1st law of thermodynamics
Discussion
Energy is conserved - can only be transformed form one from to another.
We are all well versed with this law as it crops up in GCSE science, if I can remember that far back.
BUT its not exactly true.
Take light for example. When light travels across the universe from galaxies that are 'moving' away from us, the light is red shifted.
There is an energy change between the original light and the red-shifted light, but where does this energy go (or come from if the galaxies were moving towards us)?
The surprising thing is that it is "just lost".
I watched a youtube video recently (Physics Girl), whereby a fairly brief mention was made regarding the 'lost' energy. But I am struggling to understand 'lost' in this context.
We are all well versed with this law as it crops up in GCSE science, if I can remember that far back.
BUT its not exactly true.
Take light for example. When light travels across the universe from galaxies that are 'moving' away from us, the light is red shifted.
There is an energy change between the original light and the red-shifted light, but where does this energy go (or come from if the galaxies were moving towards us)?
The surprising thing is that it is "just lost".
I watched a youtube video recently (Physics Girl), whereby a fairly brief mention was made regarding the 'lost' energy. But I am struggling to understand 'lost' in this context.
I am drawn to this question as an advocate of both the first and particularly the second law, not being a quantum scientist however please take my thoughts as just that, and not Gospel.
The given ( simple ) answer is that as the star emitting the photon is moving at a high relativistic speed the emitter and receiver are in different reference frames, and as such the conservation of energy doesn't apply.....
I guess applying Newtonian physics ( readily understandable, up / down / forces and reactions etc ) ) to try to explain a quantum effect ( speed of light, Lorentz contraction, duality etc ) is always going to fall short.
You'll understand why there is red and blue shift, the speed of light being constant the photon wavelength has to adjust to compensate for the speed of the emitter ( star )
As for different reference frames I liken it to a passenger on a train at high speed going through a station where somebody is standing, to you on the seemingly stationary train the standing man shoots past, to the man on the platform you on the train shoot past, now, because we have experience of trains we know who's moving, but if we didn't have experience it would be difficult to tell ( not my theory , mr Einstein was way ahead of the game ), not really relevant to the photon energy question but shows a relativity effect ....ie, who's actually moving ? Not really an example of different reference frames in the relativistic sense but you'll get my drift.
There's quite a few pesky things about photons, eg they have energy but zero rest mass, and as mentioned above re Lorentz contraction, try getting your head around that
The given ( simple ) answer is that as the star emitting the photon is moving at a high relativistic speed the emitter and receiver are in different reference frames, and as such the conservation of energy doesn't apply.....
I guess applying Newtonian physics ( readily understandable, up / down / forces and reactions etc ) ) to try to explain a quantum effect ( speed of light, Lorentz contraction, duality etc ) is always going to fall short.
You'll understand why there is red and blue shift, the speed of light being constant the photon wavelength has to adjust to compensate for the speed of the emitter ( star )
As for different reference frames I liken it to a passenger on a train at high speed going through a station where somebody is standing, to you on the seemingly stationary train the standing man shoots past, to the man on the platform you on the train shoot past, now, because we have experience of trains we know who's moving, but if we didn't have experience it would be difficult to tell ( not my theory , mr Einstein was way ahead of the game ), not really relevant to the photon energy question but shows a relativity effect ....ie, who's actually moving ? Not really an example of different reference frames in the relativistic sense but you'll get my drift.
There's quite a few pesky things about photons, eg they have energy but zero rest mass, and as mentioned above re Lorentz contraction, try getting your head around that
Why should energy be lost at all? If the distance between us and the light source is increasing, at whatever speed, the light waves will be further apart when they reach us therefore lower frequency. If the pitch of an ambulance siren drops as it passes us for exactly the same reason, is energy being lost?
The Second Law explained https://www.youtube.com/watch?v=EF_xdvn52As unsustainable https://www.youtube.com/watch?v=VXPoJAyeF8k Isolated system 
Dr Jekyll said:
Why should energy be lost at all? If the distance between us and the light source is increasing, at whatever speed, the light waves will be further apart when they reach us therefore lower frequency. If the pitch of an ambulance siren drops as it passes us for exactly the same reason, is energy being lost?
In our non quantum world there is no correlation between wave frequency and wave energy, they are independent of each other, However, in the quantum world frequency is energy correlated, high frequency is high energy, low frequency lower energy, Pobolycwm said:
Dr Jekyll said:
Why should energy be lost at all? If the distance between us and the light source is increasing, at whatever speed, the light waves will be further apart when they reach us therefore lower frequency. If the pitch of an ambulance siren drops as it passes us for exactly the same reason, is energy being lost?
In our non quantum world there is no correlation between wave frequency and wave energy, they are independent of each other, However, in the quantum world frequency is energy correlated, high frequency is high energy, low frequency lower energy, Simpo Two said:
Pobolycwm said:
Dr Jekyll said:
Why should energy be lost at all? If the distance between us and the light source is increasing, at whatever speed, the light waves will be further apart when they reach us therefore lower frequency. If the pitch of an ambulance siren drops as it passes us for exactly the same reason, is energy being lost?
In our non quantum world there is no correlation between wave frequency and wave energy, they are independent of each other, However, in the quantum world frequency is energy correlated, high frequency is high energy, low frequency lower energy, Pobolycwm said:
No, you're trying to use " non quantum world " ie what we see around us type reasoning to explain quantum effects, it'll end in tears.
Yep, an analogue analogy to illustrate a digital phenomenon. Mind you as Carl Sagan used 'Flatland' to explain to normal people what a fourth dimension is like, I'm not unhappy 
Simpo Two said:
Pobolycwm said:
No, you're trying to use " non quantum world " ie what we see around us type reasoning to explain quantum effects, it'll end in tears.
Yep, an analogue analogy to illustrate a digital phenomenon. Mind you as Carl Sagan used 'Flatland' to explain to normal people what a fourth dimension is like, I'm not unhappy Simpo Two said:
Pobolycwm said:
No, you're trying to use " non quantum world " ie what we see around us type reasoning to explain quantum effects, it'll end in tears.
Yep, an analogue analogy to illustrate a digital phenomenon. Mind you as Carl Sagan used 'Flatland' to explain to normal people what a fourth dimension is like, I'm not unhappy The Energy is not lost. The photon will keep its frequency all along its travel.
What happens however is an effect called the Doppler effect. It's the same law that allows speed camera to measure vehicle speed.
You look at photons being emitted towards you. Their light will reach your eye with a set frequency. But a moment later, it will have travelled some distance and the next wave will reach you a bit sooner. Thus altering the peak to peak wavelength and increasing the frequency of the signal received. Effectively shifting the apparent colour of the light to the blue.
The opposit happens when a galaxy moves away from you: the peak to peak increases, shifting the apparent light colour to the red.
What happens however is an effect called the Doppler effect. It's the same law that allows speed camera to measure vehicle speed.
You look at photons being emitted towards you. Their light will reach your eye with a set frequency. But a moment later, it will have travelled some distance and the next wave will reach you a bit sooner. Thus altering the peak to peak wavelength and increasing the frequency of the signal received. Effectively shifting the apparent colour of the light to the blue.
The opposit happens when a galaxy moves away from you: the peak to peak increases, shifting the apparent light colour to the red.
With the caveat that I only vaguely understood some of the maths of Gen Rel 25 years ago and have forgotten it all ...
Forget the expanding universe red shift and just consider any red shift for the moment. Let's say some photon is emitted towards you. If you just sit there and wait until it hits you, you'll say it had wavelength X and energy 1/X. If instead you run away from the photon, when it hits you you'd perceive a longer wave length and a lower energy. The energy of a photon is NOT a conserved quantity. At any instant there is no single "correct" measure of its energy. It doesn't make sense to talk about "its" energy. The photon only has energy with respect to an inertial reference frame in which the energy is being measured. So what do we take away from this so far? Energy is perhaps a little less special than we thought, and a property of a system, not an object.
Conservation laws are equivalent to statements of the symmetry exhibited by spacetime. So if you start be defining how geometry works, you then derive what quantities get conserved ... so our classical ideas of conserved quantities such as distance, momentum, energy and angular momentum are a product of thinking of space-time in the classical way; a flat 3D space and time. If you change your definition of spacetime's geometry, there is no need for the same measures to remain conserved. If your new spacetime has different symmetries, then different measures will be conserved.
If you chose to inflate a 3D classical spacetime, energy would not be conserved. If we're saying distance isn't conserved, frankly why would it be surprising if momentum and energy weren't either?
In Einstein's spacetime geometry, the conserved quantity is some horrendous energy/momentum/stress tensor. I have as much intuitive insight into it as a goldfish does a Rolex. But in the limit it looks like classical energy, momentum etc. The point is that it is that tensor quantity that will be conserved by spacetime inflation.
Forget the expanding universe red shift and just consider any red shift for the moment. Let's say some photon is emitted towards you. If you just sit there and wait until it hits you, you'll say it had wavelength X and energy 1/X. If instead you run away from the photon, when it hits you you'd perceive a longer wave length and a lower energy. The energy of a photon is NOT a conserved quantity. At any instant there is no single "correct" measure of its energy. It doesn't make sense to talk about "its" energy. The photon only has energy with respect to an inertial reference frame in which the energy is being measured. So what do we take away from this so far? Energy is perhaps a little less special than we thought, and a property of a system, not an object.
Conservation laws are equivalent to statements of the symmetry exhibited by spacetime. So if you start be defining how geometry works, you then derive what quantities get conserved ... so our classical ideas of conserved quantities such as distance, momentum, energy and angular momentum are a product of thinking of space-time in the classical way; a flat 3D space and time. If you change your definition of spacetime's geometry, there is no need for the same measures to remain conserved. If your new spacetime has different symmetries, then different measures will be conserved.
If you chose to inflate a 3D classical spacetime, energy would not be conserved. If we're saying distance isn't conserved, frankly why would it be surprising if momentum and energy weren't either?
In Einstein's spacetime geometry, the conserved quantity is some horrendous energy/momentum/stress tensor. I have as much intuitive insight into it as a goldfish does a Rolex. But in the limit it looks like classical energy, momentum etc. The point is that it is that tensor quantity that will be conserved by spacetime inflation.
Edited by ATG on Saturday 26th March 11:43
I'll post up the video that I was referring to in my OP.
https://www.youtube.com/watch?v=GHCc9b2phn0
It does say that energy is simply 'lost'.
I can understand that our current laws of physics apply to many things that apply to our local environments and local 'spaces', but then become approximated and even fall over when looking at the cosmological scale of things. (ie. black holes, dark energy, t=0 prior the the big bang etc. etc.)
But simply that energy is 'lost' doesn't compute.
But then having said that anti-matter particles suddenly form prior to the existence of matter particles in the quantum world. If such things are said to happen then I guess I can just put it down to "not yet explained".
https://www.youtube.com/watch?v=GHCc9b2phn0
It does say that energy is simply 'lost'.
I can understand that our current laws of physics apply to many things that apply to our local environments and local 'spaces', but then become approximated and even fall over when looking at the cosmological scale of things. (ie. black holes, dark energy, t=0 prior the the big bang etc. etc.)
But simply that energy is 'lost' doesn't compute.
But then having said that anti-matter particles suddenly form prior to the existence of matter particles in the quantum world. If such things are said to happen then I guess I can just put it down to "not yet explained".
ALT F4 said:
I'll post up the video that I was referring to in my OP.
https://www.youtube.com/watch?v=GHCc9b2phn0
It does say that energy is simply 'lost'.
I can understand that our current laws of physics apply to many things that apply to our local environments and local 'spaces', but then become approximated and even fall over when looking at the cosmological scale of things. (ie. black holes, dark energy, t=0 prior the the big bang etc. etc.)
But simply that energy is 'lost' doesn't compute.
But then having said that anti-matter particles suddenly form prior to the existence of matter particles in the quantum world. If such things are said to happen then I guess I can just put it down to "not yet explained".
Yes, energy is lost. It is not a conserved quantity in the spacetime of general relativity. The symmetries of GR spacetime mean the bonkers tensor thing is conserved, not classical energy.https://www.youtube.com/watch?v=GHCc9b2phn0
It does say that energy is simply 'lost'.
I can understand that our current laws of physics apply to many things that apply to our local environments and local 'spaces', but then become approximated and even fall over when looking at the cosmological scale of things. (ie. black holes, dark energy, t=0 prior the the big bang etc. etc.)
But simply that energy is 'lost' doesn't compute.
But then having said that anti-matter particles suddenly form prior to the existence of matter particles in the quantum world. If such things are said to happen then I guess I can just put it down to "not yet explained".
You say that current physics falls over on cosmological scales. It really doesn't. General Relativity works astonishingly well and allows us (and when I say "us" I mean us collectively, sure as hell not me as an individual) to predict weird s
t that is verifiably correct to a very very high degree of precision. Clearly there is something huge missing from our understanding since QM and GR are entirely independent of each other and yet are supposed to be describing exactly the same stuff, and we've got some observations that suggest there must be dark matter and energy out there and currently they haven't found an agreed home in the models. But whatever refinements take place to our models, it strikes me as highly unlikely that Newtonian energy will become conserved on the cosmic scale. Just as classical Newtonian physics turned out to be a special case of Gen Rel physics, I'd expect Gen Rel to turn out to be a special case within some bigger theory we've yet to invent/discover. It'll sort edge cases. It won't tear up its key behaviour.
Gassing Station | Science! | Top of Page | What's New | My Stuff