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mattbrowne's avatar

Astrophysics question: Are zero braking distance collisions and infinitive forces possible?

Asked by mattbrowne (31449 points ) June 21st, 2012

Whenever we hit the brakes while driving, it will take some time and distance until we come to a stop. Even during a head-on collision the crumple zones lead to a small, but non-zero braking distance. The same applies when we trip and hit our head on a hardwood floor. Even when two metal spheres collide, their electron clouds prevent an instantaneous stop. The braking distance is never zero.

Now consider two “small” neutron stars with their combined mass not being heavy enough to form a black hole.

Suppose they are on a collision course. It’s rare, but it can happen (on a cosmic time-scale).

When the collision occurs, there are no electron clouds to act as a buffer, no electromagnetic forces that cause a non-zero braking distance. All there is are neutrons, tightly packed. Even the nuclear forces are powerfully attractive, not repellent.

Would this lead to an instantaneous stop of both stars?
Is there an infinite force and infinite deceleration involved?
Would the collision destroy both neutron stars creating numerous smaller fragments?
Or would a single new neutron star with the combined mass be formed?

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20 Answers

Response moderated (Off-Topic)
ragingloli's avatar

No, it is not.
F=m*a, where a is acceleration expressed in m/s/s. zero braking distance also means zero deceleration time. deceleration is negative acceleration. So zero deceleration time turns it into m/s/0. Division by zero.

mattbrowne's avatar

Yes, division by zero, therefore I mentioned the infinite force. But that’s hard to imagine.

ragingloli's avatar

And that is why it is impossible. Infinite force and by necessity infinite energy does not exist in the real world. That is also why reaching the speed of light for any object with mass is impossible.

mattbrowne's avatar

So what happens the instant the two stars “touch” each other? You can’t squeeze a bunch of neutrons.

cockswain's avatar

Here’s a link that shows loosely what a computer simulation suggests.

This link suggests they become a black hole.

mattbrowne's avatar

@cockswain – Thanks for the link. Yes, I’ve heard about the gamma ray bursts that are involved, but the magnetic fields part is puzzling. Neutrons carry no electric charge, so they are not influenced by the magnetic fields, right? And the article mentions the phase one or two milliseconds after the stars hit each other. What happens right at the moment of the collision? Or a millisecond before the stars hit each other? How does the deceleration work in such a case?

cockswain's avatar

I’d be surprised if you can get the answer on fluther, but it’s an intriguing one. I’m also puzzled as to why non-charged particles generate a super-magnetic field, yet their simulation suggests that’s what occurs. Clearly we’re lacking something fundamental in our understanding of the physics that limits our ability to make the relevant assumptions. But I would like to know too. I’m guessing that our common understanding of the macro principles of kinematic equations are being influenced by quantum effects in which we (we being you and I) aren’t fluent.

Maybe you should contact the author’s of the program that found the magnetic field is generated. I bet they love to talk about this stuff.

ragingloli's avatar

Also, are you sure that neutron stars can not be compressed? If that were true, they could not collapse into black holes.

mattbrowne's avatar

@ragingloli – They can be, yes, with sufficient mass, and then form a black hole. Therefore my premise: two “small” neutron stars.

Response moderated (Spam)
Bill1939's avatar

A “non-zero braking distance” would require the masses colliding were incompressible, in which case the kinetic energy would be instantly changed into other forms of energy, such as the magnetic field that @cockswain “Here’s” link describes. This might happen if the colliding objects were black holes. I imagine that the end product would be a larger black hole.

dabbler's avatar

Zero-braking distance also implies the neutron stars are not deformable which there’s no reason to think. ... They wouldn’t stay spherical, the neutrons at the surfaces where they first touch would experience sudden deceleration while the rest of neutrons in both of them come out of spherical formation and splash into each other.

Also I don’t think there’s any reason to think neutrons are uncompressible, are they? At least with enough force they’d disintegrate into component quarks and bosons and space junk.

augustlan's avatar

[mod says] This is our Question of the Day!

mattbrowne's avatar

What about the neutron degeneracy pressure and the Tolman-Oppenheimer-Volkoff limit?

dabbler's avatar

Well, we started out with “small” neutron stars so the TOV limit doesn’t apply.

From what I can tell neutron degeneracy pressure is a factor but won’t preclude some extraordinary stuff happening if you collide the two stars together. That pressure will assure the neutrons won’t get too dense so the original spheres will distort and flow and scatter, in fact this pressure will act a bit like repelling common charge though at much shorter distances. Even if the neutrons aren’t compressible this gives them some elasticity against each other.

And I still think if we’re talking very-high-speed-collision then the neutrons that collide head-on can decompose into their component parts.

mattbrowne's avatar

Thanks, @dabbler. Well, my thought was that the enormous force created by the high-speed collision could add to the existing neutron degeneracy pressure at the point of the collision and this might locally exceed the TOV limit.

hiphiphopflipflapflop's avatar

Various regions within a neutron star have been conjectured to be superfluid, which means it will not support shear stresses, and to my knowledge they are not infinitely incompressible either. They are not solid billiard balls. Note that the local density increases with decreasing distance from the center. This is consistent with the materials compressing further under hydrostatic stress.

Yetanotheruser's avatar

Hold on! I’m learning something here!

mattbrowne's avatar

Thanks, @hiphiphopflipflapflop for the explanation. The article also reminded me that while neutron stars are composed almost entirely of neutrons, some charged particles are present as well.

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