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

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

1. ### KittamaruNever cruel nor cowardly...Staff Member

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1) Who is saying it "doesn't matter" as a blanket statement?
2) Who is saying it "cannot be captured" as a blanket statement?
3) Can you provide any evidence to actually support your claims?

Let me be blunt - you have spent so long on SciForums spewing unsupported nonsense that you have, literally, no "good will" here to fall back on. Nobody is going to give you the benefit of the doubt just on your say so. If you want to make a point, you will need supporting evidence, as the rules dictate. Furthermore, if you wish to make an extraordinary claim, then you will require extraordinary evidence.

Period.

Now, the way I see it, you have two viable options:

1) Provide the requisite supporting evidence.
2) Disengage from the conversation

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3. ### DaveC426913Valued Senior Member

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[ Deleted. It was just troll-feeding. Deleted for the benefit of the thread.]

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5. ### uhClemRegistered Member

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As a natural skeptic I can appreciate your stance. Here is some corroboration.
https://ocw.mit.edu/courses/aeronau...fall-2009/lecture-notes/MIT16_07F09_Lec16.pdf

On page 4 under the subheading of Hyperbolic Trajectory (e>1) there is an exact statement right after equation (11).

7. ### The GodValued Senior Member

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So far you have have miserably failed to pin point even a single incorrect statement by me. You are just making noise, give a single evidence of incorrect statement by me or retract. Can you? You have a task cut out for you Kittamaru, rise above the normal venom and prove any one of my statement wrong.

PS: it is laughable to see how you are supporting inaccuracies by saying that they did not make a blanket statement like this. This is precisely what I said that "in general" whatever claimed by DaveC was not correct.

And what makes you think I need goodwill from likes of origins or DaveCs or sweetpeas etc...I am never a part of mediocre coterie. I am here to see that they do not spread inaccuracies, it's a matter of concern that we have poor mod contribution here in putting the science in right perspective.

8. ### Confused2Registered Senior Member

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Thanks for that. I think the definition of 'capture' has to include starting at infinity and it isn't goint to happen. Setting up starting conditions inside an elliptic orbit is (unintentionally) cheating.

9. ### uhClemRegistered Member

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Well, the model is an abstraction. Our universe is not a two body system. Also in the model they actually use the concept of a "central force" rather than an actual physical body at the center. The central force abstraction allows that if there is no angular momentum, then the second body at infinity will drop straight to the center and out the other side without hitting a physical object. And even time is abstracted to allow infinite past and future. For example, what would it mean to travel to infinity and still that some excess velocity? As you say, its not going to happen. In practice, you start with a moving object, measure it's direction and velocity at some time, and project the ideal trajectory back and forth in time to infinity.

Also it was Newton that proved that ideal 2 body trajectories are always conic sections. So there are only the 4 possible trajectories: circle, ellipse, parabola and hyperbola. No half hyperbola that changes into an ellipse near the center. And no two body elliptical trajectories that finds an escape. Newton was just describing the models that seem to fit observations, he was not saying they explained how the world worked. He never explained gravity, he only described it.

Last edited: Aug 21, 2017
10. ### KittamaruNever cruel nor cowardly...Staff Member

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Given that TheGod has continued to fail to back his claims, whilst simultaneously becoming increasingly hostile and derogatory to the other participants in the thread, he has been issued a two week reply ban. I am going to split off his off-topic whining as well as any replies to said whining to clean the thread and get it back on-topic.

EDIT - In retrospect, attempting to purge the off-topic chatter would further annihilate the thread. Instead, I think, it should just remain and allow those who actually value science to draw their own conclusions from it.

11. ### originTrump is the best argument against a democracy.Valued Senior Member

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No, that is not necessary and is just a road block to understanding the situation.
For instance using Newton's law of gravity you can find that the force of gravity from Alpha Centauri on you is about $8.6 x 10^{-12}$N. The force of gravity from an automobile (~1000kg) 10m away from you is about $6.7 x 10^{-8}$N. So the car has 8000x more gravitational force on you than Alpha Centauri .
Of course Alpha Centauri, the second closest star to earth, is 4.5 LY away. Let's say we were 'only' 1/2 ly year away from Alpha Centauri then the force of gravity from Alpha Centauri would be $6.4 x 10^{-8}$N. So at that distance the car at 10m would still have a slightly stronger gravitational force on you.

So the point is you should not waste your time with the 'infinite' distance argument unless your goal is to minimize your chances of learning more science or disrupting the thread.

12. ### James RJust this guy, you know?Staff Member

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The God:

I'd like to address two things: (1) the physics, (2) your latest hissy fit.

See the quotes below for your "single line".

Clearly photons can escape from the surface of a planet, or from a star. But the bulk of your argument was not about that. You are arguing, for example, that in a two-body system, such as the Sun and a comet, it is possible for one body to "capture" the other, when they were initially unbound. That is impossible. Again, in this regard you introduce tangential matters like the following:
Here you introduce black holes. A black hole can "capture" photons that pass within the event horizon, but that is quite a different process than the "gravity assist" manoeuvre that was originally being discussed. You made a mistake early in the discussion, and you've been trying to introduce irrelevancies to avoid having to face that error ever since then.
Does this count as an example of a "single line" error of yours that proves you wrong?

An asteroid in a hyperbolic orbit cannot be captured by a planet if only those two bodies are involved and the asteroid does not actually crash into the planet. Certainly it is impossible for the asteroid to start orbiting the planet, as you claim it can do.

It's a simple matter of conservation of energy. Gravity is a conservative force. If the total energy of the asteroid (kinetic plus gravitational potential energy) is larger than a certain value, then a bound orbit is impossible. The only way to "capture" the asteroid is to dissipate some of its energy somehow - and gravity from the planet alone won't (can't) do that.

Ooh. Big bold writing. This must be important.

I suggest you try searching my prior posts on this forum for the terms "photon", "zero rest mass" and "momentum". Then you can apologise to me for making false claims about what I do and don't know. Clearly you were ignorant and made a rash statement without knowledge. Let's see if you can bring yourself to admit you were wrong about this. Let us also hope that you can avoid making similar statements about me or about other people in future, without first checking the available facts.

It's only nonsense to you because you so clearly do not understand why it is important. Maybe you'd do better to take some time out to research this matter yourself, before you attempt to discuss it further.

Just to emphasise what I wrote above: they can't "get into orbital motion" if they aren't already in it. Not with two bodies. You can't turn an unbound trajectory into a bound orbit without losing some mechanical energy, either to a third body, or via a dissipative force of some kind.

Then I look forward to your explanation, based on momentum and energy conservation, of how your two bodies can "get into orbital motion", having previously been unbound to one another gravitationally.

But then again, I also wonder why, if you have such an explanation, you failed to post it earlier in the thread.
---

With the physics part done, let's turn to a brief discussion of the hissy fit.

You are acting like somebody who has trouble accepting constructive criticism from other people. Instead of admitting you made a mistake, you dug your heels in and started ranting at the people who pointed out your error. A more mature person would have admitted his error, and perhaps have thanked those who pointed it out to him. Refusing to accept that you might be wrong about things actually impedes your learning in the long run, because we often learn best from our own mistakes and misconceptions.

You are also acting like somebody who has a chip on his shoulder against certain moderators. The result is that what could have been a perfectly civil discussion about physics has turned into a childish tantrum on your part. When several members suggested that you tone things down, instead you escalated your inappropriate behaviour. When you were moderated for that, as you must have known you would be once reports were filed, instead of backing off you decided to escalate things further by questioning the moderation. And in the end, the only person you ended up hurting was yourself.

Realise this first: no member requires your permission or invitation to post something in a thread.

And this second: your labelling of another member's posts as "pathetic shit" does nothing to address the content. In fact, it suggests that you have no adequate response to the content, so you felt you could only resort to name-calling. This makes you look childish.

Really? All this over one post in which I commented on some incorrect physics of yours?

You'd have done better do show why I was wrong. That's if you could actually point out an error.

There's that big, bold statement again. But up to the point where you wrote that, I had actually posted nothing about momentum conservation during the emission of photons in this thread. So there could be no "casual nonsense" from me on that topic.
It's so dreadfully important to correct people who are wrong on the internet, isn't it? There really should be internet police to come and arrest people who get their physics wrong on web forums.

Until they get the Internet Physics Police Force up and running, I guess it's up to good people like you to just grit your teeth and try to correct as many people as possible, no matter how much of a burden it is and how little you enjoy going onto those forums to post the necessary corrections.

Sure, The God. Mediocre coteries are not for you. You're a true God, crusading for Righteous Physics across the wilds of the interwebs. It's not an easy job, but somebody has to do it. Somebody has to be a hero, and who better than The God?

13. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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It actually is symmetrical, in that neither gains or loses unless they collide. Assuming they are entering hyperbolic trajectories with respect to each other, and in the absence of any third body or atmospheric interference etc, the magnitude of the momentum each body gains on the way toward closest approach is equally offset during their departure. What will change is their directions.

Last edited: Aug 23, 2017
14. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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It is not possible, in the absence of a third body or due to atmospheric braking (that will ultimately lead to collision), for one body to orbitally capture another body that approaches on a hyperbolic trajectory.

Ignoring atmosphere, assume the smaller body enters the sphere of influence of the larger with a speed of V, with V necessarily being in excess of the escape velocity (otherwise it would not be a hyperbolic trajectory but already orbital). In the absence of any third body we can treat the larger body as stationary, as all motion is then relative to that single body.
As the smaller body approaches its velocity increases under the effect of the gravity.
It will either strike the larger body or it will slingshot around and away, at all times with speed of greater than V while in the larger body's sphere of influence.

In such a scenario it is simply not possible for the smaller object to be captured into an orbit around the larger without some additional force acting upon it.
In nature this additional force is a third body such as another planet, an existing satellite, or the star etc.

In the presence of an atmosphere around the larger body, the smaller body may brush through the atmosphere and slow down. In doing so it's speed may be such that it now dips below escape velocity, the atmosphere providing the additional force necessary. However, in doing so that first orbit will again bring the smaller body to brush the atmosphere and slow it further, thus resulting in eventual, inevitable impact.

When we talk about orbital capture, however, it is generally assumed to be capture into a stable orbit, not one that quickly results in collision.

That at least is my understanding, but I'm interested to hear on what grounds you are claiming #2 to be possible (as in capture into a stable orbit)? You say to think otherwise is "propagating bad science", so what are you suggesting is the way that it might possibly occur? Where does the additional necessary force come from?

15. ### Write4UValued Senior Member

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question: Is there a fundamental mechanical difference between the terms "captured" and "assisted" ?

Seems to me that a captured object is incorporated in the orbital system, whereas, an assisted object may well be expelled from the orbital system, depending of the angle of approach. Is this too simplified?

16. ### originTrump is the best argument against a democracy.Valued Senior Member

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Yes. Captured means that the object will orbit the central mass. Assisted is a maneuver that is preformed by a space craft to speed up, slow down change the direction of the craft or a combination speed and direction change.

Yes. As has been stated multiple times, if an object not in already in orbit approaches another isolated mass like a planet it will not get captured regardless of the angle, UNLESS the object losses some of its momentum. Methods to lose momentum are interaction with the atmosphere, hitting the planet, or an interaction with another body. For the Apollo missions they went into an orbit around the moon by losing momentum thorough deceleration burns.

17. ### DaveC426913Valued Senior Member

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They are actually time-reversed (i.e. symmetrical) paths. I'm not just blowin' smoke - it's very informative to know this.

We have orbital simulators that can project a star system forward in time millions of years or backward in time. If you came upon a simulator in the middle of running a sim, you would not be able to tell, from examining the paths of the objects (short of disintegration) whether the sim was running in forward or in reverse.

Run forward long enough and you might see an infalling body get captured due to a close encounter with a planet. Stop the simulation and run it backward and presto! - you will see a body having a close encounter with a planet and getting ejected. The orbital physics is perfectly valid and symmetrical.
(Ultimately, all this proves is that Conservation of Momentum works).

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18. ### hansdaValued Senior Member

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I dont think particle photon curves space-time. Particle photon follows space-time. Otherwise, how you can explain curvature of starlight around the Sun or Gravitational Lensing.

19. ### hansdaValued Senior Member

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So, you mean to say that mathematics of spacetime is a field theory.

The energy-momentum field as you mentioned above must be different from space and time. What is metric field? What is curvature field?

Einstein's Field Equations, which explains the math of spacetime; is not a mystery, I believe.

20. ### DaveC426913Valued Senior Member

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It does both.

"Spacetime tells matter how to move; matter tells spacetime how to curve."
- John Archibald Wheeler, theoretical physicist and Einstein collaborator.

(That quote would be upgraded now, to say mass, rather than matter).

Last edited: Aug 24, 2017
21. ### Write4UValued Senior Member

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As far as I understand it is indeed a geometric (mathematically measurable) field .
https://en.wikipedia.org/wiki/Spacetime

22. ### SchmelzerValued Senior Member

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Technically, there is a difference between GR and a field theory.

In a field theory, you have a field on a fixed background. Say, $\varphi(x,t)$ is defined for every x and changes in time t. Then, for each value of x, the $\varphi(x)$ is a configuration variable. The x is nothing but an index of the coordinate in the configuration space. So, the usual n-dimensional configuration space Q with n coordinates $q^i$ is replaced by some infinite-dimensional space with an infinite number of coordinates, which would be analogically $q^x$.

In GR, it naively looks similar, we can have 10 different fields $g_{\mu\nu}(x,t)$, not? Not, because for two different gravitational fields it makes no sense to ask "what are the values of $g^1_{\mu\nu}(x,t)$ and $g^2_{\mu\nu}(x,t)$?", because different gravitational fields define different spaces, and there is nothing which tells which point on one solution corresponds to which point on the other solution.

But there is also a field-theoretic approach to GR. This approach assumes a Minkowski metric $\eta_{\mu\nu}$ as a given vacuum, and has, then, to define which point of this Minkowski background corresponds to which point of the metric. Then, the metric can be splitted into the Minkowski metric and the gravitational field: $g^{\mu\nu}(x,t) = \eta^{\mu\nu} + h^{\mu\nu}(x,t)$ or so. This approach breaks the "gauge invariance" of GR, what is from point of view of the spacetime interpretation "the same" metric could be described by quite different fields $h^{\mu\nu}(x,t)$. To obtain meaningful evolution equations for such a field theory, one has to break this symmetry with some coordinate condition.

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