# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Bill S on 20/12/2017 22:08:39

Title: Where does the energy come from?
Post by: Bill S on 20/12/2017 22:08:39
I found this question asked by a long since vanished poster in another forum.

It was not answered, and as I no longer post in that forum, I am bringing it here.

My initial thought is that if an object is moving it has kinetic energy, so any radiation would decrease its kinetic energy.

Am I on the right track?
Title: Re: Where does the energy come from?
Post by: chiralSPO on 20/12/2017 22:24:45
Do you mean radiation from the object, or radiation from the environment onto the object?

If the object is emitting EM radiation then it must be losing energy, but it doesn't necessarily follow that the velocity must change. A red hot cannonball flying through space has a combination of kinetic, potential (gravitational), and thermal energy. The kinetic and potential energies will match Newtonian models very well, and the cooling of the cannon ball will have no significant effect on the trajectory, even if it represents a significant change in total energy of the projectile. (a 10 kg iron ball moving at 100 m/s has a kinetic energy of 50 kJ, and if it cooled from 600 °C to 200 °C that would require radiation of about 1800 kJ of energy. Where did it come from? Thermal energy (which one could argue is a type of kinetic energy, but it is internal energy).)

Radiation from the environment can be used to slow particles down (https://en.wikipedia.org/wiki/Doppler_cooling), but I don't know how effective that would be on macroscopic objects. EM radiation can also be used to speed things up (https://en.wikipedia.org/wiki/Laser_ablation or https://en.wikipedia.org/wiki/Solar_sail)
Title: Re: Where does the energy come from?
Post by: Bill S on 20/12/2017 22:48:09
Quote
Do you mean radiation from the object, or radiation from the environment onto the object?

Unfortunately, I don't know what the OP had in mind.  I assumed that as he was referring to moving objects kinetic energy had a particular significance.  I'm probably guilty of over-simplification.

Title: Re: Where does the energy come from?
Post by: Colin2B on 21/12/2017 09:15:30
Without detail of what energy it is difficult to tell.
Title: Re: Where does the energy come from?
Post by: evan_au on 21/12/2017 10:33:40
Quote from: OP
if an object is moving it has kinetic energy, so any radiation would decrease its kinetic energy.
Black-body radiation due to a temperature above absolute zero will exert a radiation pressure on the object.

If the object has a uniform temperature and a uniform surface finish, then the thermal radiation from the object will be equal in all directions, and won't accelerate the object at all.

If, however, the object has an unequal temperature distribution, or one side that radiates more effectively than another, there will be a tiny net force on the object, which could accelerate or decelerate the object (depending on your frame of reference).

A tiny acceleration like this was detected from the electronics module of the Pioneer spacecraft. Although the electronics module had a uniform temperature and surface finish, it was mounted behind the radio antenna, so radiation in one direction could be emitted into space, while radiation in the other direction was scattered by the back of the antenna.

This resulted in an acceleration of around 10-10 of Earth's surface gravity.

See: https://en.wikipedia.org/wiki/Pioneer_anomaly

Similarly, if the ambient radiation is the same from all directions, the object won't accelerate at all.

However, if the Sun is on one side, and deep space on the other, you have invented a solar sail.

See: https://en.wikipedia.org/wiki/Solar_sail
Title: Re: Where does the energy come from?
Post by: Colin2B on 21/12/2017 11:33:06

Unfortunately, I don't know what the OP had in mind.  I assumed that as he was referring to moving objects kinetic energy had a particular significance.
That would still leave the question of where the KE came from.
I can imagine the original op might ask if the total amount of energy in the universe is finite and we cannot create energy, where did it come from? Other than the get out of jail card “big bang”.
Title: Re: Where does the energy come from?
Post by: jeffreyH on 21/12/2017 12:27:52
Potential energy is also to some extent a get out of jail card. It is field energy density that accounts for this energy. When we say an object acquires potential energy is this not misleading? If the gravitational field were to suddenly vanish the object would not maintain this extra energy.
Title: Re: Where does the energy come from?
Post by: chiralSPO on 21/12/2017 16:12:12
Potential energy is also to some extent a get out of jail card. It is field energy density that accounts for this energy. When we say an object acquires potential energy is this not misleading? If the gravitational field were to suddenly vanish the object would not maintain this extra energy.
(bold added for emphasis)

This may be somewhat pedantic, but find me a mechanism for a gravitational field to just vanish that doesn't impart kinetic energy to an orbiting body. Since we know of no way that a gravitational field could just vanish instantaneously, there is no way to predict what would happen to other bodies (or how much energy would need to be put in to make it happen).

I do agree, though, that it is important to talk about the potential energy of a system, rather than just a single object (it is a function of the interactions of multiple objects and/or fields)
Title: Re: Where does the energy come from?
Post by: Colin2B on 21/12/2017 16:52:00
When we say an object acquires potential energy is this not misleading?
I think the shorthand way we use energy to describe the bookeeping can be confusing to some people. It implies energy is a thing.
Title: Re: Where does the energy come from?
Post by: jeffreyH on 21/12/2017 17:22:25
Potential energy is also to some extent a get out of jail card. It is field energy density that accounts for this energy. When we say an object acquires potential energy is this not misleading? If the gravitational field were to suddenly vanish the object would not maintain this extra energy.
(bold added for emphasis)

This may be somewhat pedantic, but find me a mechanism for a gravitational field to just vanish that doesn't impart kinetic energy to an orbiting body. Since we know of no way that a gravitational field could just vanish instantaneously, there is no way to predict what would happen to other bodies (or how much energy would need to be put in to make it happen).

I do agree, though, that it is important to talk about the potential energy of a system, rather than just a single object (it is a function of the interactions of multiple objects and/or fields)

There you go. Spoiling my fun with actual science. LOL
Title: Re: Where does the energy come from?
Post by: yor_on on 22/12/2017 07:52:53
You could look at it slightly differently.

A object following a geodesic (uniform motion), then 'gravity' disappear (speed of it doesn't matter for this one), does it 'accelerate' or does it just follow a `'new' geodesic. If you think it accelerate please show me how that is supposed to work?
=

You can always ask yourself how / if you would be able to measure a 'gravity', presuming us to be inside 'a black box'. A acceleration is measurable. We can reverse it by using a constant uniform acceleration, which then would be the equivalent to 'gravity' inside some rocket, then let a ball fall to the 'ground' and watch what happens as we at the same time turn of the acceleration.

Another question to be made in this might be two black holes/neutron stars 'falling into each other'. If there was kinetic energy imparted by this, could they still be said to be following geodesics? If you impart a kinetic energy on something, as with a bullet being propelled by its gases expanding, then that is a acceleration, is it not?
Title: Re: Where does the energy come from?
Post by: evan_au on 22/12/2017 09:08:54
Quote from: yor_on
Another question to be made in this might be two black holes/neutron stars 'falling into each other'. If there was kinetic energy imparted by this, could they still be said to be following geodesics?
The kinetic energy of approaching black holes comes from their initial gravitational potential energy (minus a bit radiated as gravitational waves).

When they are far enough apart to be considered as point masses, then they will follow a geodesic in spacetime.
When their event horizons are almost touching, the event horizon is distorted from the usual ellipsoidal shape, but the singularity would still follow a geodesic (if anything at all can be said about what happens inside the event horizon...).

Quote
If you impart a kinetic energy on something, as with a bullet being propelled by its gases expanding, then that is a acceleration, is it not?
A bullet driven by expanding gases accelerates down the barrel, and is not in free fall, and is not following a geodesic - at least until it is traveling through a vacuum, when it will follow a geodesic.

If you let a bullet drop into a black hole, then it will follow a geodesic.