# Can you destroy energy?

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#### charliebigspuds

##### Guest
Well I know the anwser is no, I just don't understand why.

I get that if you start a fire the energy is transformed into heat and light, but surely the heat will cool down so where has the energy gone then? And where does the light go to?

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#### theridane

##### Guest
Heat doesn't cool down, it just spreads out. The universe is a really cold and large place, so even if you create a source of heat, like a fire, the energy (heat) will spread out into the colder places (by convection and radiation). Because the space is so vast, its heat capacity is also enormous, and from a puny human being's point of view it looks like the heat just disappeared.

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#### yevaud

##### Guest
The total mass/energy of the universe is a constant, and while it moves (so to speak) between mass and energy (depending), it never actually changes. All of the mass and all of the energy would always total exactly the same as it was 13.7 billion years ago regardless of whatever form it was currently held in.

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#### nimbus

##### Guest
The final state of the universe as Yevaud describes is then entropy. It's not energy seeping out as sand between your fingers so much as a castle of cards crumbling: the cards are at their lowest energy state, flat on the ground but still there.

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#### neilsox

##### Guest
Energy to matter conversions have been rare the past 13 billion years, except matter absorbs energy and gets warmer and emits energy as it cools, usually infrared photons. Neil

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#### Gravity_Ray

##### Guest
yevaud":1tuoyq7x said:
The total mass/energy of the universe is a constant, and while it moves (so to speak) between mass and energy (depending), it never actually changes. All of the mass and all of the energy would always total exactly the same as it was 13.7 billion years ago regardless of whatever form it was currently held in.

I dont know... Your answer is technically correct, but seems off to me. Hear me out.

The Law of Conservation of Energy cannot be applied to the universe as a whole, because there is no way to count up all the energies inside of it (unless you know something the rest of us dont know). Energy depends on your reference frame. For instance, in physics you talk about gravitational potential energy. That energy depends on where you put the origin of your coordinate system. How much energy does this book have resting on the table? Zero, if your origin is the table, but m*g*h if your origin is the floor.

Kinetic energy also depends on reference frame. How much energy is in your car on the freeway? 1/2 m v^2 to a person standing on the side of the road, but zero if you are in the car, because then your reference frame is that of the car, and you and the car have the same velocity.

Because spacetime is curved by gravity, no reference frame can contain the whole thing. Therefore you can't apply conservation of energy because you can't determine what the total energy is. In General Relativity, the energy of the universe is undefined. Unless the universe is flat (has no curvature), in which case the total energy is zero. The universe is extremely close to being, and in fact might be flat.

So to the OP; The energy is first stored in the wood, then it moves to heat and light, it stays that way, until it is captured by another tree and turned back into energy in the wood (with a bit of loss due to inefficiency).

Well that’s what I think anyway.

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#### yevaud

##### Guest
The simple response is, "then where would the mass/energy of the universe obtain more mass/energy than it began with?" The answer is, simply, it cannot.

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#### Gravity_Ray

##### Guest
The simple response is, "then where would the mass/energy of the universe obtain more mass/energy than it began with?" The answer is, simply, it cannot.

Well simply put I dont know! And neither does anybody else. We dont even know "what" the Universe is?? Until we have a better handle on Dark Energy and Dark Matter, we are just guessing. I mean they are good scientific guesses, but guesses never the less.

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#### yevaud

##### Guest
Well, you see the problem? To consider the universe as anything but a closed system would violate Godel and several other very basic physical tenets, and allow for a wide range of events that appear to be forbidden (time travel for one).

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#### nimbus

##### Guest
For all practical intents and purposes, the answer to the OP is what's consistent with all empirical evidence so far. Conservation of mass/energy, and overall entropy. That answer'll probably be effectively true for all of the OP's life.

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#### darkmatter4brains

##### Guest
.... to make things even more interesting, on the small scale (in both size and time) the conservation of energy is essentially being violated all the time in vacuum fluctuations. The trick that allows it is the uncertainty principle between time and energy.

makes one wonder where those virtual pairs come from that are popping into and out of existence all the time ....

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#### yevaud

##### Guest
Well, in actuality, those random quantum fluctuations - the creation and annihilation of particle/anti-particle pairs - are considered to be important wrt the Conservation of Energy. They've been described as countless tiny additions or deletions from the total mass/energy of the universe, always keeping it the same. picture it as an infinite number of bank tellers making or withdrawing tiny, tiny amounts to/from your bank account, such that it always balances perfectly.

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#### darkmatter4brains

##### Guest
I've never heard of that before. In fact when the pairs become real, like in Hawking radiation, certain conditions must be met so that they don't violate any conservation laws. It's commonly stated in QM and QFT text books that vacuum fluctuations violate the conservation of energy, but it's okay due to the energy time uncertainty relation. So, I'm not sure where you heard that before??

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#### a_lost_packet_

##### Guest
darkmatter4brains":1kvk0jav said:
I've never heard of that before. In fact when the pairs become real, like in Hawking radiation, certain conditions must be met so that they don't violate any conservation laws. It's commonly stated in QM and QFT text books that vacuum fluctuations violate the conservation of energy, but it's okay due to the energy time uncertainty relation. So, I'm not sure where you heard that before??

The way I've read it described conversationally is that while vacuum energy would seemingly violate conservation of energy the energy "created" by it can not be converted for use. Otherwise, the entire Universe would be heated to absurd temperatures due to vacuum energy conversion.

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#### darkmatter4brains

##### Guest
a_lost_packet_":hf2mo2ln said:
darkmatter4brains":hf2mo2ln said:
I've never heard of that before. In fact when the pairs become real, like in Hawking radiation, certain conditions must be met so that they don't violate any conservation laws. It's commonly stated in QM and QFT text books that vacuum fluctuations violate the conservation of energy, but it's okay due to the energy time uncertainty relation. So, I'm not sure where you heard that before??

The way I've read it described conversationally is that while vacuum energy would seemingly violate conservation of energy the energy "created" by it can not be converted for use. Otherwise, the entire Universe would be heated to absurd temperatures due to vacuum energy conversion.

yep, that's part of what I meant by the violation being allowed ONLY within the uncertainty principle. And, also why I stated that when they become real, there are certain conditions that must be met.

I mean think about it , the conservation of energy was around for like centuries, getting along just fine, before the "invention/discovery" of virtual particles, so I'm not sure how the concept of virtual partciles can somehow be important, or vital, to conservation of energy.

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#### darkmatter4brains

##### Guest
Here, this is from Schutz's General Relativity textbook, in his chapter on the Hawking process:

"according to quantum field theory, ordinary space is filled with vacuum fluctations in electromagnetic fields, which consist of pairs of photons being produced at one event and recombining at another. Such pairs violate conservation of energy, but if they last less than delta_T = hbar/delta_E, where delta_E is the amount of the violation, they violate no physical law. Thus, in the large, energy conservation holds rigorously, while on the small scale, it is always being violated"

I figured Schutz can explain it better than me

My quantum mechanics texts and quantum field theory texts say things along similar lines.

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#### darkmatter4brains

##### Guest
Gravity_Ray":32jl8pqk said:
Because spacetime is curved by gravity, no reference frame can contain the whole thing. Therefore you can't apply conservation of energy because you can't determine what the total energy is. In General Relativity, the energy of the universe is undefined. Unless the universe is flat (has no curvature), in which case the total energy is zero. The universe is extremely close to being, and in fact might be flat.

I always thought this was interesting and it's also related to another common misconception - that the redshifted light we receive from distant galaxies comes about because those galaxies are receding away from us, i.e. its a doppler effect from relative velocities. This, in fact, is not technically correct. For the same reason you can't construct that "global" frame you mention above to define total energy, you cannot construct a global frame to talk about relative velocities either. Therefore, talking about a distant galaxy receding from us, or having a relative velocity with respect to us, has no real meaning. in General Relativity. The redshift we see is a cosmological redshift, which comes from the fact that the metric of spacetime is changing. The classic example is to picture two galaxies at rest with respect to each other in flat space, and have one emit some light towards the other. No relative velocity, thus no Doppler redshift. Now, some time after, while the light is in transit, expand the spacetime in between the galaxies (i.e. change the metric). Yep, you guessed it, the light will now be cosmologically redshifted from the changing metric.

Anyhow, it's just interesting how all these concepts get very subtle the further you get into them

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#### yevaud

##### Guest
Well, I've read of this in a number of texts on Cosmology and QM, as well as read it referenced by Hawking and Kaku, so they seem to believe this is so. I'd think they had to account for the virtual pair production/annihilation in terms of Thermodynamics on a grand scale.

And to contradict something mentioned, those virtual pairs can become real. I don't recollect the precise mechanism. Although certainly Hawking Radiation would be one such example.

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#### darkmatter4brains

##### Guest
yevaud":15477xbq said:
Well, I've read of this in a number of texts on Cosmology and QM, as well as read it referenced by Hawking and Kaku, so they seem to believe this is so. I'd think they had to account for the virtual pair production/annihilation in terms of Thermodynamics on a grand scale.

Can you reference, or even quote, just one? I'd be interested to see the context. I mean, I know they fit into the scheme of overall energy conservation, just like anything in the Universe has to ... but, I've never heard it mentioned that energy conservation actually depends on their existence. I'd be curious to learn more if that is indeed true.

yevaud":15477xbq said:
And to contradict something mentioned, those virtual pairs can become real. I don't recollect the precise mechanism. Although certainly Hawking Radiation would be one such example.

I don't think anybody said they couldn't become real?

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#### yevaud

##### Guest
The reference to virtual pairs becoming real was in response to the comment on vacuum fields. It seemed to imply that there was no mechanism for virtual pairs becoming real, so I thought I'd mention it.

I do recollect the (paraphrased) line about the "infinite number of bank tellers making tiny corrections" from Kaku.

Edit: ah, I think I recollect what Kaku/Hawking were getting at.

The universe is a closed system, so it's total mass/energy must remain the same. Yet there are some mechanisms that can take away energy from the system - Singularities coming immediately to mind. The occasional virtual particle pair made real by a chance interaction with a photon tends to correct this, and so mass/energy is conserved.

Dunno if my memory is perfect on that, but it fits.

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#### darkmatter4brains

##### Guest
yevaud":2x03p7u9 said:
The reference to virtual pairs becoming real was in response to the comment on vacuum fields. It seemed to imply that there was no mechanism for virtual pairs becoming real, so I thought I'd mention it.

I do recollect the (paraphrased) line about the "infinite number of bank tellers making tiny corrections" from Kaku.

Yeah, that sounds like Kaku. I can hear his goofy voice in my head right now :lol: But, doesn't he have like a 1000 books by now. Any idea which one you read that in? I've been meaning to read a kaku book - I've heard they're pretty good.

yevaud":2x03p7u9 said:
Edit: ah, I think I recollect what Kaku/Hawking were getting at.

The universe is a closed system, so it's total mass/energy must remain the same. Yet there are some mechanisms that can take away energy from the system - Singularities coming immediately to mind. The occasional virtual particle pair made real by a chance interaction with a photon tends to correct this, and so mass/energy is conserved.

Dunno if my memory is perfect on that, but it fits.

I wonder if you're thinking of Black Holes and the 2nd Law of Thermodynamics. Basically, if black holes had no entropy, which is what we thought for a while, they could violate the 2nd law by swallowing up mass. But, if you admit they have entropy, then an increase in its entropy can make up for the decrease from swallowing the mass. When completing the picture of black hole thermodynamics, virutal particles do a play a role, including giving the black hole a temperature. But, they really only play this role by becoming real particles. When the virtual particles become real near a black hole, the one entering the black hole has a negative energy, so from an outside observer it looks like the Black Hole emitted a (positive energy) particle and then lost mass as a result. So, from that regard, they do fit into the scheme of energy conservation for black holes.

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#### csmyth3025

##### Guest
This whole discussion is pretty much over my head. As I understand it, the notion that the matter/energy content of the universe is constant is one of the pillars on which our physics is based. I know that there have been many laboratory-scale experiments to validate this as it pertains to closed laboratory experiments. Is there evidence on a cosmological scale that supports this?

On the matter of Hawking radiation, is there some mechanism by which it's more likey that with a given matter-antimatter pair near the event horizon, the normal matter particle would be preferentially ejected and the antimatter particle preferentially fall past the event horizon? If this event has a 50/50 chance of happening either way, would ejecting the animatter component still contribute to the entropy of the black hole? I wonder about this because an ejected matter particle can be seen as "losing mass". An ejected antimatter particle will most likely encounter its matter counterpart and produce a flash of gamma rays whose energy will be equivalent to the rest mass/energy of both particles. In this way the black hole can be seen as "losing energy".

What puzzles me about this type of interaction is what happens on the other side of the event horizon. Within the event horizon doesn't the same thing happen - except that the black hole is "gaining mass" or "gaining energy"?

I don't know what a "negative energy" particle would be.

Chris

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#### stemcelltherapy

##### Guest
Energy can not be destroyed. It can transform one state to another state. The total amount of energy from the beginning of world will remain same at end of the world.

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#### csmyth3025

##### Guest
stemcelltherapy":3jt8z4id said:
Energy can not be destroyed. It can transform one state to another state. The total amount of energy from the beginning of world will remain same at end of the world.

I agree with your statement with the exception that I would substitute "mass/energy" for "energy" based on the equivalence principle.

I'm still perplexed by the mechanism by which Hawking radiation is viewed as a mechanism by which black holes loose mass/energy.

Near an event horizon one of the virtual particles in a particle/anti-particle pair falls into the black hole and one is radiated away, thus producing two real particles. One adds to the mass/energy of the black hole and one adds to the mass/energy of the world outside the event horizon. I'm guessing that the creation of these two real particles is balanced by a reduction in the presumed vacuum energy of "space".

Is there a process by which the virtual particle that falls into the black hole "disappears"?

Chris

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#### darkmatter4brains

##### Guest
csmyth3025":1s3kx6b4 said:
stemcelltherapy":1s3kx6b4 said:
Energy can not be destroyed. It can transform one state to another state. The total amount of energy from the beginning of world will remain same at end of the world.

I agree with your statement with the exception that I would substitute "mass/energy" for "energy" based on the equivalence principle.

I'm still perplexed by the mechanism by which Hawking radiation is viewed as a mechanism by which black holes loose mass/energy.

Near an event horizon one of the virtual particles in a particle/anti-particle pair falls into the black hole and one is radiated away, thus producing two real particles. One adds to the mass/energy of the black hole and one adds to the mass/energy of the world outside the event horizon. I'm guessing that the creation of these two real particles is balanced by a reduction in the presumed vacuum energy of "space".

Is there a process by which the virtual particle that falls into the black hole "disappears"?

Chris

the one that falls into the hole is actually a particle with negative energy ... so from the outside it appears the black hole loses mass.

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