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

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

  1. James R Just this guy, you know? Staff Member

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    river:

    No, it really isn't.

    Well, there are two possibilities:

    1. Our understanding of gravity works pretty well, which means there's something we can't directly see at the galactic centre that has the mass of a million Suns.

    or

    2. Our understanding of gravity is deeply flawed - so much so that we are completely mistaken about why one object orbits another object.

    Which do you think is more plausible?

    Bear in mind that we use gravitational physics successfully for all kinds of things every day.
     
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  3. DaveC426913 Valued Senior Member

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    It is not a matter of perspective; it is a matter of empirical observation matching a theory's predictions.

    We are able to observe stars in orbit at the galactic core.
    Because we know the period of their orbit, we know the mass of the central object.
    Their perihelion (their closest point to the central object) puts an upper limit on the physical diameter of the central object. It is too small to be any known object with that mass.
    We have also been able to observe the precession of the orbits of these stars (due to very tight orbits around very massive objects), further cementing Einstein's predictions.
     
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  5. James R Just this guy, you know? Staff Member

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    river:

    Also, can you give any example of something being "thrown out" by gravity? Gravity is an attractive force.
     
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  7. hansda Valued Senior Member

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    Are you implying some thing additional to mass, with the term "matter"?

    What is "Massless matter fields (like the EM field)"? Are you implying particle photon? I dont think particle photon curves spacetime. It follows spacetime.


    But you used this concept in GR.

    Force is a concept of Newtonian Model. In GR, force is not considered; so it does not exist in GR.
     
  8. QuarkHead Remedial Math Student Valued Senior Member

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    All of it, at least in its representation
    Neither .Spacetime is a 4-manifold, not a field, and the mathematics that describes the best available theory of gravitation is a field theory. That is, since any manifold is entitled to a tensor space at every point, then the ensemble of such tensor spaces is field (loosely speaking).

    There is no question that the General Theory is a field theory - the only sensible thing you can say about gravitation (at present) is that there exists an energy-momentum field, a metric field and a curvature field derived from it (the scalar field comes for free) and that there exists some sort of equality between the first and the second.

    The existence of matter and energy about which nothing is known (dark), means that the energy-momentum tensor field is something of a mystery.
     
  9. The God Valued Senior Member

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    Energy momentum field??
    Energy momentum tensor is mystery?

    I am afraid, there is nothing sensible about your post, that too when you are acting as expert, its all hotch potch. I suggest get the definition of 'field' cleared first.
     
  10. Write4U Valued Senior Member

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    When an object enters the gravity of a star from outside its sphere of influence at a certain angle
    , it gains speed and then can be thrown out. As I understand it, it is how we use gravitational pulls to propel our spacecraft past a massive planet while taking close-up pictures
     
  11. The God Valued Senior Member

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    Context as such is that gravity is attractive or inwards curving, you do not have gravity repulsing something. The motion of a particle depends on the initial condition also, you cannot propel out anything with the help of gravity alone if initial conditions are not conducive.

    Another example, although still does not support repulsion, is ejection of jets around Black Holes. The language says that jets have ejected out, giving some kind of false gravitational repulsion impression, but the science is perfectly as per gravitational accepted theory.
     
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  12. Write4U Valued Senior Member

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    Yes, I was just giving a scenario where an object may gain sufficient speed to escape the gravitational field.
     
  13. Schmelzer Valued Senior Member

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    Yes. Matter does not necessarily have mass.
    That the EM field (that means, photons) curves spacetime too is standard GR.
    I may use it in informal forum postings, where the aim is to explain something to laymen.
    Yes, in the spacetime interpretation of GR it does not exist.
     
  14. origin Trump is the best argument against a democracy. Valued Senior Member

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    You do not have this scenario exactly right. It is not possible for a mass to gain speed only due to moving into and out of a gravitational well such as a planet regardless of the angle. If a mass is moving at say 10,000 kph and is 'pulled' towards the planet by gravity it will accelerate towards the planet and then when it leaves the planet it will decelerate to the 10,000 kph and continue on its way.
    What you are thinking of is a gravity assisted acceleration. This is done by using the orbital velocity of say a planet to sling shot a satellite to a higher speed. If the satellite swings around the planet and leaves in the direction of the planets orbit it will increase the satellites speed, if the satellite swings around the planet in the opposite direction of the planets orbit it will decrease the satellites speed. Using a planet in this way will speed up or slow down the planet due to the conservation of momentum - of course the speed change is imperceptible.

    Gravity Assist.
     
  15. DaveC426913 Valued Senior Member

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    That's actually the default case. It will always leave the system, and do so with the same velocity it entered the system.

    There are two edge cases, both rare:
    1] influence by another body in the system causes it to be captured
    2] its hyperbolic trajectory actually intersects the surface of the star.


    This has nothing to do with the Event Horzion of a BH though. An infalling body cannot gain enough speed to escape once it has crossed the EH.
     
  16. Write4U Valued Senior Member

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    Yes, that's why I qualified it with an object entering the gravity field at a "certain angle", it can use the gravitational acceleration to slingshot back out.
    t is not an example of repellent gravity, but of using a planets gravity to accelerate, so that it gets slung out of orbit, I guess by the centrifugal force
     
  17. origin Trump is the best argument against a democracy. Valued Senior Member

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    Either you did not understand the explanation of gravity assisted acceleration or you do not know how to adequately express your understanding.
     
  18. DaveC426913 Valued Senior Member

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    Still no.

    The small mass does not gain velocity with respect to the large mass.
    If it were on a hyperbolic trajectory on the way in, the will always escape.
    If it does not escape it's because it was in orbit in the first place.

    It is important to understand that gravity assist only changes the velocity of the object in a frame of reference outside the large mass.

    1]
    A satellite falling toward Jupiter will exit Jupiter's gravity well with exactly the same speed (with respect to Jupiter) as it started. Always.

    The gravity assist it gets is with respect to the rest of the solar system.


    2]
    This is why it is pointless to use the sun as a slingshot (despite Kirk's and Spock's insistence). Whatever speed you start with when falling toward the sun is exactly the same speed you will end up with when rising away from the sun.

    The only time you would use the sun for gravity assist is when you are trying to move a craft through the galaxy - i.e. outside the solar system.
     
    Last edited: Aug 16, 2017
  19. Write4U Valued Senior Member

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    I agree, and I was not addressing an infalling body in the example. Just wanted to clarify that a passing object can gain speed , sufficient to escape the pull of gravity. I may leave the field at the same speed it entered, but it was accelerated by the gravitational pull. Visualize a golf ball actually acceleration out of the cup , when it skirts just along the edge.
    But I stipulate this has nothing to do with any gravitational repulsion, just with a centrifugal force overcoming the gravitational attraction.
     
  20. DaveC426913 Valued Senior Member

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    This is still visualizing it wrong. It always had enough speed.
     
  21. Write4U Valued Senior Member

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    Then why do we use the term "slingshot effect" at all ? Are our astronauts crazy? Is there no gravitational acceleration involved at all? I'll repeat
    https://en.wikipedia.org/wiki/Gravity_assist

    Obviously these spacecraft used the gravitational pull of these planets. A little less original speed and they would have fallen into the gravitational well or ended up in orbit. Is that not what our satellites use to stay in orbit?
     
    Last edited: Aug 16, 2017
  22. DaveC426913 Valued Senior Member

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    The slingshot effect must involve a massive body (other than the sun) within our solar system, but the velocity gained is not with respect to that body; it is with respect to the solar system.

    So, in effect, you don't get any slingshot effect when observing only two bodies interacting.
    Which is why it is meaningless to say the small object "gains enough speed to escape the pull of gravity".

    In your original scenario, you would need to invoke a third mass (C) - orbiting near the largest mass (A) - from which your target mass (B) can steal potential energy, to escape A. The third mass C will fall in further toward A.

    When spacecraft use Jupiter for gravity assist, Jupiter slows down and falls inward while the craft speeds up with respect to the Solar system.
     
    Last edited: Aug 16, 2017
  23. Write4U Valued Senior Member

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    So you are saying we cannot escape the gravitation of the sun, ever? Well there goes our dream of "interstellar travel". It's simply not possible.

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    Last edited: Aug 16, 2017

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