Is Brian Cox right to claim that Gravity is a strong force for large masses?

cdux

I watched a program of his in which it was claimed that since mass bends space in accordance to General Relativity, then in the case of very large stars it becomes a strong force to the point of being able to crush a star to a single nucleus (Neutron Stars) or less (Black Holes).

His argument is that Gravity is a force that scales and that it is not simply a matter of adding individual components and hence to claim it's weak, but that since space is bent in those areas, then gravity as a fundamental force of nature becomes stronger.

Now, I wonder not only about the claim's accuracy, but also if it's only a matter of interpretation and nobody is really wrong or right, as long as the discussion is framed properly.

PeterDonis

This is not a very good description of a neutron star. A neutron star is not "a single nucleus"; it's much larger than a nucleus, both in terms of mass (a typical neutron star has a mass somewhat larger than the Sun) and in terms of size (a typical neutron star has a diameter of tens to hundreds of kilometers). Also, a neutron star is all neutrons; an atomic nucleus is a mixture of neutrons and protons. The only real similarity between a neutron star and an atomic nucleus is that both have approximately the same density.

This depends on how you define "stronger". A very compact object like a neutron star has a much larger "acceleration due to gravity" at its surface than an ordinary star like the Sun (more than a billion times larger). But that's just because the same mass (approximately) is packed into a much smaller volume; it isn't due to any change in, for example, Newton's gravitational constant, G, which is the closest thing we have, classically speaking, to a measure of the strength of gravity "as a fundamental force of nature".

Even inside a black hole, G is the same, so gravity as a fundamental force is the same. Spacetime curvature becomes very strong as you get close to the singularity at the center of the hole, but that's just because the object that collapsed to form the hole left behind strong curvature; it's not due to any change in the "fundamental force" itself. At least, that's how I see it.

There is one possible thing he could mean that is true: when he says that gravity scales and that it is not simply a matter of adding individual components, he could mean that gravity is nonlinear; that is, if you have multiple gravitating objects, you can't determine the total field due to all of them by just adding together the individual fields of each object taken in isolation. However, if that's what he means, he's not making it very clear, IMO.

It looks to me like the claim is based on using vague terminology and not looking very closely at the actual details of the physics.

phinds

Never, ever, ever, ever, EVER take anything about science seriously when you see it on TV. I have seen pretty much every modern scientist who is well known in the science community appear on TV and make some outrageously stupid statement that in most cases I'm SURE they know better. I think it's part of their contract that they HAVE to dumb it down, although possibly sometimes it's just because of sloppy terminology.

Science editors for these program seem to be either non-existent or morons.

I should add that there is one exception to this and that's Neil deGrasse Tyson. I can't remember ever having heard him say anything stupid.

George Jones

Which program?

DiracPool

Yeah, do you have a link or name of the show? I'd like to see it in context.

D H

Apparently this one: http://www.bbc.co.uk/programmes/b00zv39p. In one of the clips, Cox starts talking about how "gravity scales" just before the clip ends.

I'm not a big fan of Brian Cox. Instead of explaining physics he tends to mystify it.

Naty1

I'd say this part is pretty good

if a he adds something like "as the pressure of thermonuclear reactions which oppose gravity begin to run out of fuel...'

I get bored with Cox's shows because there are long winded visuals which explain little...seems like a cool guy though. [Probably gets more dates than the guys on BIG BANG THEORY [LOL].

If Cox tried to explain to the general public something like "....a neutron star is all neutrons" because electrons are forced into the nucleus where degeneracy pressure now oppose further collapse.."
eyes of viewers enjoying a beer would glaze over even more!!!

All these shows do have one benefit: if something is discussed which seems interesting, a different concept you haven't heard about previously, these forums and Wikipedia are a convenient place to follow up and get a more factual, detailed understanding.

mfb

All massive objects are nearly neutral in terms of their electric charge, and completely neutral in terms of their color charge. They cannot be neutral in terms of their mass, however. If you add more stuff to it, the total electric charge stays nearly zero, while the mass increases.
In that way, gravitational force scales with mass (it is proportional to mass, unless you consider black holes), while the other forces do not scale that way in realistic setups.

schaefera

"Excuuuuse me?!" said the superposition principle. Of course you can!

haushofer

The superposition only holds for linear differential equations like the Poisson equation, Schrödinger equation or the Maxwell equations. For the Einstein equations of GR however you cannot apply the superposition principle due to their non-linear nature. This becomes apparent when you are bored and try to write out the Einstein equations in terms of the metric and its derivatives.

Physically the failing of the superposition principle is because gravitational waves carry energy, and thus self-interact.

mfb

Gravity is not a linear force - in general (in general relativity), you cannot. For weak fields, the nonlinear effects can be neglected and superposition is a good approximation.

Nugatory

Not just "bored", but "very bored"
Is that complete expansion online somewhere?

PeterDonis

The superposition principle only works for theories with a linear field equation. The Einstein Field Equation is not linear.

haushofer

I'm not sure, but you could try reading Einstein's original papers; it took some time for him to adopt differential geometry as we know it.

Or perhaps try a nerdy wallpapershop. I've always wanted to have six loop N=8 SUGRA wallpaper, but never found it.

DiracPool

I think the only guy's eyes who are glazed over is Brian himself, and it aint from drinking a beer.