Does e=mc2 violate thermodynamics?
- Thread starter Dustin Kesselberg
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Dustin said:
Okay, here's a simple example:
You look at a electron with very little energy (it's not moving around too much, maybe it's part of your arm), and you will find that its mass is very small.
You look at an electron with alot of energy, this energy could be coming from its increadible speed as it hurtles toward the earth, and you find that it's energy is much larger.
Both are electrons. But one has more mass because it has more energy.
And this doesn't only apply to kinetic energy either. Any form of increase to the energy of an object or system will increase it's mass in a dirrect way. Namely following the formula E-mc squared.
For instance, if you heat up a bot of water, with the lid on so that no water inside will escape, it's mass will increase as well. This is because the thermal energy increased, but you can also look at is as the average kinetic energy of each particle having increased.
If you climb a mountain you will actually have greater mass. This is because your potential energy will have increased.
Any normal scale that could detect the differance (yeah right) would actually show you as weighing less than it showed you at sea level, because you're further from the center of the earth, but your mass will have increased, even if to a miniscule extent.
Pragmatist
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Dustin said:
No, that is not what I am saying. Firstly you need to define "motion". Motion is a purely relative term. There is no way of defining some absolute motion because there is no absolute frame of reference. If an object is found in empty space, is it in motion or not? There is no way to tell in any absolute sense - the only motion we can speak about meaningfully is motion relative to us as observers. Therefore understanding that motion is relative (and therefore so is mass, energy and momentum) is key.
As I mentioned in another thread, all objects seem to have some sort of internal periodic motion associated with them. For example at the quantum level we talk about the wavelength of particles - this implies a periodic motion otherwise there wouldn't be a "wave" that we could meaningfully talk about the "wavelength" of!
But, I don't need to even start from that - all I need to do is define an abstract quality called "momentum". You have to let go of classical ideas first. Stop thinking of momentum as being a quantity of mass in motion, and start from the assumption that there is some totally unknown property that we will call "momentum". It is from there that you build the other definitions.
A photon, by definition, has energy but has no mass. A photon is always (relatively) in motion with respect to any observer - basic relativity.
I did define energy (look again).
Energy and mass are not "created". They are not substances as I said. They are properties we associate with an object depending on how we look at it (from a relative state of rest or motion). For example, if you look at a movie, frame by frame, you have a set of static pictures. A person in that movie appears to have no quality which we would call "life" in the individual static frames. But when we run the movie reel through a camera, the rapid motion of frames appears to us to be a "live" event, and the person is no longer a static, lifeless, object, but rather appears to be just like a living person. The illusion of continuity arises because the frames are relatively in motion with respect to us. Therefore with regard to a movie, we could define a person in a static frame as having "presence". But the person in the moving frame appears to have "life". The qualities we call "presence" and "life" are not objective physical entities, they are subjective impressions that depend on our state of motion relative to a movie frame.
In the same way, the qualities we call "mass" and "energy" are subjective qualities that appear depending on our state of motion relative to another body. The only fundamental property we require the body to have (in a physical sense) is a property called "momentum".
Like I said before, think about it. That goes for everybody!
CurtC said:
Not that it really matters much, but electricity is made up of streams of electrons. The reason they can be used for energy is because they all have the same electrical charge.
They repulse each other and are attacted to things with positive charges.
Also, moving electrical fields produce magnetic fields, and vice versa. I think...
Anyway, that's why you can use electricity to do work.
Electrons do have mass, but their mass will actually increase just based upon what sort of electrical potential builds up. Basically, electrical energy = mass (c)(c).
To put it another way, the mass of a battery will decrease as it runs down, even though it has the same amount of matter in it. The actual amount of that decrease is very very small though, becasue the energy isn't that large.
To make something's mass increase by 1kg you'd have to add
90,000,000,000,000,000 joules of energy. I think that's the right number.
But no matter what you put that energy into, it's mass will still be equal to 1kg. It will still cause that same gravitational field.
For instance, say you explode an atomic bomb inside a large sphere in space. the sphere is large enough and made of something strong enough that the whole explosion will be contained.
Question, will the gravitational field produced by the sphere decrease? After all you just converted some mass into energy, right?
Answer: No. The "new" energy IS mass, and whatever form it is now taking (increase in temperature for instance) it should still create a gravitational field equal to the one it had before.
I'm pretty sure that's right.
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Some clarification.
The point is that mass, not matter, is equivalent to energy. Typically matter is anything which has non-zero rest-mass. This rest-mass is a form of mass/energy, as are kinetic energy and potential energy. Indeed QM indicates that rest-mass itself is a form of potential energy, associated with the Higgs field.
That said, there is absolutely nothing wrong with saying that energy is conserved. Turning matter into energy does not create energy. It just changes the form of the energy from rest-mass to something else (usually kinetic energy). Note that kinetic energy has just as much mass as an equivalent amount of rest-mass.
So the total mass/energy of the Universe does not change. You just sometimes turn rest-mass into other forms of mass, such as kinetic energy.
You just have to remember that mass is energy, and energy is mass. Mass is not a form of energy, it is energy. There is no form of energy which does not have mass given by M=E/C^2, and there is no form of mass which does not have energy given by E=MC^2.
Dr. Stupid
The point is that mass, not matter, is equivalent to energy. Typically matter is anything which has non-zero rest-mass. This rest-mass is a form of mass/energy, as are kinetic energy and potential energy. Indeed QM indicates that rest-mass itself is a form of potential energy, associated with the Higgs field.
That said, there is absolutely nothing wrong with saying that energy is conserved. Turning matter into energy does not create energy. It just changes the form of the energy from rest-mass to something else (usually kinetic energy). Note that kinetic energy has just as much mass as an equivalent amount of rest-mass.
So the total mass/energy of the Universe does not change. You just sometimes turn rest-mass into other forms of mass, such as kinetic energy.
You just have to remember that mass is energy, and energy is mass. Mass is not a form of energy, it is energy. There is no form of energy which does not have mass given by M=E/C^2, and there is no form of mass which does not have energy given by E=MC^2.
Dr. Stupid
CurtC
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That's not a very good way of thinking about electricity. It's kind of what most kids learn in fifth grade, but beyond that it's not very helpful.Roboramma said:
Think of electricity instead as the intensity of the electric field. This field can cause electrons to move, but moving electrons themselves are not "electricity." You can have other forms, such as a propagating EM wave, which moves no electrons, and you can have a stream of protons moving due to the electric field, no electrons required. And the electric field is carried by photons, which was my point in the first place.
patnray said:
Yes, that question ("how is an atomic bomb small scale?") confused me in the context of chemical reactions also.
However, an atomic bomb IS small scale in discussion of conversion of matter to energy. Most of the particles that were there before an atomic reaction are still there afterward. Only a very tiny fraction of mc^2 is released. A one kiloton explosion is the energy equivalent of 47 mg of mass. The Hiroshima explosion was about 15 kT, or about 0.7 gm mass equivalent.
Badly Shaved Monkey
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Roboramma said:
Um...I think you've got a bit muddled there. A bettery doesn't contain energy (and therefore a little more mass) because it has been stuffed with electrons. That would give it an overall -ve electrical charge.
A battery contains energy (and therefore a little more mass) as electrical potential because electrical charges have been separated.
RussDill
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Dustin said:
There is a great thought experiment for this. Once many people realize that energy can be converted to mass, and vice versa, they make a thought experiment for a perpetual motion machine. At the bottom, put something that converts water into energy, just a little upstream from that, put a turbine that converts the kenetic energy of the water into energy as well. Beam the photons up to a receiver. The receiver converts the photons back into water, and the water travels downhill to the turbine.
You are getting the output of the turbine for free, right? Wrong. When you beam the photons up, they lose energy due to redshift. The amount of energy they lose is equal to the kenetic energy the water would have gained.
RussDill
Philosopher
Dustin said:
Within an atom bomb, each atom of uranium is not converted into energy. If it did, we'd be in really, really, big trouble. No, in an atom bomb, uranium is decayed via chain reaction (the atom is split). The post decayed state has significantly less energy, and also less mass, thus the conversion of mass to energy.
RussDill
Philosopher
CurtC said:
aye, and they are tricky buggers too, bouncing off any change in impedence and making reflections, etc.
egslim
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Because it is very much simplified, highschool physics contains lots of small errors. In the context in which they are presented they're usually correct, but when applied to other fields one gets into trouble.
Take for example the definition of energy as: "the ability to do work".
I would prefer this: "Energy is a mathematical quantity, existing in different forms. (For example kinetic energy, gravitational energy, heat and mass.) The sum of all these energies in a closed system is constant.
When one gets to modern physics, classical definitions lose their value and can actually cause a lot of confusion. Take for example "wave" and "particle". In quantum mechanics, every particle is a wave and every wave a particle. Which is impossible with the classic definitions, yet works perfect for quantum mechanical calculations.
This is what makes modern physics very abstract and thus a lot harder (if not impossible) to comprehend.
Note the difference between comprehension en the ability to describe.
Take for example the definition of energy as: "the ability to do work".
I would prefer this: "Energy is a mathematical quantity, existing in different forms. (For example kinetic energy, gravitational energy, heat and mass.) The sum of all these energies in a closed system is constant.
When one gets to modern physics, classical definitions lose their value and can actually cause a lot of confusion. Take for example "wave" and "particle". In quantum mechanics, every particle is a wave and every wave a particle. Which is impossible with the classic definitions, yet works perfect for quantum mechanical calculations.
This is what makes modern physics very abstract and thus a lot harder (if not impossible) to comprehend.
Note the difference between comprehension en the ability to describe.
Badly Shaved Monkey said:
I'm not quite sure what you mean... I didn't say it contained energy because it was stuffed with electrons.
All I said (maybe not in the clearest way) was that even though there may be the same amount of matter present from one moment to the next, the total mass of the battery is slightly less when it has less energy.
I'm pretty sure that's correct, regardless of the internal workings of batteries which I know very little about.
Badly Shaved Monkey
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Roboramma said:
OK, I can see what you were trying to say now.
I don't understand how you're looking at this.
The momentum of an object depends on the observer's state of motion, whereas its mass doesn't. At least, its rest mass doesn't, which seems to be the sort of mass you're talking about, because you say that photons are massless.
And the other sort of mass---namely, relativistic mass---is identical to energy, not complementary to it as you seem to be saying.
If by "fundamental" you mean "invariant" (i.e., independent of the observer's state of motion), then rest mass is and momentum and energy are not.