when given no rest mass, and if it has to move a speed ,why wouldnt it be like 399,987,362 m/s.
  8. Feb 2, 2012 #7
    Rest mass 0 + energy is why I think it travels at c.

    Look into how the meter & second are defined for why the value is what it is.

    Lastly, it is what is observed/measured.
    Last edited: Feb 2, 2012
  9. Feb 2, 2012 #8


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    The short answer is the speed of light is determined by the permittivity and permeability of the vacuum. Permittivity is the resistance to forming an electric field in a medium, permeability is the resistance to forming a magnetic field in a medium. Now you are probably wondering why the classical vacuum has any resistance to an electric or magnetic field. In QED it is all blamed on vacuum energy. Virtual particles in the vacuum absorb and reemit photons just like the atoms in a pane of glass, only much faster. Without vacuum energy, the speed of light would theoretically be infinite.
  10. Feb 2, 2012 #9
    I've only been exploring SR.

    So an answer is because EM experiences a uniform resistance from space? Then is that resistance not equivalent to an at rest aether?

    Is this "vacuum energy" isotropic too?
  11. Feb 2, 2012 #10
    Must be something special or complex about how permittivity and permeability determine c?

    If they are a "drag" that limits c, how is that not like free fall terminal velocity in air (where the falling object enjoys a constant gravitational acceleration for the "drag" to offset)?

    What is the energy source that keeps the photons pressing against the "drag" of permittivity and permeability to maintain c, rather than slow down...?

    I know photons are not assigned volume, mass, quantum numbers, or a reference frame... so conceptually this is hard... but if a property of vacuum is holding their speed down, wouldn't they need to have a source of constant energy to apply against that resistance or drag (permittivity and permeability)?
  12. Feb 2, 2012 #11
    It does " spend energy " moving through vacuum or another substance, thus I think it's frequency becomes larger, thus it moves to the Blue color. It's energy transfers to frequency.
  13. Feb 2, 2012 #12


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  14. Feb 3, 2012 #13
    The suspense is killing me!

    Dave, were you waiting for Chronos to address this before elaborating more yourself?
  15. Feb 3, 2012 #14


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    What's to address? emirhasa opened this thread with some confused ideas about how photons travel. We corrected him.

    Then for some reason, he went on to make more false claims; I have no idea why. They're not true. Nor do I know why he said them.

    Permissivity does not mean the same thing as friction or drag. No force is acting to slow photons down. Like every other massive or massless thing in a vacuum, they need no energy to maintain a constant velocity (Newton's First Law).
  16. Feb 3, 2012 #15


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    Here is a way to think of c as a limit.

    Consider space and time like two directions on the surface of the Earth. North is space, West is time. All objects move at 60 mph, the rate at which time passes.

    A car "at rest" is actually moving through time (West) at 60mph, but moving through space (North) at 0mph. As the car accelerates in space, it cannot move faster than 60mph, so what happens is its direction change from due West to Northwest. It is now moving through both time and space.

    Notice now, the faster it moves through space (Northward), the slower it moves through time (Westward). This is time dilation - the people in the car would appear to be aging slower to those outside.

    Now the car accelerates faster and faster (more of it velocity is devoted to spatial movement (North), which means less of its velocity is devoted to time movement (West)) The car turns ever Northward. Take this to its limit (the car accelerates without limit), and you see that, at some point, the car is travelling due North with no Westward component at all.

    What if the car tried to accelerate even more (turned even more Northward)? It can't. Not because something is stopping it, but because there is no more Northwardly than due West. See?

    Objects moving at c are moving at the maximum velocity through space, and thus are not moving through time at all. (Photons do not experience time.)

    Likewise, in an unrelated analogy, trying to go faster than c is like gonig to the North pole then trying to go more North. You're not "stopped" from going, there just isn't a "more North" than the North pole.
    Last edited: Feb 3, 2012
  17. Feb 3, 2012 #16
    Well first of all, Dave, I wasn't the only one stating false statements. You accused Chronos of such. On what fact? And what physics forum is it if you can't ask some questions in it. After all, this world consists out of physics and it's laws and there is so much we don't know that it is even senseless to ask someone why he made a false statement. Scientists of any kind have been making false statements from the beginning of the human time. Perhaps in a distant or near future we might just find out about some major flaws in Einstein's theories. No need to show arrogancy, whether you judge your post to be arrogant or not it simply implies so.

    Second, what you explained I already knew. And the elaboration of the last post isn't sufficient enough. You simply say for e.g. "you can't go northwardly more".
    You made a link between speed and time. I already knew photons do not "experience" time, if they could experience it. So, your explanation is pretty bad. What I asked is what actually defined the limit for the speed of light... I believe permittivity and permeability is more of a sufficient answer, even if it's not right.

    Because it's like I asked why can't some Porsche car move faster than for instance 200 mph.

    And you(Dave) said because the Porsche car is limited to a speed of 200 mph.
    Then you made a parallel with something else with that, and you say: "think of it as for example: a Ferrari car moves at a top speed of 190 mph. It's because it simply can't go faster than that.

    Well HECK thanks for actually not clearing anything up.

    The other explanation though, is more clearer and gives me a picture of something, let it even be WRONG, he at least UNDERSTOOD what I was ACTUALLY ASKING.

    So the right explanation for the above example would be:
    The Porsche can't go faster than 200 mph because it's limited to it's engine capabilities which has 155 Horsepower, or nowadays roads aren't good enough for it to go faster than 200 mph or some system has a false in it or BLA BLA BLA.

    I hope that, with this thorough, third time explanation people will get a picture of what kind of answer I am looking for.

    Thank you
    Last edited: Feb 3, 2012
  18. Feb 3, 2012 #17
    Chronos:"The short answer is the speed of light is determined by the permittivity and permeability of the vacuum...Without vacuum energy, the speed of light would theoretically be infinite."

    I was waiting for a clarification of the post from Chronos - the last sentence seems to suggest some drag effect holding the speed of light down to c...

    Dave:"As the car accelerates in space, it cannot move faster than 60mph..."

    So, you assume the conclusion? How does this explain?

    [edit - just saw emirhasa's last note after writing this... Dave, I'm not meaning to pile on, leaving my questions as written :)]

    In your scenario, time passing is westward movement, but the independent westward component of the car's movement continues to be 60mph when the car travels north. If the stipulation is that the resultant movement of the car may not exceed 60mph, then is this not begging the question? I'm not seeing how you can offer a way of thinking about c as a limit by stating that nothing can move faster than c...?

    "...the faster it moves through space (Northward), the slower it moves through time (Westward)."

    I'm not seeing this either - unless you are including this restriction that motion of the car in the direction of the resultant of west and north legs must always result in not exceeding 60mph. Again, that stipulation looks like it is putting the cart before the horse - the limit for c is being "built into' the example meant to show why the limit is c.

    Maybe a better example is possible; this one seems to assume from the start that

    SQRT( deltaWest)^2 + deltaNorth^2 ) <= 60mph = c

    In other words, if the west and north directions are independent, then the westward component is always 60mph and the northward component is any value -60 to 60mph. If the car goes north at 60mph, then the resultant "c" is just under 85mph.

    I can see how using the resultant of west and north demonstrates the c limit after making the assumption that it is this resultant that is assigned to the "fastest speed", but that does seem to me to be loading this example with the outcome in question.
  19. Feb 3, 2012 #18


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    You started asking questions, yes. But then you made some claims in post 11. How could you insist on these claims when they were about something you didn't understand?
  20. Feb 3, 2012 #19


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    OK, I am guilty of making the wrong assumptions about who I'm addressing.

    It is very common on PF to get members from all walks of life asking questions, and every question requires some assumptions about the level of knowledge of the asker (this has to be done, there's no universal way to address everyone).

    Based on the ideas put forth in the opening post, I made an educated guess that emirhasa was fairly unknowledgeable about photon propagation. (Sorry emirhasa, but your first post was pretty off). I chose to answer the question in a way that a layperson would understand. We do this all the time on PF.

    The car analogy is a poor analogy but it is often enough to make it clear to laypeople why c is not so much an exceedible 'speed limit' as an ultimate 'end of the line'.

    Apologies to both of you.
  21. Feb 3, 2012 #20


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    I suspect that this will not satisfy you, but the answer for this question is simply that we have defined meters and seconds such that the speed of light is 299 792 458 m/s.


    The value of any dimensionful universal constant is merely an artifact of our units. We can make the speed of light have any numerical value, such as 399,987,362 simply by choosing our units such that it is true. There is nothing more to the value of a dimensionful constant than that. For convenience, we usually use units where c=1.

    I suspect what you are more interested in is why the fine structure constant has the value it does. That is unknown at this time:

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