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I understand that the Laws of Thermodynamics imply that no form of perpetual motion machine is possible in an isolated system.

But, isn't it the case that light seems to go on forever. The light from stars millions of light years away, millions of light years ago, bring us energy and momentum today. This seems quite contradictory to me.

Explanation:

This is my understanding of the fact that the Laws of Thermodynamics imply no perpetual motion: Every form of energy ends up as internal energy of astronomical bodies and there is no way to convert it back to other forms with 100% efficiency. If this is true, then this is the contradiction: It seems light will rarely end up as internal energy, compared to other forms at least. Or we would not have detected light from stars millions of light years away, millions of light years ago.

  • Thus if u use light to power something, will perpetual motion be possible?
  • What is the reason for light's high resilience to end up as internal energy ( at least as compared to other forms of energy )? How does this relate to QED's way of treating matter-light interactions?
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  • $\begingroup$ Light coming from remote stars is heat. Heat is energy transfer process, not something contained. Sunlight heats (transfers energy to) the Earth. So does starlight, but by an immeasurably small amount. $\endgroup$
    – David Hammen
    Commented May 26, 2017 at 7:39
  • $\begingroup$ By heat I meant energy due to internal motion of the molecules/atoms $\endgroup$
    – PhyEnthusiast
    Commented May 26, 2017 at 7:49
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    $\begingroup$ Don't confuse the two. Internal energy and kinetic energy are state variables of an object or system of objects. Heat and work are processes that change those state variables. $\endgroup$
    – David Hammen
    Commented May 26, 2017 at 8:20

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Pioneer 10, a man-made object, was launched in March 1972. It's currently travelling at 12.5 kilometers per second in the outer space, after having escaped the Solar System. Unless something stops it, it will travel forever in this galaxy.

Your statement, that the second law of thermodynamics implies no perpetual motion, is incorrect. In fact, it contradicts Newton's first law:

In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

There is no friction in space. There are no forces opposing motion. Therefore, not only light, but any object, can travel indefinitely.

The second law of thermodynamics is more of a "no free lunch" statement. It means we cannot build a perpetual motion machine of the second kind, that is (from wikipedia):

[...] a machine which spontaneously converts thermal energy into mechanical work. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However, it does violate the more subtle second law of thermodynamics (see also entropy).

Needless to say, perpetual motion of an untouched body is useless in terms of extraction of mechanical energy.

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The Second Law of Thermodynamics requires that entropy stay the same or increase. Light travelling without interacting is a process for which entropy is a constant.

Perpetual motion machines generally refer to systems from which we can extract usable work without inputting any energy. In the example of light propagation in vacuum, the light isn't outputting any work while travelling through space.

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Cosmic microwave background CMB Initial light is changed from high frequency to lower frequency. Deterioration goes on as the have become longer and contain less energy

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