This is a question with a multiple-part answer. The first part goes as follows: we have different domains of applicability. The first law of thermodynamics, which deals with the conservation of energy, is fundamentally about quantifying the exchange of energy (heat, work, internal energy). It gives the correct calculations of energy changes in a system but does not address the directionality of processes.

The second law of thermodynamics introduces the concept of irreversibility and the arrow of time. It governs the flow of energy in processes like heat engines and establishes that certain processes, such as the conversion of heat to work, are fundamentally limited by natural irreversibility.

The second part deals with dissipation and entropy; the first law of thermodynamics can factor in energy losses (like heat or friction) by adding specific terms to the equations. However, it doesn’t tell us why these losses happen or why some processes can't be reversed. This is where the second law comes in.

The second law explains that energy naturally tends to spread out and become less useful for doing work. This spreading out is called entropy, and it's why we can never make a machine perfectly efficient—there will always be some energy lost to things like friction and heat. The second law sets the limits on how efficiently we can make things like engines and reminds us that some losses are simply unavoidable in real life.

The third part deals with the importance of Carnot Efficiency; the second law of thermodynamics tells us the maximum possible efficiency that any system, like a heat engine, can achieve—this is called Carnot efficiency. While the first law can calculate the energy being used or lost, it doesn’t set limits on how efficient a system can be. Without the second law, we wouldn’t know how close a real system is to being as efficient as it could be, which is essential when designing engines, power plants, and other energy systems.

Even though the second law’s efficiency predictions might seem based on observations, they aren’t just guesses. They are rooted in the well-known fact that entropy (the natural spreading out of energy) always increases in isolated systems. These observations form the foundation of how we understand energy and its behavior, so they are far from speculative—they’re crucial for understanding real-world limitations in engineering and science.

Some people think of the second law of thermodynamics as more theoretical or abstract because it deals with things like entropy and the fact that certain processes can’t be reversed. This makes it feel less concrete than simply tracking energy, as we do with the first law.

But actually, entropy can be measured, and the second law can be used to make precise predictions about how far a system is from being perfectly efficient. Saying that the second law is speculative overlooks the fact that it's been proven over and over in a wide range of real-world systems. It’s not just a theory—it’s backed by solid evidence and helps us understand the natural limits we face in engineering and science.

The last part is the first and second laws of thermodynamics work together, and you can’t swap one for the other. The first law focuses on energy conservation—it tells us how much energy is moving around, whether it's being used or stored. The second law, however, tells us about the quality of that energy—whether it can still be used to do work, the natural direction of processes, and why energy is always lost in the form of heat or friction.

For example, in a heat engine, the first law can tell us how much energy is being used or lost, but only the second law can explain why the engine can never be 100% efficient and why some energy is always wasted. Ignoring the second law would mean losing these important insights into how real-world processes work.

These lengthy articles can easily confuse scientists. Judging the direction is not a big deal, the key is to make quantitative calculations that are consistent with experiments. The second law of thermodynamics is far from perfect in this regard. The dashed arrows in the picture indicate a lack of quantitative calculations.

In fact, Wh (dissipation term) in the formula can indicate the direction of dynamics. Of course, the free energy G can also replace entropy to explain the thermodynamic direction.

The second law of thermodynamics still has contradictions, as shown in the picture.

Bo Miao Antonios Valamontes

The second law of thermodynamics has always sparked intriguing debates, and I appreciate the challenge you've raised regarding Kelvin's argument and the concept of perpetual motion. The law, which prohibits the second type of perpetual motion machine, is often seen as limiting, yet it is foundational to our understanding of energy transfer and entropy. It's not simply about asserting that perpetual motion machines are impossible, but about the universal principle that systems tend toward increasing disorder or entropy.

To expand the discussion, it's worth considering how modern science continually explores the boundaries of these laws. For instance, recent advancements in quantum mechanics and nanoscale thermodynamics are beginning to question how these principles apply in extreme or non-classical scenarios. Though we aren't overturning the second law anytime soon, it's fascinating to think about where these fields might lead in terms of energy efficiency or new forms of energy systems.

This leads to a larger, global question: Could breakthroughs in fields like nanotechnology or renewable energy ever challenge our classical understanding of thermodynamics? It's essential to remain open to new possibilities, even while respecting the fundamental laws that have guided science for so long.

I'd love to hear more thoughts on how current technological advancements might evolve our understanding of energy systems! Let's keep pushing the boundaries of what's possible.

Thank you for your attention to this issue.

Firstly, it is necessary to follow the general principles of science: logicality and empiricism.

Secondly, the irreversibility of the second law of thermodynamics and Newton's law of relativity are proud, while the reversibility of quantum mechanics is violated. Scientists are actually proud of this violation.

I only discuss basic thermodynamic problems, and I am not familiar with some cutting-edge technologies. I am willing to see familiar people express their opinions,

Bo Miao

Please have a look at the JANAF Thermochemical Tables. You fill find there entropies as a function of temperature tabulated for many substances. There is a lot of experimental research to this end. Do you mean that these all experiments are wrong? By the way, JANAF means Joint Army and Air Force, as this tables have been created to help to compute the rocket propulsion. Try to do it without the Second Law.