# Conservation of mass/energy

1. Jan 28, 2007

### Robert J. Grave

Isn't the big bang, the universe from nothing, a violation of the consevation of mass/energy since mass/energy can not be created or destroyed?

2. Jan 28, 2007

### Trail_Builder

i'm a super newbie to physics but i thought there wasnt a conservation of mass concept, because doesnt mass increase with speed (but only noticable at near speed of light speeds)?

also, i not sure on the whole big band thing, but i recall hearing something about the laws of physics breaking down at a singularity...

please,someone correct me if im wrong, since i only 16 and done school based physics.

3. Jan 28, 2007

### hellfire

May be you have heard about Alan Guth's quote that the universe might be "the ultimate free lunch". He proposed the idea that the energy of matter in the universe may be compensated by the potential energy of the gravitational field (which is negative for newtonian potentials). This, however, is a very speculative idea and cannot be rigorously formulated, basically due to one main problem: we still lack of a clear definition of energy of the gravitational field in general relativity. Eventually, a theory that describes the origins of the universe and accounts for the true degrees of freedom of gravitation (beyond its classical regime) might provide an answer to that question.

4. Jan 28, 2007

### EL

The Big Bang theory says that our universe once was in a very hot and dense state. What happened before that is open for speculation, since we do not know what physical laws hold under such extreme conditions. The quote "universe from nothing" mearly sounds like some creationism propaganda (i.e. something made up by people who have no clue what the Big Bang theory is about...)

5. Jan 28, 2007

### Robert J. Grave

The big bang therory, I understand, states that the universe began as a singularity at extreme tempatures and expanded from there. If it 'began' than it came into being. Isn't that something from nothing? I have a problem with the idea of something from nothing. It violates the basic physical experience of no creation and no destruction of mass/energy. The conservation of mass/energy.
The 'free lunch' is the idea that gravity exactly balances all other energy in the opposing direction making the net energy of the universe = to 0, an idea I feel may be correct. The expansion barrowed it's energy from gravity. Gravity would cause the universe to contract back to the same state as the 'big bang' after the stars burnt out and the expansion stopped leaving the only enegy gravity. And ,maybe at some critical density, the expansion would began again, resulting in an endless cycle of expansion followed by contraction and so on. That would end the problem of 'something from nothing' since it would be a continuance, not a beginning. Maybe that is what the universe is. Hawking's ' No Boundry Proposal ' has the collapse following the end of the expansion as part of his therory but does not propose the occilation of the universe, but others have. Who knows? I just like to see the 'laws' we observe not be violated in any therory. So, the universe 'begining' at the big bang sounds like a violation of the conservation of mass/energy. Can anyone explane why it is not?
In relativity the mass increase is the observed mass between frames of referance in relative mothion and is recipical. The rest mass remains constant. There has never been an observed violation of conservation nor a theoretical demonstration of it.

6. Jan 29, 2007

### EL

The singularity is a consequence of GR, which we know is not a correct description of gravity under such extreme conditions. For that we need a quantum gravity theory, something we're still looking for. The singularity is hence just an artefact of applying a physical theory (GR) in a place (early universe) where it does not hold very good.
For more about Big Bang see e.g. http://en.wikipedia.org/wiki/Big_Bang

7. Feb 1, 2007

### heusdens

Unfortunately however, it has slipped into physics/cosmology, and was introduced by Vilenkin as a 'viable model' (tunneling of the universe from "nothing").

I can't provide a link to the original paper by Vilenkin (which is: "Vilenkin, A. (1982), "Creation of Universes from Nothing", Physical Letters 117B"), it doesn't appear to exist on the internet.

Philosophically speaking, it makes no sense(*). I would argue also, it is not physics, since physics describes how the universe from one state of existence transforms into another state of existence, and always describes an existing universe (which is some form of matter in motion requiring space and time).

Further, one can not call this (the supposed 'becoming' of the universe "from nothing") a begin, since a begin (of something) is just comparing two existing states, one after another, and claim that in the first state that something did not exist, and did exist in the second state. Since there is no prior state, the whole thing becomes meaningless, and contains an impossibility. (see also the philosophical note below).

---------------------------

(*)
The reasoning behind it is that - as Hegel showed - being and non-being can not be treated as absolutely seperate notions, but always and only belong together in their (dialectical) union which is becoming (or ceasing to be). What we always see is that in any process, in which something (some configuration or state of matter) changes into something else, being and non-being belong to each other, since one changes into the other. Like for example a massive gas cloud of Hydrogen, due to gravitational collapse, become a new star, and the being of the star anulls it's non-being, and at the same time the being of the gas-cloud is anulled (since it is no longer a gas cloud but a star).

If you treat them (being and non-being) as totally seperate notions (having nothing to do with each other, that is: denying that they are effectively together and can not be seperated), then the becoming is anulled (made impossible). So a becoming or begin of something in or from nothing is therefore an impossibility. See: Incomprehensibility of the Beginning.

------------------------

And another note:

What do we mean by "existence"? It can be shown that there is no such thing as "absolute" existence, all existence is relative. What I mean is this: for me (an observer) an apple exist, and for the apple it is the case that I exist, since there can be objective relations between me and the apple (the apple can fall on my head/I can eat the apple).
But for the universe, it is obvious that there can not be anything outside and apart of it, the universe has no object outside of it, neither is there an object that has the universe outside and apart of it.
This therefore means that it is meaningless to state that the universe exists in the objective sense, since there can not be any objective relations for the universe.

[the only way in which it can be done, is by just postulating that there is a multiverse or higher order universe, of which the universe is part, but that is only shifting the problem to some higher realm, the issue remains effectively the same].

Last edited: Feb 1, 2007
8. Feb 1, 2007

### hellfire

heusdens, Vilenkin does not use the term "nothing" to mean nothing at all (as done in philosophy), but to describe a state of vanishing classical space. Take a look to this post and the link there.

Last edited: Feb 1, 2007
9. Feb 1, 2007

### heusdens

I can't access the original paper, so I don't know what he means with "nothing".

A "nothing" as a physical state would mean the absence of a topology, not even having points in space....

Well, I only reason that it would not make any sense to assume a universe "coming into existence" from such an absent state.

PS. Does anyone have this original paper by Vilenkin (1982).

If "nothing" realy means a (classical or whatever) space with a defined metric and with some fields in them existing, then such is not "nothing", and it would be confusing to call that state "nothing".

Last edited: Feb 1, 2007
10. Feb 1, 2007

### wxrocks

My 2 cents -- Really -- the first 100 to 1000 seconds of existence are pure theory/speculation anyway. Anyone who is trying to explain the universe before that time is selling something! Because we don't have a good understanding of the physics at those extreme conditions, there is no way to say what the universe was doing before that point. Was it nothing, was it something, was it whatever -- all these questions can be asked, but to say for sure it was one way or another is pretty much a guess.

11. Feb 2, 2007

### EL

I think you're way too pessimistic here. I would say we have very good control of what's happening from 1 second and on, when the BBN started.
From the electroweak epoch (~10^-12 seconds) we have fairly well motivated theories, even though the details are unknown. The inflation scenario is more or less speculation, even though there are good reasons that it must have occured. What happened before inflation is mainly speculations.
For a timeline see:
http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang

12. Feb 3, 2007

### heusdens

Inflation is past time eternal in the sense that 'once' it occurs, it can go on forever, which also leads to the conclusion, it didn't have to start at some time.

13. Feb 3, 2007

### EL

True, and that's why it has become common to set t=0 at the end of inflation, when all matter was created through the reheating. When we say the universe is 13,7 billion years old, we really mean inflation (or some other process creating all matter) ended 13,7 billion years ago.

14. Feb 4, 2007

### heusdens

But to go back to the topic (conservation of mass/energy) how does inflation treat these? I assume the conservation of mass/energy is still valid under inflation?

15. Feb 4, 2007

### hellfire

In general, the conservation equation that applies in cosmology follows from the divergenceless energy-momentum tensor of a perfect fluid that describes some of the components of the universe:

$$\partial_0 \rho + 3H(\rho + p) = 0$$

During inflation the energy density of the scalar field that drives inflation remains constant, but [itex]p = - \rho[/tex] and the equation is satisfied.

16. Feb 4, 2007

### heusdens

Yes, but if an inflating region keeps it energy density while growing exponentially, this means that there is going to be more energy.

17. Feb 5, 2007

### hellfire

This is correct, but as mentioned in general relativity the conservation law applies for the energy-momentum tensor but not for always for its 00 component. There are very long old threads about this topic of energy conservation in GR here in PF.

Last edited: Feb 5, 2007
18. Feb 5, 2007

### EL

I've never really got why the concept of energy conservation is that important. After all, "energy" is a quantity we define in such a way it will be conserved!
For example:
We first define energy of an object to be mv^2/2.
Then we discover this quanitity isn't really conserved when the object is moving in a gravitational field. Hence we redifine energy by adding a gravtitional potential so that energy indeed will be conserved.
After being happy with that for a while, we notice friction, and once again redefine energy by including heat, etc.
In the same manner we go on including electromagnetic field energy, sound energy, chemical energy, and so on...
Now, when looking at GR, we notice that energy (= 00-copmponent) isn't a conserved quantity, but instead we have a conservation law including the full energy-momentum tensor, i.e. the formula hellfire showed.
Since we really do not know what physical quantities are conserved at the scales of the extremely early universe, we don't know how to define energy at that time.
In my eyes, the question is hence not if energy is conserved, but how to define it to make it a conserved quantity...

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