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Photon acceleration

  1. Mar 4, 2015 #1
    (Moderator's Note: this thread was split from https://www.physicsforums.com/threads/no-such-thing-as-instantaneous-speed.800941/)

    Does a particle reach its terminal speed instantaneously or not? Specifically a massless photon (restmass). Without mass there is no inertia. Without inertia, how can there be acceleration?

    What then prevents a massless particle (such as a photon) from starting at SOL and forces it to consume time in order to accelerate until it reaches SOL?

    IMO, length of travel, trajectory, time required are irrelevant to the question.
    Last edited by a moderator: Mar 8, 2015
  2. jcsd
  3. Mar 4, 2015 #2
    Terminal velocity is usually referencing falling objects and would not apply to photons. Newton's second law also does not apply to photons.
    Last edited by a moderator: Mar 8, 2015
  4. Mar 4, 2015 #3
    Thanks for the reply. I understand the meaning of terminal velocity, I just used the more obscure definition:

    But I am confused by the OP Title : "No such thing as instantaneous speed?" If an example demonstrates that instantaneous speed can be obtained then would that not answer to the question?

    Moreover, as I understand it, Newton's second law only applies to massive objects.
    This is why I chose an inherently massless photon, which appears to fall outside of Newton's law and thus may not necessarily obey Newton's law.

    As layman, I am completely open to correction. All I need to know if a photon reaches its speed instantaneously or not. An easy question to answer for a knowledgeable person, I should think.

    Last edited: Mar 4, 2015
  5. Mar 4, 2015 #4
    When a photon is emitted it is emitted at the speed of light. Photons travel with uniform velocity in a given medium.
    Last edited by a moderator: Mar 8, 2015
  6. Mar 4, 2015 #5
    Can I take that as confirmation that photons do not accelerate, but achieve their speed instantaneously?
  7. Mar 4, 2015 #6


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    Staff: Mentor

    Yes, that is an easy question. The answer is "Neither".

    The problem here is you're talking as if a photon is like a little tiny bullet or other small solid object travelling from a light source to whatever is being illuminated by the light.
  8. Mar 4, 2015 #7
    That is an assumption, which I did not consider. I was speaking of a radiant object emitting photons. Photons have a source. In this case I visualized a radiant source. I did specifically
    choose the photon as it has zero rest mass and exhibits dual properties while in transit, which cannot be said of a bullet or any other massive object which must obey the laws of inertia. As I understand it, the mass of a photon only exists at SOL. So how fast does a photon (reach) accelerates to SOL? Does it take time (of any duration) or is it instantaneous?

    I am not trying to argue, but only clarify a specific circumstance which might possibly be used to falsify the proposition that "there is no such thing as instantaneous acceleration", even as that is true of all physical and massive objects.

    I am just probing for a definitive answer, if one exists. I wish I could accept the use of "Neither" as a definitive answer, but that logically leads to even more questions.

    Can you clarify your use of "neither"? I hope I am not seen as trolling the subject. Please bear with me.
    Last edited: Mar 4, 2015
  9. Mar 4, 2015 #8
    Here is something I feel compelled to share.

    I used to try to understand science concept for concept instead of doing it the hard way and learning what leads to the concepts. But one day I got tired of asking questions that really led to more questions, so I decided to study science. To my delight, the small stuff that led to my big questions became just as interesting as the more advanced concepts.

    It is a journey that I am so glad I chose to go on at a relatively late age of 32. I upgraded all the way from grade 10 just to make sure I understood as much as possible going into university. I am also glad I did that because I learnt much more that just science.
  10. Mar 4, 2015 #9
    Unfortunately I am 76 years old and time constrains force me to resort to specific questions, which hopefully elicit explanations of the basics and specific links addressing my area of inquirty. I do wish I had started earlier, but life got in the way.
  11. Mar 4, 2015 #10


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    Staff: Mentor

    Let me repeat the previous answer, but with a slight twist: The answer is "Neither".

    Photons do not exist at any other state than traveling at C. It is therefore meaningless to talk about acceleration or time to reach C because they never travel at any speed but C.
  12. Mar 5, 2015 #11
    I don't mean to harp on this, but you can learn a lot in three years studying part-time. Take two years to review high school concepts if you don't know them thoroughly (grades 11 and 12). Then the first-year material for physics, chemistry and calculus can give you a really full and satisfying array of knowledge.

    For high school upgrading, http://www.khanacademy.org/ (Khan Academy), long distance learning courses, college high school upgrading, tutors (usually about $10 to $15 an hour), etc. can help you be ready for the university material. But just make sure you know the high school material first before studying from the following links below.

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    Last edited by a moderator: May 7, 2017
  13. Mar 5, 2015 #12
    Thank you Russ for the follow up.

    I know free photons only travel at C, but their origin may not. This is the reason I picked them. As far as I know new photons are constantly created and emitted from subluminal or even stationary sources (such as a photon gun in the double slit experiment).

    But if they leave the source at C, then that would constitute instantaneous speed without acceleration and provides an answer to the OP question.
    IMHO, unless I am missing an important detail, this seems a logically conclusion.

    Oh jeez, now it just occurred to me that if a photon travels as a wave, if it may be the wave which travels at C and the photon only materializes as a particle when striking and is absorbed by its target, such as a photographic plate.

    Is this merely a rambling or worthy of consideration?
    Last edited: Mar 5, 2015
  14. Mar 5, 2015 #13
    Well I have frequented science fora and researched a myriad of subjects from reputable links provided by "learned fellows, for the past 10 years. I consider that as good as trying to absorb structured curricula. The subject is so enormous and involves learning an entirely new language and maths, that, even as a bookkeeper, I rather address my perceprtions of how the universe works in a more informal narrative manner, which allows me time to research parts of the whole at my leisure. It has served me well, so far.
    Last edited by a moderator: May 7, 2017
  15. Mar 8, 2015 #14
    [QUOTE="es a particle reach its terminal speed instantaneously or not? In my previous post I cited a massless photon (restmass). Without mass there is no inertia. Without inertia, how can there be acceleration?/QUOTE].

    If these things bother you like they do me - perhaps you may find solace in the words of Einstein: "All these years of conscious brooding about the photon have brought me no closer to the truth. Nowadays every Tom, Dick and Harry thinks he knows the answer, but he is wrong.

    From one old guy to another, the rationale may lie in considering a transition of one form of energy to another as an instantaneous process, the change of state from the merger of an electron and positron for example carries with it as an adjunct of the instantly created gamma ray photons, new physical structures where 3-D angular momentum is merged into a single axial direction (which may be unaffected by the characteristic spatial impedance of free space). Obviously, I fit the category of one of the three referred to by Einstein, probably "Dick."
  16. Mar 8, 2015 #15
    This is such a bad attitude for so many reasons. Einstein probably said this in his later years when he showed a baffling amount of ignorance and arrogance.
  17. Mar 8, 2015 #16


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    Staff: Mentor

    You might want to consider upgrading your spell-check software. It seems to have made a terrible hash out of the phrase "deep appreciation of the philosophical difficulties that are hidden behind the undisputed and unprecedented empirical successes of quantum mechanics". :smile:

    And, kidding aside, Einstein wasn't alone here. For example https://www.physicsforums.com/threads/what-did-feynman-really-mean.801785
  18. Mar 8, 2015 #17
    I hate that I am a student criticizing Einstein because we get the luxury of hindsight and the opportunity to stand on the shoulders of giants of the past 100 years.

  19. Mar 8, 2015 #18
    In the same line of thought, you might to want to consider how long it takes for silence to become sound when you knock a door. Or how long it takes for falling dominos to reach constant velocity.

    An equivalent way to view this with photons is the following. A photon is emitted when an electron falls from one energy state to a lower energy state, so how long does it take for an electron to change state? I believe an upper limit can be deduced, and is most likely around 10 to the -17 s (0.00000000000000001 s) at most, but this would have to be confirmed.
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