Bjarne on: Dark Energy, only an illusion.

[sol invictus]

Yes, quite simply. That's what I meant by time/space/matter all expanding together. Gravity increasing with mass according to the gravitational constant.

Sorry, I don't understand you. Suppose we go back to a time when the earth had 1/2 its current radius. What was its mass? And if Newton's "constant" changed, what was it at that time?

Right you are, and so are the Voyager probes. At the very least something is going on that we don't understand. If I were a betting man, I'd bet a dollar that the dark matter (sucking everything outwards) theory will look really silly in about 50 years.

Why do you think dark matter "sucks everything outwards"?

As for looking silly, no, certainly not. Our current models for cosmology are based on the work of thousands of very bright professionals using the best available data and the best available theories. Obviously the models may be wrong, but they certainly aren't "silly".

 

[Landrew]

Sorry, I don't understand you. Suppose we go back to a time when the earth had 1/2 its current radius. What was its mass? And if Newton's "constant" changed, what was it at that time?

Well, I can't do the math in my head, but I never said the relationship between mass and gravity would change. Not that I know, but it's just a different slant on looking at the evidence.

Why do you think dark matter "sucks everything outwards"?

That's the way I understand the explanation for the way the universe appears to be accelerating in its expansion.

As for looking silly, no, certainly not. Our current models for cosmology are based on the work of thousands of very bright professionals using the best available data and the best available theories. Obviously the models may be wrong, but they certainly aren't "silly".

Silly is just a subjective label. I happen to think the flat earth theory was a bit silly, and who's to say the dark matter theory might not be regarded as a bit silly some day?

 

[edd]

Dark matter is not dark energy.

Also, there's a difference between thinking a theory is silly given current evidence, and thinking it is silly given past evidence.

The idea of a flat Earth for example, is only really silly when you possess evidence that it isn't flat. If you'd lived your life in a small box you'd possibly have a hard time coming up with a reason why the outside world was not as flat as the bottom of the box in which you sat.

It may turn out that the idea of dark matter is later seen as 'silly' in the light of future evidence, but I would say it will never be seen as silly for us to believe it given our current evidence, and I think that's what sol is saying too.

 

[wollery]

Did you see the astronauts bounding along on the moon, and hitting a golf ball out of sight? The moon is 1/6 the mass of the earth, and has 1/6 the gravity.

Mass. Not size, mass. If the Earth stays the same mass and expands then in the past it had the same mass but a smaller radius. This means that the surface is closer to the centre of mass. The force of gravity is proportional to the mass and inversely proportional to the square of the distance, F=GMm/R², so if you have two identical objects (mass m) on two different planets with the same mass but different radii the one on the smaller planet will experience a stronger gravitational force.

This means that if the Earth is expanding at the same rate as space then its surface gravity must be dropping.

The only solution is to alter either the mass or the gravity, but this would have noticeable effects on distant objects, such as galaxies, because we'd be seeing them as they were when they had different gravitational constants or masses.

The Voyager probes have a 50-year battery and send to earth their position calculated by their relative position to the planets and stars. If they are farther away from earth than their speed should place them, you could either hypothesize that dark matter is pulling the probe away faster, or space itself has grown larger since it was launched in 1977. If there are other explanations, I'd like to read them.

But if our measuring devices are growing larger by the same proportion as the rest of space then we would not be able to measure the extra distance, because that "extra" distance would look like it was exactly the distance we expected.

Let me illustrate that.

Suppose that space expands by 10% every year and the probe travel 1 billion km in that time. Well, after 1 year it will actually have travelled 1.1 billion km due to the expansion of space. But during that same year our measuring devices have also expanded by 10%, or, to put it another way, our metre sticks are now 1.1 metre long. This means that the probe, which is actually 1.1 billion km away appears to be 1 billion km away, because our metre stick is also 10% longer. Hence we don't measure any expansion.

 

[phunk]

Did you see the astronauts bounding along on the moon, and hitting a golf ball out of sight? The moon is 1/6 the mass of the earth, and has 1/6 the gravity.

Actually the mass of the Moon is only a little over 1% of the mass of the Earth.

 

[Toke]

Actually the mass of the Moon is only a little over 1% of the mass of the Earth.

 

[edd]

He's right. 1.2% of the mass, 27% of the radius.

 

[phunk]

Gravity is proportional to mass but inversely proportional to distance squared. 1% of the mass means 1% of the gravity... but 1/4 of the radius means 16x the gravity... The result is around 16%, or 1/6.

 

[Toke]

Gravity is proportional to mass but inversely proportional to distance squared. 1% of the mass means 1% of the gravity... but 1/4 of the radius means 16x the gravity... The result is around 16%, or 1/6.

I see.
I had not considered how big the effect of radius were.
Thanks.

 

[Belz...]

Did you see the astronauts bounding along on the moon, and hitting a golf ball out of sight? The moon is 1/6 the mass of the earth, and has 1/6 the gravity.

Really ? The moon is 0.012 Earth masses, actually. Your calculation is a "little" off.

 

[Landrew]

Really ? The moon is 0.012 Earth masses, actually. Your calculation is a "little" off.

Well, I'm not an astrophysicist, so my calculation doesn't count. The point however, is that the smaller the body, the less the gravity. That's it.

 

[MRC_Hans]

Well, I'm not an astrophysicist, so my calculation doesn't count. The point however, is that the smaller the body, the less the gravity. That's it.

Excuse me, but the point seems to be that you are speaking without thinking. I think we already have one too many on this thread doing that, so please don't join Bjarne. If you want to know stuff like the relative masses of the moon and Earth, Google is your friend.

Hans

 

[sol invictus]

Well, I'm not an astrophysicist, so my calculation doesn't count. The point however, is that the smaller the body, the less the gravity. That's it.

But that isn't necessarily true, as people keep pointing out.

 

[Dancing David]

Gravity is proportional to mass but inversely proportional to distance squared. 1% of the mass means 1% of the gravity... but 1/4 of the radius means 16x the gravity... The result is around 16%, or 1/6.

Thanks.

 

[wollery]

Well, I'm not an astrophysicist, so my calculation doesn't count. The point however, is that the smaller the body, the less the gravity. That's it.

No, that isn't it.

It's a very naive assumption to simply equate size with gravity. I showed you why in my last post, gravity is proportional to mass, but inversely proportional to the square of the distance. This means that for a smaller body you are nearer the centre of mass, and the gravity is therefore stronger.

But don't just take my word for it, you can do calculations of your own.

The formula is F=GMm/R².

If you want to make it easier you can calculate the force on a 1kg test mass at the surface. Since it's 1kg it doesn't affect the maths, and can effectively be ignored. Alternatively you can calculate the acceleration due to gravity. Since F=ma, it follows that a=F/m and therefore that a=GM/R² (the mass of the test mass cancels out).

Some examples for you.

Earth

We can measure this directly.

a=9.81ms^-2
Sun

M=1.9891×10³⁰ kg
R=6.955×10⁸ m

a=GM/r²a=6.67259×10^-11 x 1.9891×10³⁰ / (6.955×10⁸)²a=1.3272×10²⁰ / 4.8372×10¹⁷a=274.4 ms^-2
Jupiter

M=1.8986×10²⁷ kg
R=71,492,000 m

a=24.7 ms^-2
Brown dwarf

M=1.5×10²⁹ kg
R=71,492,000 m

a=1958.3 ms^-2
So, the Sun is a hell of a lot more massive than Earth, but it's also has a much larger radius, so it's surface gravity is almost 30 times stronger. Yet a brown dwarf has, at most, about 8% of the Sun's mass, but due to its small radius has a surface gravity more than 7 times larger than the Sun. All of these numbers are readily available on the internet.

Size isn't everything.

 

[Belz...]

Well, I can't do the math in my head, but I never said the relationship between mass and gravity would change. Not that I know, but it's just a different slant on looking at the evidence.

Uh-huh.

Silly is just a subjective label.

Do you think that the theory of humours was silly ?

I happen to think the flat earth theory was a bit silly

That's because it's been almost 2500 years since it's been proven wrong.

 

[Belz...]

Well, I'm not an astrophysicist, so my calculation doesn't count. The point however, is that the smaller the body, the less the gravity. That's it.

It's not a calculation, I simply looked it up. You, however, simply made it up in order to fit with the 1/6th gravity.

And you're still wrong. The less MASSIVE the body, or the FURTHER away from the center, the less the gravity. So that's certainly not "it".

 

[Landrew]

My only point was that a smaller planet should have less gravity. If I'm wrong about that, then I really don't know much about physics and I'll withdraw my whole point.

 

[sol invictus]

My only point was that a smaller planet should have less gravity. If I'm wrong about that, then I really don't know much about physics and I'll withdraw my whole point.

You're wrong about that.

For example, Europa (radius 1600km) is significantly smaller than Callisto (radius 2400km), but Europa has stronger surface gravity.

It's ok to be wrong - now try to understand why you were wrong and learn from it (hint: read the posts in this thread).

 

[Belz...]

My only point was that a smaller planet should have less gravity. If I'm wrong about that, then I really don't know much about physics and I'll withdraw my whole point.

Ok, you were wrong about that, then. For the same mass, a smaller planet would have MORE gravity.