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

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

1. ### DaveC426913Valued Senior Member

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No. For starters, escape from the SS does not depend on gravity assist. So we're not trapped. Nice try.

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What I am saying is:

1] The sun cannot be used as a gravity assist. Any speed we might gain falling toward it is exactly lost when we rise away from it. So it does not do anything for us with respect to escaping the solar system.

2] We can use the planets to change our velocity with respect to the SS. Which is what all our satellites do.

3] But gravity assist is not applicable in the two body scenario you posited many posts ago.

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3. ### Write4UValued Senior Member

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So it gains speed? You are making it more difficult than the simple concept of using gravitation to increase the speed of the much smaller object, which has no noticeable effect on the massive body that is creating the gravitational well. I realize there are exact mathematical trajectories required to achieve the desired effects, but somehow we are making it work, or is all this just fairy tale?

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

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You still do not seem to get it. Since the moon is orbiting the earth we can use the ORBIT of the moon to get a gravity assist. We can pick up speed by the 'sideways' movement of the moon. If somehow the moon was stationary relative to the earth there would be no way to increase our speed by doing a moon flyby. We would increase our speed as we approached the moon and decrease our speed as we moved away from the moon. The net effect would be that there would be no change in speed, just a change in directions. Savvy?

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7. ### DaveC426913Valued Senior Member

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Key: that speed is relative to Earth. Not relative to the Moon. That's important regarding your original scenario.

The upshot is that an object falling toward a black hole (or any other body) does not get a boost from the body's gravity that allows it to escape the body.

8. ### DaveC426913Valued Senior Member

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Here's a diagram that illustrates the concept.

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Look at a scenario that involves only the train and the baseball.

Train sees baseball coming at it at 80mph (30+50), and then (due to a perfectly elastic collision) leaving at 80mph.
As far as the train is concerned, the velocity of the ball has not changed. It got no assist.

Baseball : Train :: Spacecraft : Jupiter
or
Baseball : Train :: Particle : Black hole

That was the scenario you first proposed.

In order to enjoy an assist, one must use the boy's frame of reference.

Ball heads toward train at 30mph, but leaves at 130mph (30+50+50).
With respect to that third vantage point, the ball has gotten an assist from the train.
i.e. boy sees asist, train does not.

Baseball : Train : Boy :: Spacecraft : Jupiter : Solar System
Compare to
Baseball : Train : Boy :: Particle : Black hole : <null!>

There is no external frame of reference in which the particle has gotten any boost.

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9. ### QuarkHeadRemedial Math StudentValued Senior Member

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Yes, for sure. That is what I said and that is what I meant. The theory of gravitation as proposed by Einstein is a field theory - every element in his seemingly simple equation (actually it is 10 equations) is a field.
It is. If you know what dark matter and dark energy are (both of which contribute to the gravitational field) please share it with us

I see no need to be impolite. I CAN give a definition of a field - but I have seen no evidence here from you (or others) that the mathematical exposition would be appreciated or understood.

Let me know if you want the definition (in fact there are two, depending on context)

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Yep.

Yep.

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QM?

12. ### Write4UValued Senior Member

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I never claimed anything like it. I claimed that an object can experience a slingshot effect from a gravity field, IF it enters that field at a specific angle.

All elliptical orbits produce a slingshot effect, the difference is that these orbits are stabilized. The object speeds up as it approaches the planet, passes (slingshots) by and then begins to slow down until the gravitational effect draws it back to the planet (or sun), begins to speed up again and then slingshots past the planet again in an endless elliptical orbit.
This process does also affect the planet it self. This interaction creates a slight wobble in the larger massive object. On earth we experience this effect from the moon's gravity, by our shifting ocean tides.

My example was for an object (meteor) coming from space and skirting the gravitational field at an angle, which creates a slingshot effect that may well throw it back into space outside the gravitational field and then it just disappears back into space, never to be seen again.

What is so difficult to understand about that? My example did not say that the object was falling anywhere. It just received a momentary boost from the gravitational field as it was passing by. A slingshot effect.
Post # 60 illustrates this effect clearly.

And I have also answered the observation that this is not the same as a repulsive gravitational effect. It is a change in momentum effect.
It was one answer to the question how an massless object can escape a deep gravitational field.
Another answer is that, photons escape from the sun all the time, because they travel at the top end of the wave spectrum. Inside the sun it may a take a photon 2000 years to escape the limiting space and energetic activity inside a sun, then it reaches earth in 8 minutes.
At least we know that under certain conditions the density of a gravitional field can be greater than the momentum of the photon and anything with a momentum of luminal speed, i.e. only permitting the functions of virtual particles. A chaotic state of matter itself from which fractal patterns form, some of which have the ability to change from virtual to causal real particles. Energy. Dark Energy?
IMO, this is why

Last edited: Aug 16, 2017
13. ### Confused2Registered Senior Member

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14. ### The GodValued Senior Member

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Just because GR is a field theory does not mean that "energy momentum" is a field. This is how you referred energy momentum and I know now you will obfuscate further with your continued word salad to support your erroneous stand. You can give both the definition and establish how energy momentum is a field, I will try to understand your maths. Hope in the process you too clear your bumps.

There is no mystery tensor, in fact there is no mystery around DM and DE, they are nice tools, we use them as and when they are required. They are now part of well established mainstream cosmology, the mystery is...ok...leave it.

Mathematical expressions got to be distinct not mysterious.

Last edited: Aug 17, 2017

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16. ### DaveC426913Valued Senior Member

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It doesn't matter what angle. See below.

No, they don't. You are misappropriating an existing term.

If it came from outside the gravity well it will return to outside the gravity well.
If it came from inside the gravity well it will stay within the gravity well.
The only way an object can be captured is by interaction with a third body (by losing energy the third object).
The only way an object can be ejected** is by interaction with a third body (by stealing energy from the third object.).

**This is what you are describing: an object starts within a gravity well, but manages to escape it. (This cannot happen without a third body.)

It's wrong. See above.

17. ### Write4UValued Senior Member

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No what I am describing is the opposite. The object starts outside the gravitational field, and falling in at a specific angle , which momentarily is aided by the gravitational pull to experience as slingshot, not much different than a centrifugal force of the fundamental function of a sling shot and then tend to go outward in a straight line where it leaves the "sling", sometimes escaping back into space, but at a different angle or wave length?

18. ### DaveC426913Valued Senior Member

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If it starts outside the gravitational well, it will end up outside the gravitational well, and it will have the same speed. Full stop.

This is called a hyperbolic trajectory.

Going back the original statements:
When an object enters the gravity of a star from outside its sphere of influence (at any angle), it gains speed and then will exit the well again, ending up with the same speed as when it first entered.

This is not an object 'gaining speed' or getting 'thrown out'. It is an object on a hyperbolic trajectory.

No. It already has sufficient potential energy even before it enters the star's well. Its escape is inevitable. And its speed will end up the same as before it entered.

Last edited: Aug 17, 2017
19. ### DaveC426913Valued Senior Member

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Then what does it have to do with the thread topic?

A particle can certainly follow a hyperbolic trajectory around a black hole. That particle already has enough potential energy to exit the gravity well, it needs (nor gets) a boost.

Unless the trajectory intersects the event horizon. Then will never escape. It's potential energy will be absorbed by the BH.

20. ### Write4UValued Senior Member

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But that's what I have been saying.
With exception that an object even with the sun's gravitational field can be "knocked out" from that field.

Saturn is a perfect example. All the rings are comprised of things with similar mass orbiting at the same speed . But occasionally collisions take place , especially at the edge of the gravity well (the gravity horizon) .
This happens much more often inside out sun itself. We call it EM radiation escaping into space.

This phenomenon is not caused by gravity but by chaotic behaviors of extremely energetic bodies. The sun is a hydrogen bomb only kept in check by its own gravity, but particles do escape, some hit the earth, others may not hit anything or be pulled in by another massive gravitational field (such as a small BH) and just keep going into outer space. If it were not for the earth's magnetic shield and atmosphere, we'd all fry, when the sun "burps" with a solar flare.

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https://en.wikipedia.org/wiki/Solar_flare

Last edited: Aug 17, 2017
21. ### originTrump is the best argument against a democracy.Valued Senior Member

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What you said is this:
That is wrong. As long as the object doesn't hit the star regardless of the angle it will go on it's merry way and it will not gain speed.
That wrong that is not how gravity assist works.
Objects have not gained speed simply by the gravity of a flyby of the star after leaving the influence of the star.

These statements were wrong, it is curious that you are not admitting that. Either you still do not understand or you are simply unwilling to admit you were incorrect.

22. ### DaveC426913Valued Senior Member

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It isn't.
"Knocked" is a very different animal. To be knocked out of orbit requires a third body. Gravity of the central mass alone cannot boost anything.

Yes. Between multiple bodies.
Gravity of the central object alone cannot boost anything.

The rest of the post is drifting even farther from the thread topic.

23. ### The GodValued Senior Member

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Speed with reference to what?

Take for example if a comet enters and leaves earth gravity, then what will be it's comparative speed with respect to sun (and earth too) while entering and leaving ? See if your assertion in the quote agrees with your answers to this question.