Is evolution enough?

[woolyhead]

Exactly the right amount of dark energy for the total energy in the universe to be zero? Does this mean that dark energy is negative energy? How is negative energy defined? Energy used to be defined in terms of the work it can do so how is negative energy defined...in terms of the work it can't do maybe? Or is there such a thing as negative work, maybe? I think I may have some of that in me...the harder I work on tidying up my bedroom the worse it gets.

Edited August 10, 2015 by woolyhead

[unbeliever]

Exactly the right amount of dark energy for the total energy in the universe to be zero? Does this mean that dark energy is negative energy? How is negative energy defined? Energy used to be defined in terms of the work it can do so how is negative energy defined...in terms of the work it can't do maybe? Or is there such a thing as negative work, maybe? I think I may have some of that in me...the harder I work on tidying up my bedroom the worse it gets.

 

Everything but gravitational potential is positive.

Dark energy is a kind of repulsive gravity which kicks in a very long range, so it goes back to being positive.

[woolyhead]

You switched from talking about energy to talking about potential. Are they the same? So shall I take it that two bodies have between them and in total, greater gravitational energy when they are further apart than when they are closer? What about the strong nuclear force having negative potential at some distance? Is that showing negative energy/potential? When you mention potential do you suggest that there is no absolute energy, only relative energy? If so what is it relative to?

Edited August 13, 2015 by woolyhead

[unbeliever]

You switched from talking about energy to talking about potential. Are they the same? So shall I take it that two bodies have between them and in total, greater gravitational energy when they are further apart than when they are closer? What about the strong nuclear force having negative potential at some distance? Is that showing negative energy/potential? When you mention potential do you suggest that there is no absolute energy, only relative energy? If so what is it relative to?

 

Yes, sorry for the ambiguity. I should have said gravitational potential energy.

 

Gravitational potential energy is given by -GMm/x

where G is a constant describing the fundamental strength of the gravitational force, M is the mass of the source of the gravity, m is the mass of the subject of the gravity and x is the distance apart. So it's a small negative number at large distances and a larger negative number at close distances.

 

Mathematically, it represents the work done to bring the mass m close to a source of gravity M from an infinite distance away. It's negative because in fact less than no work is required to achieve this, you actually gain energy from it. At infinite distance the gravitational potential energy is zero, so it is absolute rather than relative.

 

Gravity loses strength gradually with distance. So does electromagnetism.

The strong nuclear force loses strength extremely quickly with distance, but the same basic rules apply.

 

If you add up the masses of all the protons and neutrons in an atomic nucleus (except hydrogen) you'll get a number greater than the mass of the nucleus itself. The whole is less than the sum of its parts. Thats because the energy (and therefore via E=mc2 the mass) of the nucleus is reduced by the potential energy of the strong force binding the protons and neutrons together.

In order to separate the bound neutrons and protons, you have to supply enough energy to bring the total mass up to the mass of neutrons and protons and that allows them to split up.

 

The same would be true of for example the earth-moon bound system. The moon is bound to the earth by gravity in much the same way as neutrons and protons are bound together in a nucleus. In order to separate the two, you would have to provide positive energy (e.g. strap a big rocket to the moon). Once you have provided enough energy that it matches or exceeds the negative gravitational potential energy of the moon-earth bound system (so that the total energy involved goes to zero), they're free to move separately.

 

In the same way as the strong force reduces the mass/energy of a nucleus of bound protons and neutrons, gravity reduces the mass/energy of the universe. If you were somehow to stand outside the universe and measure it's mass you would find that in this case the energy of the gravity holding to together had reduced its mass/energy not only below the mass of the constituents, but all the way down to zero.

 

In the last ~15 years astronomy technology has managed to make a very convincing measurement of the total mass/energy of the universe (obviously from the inside) and the answer they get is compellingly consistent with zero.

Edited August 14, 2015 by unbeliever

[woolyhead]

Yes, sorry for the ambiguity. I should have said gravitational potential energy.

 

Gravitational potential energy is given by -GMm/x

where G is a constant describing the fundamental strength of the gravitational force, M is the mass of the source of the gravity, m is the mass of the subject of the gravity and x is the distance apart. So it's a small negative number at large distances and a larger negative number at close distances.

 

Mathematically, it represents the work done to bring the mass m close to a source of gravity M from an infinite distance away. It's negative because in fact less than no work is required to achieve this, you actually gain energy from it. At infinite distance the gravitational potential energy is zero, so it is absolute rather than relative.

 

Gravity loses strength gradually with distance. So does electromagnetism.

The strong nuclear force loses strength extremely quickly with distance, but the same basic rules apply.

 

If you add up the masses of all the protons and neutrons in an atomic nucleus (except hydrogen) you'll get a number greater than the mass of the nucleus itself. The whole is less than the sum of its parts. Thats because the energy (and therefore via E=mc2 the mass) of the nucleus is reduced by the potential energy of the strong force binding the protons and neutrons together.

In order to separate the bound neutrons and protons, you have to supply enough energy to bring the total mass up to the mass of neutrons and protons and that allows them to split up.

 

The same would be true of for example the earth-moon bound system. The moon is bound to the earth by gravity in much the same way as neutrons and protons are bound together in a nucleus. In order to separate the two, you would have to provide positive energy (e.g. strap a big rocket to the moon). Once you have provided enough energy that it matches or exceeds the negative gravitational potential energy of the moon-earth bound system (so that the total energy involved goes to zero), they're free to move separately.

 

In the same way as the strong force reduces the mass/energy of a nucleus of bound protons and neutrons, gravity reduces the mass/energy of the universe. If you were somehow to stand outside the universe and measure it's mass you would find that in this case the energy of the gravity holding to together had reduced its mass/energy not only below the mass of the constituents, but all the way down to zero.

 

In the last ~15 years astronomy technology has managed to make a very convincing measurement of the total mass/energy of the universe (obviously from the inside) and the answer they get is compellingly consistent with zero.

That's very interesting. Thank you, but why is the gravitational energy negative? That fact is a human concept, not a law of nature, isn't it? So why did someone make it look negative? Was it just to balance the books and make the total energy in the universe look like zero? And when you say "you" gain energy by bringing two massive bodies closer, do you refer to the system consisting of only these two bodies? Taking the positive convention for the numbers involved it seems to me that you end up with less energy ie a bigger negative amount. Most people would call that less of something, except for debt. But that's probably only semantics.

Edited August 14, 2015 by woolyhead

[unbeliever]

That's very interesting. Thank you, but why is the gravitational energy negative? That fact is a human concept, not a law of nature, isn't it? So why did someone make it look negative? Was it just to balance the books and make the total energy in the universe look like zero?

 

No. For one thing, it makes the maths work. And what makes the maths work is usually the truth.

It makes sense that there should be more (in terms of magnitude) gravitational potential energy when you're close to a heavy thing, like the earth, than when you're far away. If you're an infinite distance away, there would be no gravity and therefore no gravitational potential energy.

If you bring an object closer to the earth, e.g. drop something from a great height, there's an almighty crash when it hits the ground and a release of kinetic and thermal energy. But in doing so, you've brought it to where the gravity is stronger, so the gravitational energy must be greater in magnitude, but you've also clearly gained thermal and kinetic energy. The only way that can make sense is if one of the energies has a different sign to the other.

I suppose it is convention that gravitation energy is negative and mass etc are positive, you could swap it over and the maths would still work and it would still make sense, but you have to give them opposite signs.

 

It's not just gravity of course but all attractive forces. As I said before, the attractive strong force is negative energy in a nucleus and you have to deduct it from the mass of the nucleus to get the answer right. But the repulsion between the protons (as they all have the same positive charge) adds a little bit of mass back. Not as much because the strong force is much stronger than the electromagnetic force over such short distances, but it is a real effect that you can measure. So repulsive forces are positive energy.

 

So all attractive interactions via forces show up as negative energy and all repulsive ones show up as positive energy. Since gravity is always attractive by nature, its interactions show up as negative energy.

[woolyhead]

Thanks M&NIC. What a fascinating subject this is. There's just one thing about what you last said...making attractive forces negative makes the maths work out. That's what I suspected, ie it also therefore makes the energy in the universe zero, so long as the mass energy is regarded as being positive energy, doesn't it? And do you know why people say that the vacuum has energy? Is that zero also? In his book the Road To Reality Roger Penrose puts forward the idea that although quantum theory's maths works out well, there is no proper explanation as to why the maths suddenly switches from being linear during the U process to being abrupt in the R process. He goes on to propose a mechanism for this, saying that if two tiny masses were driven further apart by the wave function as it reaches them, this could account for the sudden collapse of the wave function and thereby give a mechanism for why it happens. It occurred to me that if the wave function contains energy which then drives the masses further apart then they must have absorbed this energy and therefore further apart means they have greater energy, not less. What do you reckon?

Edited August 15, 2015 by woolyhead

[unbeliever]

Thanks M&NIC. What a fascinating subject this is. There's just one thing about what you last said...making attractive forces negative makes the maths work out. That's what I suspected, ie it also therefore makes the energy in the universe zero, so long as the mass energy is regarded as being positive energy, doesn't it? And do you know why people say that the vacuum has energy? Is that zero also? In his book the Road To Reality Roger Penrose puts forward the idea that although quantum theory's maths works out well, there is no proper explanation as to why the maths suddenly switches from being linear during the U process to being abrupt in the R process. He goes on to propose a mechanism for this, saying that if two tiny masses were driven further apart by the wave function as it reaches them, this could account for the sudden collapse of the wave function and thereby give a mechanism for why it happens. It occurred to me that if the wave function contains energy which then drives the masses further apart then they must have absorbed this energy and therefore further apart means they have greater energy, not less. What do you reckon?

 

I'm afraid I'm not familiar with the more specialised ideas you refer to. I'll have a read though. Thanks for the recommendation.

 

Vacuum energy also adds up to zero. 2 equal and opposite particle are created and then annihilate with each other.

 

When I say the maths adds up, I mean the maths of every part of observed physics, not just the idea that the total energy of the universe is zero.

All sorts of things wouldn't work without the fact that attractive forces are associated with negative energy. If you flip the sign on gravitational potential energy, without also flipping the sign on mass, kinetic energy, thermal energy, etc then all the basic equations that you can easily test are simply wrong, so you can't do it.

I'm talking really basic stuff here. Like for example if an object is in the air and you drop it, if you reverse the sign of the energy of gravitational potential energy, your equations would predict that it would fly off into space. You really can't have that.

[woolyhead]

Interesting but weird. Not in line with what we were told at school and not what we see around us in the world, I would have thought. Although come to think of it what experience did we have of gravitational energy? The kinetic energy acquired by a mass when falling to earth due to gravity isn't gravitational energy is it? So its kinetic energy increases and its gravitational energy, although of negative sign, also increases. Does the one energy balance the other, giving no nett increase in energy? When you say gravitational energy increases I assume you mean the modulus increases.

[unbeliever]

Interesting but weird. Not in line with what we were told at school and not what we see around us in the world, I would have thought. Although come to think of it what experience did we have of gravitational energy? The kinetic energy acquired by a mass when falling to earth due to gravity isn't gravitational energy is it? So its kinetic energy increases and its gravitational energy, although of negative sign, also increases. Does the one energy balance the other, giving no nett increase in energy? When you say gravitational energy increases I assume you mean the modulus increases.

 

You're right on the modulus.

Energy is an absolutely conserved quantity. It can only be exchanged.

Since gravitational potential energy is negative, when you have an exchange with say kinetic energy, the modulus of both energies increases exactly the same amount.