First law of thermodynamics, mass/energy in the Universe

In summary, the first law of thermodynamics states that matter cannot be created or destroyed, and the second law states that entropy can only increase in our universe. This raises questions about how these laws can be reconciled with the Big Bang theory and whether we can ever determine the origins of mass and energy in the universe. While the law of conservation of energy does not apply to the universe as a whole, we can still ask where the energy in a localized patch of space-time came from, but this question does not apply to the universe as a whole. Additionally, the second law of thermodynamics does allow for isolated systems to increase in entropy, as demonstrated by the example of an ideal gas expanding to fill a larger volume.
  • #1
girts
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Ok, so just a quick question, first law of thermodynamics basically states that matter cannot be created nor destroyed aka it can only change states and turn into energy or vice versa, the second law states that, IIRC, for perfect isolated systems entropy remains constant but for our universe entropy can only increase not decrease, so given this information how can we reconcile these two laws with our current best theory about the origins of the universe which is the "Big Bang" theory?

Also given the fact that physical laws and our means of "seeing" in the past like EM radiation itself arose only after or with the big bang, will we ever be able with scientific experimental certainty tell how these laws go together with how the universe began, taking also into account the fact that we cannot determine the total size of the universe due to it's accelerated expansion?I have heard that typically we don't apply the first law to the universe, but if so does it help us much? because we know for a fact that the universe aka space-time comes together with a certain amount of mass/energy that we can observe within the universe so with or without the first law applied to cosmological scale do we have or will we ever have any certain knowledge of exactly where all this mass/energy came from?
 
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  • #2
girts said:
with or without the first law applied to cosmological scale do we have or will we ever have any certain knowledge of exactly where all this mass/energy came from?
The law of conservation of energy does not apply to the universe as a whole. It is a local conservation law. With no conservation law to use, the question of "where did it come from" does not arise. It does not have to come from anywhere -- it's not conserved.

If one restricts one's attention to a local patch of space-time, one can ask "where did did the energy in that patch it come from". But the answer is simple -- it was either already there or it crossed the boundary into our patch. Our model of the universe does not extend to a time before the universe existed. It does not cover a transition from nothing to something. So again, there is no problem.
 
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  • #3
girts said:
the second law states that, IIRC, for perfect isolated systems entropy remains constant but for our universe entropy can only increase not decrease

The entropy of an isolated system can increase. Suppose we have a cubical isolated container with volume of ##1m^3##, and containing an ideal gas that is for some reason all located in one half of the cube at the initial state. In a quite short time after the initial moment, the gas will be distributed to fill the whole cube evenly, while its temperature is the same as initially because expanding to vacuum does not consume energy and the temperature of an ideal gas depends only on the internal energy and number of moles. Now, as the gas is occupying a larger volume, its entropy has to definitely be larger than initially, because there's no temperature drop that would counteract the effect of the volume change on entropy.
 

What is the first law of thermodynamics?

The first law of thermodynamics states that energy can neither be created nor destroyed, but it can be transformed from one form to another.

How does the first law of thermodynamics relate to mass and energy in the Universe?

The first law of thermodynamics applies to the entire Universe, including both mass and energy. This means that the total amount of mass and energy in the Universe remains constant, but they can be converted into each other through various processes.

What are some examples of the first law of thermodynamics in action?

Some examples of the first law of thermodynamics include the conversion of chemical energy into heat and light in a fire, the conversion of food into energy in our bodies, and the conversion of nuclear energy into heat and light in the Sun.

Does the first law of thermodynamics have any exceptions?

No, the first law of thermodynamics is a fundamental law of physics and has no known exceptions. It has been extensively tested and confirmed through various experiments and observations.

How does the first law of thermodynamics relate to the concept of conservation of energy?

The first law of thermodynamics is essentially the scientific expression of the principle of conservation of energy. It states that the total energy in a closed system remains constant, which is another way of saying that energy is conserved.

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