# Causal mechanism for gravity

Discussion in 'Alternative Theories' started by RJBeery, Apr 5, 2020.

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

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Roughly speaking, it's because the energy-momentum tensor associated with them isn't big enough to create a black hole. Put another way, there isn't enough energy in a small enough space (the energy density isn't high enough).

Last edited: Apr 25, 2020

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

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If you have issues with Baez's explanation, you could take them up with him, you know.

Energy has to come from somewhere to accelerate a mass. You can't create energy out of nothing (in a fixed reference frame).

But they aren't.

That word "relativity". What do you think it means? Relativity is all about frames of reference. In GR, gravity is an effect that is perceived as a result of being in an accelerated frame of reference; there is no gravitational "force" in GR.

You say there's a contradiction in the logic, somewhere, but you don't say where. What are you talking about?

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

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Derek.H.:

It is true that if you did this experiment you would see the "same amount of time contraction" (or time dilation, more accurately). But that wouldn't tell you anything special about the CMB because you could speed yourself up to 1000 km/hr (or whatever you like) relative to the CMB, launch your two spaceships from that state of motion, and observe exactly the same result.

That doesn't work, because their "clock rates" will depend on who is observing them. If the observer is on spaceship A, he will measure spaceship B's clock as running slower than his, but if he is on spaceship B, he will measure spaceship A's clock as running slower than his.

By talking about a "slower ship" and a "faster ship" you're assuming a particular frame of reference. In fact, you specified the frame of the CMB at the start. But talking about something forming a black hole just because it moves fast brings us back to the original question. As has already been explained, black holes don't form just because mass moves fast.

Particles have been clocked at well over 99.999999% of the speed of light.

7. ### Derek.H.Registered Member

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But it could be , if we assume some super future physics tech .
So...the question stands !
D.

Last edited by a moderator: Apr 25, 2020
8. ### James RJust this guy, you know?Staff Member

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Or if we assume that magical fairies can make it happen!

9. ### Derek.H.Registered Member

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Okay , it may be unrealistic , but it's the thought experiment that counts.
The point of the "Ships" example was to point out that moving the entire system one way or another , changes the relativistic time/mass measurements significantly , with only ONE vector (velocity/direction) yielding perfectly symmetrical amounts of distortion , regardless of the direction of ships . This vector is that of the space matrix-fabric , and will vary from location to location , everywhere in the extant universe .
*It is herein postulated that space itself flows in great currents throughout the heavens , creating astronomical oddities such as the "Great-Attractor" , and the "Quasar-Enigma" .
*As was said in "Total-Rekall" :
"...they're all connected !!" .
D.H.

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

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I just explained to you why you're wrong about that. Maybe go back and read it.

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The universe/space/time is expanding over large scales.
Over smaller planetary/stellar/galactic and galactic wall systems, the gravitational effects see such expansion overcome.
There will be a time in the far distant future, when once M31 and the MW, along with the galaxies that make up the wall, all merge into one, that the far distant galaxies will have moved beyond our observable horizon. Which in fact will mean that cosmologists [if any still exist] in that far distant future, will have no observational evidence for the expanding universe.

12. ### foghornValued Senior Member

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That's the way I sort of understand it as well. The mass/energy density in a small enough area ( Schwarzschild radius) doesn't appear.
Ignoring what I just said, the other ''drawback'' to a very small mass ''black hole'', would be how long it existed due to Hawking Radiation.

13. ### RJBeeryNatural PhilosopherValued Senior Member

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From these quotes, it looks like you believe that, under the correct conditions, a black hole would be created from stress-momentum energy (mass + kinetic) in the same way that a black hole can be created from stress-angular momentum energy (mass + rotational energy). Do you believe that?

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

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If you're asking me whether I believe that black holes can form when there's enough mass in a small enough space, the answer is yes. You can use the energy-momentum tensor to describe the arrangement and show, for instance, that the Schwarzschild solution is a valid solution of the Einstein equations.

15. ### exchemistValued Senior Member

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I suspect the issue is not the mass term but the kinetic energy term. From the simple logic of the arguments put forward earlier, it seems to me that k.e., being frame-dependent, cannot contribute to whatever is needed to form a black hole. But I freely admit I have no idea at all how this is handled in the maths of GR.

16. ### foghornValued Senior Member

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Yes, I must admit I'm abit puzzled by kinetic energy being a relative quantity. If something is not a black hole in one frame, it can't be a black hole in any frame.

17. ### exchemistValued Senior Member

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Quite. GR must reach that conclusion in its maths too, it seems to me.

It rather looks to me as if RJB's question really amounts to asking how that comes out of the maths - but without doing the maths. I'm not sure anyone here will be able to help with that.

18. ### RJBeeryNatural PhilosopherValued Senior Member

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That's the logical contradiction -- momentum is a relative term, but supposed black holes are an absolute feature. We cannot rely on some energy-momentum threshold to be crossed when that energy-momentum value changes with each observer.

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

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Forget GR for a moment. Energy isn't conserved in any accelerating frame of reference.

Jump in your car and accelerate down the road. From your reference frame sitting in the driver's seat, all of the houses you're driving past are somehow gaining kinetic energy from nowhere.

Look out! You're driving straight towards a brick wall. If you collide with it in your stationary car (the car is stationary in the driver's frame of reference), the wall's kinetic energy will cause all sorts of damage to your car, even though the wall didn't get that energy from any obvious source.

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

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Sure we can, as long as when we change frames some of the terms in the energy-momentum tensor increase while others decrease, in some appropriate way.

21. ### foghornValued Senior Member

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Alas, I think those chaps have moved on. Did you notice RJB's post with a link to this Baez page:
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_fast.html

22. ### RJBeeryNatural PhilosopherValued Senior Member

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If the conditions for a black hole are related to energy density, then the (apparent) volume of an object would have to decrease as its (apparent) momentum has decreased, due to a change in observer, in order to do what you're suggesting. Do you believe this is what occurs in general relativity?

23. ### exchemistValued Senior Member

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Yes. In fact I found where Baez got it from - someone called Gibbs, here: http://sasuke.econ.hc.keio.ac.jp/~ken/physics-faq/black_fast.html

There's some extra text in this version which helps:

"In fact objects do not have any increased tendency to form black holes due to their extra energy of motion. In a frame of reference stationary with respect to the object, it has only rest mass energy and will not form a black hole unless it's rest mass is sufficient. If it is not a black hole in one reference frame then it is not a black hole in any reference frame."

However, like the version RJB quoted, it doesn't go into the derivation of the conditions for black hole formation in enough detail to show exactly why kinetic energy does not contribute, presumably due to the hairiness of the relevant maths.

foghorn likes this.