Law of Conservation of Energy.

[Qfwfq]

Ok I got one. Photons that are red-shifted due to the expansion of space, by definition E=hf, will loose energy. Where does it go?

I got another one and, before going into a long discussion of your one, :hihi: I'll set you thinking about mine.

 

I've got a little laser, it emits a very monochromatic beam so the photon energy [math]\small h\nu[/math]is quite well defined. I'm pointing it in a direction, and you are travelling along at a high velocity, let's say half c, in the same direction. What is the photon energy for you?

 

Now, as you and I are simultaneously two different observers, there's no before and after, it doesn't even make sense to ask where the energy "went", nor where it "came from". :xx:

 

:)

[Jay-qu]

Yeah, but it still boils down to the same thing, there has to be a difference in reference frames or something..

[Qfwfq]

Indeed, the value of energy depends on how one chooses references but these choices don't change the reality that is being represented.

 

According to a particle in the solar wind, it is sitting still and most of the universe is travelling at nearly c, imagine the kinetic energy! :)

[Jay-qu]

LOL good point! I think I will ask my physics teacher today, but he will most probably just scratch his head! :thumbs_up

[Farsight]

I rather think that energy can be neither created nor destroyed because it doesn't exist. Not really. It's just a property of something else. If you think of Potential Energy, there's no "energy" actually there. You might have a suspended mass or a compressed spring, but that's what you've got. You can't see the "energy", or touch it, or smell it, or weigh it. If you think of Kinetic Energy, again there's no "energy" actually there. All you've got is some mass moving at some velocity. Maybe one way to think about this is to think of another property, the property of color. Can you think of a way to increase or decrease the amount of color in the world?

[InfiniteNow]

Popular,

 

I think I understand the point you are making, however, it's the exact opposite. It's not that there is no energy, it's that EVERYTHING is energy... mass, color, velocity...

 

As for increasing color in the world, I like art. :shrug:

[GAHD]

Asking where red-shifted energy goes is kinda like asking where the energy lost to friction (heat) goes: I'd ASSume into the surrounding area, as it is captured by various feilds.

 

What about polarity? If light goes through a feild where it's polarity is changed, where did the energy to change it's polarity go? One polarity isn't more energetic than another...

[Jay-qu]

Friction losses go into the vibrations of atoms and then into infrared radiation that spreads through the surroundings.. I dont think it is quite the same.

[Erasmus00]

Yeah, but it still boils down to the same thing, there has to be a difference in reference frames or something..

 

Indeed. Consider this: energy is one component of a 4-vector in relativity. When you change reference frames by going to a moving one you mix up all the components of a 4 vector (just as when you change reference frames by rotation you mix up all the 3 vector components. When you rotate, you don't expect the x component of the vector to be the same).

 

Now, while we can't expect the components of the 4 vector to be the same after a boost, we do expect "lengths" of 4 vectors. (Just as rotations might change the components of all normal vectors, but the lengths have to say the same.). Now, we may ask, is the magnitude of the 4-vector the same? Well, from the formula:

 

[math] E^2-p^2c^2 =M_0^2c^4 [/math]

 

We see that the magnitude of the energy/momentum 4-vector is the rest mass. Since rest mass is obviously the same in all frames, this works.

-Will

[Jay-qu]

so from that equation, the momentum changes if the energy changes to keep rest mass constant?

[Erasmus00]

so from that equation, the momentum changes if the energy changes to keep rest mass constant?

 

Yes, exactly.

-Will

[Jay-qu]

so what does it mean to say that the momentum of a photon has decreased?

[learnin to learn]

Will I think I am following what you said, but could you clarify what vectors are for me. (just started studying physics)

[Erasmus00]

so what does it mean to say that the momentum of a photon has decreased?

 

Electromagnetic waves (and hence, photons) carry momentum. We can work out how to define this momentum using Maxwell's equations (and what is known as Poynting's theorem).

 

Although (perhaps a bit circular) we can pluck the end result right from the formula I've used above. For a photon, rest mass is 0, so:

 

[math]E^2=p^2c^2 [/math]

 

or [math] p=E/c [/math]

 

Will I think I am following what you said, but could you clarify what vectors are for me. (just started studying physics)

 

Vectors are simply things that have both a quantity and a direction. For example, to tell me a velocity in a coordinate systems you have to give me three components (the velocity along x, along y and along z).

 

In a four dimensional space (like those you have to deal with in relativity, three spatial dimensions, plus time) you need four components to specify a vector.

-Will

[HIENVN]

Could you expand on this "internal" vs "external" thing? Reminds me of Shingeo Shingo.

 

Bill

Your right question spent me a long time to have a right answer to you.

Internal energy and external energy are exclusive terms of an “Improved Classical Mechanics,” which is a development of Newton’s first law (principle of inertia). The “Improved Classical Mechanics” display a single secret of the universe that unified field theory need this secret to establish.

Quantum theory should be a theory that study external energy of atoms (energy that emit/ absorb from electrons) and relativity theory should be a theory study internal energy of atoms (energy of nuclei). Although scientists around the world knew a relationship between these internal and external energy, and these scientists may misunderstand this relationship; therefore, this is a reason that the recent scientists can not discover the existence of gravitational waves on the earth.

In another way, “Improved Classical Mechanics” will study the interaction between nuclei at a distance that can not be solved by quantum and relativity theory.

I think my above expansion on “internal” vs “external” thing just is a summary, because a full explanation will need my more time to complete it. I wish will invite you to some my new posts that I mind will dealt to these “internal” vs “external” thing.

[LJP07]

The Law of Conservation of Energy states that energy cannot be created or destroyed, only changed from one form into another.

 

So when the Big Bang occurred, it was initially potential energy?

[Erasmus00]

The Law of Conservation of Energy states that energy cannot be created or destroyed, only changed from one form into another.

 

So when the Big Bang occurred, it was initially potential energy?

 

The thing to understand is that the law of conservation of energy is really a physical statement that the equations of physical laws are symmetric in time. This is a fancy way of saying that the physical laws don't change with time, they were the same yesterday as today.

 

If the physical laws were changing, we wouldn't expect energy to be conserved! Now, the bigbang is a very special event. All the physical laws more or less started at the big bang, as such, the laws very much changed, and there isn't any expectation of energy conservation.

-Will