If photon is mass-less why can it be pulled into blackhole?

Discussion in 'Physics & Math' started by Saint, Aug 7, 2017.

  1. Write4U Valued Senior Member

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    In this whole rant, there is one single keyword, "effect". In fact the qualifier "slingshot" is actually a misnomer, it is just another metaphor.
    This is a slingshot;

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    But its a Handy Dandy word for acceleration by gravitational assist (of an orbiting planet). What is it you call it, oh yes, "mathspeak".
     
    Last edited: Aug 18, 2017
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  3. DaveC426913 Valued Senior Member

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    You're still trying to defend the indefensible. Scientific definitions are not personal opinions.

    I've been polite, but that's been burned up. If you must persist in making a fool of yourself for another 50 posts, you're going to have to do it on your own here on in.
     
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  5. Write4U Valued Senior Member

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    No, it was the correct response to that incomplete example, which should have read "the gravitational pull (assist) of an orbiting planet"
    As stated it had no mention of gravity, hence my kinda snarky remark. And I understand the slingshot effect very well. I am the one who posted the links to examples.

    Sorry Origin, I was just a little annoyed with the unwarranted semantic criticisms.
     
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  7. Write4U Valued Senior Member

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    When was Neptune discovered?
    Anyway, back to the OP question. I'll just sit back and listen to the scientists.
     
  8. The God Valued Senior Member

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    Now you got it.

    The momentum gain (or loss) by the spacecraft is due to orbital motion of that planet around Sun (third body).

    But of course Gravitational pull of that planet is also there, but gets nullified in get in get out process. In fact spacecraft should not enter into planet's deep atmosphere, then the drag also kills the fun.
     
  9. Write4U Valued Senior Member

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    In the cause of science, I have to disagree with the three body only theory. This is used within the solar system and orbiting planets can be used to eventually escape the sun's gravitational field.

    But in outer space a two body gravity assist can also be used for a slingshot effect as long as the massive body is in motion.
    Gravity assist

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    In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically in order to save propellant, time, and expense.
     
    Last edited: Aug 18, 2017
  10. origin Trump is the best argument against a democracy. Valued Senior Member

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    This discussion was never just about semantics.
     
  11. Write4U Valued Senior Member

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    I think so, I understand what everyone is saying, but it seems I have a problem with stating my perspective concisely and clearly., so that others understand what I am saying which is fundamentally the same but from a slightly different perspective.
     
  12. origin Trump is the best argument against a democracy. Valued Senior Member

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    I do not believe you. I think we are just seeing your reluctance to admit you were in error. It looks like from post 106 that you now understand the basics of gravity assisted maneuvers. Why not celebrate that you have gained new understanding instead of pretending you already knew it.

    I learn new stuff everyday - it is a good thing.
     
  13. exchemist Valued Senior Member

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    What you seem not to be acknowledging is that velocity always has to be measured relative to something. If you only consider 2 bodies (planet and satellite), the incoming and exit velocity of the satellite relative to the planet will be the same, as Dave originally said. However the velocity of the satellite relative to a 3rd body, say the sun, can be altered, as a result of the planet being in motion relative to the sun.
     
  14. Write4U Valued Senior Member

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    For observational and practical purposes I can see this, but is a third body absolutely necessary for a body to be in motion, independent of observation?

    The orbiting planets are handy because we know their precise behaviors and can use that knowledge to calculate trajectories, etc.

    But as I understand it as long as a massive body is in motion (momentum) it can be used to increase the speed of a bypassing spacecraft through the slingshot effect (gravitational assist)

    Is the illustration above incorrect? It merely shows a massive body in motion and a spacecraft gaining speed from the gravitational assist of that body. A two body interaction.

    Does relativity to a third body have anything to do with this physical phenomenon?

    At its fundamental level it comes down to some sum of motions and gravitational pull of the massive body and the motion of the spacecraft using that gravitational pull, no?

    The third body (like our sun) is what keeps planets in orbit, but that just means that the orbiting planets possess motion (momentum) relative to the sun, which is useful, but not absolutely necessary. Only motion and gravitational pull between two bodies are necessary for a spacecraft to alter course or increase speed. (as illustrated)

    The underlying physics is not concerned how or why a massive body has acquired its motion, as long as it is in motion and if a spacecraft can plot an in-flight precise intercepting trajectory, it should be able to use the gravity pull of that body, for use of altering course or even increase it's speed from the gravitational assist.

    The gravity of the third body (the sun) establishes predictable planetary orbits, but the slingshot effect has nothing to do with orbits themselves, but the speed and momentum and gravitational effect of a massive body are key, regardless if it is in orbit within the solar system or follows a straight path outside the solar system.

    Does this make sense?
     
    Last edited: Aug 18, 2017
  15. Confused2 Registered Senior Member

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    What you do not seem to be acknowledging is that your choice of coordinate system is up to you. For a flight from Earth to Pluto I might choose to work entirely in coordinates centred on Pluto (the destination) - is Pluto the 'third body' because of my choice of coordinates? For a flight from Earth to a polar orbit of the Sun I might choose to work entirely in coordinates centred on the Sun. The slingshot physics (a two body problem) are exactly the same but I'd be choosing different coordinate systems.
     
  16. DaveC426913 Valued Senior Member

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    The scenario that has been on the table has only two reference frames - the small body and the large body.

    Here, you have introduced a third body / reference frame. This is what Exchemist and I have been saying.

    You're sort of jumping into the middle of a sidebar, and taking comments out of context.
     
  17. Confused2 Registered Senior Member

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    I sort of have a purpose. Sadly I only have a little SR to attack the question in the OP which is a GR question. However, I will venture that the coordinate system seen by a photon travelling close to a Black Hole may not look like a straight line when seen from point far from the Black Hole though to the photon it is indeed a straight line. Your choice of coordinate system is arbitrary as long as you are prepared to reconcile what happens in your chosen coordinate system with what happens in another,
    Throw a ball up at 45 degrees to the ground. GR - straight line. Euclid - parabola. Is one 'true' and the other 'false'? Pick your coordinate system (carefully).
     
  18. Write4U Valued Senior Member

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    Interestingly you can indeed make a parabola using only straight lines.
     
  19. Write4U Valued Senior Member

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    I'm afraid, you are not understanding to point I was making.

    Moreover defending a position is a very good way to learn.
    Blind acceptance of convention, leads to stagnation.
     
  20. The God Valued Senior Member

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    Why?
    You can describe motion in any FOR. There is no such restriction. You can have milky way centre as FOR, how does it matter.
     
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  21. uhClem Registered Member

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    In a two body problem, neither body has a reference frame because they are both accelerating. Both are orbiting the center of mass of the system, unless they have hyperbolic velocities relative to the center of mass of the system. The best reference frame to use is the center of gravity of the system, though any reference frame will work. But neither body can be used as a reference frame. If two bodies are very dissimilar in mass, say a spacecraft and a star, the star's orbit around the center of mass would fit into an the size of a hydrogen atom (that is a guess, did not bother to calculate it) because the spaceship's mass is relatively insignificant compared to the star. So the star can be thought of as having reference frame which is the center of mass reference frame for all practical purposes. But the spacecraft will not have a reference frame because as it gets close to the star it will be experiencing significant acceleration.

    And because the star in this spaceship/star system is in orbit (though a very very insignificant one) in theory the star could be used to boost the spaceship, but because the star's orbit is so very small, and therefore the orbital velocity is so very small, the boost would be so very tiny that the effect is not worth considering.
     
  22. DaveC426913 Valued Senior Member

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    It doesn't matter - you cannot give the photon more potential energy than it had. In a simple two-body scenario:
    - there is no coordinate system in which the photon can escape, if it was already off within the BH's grip.
    - likewise, if it wasn't within the BH's grip, then there is no coordinate system in which it can be captured.
     
  23. uhClem Registered Member

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    Actually, I want to take back that second paragraph. There can be no boost in a 2 body scenario. The problem is that the center of mass is always between the two bodies. In order to get the boost one body would have to cross the orbit of the other body, just behind it with respect to its motion, which would require both bodies to be on the same side of the center of mass. That can never happen in a 2 body problem. It is only possible with 3 or more bodies. Sorry about that.
     

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