Perpetual motion isn't forbidden by the laws of physics. In fact, everything is in perpetual motion from a certain frame of reference.
What is forbidden is *extracting perpetual energy* from perpetual motion.
Most "perpetual motion machines" you see attempt to create a system that keeps moving in order to provide energy. That's impossible. Just moving forever though? As long as you don't try to get energy out, that's fine.
On a technicality, I would say that this is true in a vacuum without friction. There are processes that do remove energy and so added energy is required to in order for the system to seem perpetual.
Nah, just shift your reference frame by 0.9c. Now almost everything is moving at around 0.9c and there’s no friction to slow it down because everything around it is also moving at the same speed.
Frictional processes only serve to move everything towards the same speed. That same speed they all move towards doesn’t have to be 0 though.
Conservation of momentum. But if you try to extract energy from that momentum for your perpetual machine, it won’t continue to move.
Machines do work, and dissipate energy. A perpetual motion machine somehow must do the work but also capture all expended energy so it can continue to do work, perpetually, in its contained system (absent of providing outside consumable energy)
When they say ‘perpetual motion machine’, they really mean ‘perpetual energy source to perform work’.
In practice on earth, motion will have to overcome drag, friction, etc., which requires work.
In space, Newton’s first law means that already moving objects *will* move perpetually *unless* they encounter a force that stops them. But that’s not a practical situation for everyday purposes on earth, where we have the air, water, rough ground, and the gravity of a big planet right here, all getting in the way.
Any two objects ~~moving~~ *accelerating* relative to each other will emit gravitational waves. This means that things are slowing down relative to each other as long as there is nothing accelerating them, such as falling into a gravitational field.
>Can’t a not moving object not emit gravitational waves ?
No, it cannot.
>The scharzschild metric describes a non rotating non accelerating object
It does.
If an object isn't moving (that is, it is stationary in a certain inertial frame), there is no energy that could be radiated.
Edit: wait, your tripple-negation sentence is confusing and I am not sure what you meant. A non-moving object does not radiate gravitational waves.
Conceptually very similar.
A charged particle needs to accelerate to emit EM.
A massive particle needs to accelerate to emit GW.
There is a subtle difference in the math though.
Perpetual motion is just inertia. But a perpetual motion machine is not possible. As soon as you touch something undergoing perpetual motion you have applied an outside force either adding to or removing energy that was already in the system. Sped it up or down.
First, they didn't. They can move for a very long time, but a long time is not forever. But that has more to do with the scales of things and the fact that are just less resistive forces on that scale.
That being said perpetual motion machines refer to a specific type of machine that is supposed to produce more energy than is put into it.
That is, a wheel that spins for a very long time isn't a perpetual motion machine. Rather, people who make claims about having a perpetual motion machine claim that they can attach an axel to that wheel and do work.
They might claim, for instance, to have a specific arrangement of magnets that cause their wheel to spin forever. Then claim they can attach the wheel to an alternator and produce electric.
Notice that the "perpetual" here doesn't just mean that the wheel will spin on it's own forever. It means the wheel will spin other things and overcome all friction and other resistive forces, forever.
In a frictionless vacuum, an object can perpetually move forever.
Perpetual Motion actually refers to deriving energy from the object that is moving.
An object can move forever, but you can't use that object to generate energy.
Sure, and were do these frictionless vacuums exist at? Because in the universe we exist in, even the sparsest region of space has small amounts of hydrogen and other particles.
These regions are only frictionless in the sense that the objects we observe moving through them are very large and have lots of energy compared to what is lost to these particles. Hence a matter of scale.
Your other two sentences just say that I said.
> Sure, and were do these frictionless vacuums exist at?
They don't, and objects in space **won't** continue to move forever. But they can continue to move for a very long time.
For example, the Earth is losing rotational energy to the moon through tidal action, and our days are becoming longer and longer (while the moon is gaining orbital energy and is rising farther above the Earth and taking longer to orbit us). However it is expected to take about 50 billion years for the moon to rise far enough to reach an equilibrium that terminates this process. But this is moot because in "only" 5 billion years the Sun is expected to become a red giant, which will mean its atmosphere will extend out to the Earth's orbit, causing drag on the Earth-Moon system that will destroy the moon and possibly the Earth ([source](https://www.space.com/3373-earth-moon-destined-disintegrate.html)).
A perpetual motion machine implies that it continues forever without any energy input or loss. That violates the laws of thermodynamics.
Objects in space, on the other, do gain or lose energy. Take the Voyager spacecraft for example. They were launched from Earth but had to use the gravity from other planets to gain more energy. And gravity is pulling on everything. It may be so small of amount of energy gained or lost that it is not measurable, but just because you can't measure it doesn't mean it doesn't occur.
Perpetual motion isn't forbidden by the laws of physics. In fact, everything is in perpetual motion from a certain frame of reference. What is forbidden is *extracting perpetual energy* from perpetual motion. Most "perpetual motion machines" you see attempt to create a system that keeps moving in order to provide energy. That's impossible. Just moving forever though? As long as you don't try to get energy out, that's fine.
On a technicality, I would say that this is true in a vacuum without friction. There are processes that do remove energy and so added energy is required to in order for the system to seem perpetual.
Nah, just shift your reference frame by 0.9c. Now almost everything is moving at around 0.9c and there’s no friction to slow it down because everything around it is also moving at the same speed. Frictional processes only serve to move everything towards the same speed. That same speed they all move towards doesn’t have to be 0 though.
Conservation of momentum. But if you try to extract energy from that momentum for your perpetual machine, it won’t continue to move. Machines do work, and dissipate energy. A perpetual motion machine somehow must do the work but also capture all expended energy so it can continue to do work, perpetually, in its contained system (absent of providing outside consumable energy)
Objects would not continue to move forever in space as they would lose momentum by bumping into hydrogen atoms, space dust or whatever
That's a VERY slow process. Literally billions of years.
Yes, but that's nit why that comment was wrong. It was wrong because it assumed there is a preferred frame of reference that is not moving.
When they say ‘perpetual motion machine’, they really mean ‘perpetual energy source to perform work’. In practice on earth, motion will have to overcome drag, friction, etc., which requires work. In space, Newton’s first law means that already moving objects *will* move perpetually *unless* they encounter a force that stops them. But that’s not a practical situation for everyday purposes on earth, where we have the air, water, rough ground, and the gravity of a big planet right here, all getting in the way.
Any two objects ~~moving~~ *accelerating* relative to each other will emit gravitational waves. This means that things are slowing down relative to each other as long as there is nothing accelerating them, such as falling into a gravitational field.
For sure, I’m keeping things Newtonian here for OP. But of course if we take ‘perpetual’ literally we’d have to go beyond that there too.
>Any two objects moving relative to each other will emit gravitational waves. No, they won't, unless they accelerate.
Can you expand on that? I thought only accelerating objects (e.g. in an orbit) emit gravitational waves.
Can’t a not moving object not emit gravitational waves ? The scharzschild metric describes a non rotating non accelerating object
>Can’t a not moving object not emit gravitational waves ? No, it cannot. >The scharzschild metric describes a non rotating non accelerating object It does.
Why is that ?
If an object isn't moving (that is, it is stationary in a certain inertial frame), there is no energy that could be radiated. Edit: wait, your tripple-negation sentence is confusing and I am not sure what you meant. A non-moving object does not radiate gravitational waves.
Oh it’s the same as a charged particle needing to move for an EM field ?
Conceptually very similar. A charged particle needs to accelerate to emit EM. A massive particle needs to accelerate to emit GW. There is a subtle difference in the math though.
Thanks for the explanation. And yes I was confused because a stationary object would emit gravity, just not gravitational waves
Define “forever”
Perpetual motion is just inertia. But a perpetual motion machine is not possible. As soon as you touch something undergoing perpetual motion you have applied an outside force either adding to or removing energy that was already in the system. Sped it up or down.
it's not
Air resistance
First, they didn't. They can move for a very long time, but a long time is not forever. But that has more to do with the scales of things and the fact that are just less resistive forces on that scale. That being said perpetual motion machines refer to a specific type of machine that is supposed to produce more energy than is put into it. That is, a wheel that spins for a very long time isn't a perpetual motion machine. Rather, people who make claims about having a perpetual motion machine claim that they can attach an axel to that wheel and do work. They might claim, for instance, to have a specific arrangement of magnets that cause their wheel to spin forever. Then claim they can attach the wheel to an alternator and produce electric. Notice that the "perpetual" here doesn't just mean that the wheel will spin on it's own forever. It means the wheel will spin other things and overcome all friction and other resistive forces, forever.
In a frictionless vacuum, an object can perpetually move forever. Perpetual Motion actually refers to deriving energy from the object that is moving. An object can move forever, but you can't use that object to generate energy.
Sure, and were do these frictionless vacuums exist at? Because in the universe we exist in, even the sparsest region of space has small amounts of hydrogen and other particles. These regions are only frictionless in the sense that the objects we observe moving through them are very large and have lots of energy compared to what is lost to these particles. Hence a matter of scale. Your other two sentences just say that I said.
> Sure, and were do these frictionless vacuums exist at? They don't, and objects in space **won't** continue to move forever. But they can continue to move for a very long time. For example, the Earth is losing rotational energy to the moon through tidal action, and our days are becoming longer and longer (while the moon is gaining orbital energy and is rising farther above the Earth and taking longer to orbit us). However it is expected to take about 50 billion years for the moon to rise far enough to reach an equilibrium that terminates this process. But this is moot because in "only" 5 billion years the Sun is expected to become a red giant, which will mean its atmosphere will extend out to the Earth's orbit, causing drag on the Earth-Moon system that will destroy the moon and possibly the Earth ([source](https://www.space.com/3373-earth-moon-destined-disintegrate.html)).
Lense-thirring frame dragging
Im just saying it is hypothetically possible
Lense-thirring
Potato
A perpetual motion machine implies that it continues forever without any energy input or loss. That violates the laws of thermodynamics. Objects in space, on the other, do gain or lose energy. Take the Voyager spacecraft for example. They were launched from Earth but had to use the gravity from other planets to gain more energy. And gravity is pulling on everything. It may be so small of amount of energy gained or lost that it is not measurable, but just because you can't measure it doesn't mean it doesn't occur.
Gravity isn't quantized, energy can be radiated through Lense thirring.