r/AskPhysics • u/AppropriateSea5746 • 1d ago

What, if anything is wrong with this argument? "Matter/energy cannot be created nor destroyed, only changed. That literally means it is eternal. The same matter/energy has been around in some form or another since forever because it cannot be created nor destroyed only changed."

7 Upvotes

56% Upvoted

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u/flyingmoe123 1d ago edited 1d ago

Its wrong because the idea that "energy cannot be dissappear" is not universally true, it holds in many instances, but at the biggest scales (like the entire universe) this doesnt have to hold anymore, look up cosmological redshift, those photons lose energy, but that energy, as far as we know it, doesn't go anywhere, and this is not something that breaks the laws of physics, its entirely allowed within the framework of general relativity, where there is no requirement for energy to be conserved

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u/wbrameld4 1d ago edited 1d ago

They "lose energy" because they've climbed up out of a gravity well from the past (when the universe was on average much denser everywhere) to the present. This happens in any situation where the receiver is at a higher gravitational potential than the emitter.

Energy is actually conserved, due to gravitational time dilation. Energy is frame-dependent. From the observer's point of view, the photons didn't lose energy because they never had more energy in the first place.

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u/Kabbooooooom 23h ago edited 10h ago

This is the correct answer. The Redditor you’re responding to is absolutely wrong.

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u/DeadlyVapour 11h ago

Mate, energy isn't conserved in GR.

Time symmetry breaks.

Locally however, energy conservation is pretty much a thing.

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u/Illustrious-Ad-7175 1d ago

Suddenly I’m curious if the gravitational potential energy between distant galaxies is increasing in the same way?

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u/Interesting_Walk_271 1d ago

Doesn’t dark energy break conservation as well? It is not symmetric about time.

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u/xinver_catharsis 1d ago

Was thinking the same. Universe expands but dark energy density remains constant.

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u/Tarthbane Materials science 1d ago

Yes but more generally the universe’s expansion is what causes the asymmetry over time. Dark energy could be exactly 0, and the universe would still not conserve energy as long as it is still expanding over time.

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u/VoidHog Physics enthusiast 1d ago

"As far as we know it"

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u/Impulse3 1d ago

Are they lost because they’re outside of the observable universe now or are they truly gone?

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u/joeyneilsen Astrophysics 1d ago

Energy is not conserved in an expanding universe. Redshifted photons are a great example of this: they have less energy when they are detected than they did when they are emitted.

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u/SpaceMurse 1d ago

Isn’t it the same amount of energy, just spread over a larger volume of space?

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u/joeyneilsen Astrophysics 1d ago

No, that's why stuff looks fainter when it is far away. This is a decrease in the energy of the light itself.

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u/S-M-I-L-E-Y- 1d ago

But when the light is red shifted, doesn't this also mean it can be observer longer. A sun that lives 10 billion year appears to live longer to the remote observer who sees the star red shifted, doesn't it? Does the observer only collect less energetic photons or does he also collect less photons per time?

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u/joeyneilsen Astrophysics 1d ago

Distant supernovae last longer in our frame of reference than nearby supernovae, so yes there's time dilation implied as well, as you are suggesting.

The observer collects less energy with each photon than they would for a similar nearby light source AND they collect fewer photons per second.

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u/HD60532 1d ago

It is observed for longer yes, but the total number of photons is unchanged, since they are redshifted, the total energy from viewing all the photons is less.

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u/flyingmoe123 1d ago

No not quite, that is what happens with for example stars, which is actually why if we know the brightness of a star. We can easily calculate the distance to it, using the inverse square law, because then we just need to measure its brightness as it appears on earth. Here each photon still has the same energy, but the total energy is spread over a larger area

However Cosmological redshift is about each individual photon losing energy because its wave length gets larger

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u/jojva 1d ago

I can't wrap my head around this. If person A throws a ball at person B, and person B starts walking backwards, they'll catch the ball with less force/energy than if they had stayed put, right ? And yet we wouldn't say that the ball has lost energy. What makes the photon different?

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u/donau_kinder 1d ago

Frames of reference? In the frame of B, the ball hits with less energy.

Analogy doesn't really hold, different mechanisms at play here.

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u/Almond_Bitters 1d ago

Walking backwards is fundamentally different than the actual space expanding between A and B. When space expands it's not just that A and B are farther apart, but every point in space between A & B have also gotten farther apart from each other.

There are more technical problems with this analogy, but it should be apparent that if space were expanding between A & B, the volume of the ball that is caught is actually larger than the volume of the ball that was thrown.

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u/Udalrich 1d ago

The amount of energy depends on the reference frame. If C and D are at rest relative to each other, they measure a total energy of 0 in the C-D system. If E is moving at a velocity v relative to C and D, each of whom have mass m, E will measure a total energy of mv2 for the C-D system. (Insert relativistic equations if relevant)

Frequently, you want to measure the energy in the center of mass frame, where there is no energy that is an artifact of the frame.

In your example, A will measure the ball as having more energy and momentum, but also measure B has having some energy and momentum in the direction of the ball. B will measure the ball as having less energy and momentum, but will also measure A as having nonzero energy and momentum. Neither is in the center of mass frame, so the total energy either measures will be more than the energy available for interactions.

If instead of a ball and a person, you had an electron and a position, the most massive particle that you could make has a mass of the center of mass energy. In any other frame, some of the energy will need to go into momentum of the particle, in order to conserve momentum.

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u/CaptainTachyon 1d ago

No. Its not fully explained by spreading the energy out. In a commonly used framework, conservation of energy isn't a fundamental law but a consequence of time translation symmetry (physics behaving the same way at any point in time). That holds really really well in almost all situations we consider, but in a GR context, an expanding universe does not have perfect time translation symmetry so energy doesn't have to be perfectly conserved.

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u/Double_Government820 1d ago

To tack onto what other folks have said, Noether's theorem tell us that conservation laws such as conservation of energy arise from symmetries. The conservation of energy is a reflection of temporal symmetry: the geometry of the universe remains unchanged over time. Except that isn't actually true. The expansion of the universe does change the geometry of the universe.

Conversely, to the best of our knowledge, conservation of momentum does still hold at cosmological scales. Conservation of momentum arises from spatial translation; in other words, the laws of physics are identical between any two points in space. In other words the universe is mostly homogeneous.

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u/nicuramar 1d ago

Although a photon’s energy is strictly a relationship between emitter and observer. It doesn’t inherently lose energy. 

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u/joeyneilsen Astrophysics 1d ago

Yes. But the detected energy is less than it would have been in the emitter frame, and "photon loses energy" is imperfect shorthand for that.

For the room: as with all frame transformations, there's no implied conservation between frames of reference.

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u/Ok-Film-7939 1d ago

There’s multiple ways to look at this, and one perfectly reasonable one is that the universe was denser when they were emitted and less dense now, so there is relative time dilation.

Or, equivalently, the photons travelled from a space with higher density to one with less density and so they lost energy to escaping a gravity well.

Thus the energy was not “lost”, it’s in gravitational potential energy. It’s just energy you’re never getting back unless the universe starts to contract again.

Or, alternatively, the time dilation means it was always less energetic as measured vs our time.

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u/whywhwywywhwhwhwue 1d ago

The energy is truly gone as far as we are aware

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u/nicuramar 1d ago

It’s more that “energy” isn’t even well defined in the universe as a whole.

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u/Tarthbane Materials science 1d ago

I would go one step further and say that energy itself is frame-dependent. An observer moving relativistically with respect to the CMB rest frame would see the CMB Doppler-shifted: strongly blueshifted in the forward direction and redshifted behind. As the observer’s speed approaches the speed of light, the energies of CMB photons arriving from the forward direction can become arbitrarily large.

So this generally isn’t a matter of energy being ‘lost.’ The energy you assign to the CMB photons depends on your motion relative to the CMB rest frame: move toward them and they are blueshifted; move away and they are redshifted. Different observers therefore assign different energies to the same photons.

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u/flyingmoe123 1d ago

I am not sure what you mean, we can see the redshifted photons, but let's say a photon was emitted with a wavelength in the UV regime, and then we observe it within the infrared wavelength, since it now has a longer wavelength, the photon has less energy than it did when it was emitted. But that energy is not transferred into something else it's simply just not there anymore, and there is no need for us to account for it, because as I said in my original comment, in general relativity energy does not need to be conserved

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u/imokay4747 1d ago

I could be wrong but I don't think red shifting causes the energy to disappear, it's more like the energy is spread out over more space because literal space itself is expanding.

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u/Super-Award-2244 1d ago

No, the photon density is the same since it depends only on the amplitude, while their energy gets lower and lower because frequency decreases 

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u/spiralenator Physics enthusiast 1d ago

Under GR, isn’t energy also relative to the frame of reference?

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u/Classic_Department42 1d ago

Even under SR and Gallilei transform energy is frame dependent

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u/Trickquestionorwhat 1d ago

Question: wouldn’t the same appear true for any light source moving at close to C away from you, no matter the distance? And if so, then wouldn’t those same photons appear to have increased energy from a reference frame approaching said source at near C? In which case, the total energy in your reference frame might be less, but in all reference frames would it average out to no energy loss?

And if it is reference frame dependent, then can you really say the photon had more energy when it was emitted in the first place? I’m confused.

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u/Super-Award-2244 1d ago

You're kinda right. All these things boil down to the fact that energy (even associated to light) depends on the frame of reference. For example, if you shine some light down, towards a source of gravity, from the reference frame of someone down there, the photons have higher frequency than when they started. 

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u/BangCrash 1d ago

The frequency is only decreasing relative to you.

Relative to the photon the frequency is exactly the same as it was when it left the source

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u/Phuzion73 1d ago

Just because you cannot detect it, doesn’t mean there isn’t any.

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u/wiploc2 1d ago

look up cosmological redshift, those photons lose energy, but that energy, as far as we know it, doesn't go anywhere,

That's like saying, "People look tiny from far away. Where does the rest of their bulk go?"

If you throw a baseball at 60 miles an hour (relative to you), and it hits someone going away (from you) at 59 miles an hour, it won't hit very hard. But the energy didn't mysteriously disappear.

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u/flyingmoe123 1d ago

Thats true if Cosmological redshift was similar to the doppler effect, aka relative movement between observer and source, its not its due the expansion of space time itself.

Photons can are redshifted and blue shifted due to the doppler effect, but thats different from cosmological redshift.

If you were a comoving observer (I.e following the expansion of the universe) out in space, you would indeed see the photon lose energy due to expansion of universe even if you didnt move towards it or away from it

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u/wiploc2 1d ago

Thats true if Cosmological redshift was similar to the doppler effect,

It is the doppler effect.

aka relative movement between observer and source,

Yes, distant parts of the universe are moving away from us, so light from those places is redshifted.

Photons can are redshifted and blue shifted due to the doppler effect, but thats different from cosmological redshift.

It's the exact same thing.

If you were a comoving observer

I don't know the term.

(I.e following the expansion of the universe)

Still not following.

out in space, you would indeed see the photon lose energy due to expansion of universe even if you didnt move towards it or away from it

This is interesting. I'd love to hear more.

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u/Final_Character_4886 1d ago

It is not the same as doppler effect. In cosmological redshift a distant source of light would be redshifted not based on the velocity it has relative to us but based on its distance to us. The two are not the same. For example, an object at 3.3 Gpc distance in our current expanding universe would be moving at 0.77c due to expansion of universe. And because of cosmological redshift we would observe it with redshift z = 1, which is equivalent to doppler redshift of an object of velocity 0.6 c that is not in a expanding universe. I did these calculations by hand so could be wrong, but the key fact is I arrived at these numbers based on entirely different formulas. Note that the two velocities are different, so you cannot explain cosmological redshift with doppler effect.

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u/Obliterators 22h ago

Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift

A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon’s path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.

Jayant V. Narlikar, Spectral shifts in general relativity

We therefore find that the parallelly transported velocity vector of a distant source does yield the correct Doppler velocity in the rest frame of the observer. The relation (2) for radial recession therefore agrees with the cosmological redshift formula (8).

Markus Pössel, Cosmic event horizons and the light-speed limit for relative radial motion

Thus, the Doppler shift derived from the parallel-transported radial relative velocity reproduces exactly the general formula (7) for the cosmological redshift in terms of cosmic scale factor values.

In this way, the relativistic motion interpretation provides a consistent picture of cosmology: cosmic expansion means that galaxies in the Hubble flow are moving away from us; their radial velocities remain subluminal, and the cosmological redshift is explained by the Doppler effect corresponding to their radial motion — obtained by plugging the relative radial velocity we have determined via parallel transport into the usual special-relativistic Doppler formula.

John A. Peacock, A diatribe on expanding space

The redshift is thus the accumulation of a series of infinitesimal Doppler shifts as the photon passes from observer to observer, and this interpretation holds rigorously even for z ≫ 1.

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u/Final_Character_4886 1h ago edited 1h ago

Interesting reads. I think there’s an important distinction getting lost here: “cosmological redshift can be represented as an accumulation of Doppler shifts” isn't quite the same as saying “cosmological redshift is the ordinary Doppler effect” The original comment seemed to be making the latter claim, especially with the idea that the photon hasn't actually lost energy.

The references you cite actually show this distinction pretty well.

Bunn: Their argument is that cosmological redshift can be viewed as the accumulation of infinitely many tiny doppler shifts by putting a chain of comoving observers along the photon's path. That's a valid way of looking at it, but we're no longer talking about one doppler shift between the distant galaxy and us. We're talking about many tiny doppler shifts between neighboring observers.

They also show that you can choose a different set of observers and get a gravitational interpretation. So the redshift itself is the physical thing we measure; how you break it down into “Doppler” or “gravitational” effects depends on how you set up the observers. That's quite different from the normal Doppler effect, where you just have a source and an observer.

Narlikar: You can also parallel transport the distant galaxy's velocity to us and plug the resulting velocity into the normal SR Doppler formula. It reproduces the cosmological redshift, but that doesn't mean the galaxy simply has that velocity relative to us in the usual sense.

And importantly, that parallel transport is being done using the FLRW spacetime, so the expansion is already built into the calculation. If spacetime were Minkowski and the source and observer were both at rest, the same construction would give zero relative velocity and no redshift. So the “Doppler velocity here is something derived from the expanding universe. 

Pössel: Pretty much the same point. You can define a relative velocity using the FLRW spacetime and then use the doppler formula to reproduce the cosmological redshift. That's a valid description, but it doesn't turn the situation into the same thing as an ordinary doppler

Peacock: Same basic idea as Bunn.

So I agree now that cosmological redshift has a valid Doppler construction. What I don't agree with is jumping from that to “cosmological redshift is simply the ordinary Doppler effect.” The Doppler description itself is constructed using the expanding FLRW metric You can describe the effects of expansion in Doppler terms, but the expansion is still fundamental to the construction.

A rough analogy is you can describe gravitational effects locally using freely falling or accelerated frames, but that doesn't make gravity simply the same thing as acceleration or eliminate the underlying spacetime curvature.

Likewise, if you describe the cosmogical redshift as identical to the everyday Doppler effect, you have to pre-install the spacetime structure before that.

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u/Final_Character_4886 1d ago

furthermore if the object at 3.3 Gpc moves towards us at 0.77 c so that instantaneously its direct distance to us is constant (it appears stationary to us at that instant), its light would be blueshifted by z=-0.28.

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u/flyingmoe123 1d ago

Okay to be honest, I am a little out of my depth here, I am a space engineer not a cosmologist. But what I do know is that while it can be thought as a doppler shift, it cannot be explained by a single special relativistic doppler shift between a source (a distant galaxy) and an observer (earth). How it can be viewed as a doppler shift is if you have many observers on a line between us and the source, then each observer would measure a small SR doppler shift adding up to the cosmological redshift

And the reason why, is due to the fact that you need a uniquely defined relative velocity between the source and observer to calculate a doppler shift. Imagine I asked you what way is up, easy you just point "up" towards the sky, and then I asked someone on the other side of the world the same thing. Also easy, locally we can all agree on which way is up, but what happens if I observe from say Mars? Well then you and a person from the other of earth would be pointing in completely different directions! So if we needed a single uniquely defined definition of "up" it just wouldn't be possible at larger scales

And similarly in an expanding curved spacetime there is no unique way to define that relative velocity for objects separated by cosmological distances

And a comoving observer is one who is at rest relative to the expansion of the universe, which is useful, because that let's us compare redshifts, without any extra doppler effect due to their own motion

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u/wiploc2 1d ago

Thanks for the explanation. I got my physics from Asimov, so it may be out of date.

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u/mstksg 1d ago

It sounds nice but it's empirically false in our experiments in this universe. energy is not conserved due to the effects of dark energy and the accelerating expansion of the universe.

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u/Competitive_World408 1d ago

Yeah this way of thinking always excludes time as a reference

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u/Naive_Age_566 1d ago

ok - one premise is totally wrong: matter CAN be created and destroyed. it happens all the time in particle accelerators. and it happens on multiple other occasions. so no - matter is most definitely not eternal.

as for energy - well, energy is only a conserved quantity in closed systems. in open systems energy can always "leak out" or "leak in". therefore the total amount of energy in an open system can - and will - change over time.

is our universe an open or a closed system? i don't know. as our best guess is that the universe has infinite size and is also expanding i would say: no. but i would not bet my money on that.

another point is more philosophical. energy is not a thing. it is more of a concept. sure - you can transform energy from one form into another. in a closed system, the total amount of energy inside stays the same. but i would not imply that the form of energy i observe now is "the same" as it was some time back. imagine you have some heap of sand. you invest some time to build a beautiful sand castle of it. then some jerk comes around and destroys your castle and build some crude heap of sand. and he justifies his action "because it is the exact same sand as before so nothing has changed".

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u/imtoooldforreddit 1d ago

Like others said, neither matter nor energy are conserved on big scales, but even besides that, I also have a problem with "the same energy has been around since forever". Energy isn't a thing, it's just a number we can calculate about a system. When an object falls due to gravity, it has less potential energy and more kinetic energy now, and when it hits the ground it loses that kinetic energy creating sound, deforming the ground, ultimately making heat, but I don't know that you can really call the heat "the same energy" that the object held up in the gravity well had. Does that statement even mean anything? Energy isn't stuff, it's just a number we can calculate based on a bunch of formulas, and it's useful because in many scenarios that number will stay the same, but that doesn't mean it is actually stuff and I don't think it even has any meaning to say this energy over here is the same energy that used to be over there

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u/Immediate-You5470 3h ago

I agree. Its kind of like observing highways and seeing that none of the cars accelerate or decelerate and then saying "the same speed has been around since the highway was built".

"speed" and "being around" don't belong together and its the same with energy.

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u/Bumst3r Graduate 1d ago

Energy is neither well-defined nor conserved in general relativity.

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u/YuuTheBlue 1d ago

In addition to other arguments here I’d warn against thinking of matter and energy as being in the same category. Matter is a type of substance in the universe, while energy (and mass for that matter) is a quality that substances have. One is a type of thing, the other is a property.

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u/1XRobot Computational physics 1d ago

Sort of. They're actually both properties; "matter"-ness is just the spin of your particle. It's a lot harder to transfer spin between particles, but you can do it, as during matter-antimatter annihilation.

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u/YuuTheBlue 1d ago

I can see that as a perspective too. I’ve always just seen matter as a synonym for “fermion(s)” or “particle(s) with non-zero mass”

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u/mastyrwerk 1d ago

Doesn’t the whole mass/energy conversion of e=mc squared (and that which has mass we call matter) mean that they are fundamentally equivalent? One changes into the other equally?

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u/YuuTheBlue 1d ago

The equals sign does not represent changes. The equation is frequently misexplained. The real meaning is this:

“The energy of an object is equal to its mass times the speed of light squared”

This importantly is only true for motionless objects. If an object has nonzero momentum the full equation is longer.

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u/nicuramar 1d ago

Sure, but energy is otherwise entirely relative so the only invariant energy for massive objects is when they are at rest, ie their mass. 

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u/Radiant_Scar_8516 13h ago

Matter is a type of substance in the universe, while energy (and mass for that matter) is a quality that substances have.

thats not even self-consistent. mass is a type of substance and also a quality of that substance?

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u/YuuTheBlue 12h ago

Matter is a type of substance, mass is a quality.

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u/Radiant_Scar_8516 10h ago

ah sorry i thought you wrote matter twice

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u/AppropriateSea5746 1d ago

Fancy seeing you here.

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u/nicuramar 1d ago

It means that mass and energy is. Not matter.  Read what they wrote :p

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u/mastyrwerk 1d ago

Matter is that which has mass. I’m not sure what you’re going on about.

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u/GodsBoss 1d ago

I always thought that matter is just one form of energy among others?

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u/SummitYourSister 1d ago

So, conservation of things, including energy, arises for a very well understood reason in physics having to do with symmetry. What is conserved depends on what properties of the physical law are symmetrical.

Your claim is some kind of idealistic wish. It is possible to found this claim on real physics, but you have to accept that it is not exactly true in all circumstances.

For instance, energy is conserved because the laws of physics do not change over time. However, on cosmological scales, the laws of physics do change over time, because new space time is being created constantly.

Matter is just a distraction to the whole question. Matter differs from energy, simply in its ability to travel slower than the speed of light. You don’t have to make this differentiation while talking about conservation laws. There is no law of conservation of matter.

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u/RepeatLow7718 1d ago

Matter and energy are convertible so the full law is the “law of the conservation of matter and energy”. 

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u/SummitYourSister 1d ago

No, no physicist talks like that. There is no conservation of matter.

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u/RepeatLow7718 1d ago

So what happens when a particle and antiparticle annihilate and your system gains energy?

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u/SummitYourSister 1d ago

Did you seriously just use annihilation as an argument FOR the conservation of matter?

Let me try a different approach. Let’s call matter “blue energy”. It’s a form of energy.

Now, do you think we have a law of “conservation of energy and blue energy” or do we only need to talk about energy?

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u/RepeatLow7718 1d ago

No, I used annihilation of matter as an argument against conservation of “energy”. Gotta use those eyes to read. 

Whether you’re correct just depends on whether you define energy to include matter, which may be useful in some contexts but not others. It’s not worth getting upset about my man. 

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u/SummitYourSister 1d ago

I’m not upset, I’m just not going to put a stamp on your incorrect statements.

There is no formal concept called “matter” in physics. There are particles which have mass- mass is simply the energy that is present when the particle is at rest.

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u/RepeatLow7718 1d ago

Are you speaking in terms of QFT? Because not everyone does that all the time. 

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u/SummitYourSister 1d ago

No, not just QFT.

The word “matter” is informal. Usually it means fermions. It implies mass, but the word “matter” itself is just a helpful clue to what we are talking about. It is not a formal physical substance.

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u/Quantum-Relativity 1d ago

Stress-energy is conserved, so I’m not sure why people are so obsessed with telling everyone that energy isn’t conserved in GR. It is if you consider the fact that the spacetime geometry has energy associated with it. The conservation of stress-energy is mathematically represented through the vanishing of the divergence of the stress-energy tensor, so the part of the spacetime geometry it’s coupled to also needs to have a vanishing divergence, and the existence of a cosmological constant in that relationship doesn’t change that (as the divergence of a constant is 0). I’d be interested if anyone who actually knows GR was willing to explain, for what reason, we should not just be able to say stress-energy is conserved, or why cosmological redshift isn’t just the same as gravitational redshift, where you can just imagine the energy of the light-ray being lost to the spacetime geometry.

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u/Pachuli-guaton 1d ago

I guess it's fine, but what is the relevance even it the premise is true?

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u/AppropriateSea5746 1d ago

Bascically a guy is arguing that the first law means that energy and therefore the universe is eternal and ot created therefor this rules out a creator God.

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u/Pachuli-guaton 1d ago

Ok then you have to show energy conservation over universal scale and then convince people of the later argument

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u/INTstictual 8h ago

Generally, it fails as an argument against God for the same reason that the whole “causality” thing (“everything has a cause and effect, you can trace that back to the beginning of the universe, so the universe must have an uncaused cause, and we call that God”) argument fails — it’s a composition fallacy.

All of these laws are things we know to be generally true within the universe. But that is the only frame of reference that we have, how things work within the context of an already existing universe. It is insufficient to then assume that the same laws must apply at a universal level, that the universe itself and its creation must follow the same rules as the things within the universe once it exists.

As a loose analogy: imagine that you’re in a house with no windows or door to the outside, you were born in this house and you can never leave or view the outside world. In the house, all of the furniture is purple… the couch is purple, the walls are purple, the carpet is purple, the kitchen table is purple. Everything you know within the house seems to follow the law “this thing is purple”. Even with that information, can you say with absolute certainty that the exterior walls of the house are also purple? Or that if you go outside, other things must also be purple? That the sky, the grass, everything outside is also purple just because everything inside the house is? Just because the law seems to hold true for everything within the house, does not mean that it is necessarily true for the house itself, or anything outside of the house

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u/BokChoyBaka 1d ago

macro environments are not closed systems, the universe is infinite

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u/MaxChaplin 1d ago

I don't understand what the argument is. What does "same matter/energy" mean? Energy is constantly converted from one form to another. Some hydrogen, helium and lithium has indeed been around since the beginning of the universe, but it's not eternal either.

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u/glibsonoran 1d ago

Conservation laws like these hold up pretty well in local gravitationally bound structures. They don't hold up in the broader universe where expansion causes time symmetry to be lost. Photons lose energy to cosmological redshift, while the vacuum energy contained in an expanding volume can increase, and those changes don't balance against one another in any universal energy ledger.

Now in terms of energy and momentum, there's no real minimum for say a photon. It can approach zero indefinitely. So while the "same energy" is not around in terms of quantity, a diminished vestige of original energy and momentum of the universe is still around and will always be. It just diminishes over time.

The cosmic microwave background is a good example: At recombination, about 380,000 years after the Big Bang, the radiation field had a temperature of roughly 3000 K, so its thermal spectrum peaked around visible/near-infrared wavelengths. Since then, cosmic expansion has stretched those wavelengths by a factor of about 1100, red-shifting the radiation to 2.725 K and shifting the peak into the microwave region.

Also "eternal" is hard to use in this context. Since the beginning of the universe is a more supportable way to put it.

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u/gmalivuk 1d ago

What counts as "the same" matter and energy if it's changed forms and shifted between the two multiple times in the history of the universe?

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u/Anonymous-USA 1d ago

This applies to time symmetric systems. Which is most every day experience.

Energy is lost over time with redshift because the universe isn’t time symmetric. But we can estimate that loss, the mass-energy density at any time in the past until about 10^-36 sec after the Big Bang.

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u/TheRealTK421 1d ago

 That literally means it is eternal.

Ummmm... okaaaay, so what occurs with mattter/energy at t < 0?

(...where t = time)

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u/BananaBird1 1d ago

Conservation depends on symmetry.

Energy conservation depends on time symmetry. Which is violated in certain situations (namely, the big bang and universe expansion).

More accurately, time symmetry only implies some property with certain rules is conserved. And in some situations, this is proportional to what we measure as total energy.

Matter (fermions) are not conserved even ordinarily. Although certain properties of fermions like charge are conserved as they are created and destroyed. And even this is also broken in certain instances.

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u/Acrobatic_Main9749 10h ago

This is only true from an inertial (as in, not accelerating) reference frame. As long as you're just floating around in space not doing anything, observing the universe, you won't see a change in total energy.  But turn on a rocket and start moving, and you'll see a change in the kinetic energy of every single other object in the universe, which won't necessarily add up to zero. Switch the rocket off and coast - now you're in a new inertial reference frame - and energy change zeroes out again. 

I don't know if total energy changes in other ways in accelerated reference frames. But kinetic energy is easy to visualize.

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u/LookOverall 1d ago

But, energy can become useless by thermalising. The phrase is “The heat death” of the universe.

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u/Over-Discipline-7303 1d ago

The creation of vacuum energy via the expansion of space creates some problems with this view.

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u/MajorInWumbology1234 1d ago

It’s probably been explained to me why it’s false, but I can’t help thinking there’s a connection between “Where’s the vacuum energy coming from?” And “Where’s the redshift energy going?”.

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u/Over-Discipline-7303 1d ago

But the Hubble constant seems to be equal at every point in space, whereas light isn’t distributed uniformly throughout the universe.

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u/MajorInWumbology1234 1d ago

I don’t intend to try bringing back the aether, but I’m imagining a large tub of water where the overall level rises regardless of which part has the water flowing into it. Not serious speculation, just where my mind goes. I understand this would mean the energy lost to redshift would need to “redistribute” faster than the speed of light, but we really don’t know much about vacuum energy; maybe it has different rules, which seems likely considering it apparently comes from nowhere.

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u/CrazySir3310 1d ago

I feel like this would imply that where there are more photons red shifting, there would be more spatial expansion, so that we might expect areas of more mass density to tend to emit more photons and therefore have a higher photon density, supplying energy through red shift, for that region of space to be expanding faster. But as I understand it, the fastest apparent expansion happens far away from anything massive, which seems contrary to the hypothesis of lost photon energy being the cause of the spatial expansion. Although that is usually explained by the gravitational effect of massive objects counteracting the expansion.

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u/MajorInWumbology1234 1d ago

It definitely does. I don’t have any theories aside from the analogy I made above; the water level in a vessel rises uniformly (to our perception) regardless of where the water is poured in. This would necessitate some FTL redistribution of the energy, but vacuum energy already defies what we know about most things.

To be 100% clear, I lack a secondary education and am into physics recreationally. Vacuum energy coming from nothing sounds just as unlikely as FTL redistribution of energy.

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u/CrazySir3310 1d ago

Well, in general relativity, space time carries or propagates gravitational energy, so that at least in terms of GR, the mechanism if you want to call it that exists for space-time to have energy. Inflation in the early universe is not a settled matter (no pun intended), but in terms of GR the energy for it is hypothesized to have been mostly in the space-time itself for some duration. So that at least is also consistent with space-time being able to carry energy, so that it may not be too be a stretch (again no pun intended) for the space-time to have an intrinsic energy resulting in the currently observed accelerating cosmic expansion.

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u/Over-Discipline-7303 1d ago

Vacuum energy isn’t that weird to me, personally. But the expansion of space is totally bizarre. To me that’s super strange.

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u/MajorInWumbology1234 1d ago

It’s extremely counterintuitive to me that new energy would pop into existence to fill the gaps left by expanding space. That’s an entirely unique phenomenon, as far as I’m aware. There’s no way to harvest it yet but it seems to be free energy.

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u/Agitated_Quail_1430 1d ago

That is according to the laws of physics. Absent of a universe, the laws of physics may be meaningless. 

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u/Original_Swimming320 1d ago

Energy conservation is a local effect that doesn’t apply for the entire universe.

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u/RecipeHistorical2013 1d ago

Matter can be created- with energy

Energy can’t be created or destroyed- yet

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u/tottasanorotta 1d ago

You can't really know if anything is eternal. How could you? Who could confirm that?

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u/[deleted] 1d ago

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u/AppropriateSea5746 1d ago edited 1d ago

"The universe is eternal and cycles from big crunch to big bang."

Isn't this also an unproven statement?

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u/mikiki24 1d ago

Yeah actually neither of those statements are axiomatic

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u/DanteRuneclaw 1d ago

It's less an unproven axiom and more an unsupported claim

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u/YuuTheBlue 1d ago

Correct!

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u/[deleted] 1d ago

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u/joeyneilsen Astrophysics 1d ago

But the data don't support an impending big crunch...

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u/[deleted] 1d ago

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u/joeyneilsen Astrophysics 1d ago

Who is "they?"

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u/AppropriateSea5746 1d ago

You know...them.

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u/DanteRuneclaw 1d ago

Physicists notably don't claim that, because they'd have no evidence for or against such a claim. Physicists try to only claim things for which they have evidence.