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FSW parameters to fabricate the dissimilar materials of Brass-Copper joints without any defects. It evaluated by microstructural characteristics, mechanical behaviour of the welded joints. In this process tool cylindrical shoulder produced frictional heat among tool shoulder and base materials. Commercially size of plates 100 × 50 × 6 mm Br and Cu were welded by machine parameters of rotational speed 1200 rpm travel speed of 50 mm/min and an axial force of 8 kN without any tilt. The made-up of FSW specimens characterized microstructures by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM). Observed microhardness by Vickers hardness tester. The observed stir zone area properly joined in dissimilar metals without any defects and enhanced the microhardness value.

This chapter deals with the discovery of the positron, the antiparticle of the electron. The positron was first discovered and reported by Carl Anderson and soon thereafter by Patrick Blackett and Giuseppe Occhialini. Neither of these experimental investigations were motivated by Paul Dirac’s earlier prediction of the positron, but rather by more straightforward concerns about the absorption and production of cosmic ray particles. With respect to the experiments of Anderson, and Blackett and Occhialini, once was enough to establish the existence of the positron and to justify in particular the pursuit of gamma ray experiments in an attempt to understand the production of positrons.

This chapter will discuss the discovery of parity nonconservation. In the early 1950s the physics community was faced with a vexing problem, the θ − τ puzzle. There were two elementary particles which, on one set of criteria, mass and lifetime, seemed to be the same particle. On another set of criteria, spin and parity, they seemed to be different particles. Solutions using accepted physics failed to solve the problem. In 1956, T.D. Lee and C.N. Yang suggested that the violation of parity symmetry, a radical solution, would solve the problem. They proposed several experiments that would test their hypothesis. The first, the β decay of oriented nuclei was performed by C.S. Wu and her collaborators. They observed an asymmetry in the decay, more electrons were emitted opposite to the nuclear spin than along the spin direction, thus, demonstrating parity nonconservation. Similar results were obtained in a second set of experiments on π → μ → e decay by Garwin, Lederman, and Weinrich and by Friedman and Telegdi. The question of whether just one of these experiments would have sufficed to demonstrate parity violation will be discussed.

In this chapter we will examine the Meselson-Stahl experiment which decided the issue between two models of DNA replication. The experimental result was held not to require replication. A second experiment of a rather different nature was used to eliminate a third possibility, and whereby contrast that experiment was further refined so as to yield a more accurate result. These experiments set the stage for the creation of new competing accounts of DNA replication all of which—given the framework established by the Meselson-Stahl experiments—were eliminated by experimental tests leaving only the Watson-Crick model in play.

At the time of Mendel’s experiments on plant hybridization in pea plants, there was no existing theory of hybridization. It is unclear whether Mendel had his reproductive and inheritance model, which included dominant and recessive characters, in mind when he began his work, or whether it was suggested to him by his early results. His experiments on pea plants not only suggested the laws of segregation and independent assortment, which are the basis of modern genetics, but also provided strong evidence for those laws. The experiments also provided evidence that those factors, which we now associate on a deeper biological level with genes, come in two types, dominant and recessive. And while Mendel’s results once they had been rediscovered were in some respects experimentally duplicated, those duplications were of only limited importance since by far the dominant effort was to expand on Mendel’s results by extending their application and tying Mendel’s factors more specifically to their biological underpinnings. In short, once was very much enough to get a major experimental and theoretical endeavor under way.

In this chapter we will discuss the experiments of Richard Cox and his collaborators, as well as those of Carl Chase, Cox’s student, which, at least in retrospect, showed that parity symmetry, (space-reflection symmetry), was violated. The experiments involved the double scattering of electrons from β decay. The second scattering showed a left-right (90°/270°) asymmetry. At the time, no one, including the experimenters themselves, realized the significance of these results. We will discuss whether these results did, in fact, demonstrate parity nonconservation and why they were overlooked.

In 2016 the LIGO-Virgo collaboration announced “the first direct detection of gravitational waves.” This was to distinguish their result from the indirect observation of Russell Hulse, Joel Weisberg, and Joseph Taylor, which used the decrease in the period of a binary pulsar to “establish, with a high degree of confidence the existence of gravitational radiation as predicted by general relativity.” This raises several interesting questions. One might ask how one can distinguish between direct and indirect observation and whether that distinction is exemplified in the practice of science. One might also ask whether a direct observation has more epistemic weight than an indirect observation. In this essay, I briefly discuss several episodes from the history of modern physics in an attempt to answer those questions. These episodes include Galileo and falling bodies, the discovery of the neutrino, the Higgs boson, and gravitational radiation.

Anyone who studies the history of physics quickly realizes that the history presented in physics textbooks is often inaccurate. I will discuss three episodes from the history of modern physics: (1) Robert Millikan’s experiments on the photoelectric effect, (2) the Michelson-Morley experiment, and (3) the Ellis-Wooster experiment on the energy spectrum in β decay. Everyone knows that Millikan’s work established the photon theory of light and that the Michelson-Morley experiment was crucial in the genesis of Albert Einstein’s special theory of relativity. The problem is that what everyone knows is wrong. Neither experiment played the role assigned to it by physics textbooks. The Ellis-Wooster experiment, on the other hand, is rarely discussed in physics texts, but it should be. It led to Wolfgang Pauli’s suggestion of the neutrino. I will present a more accurate history of these three experiments than those given in physics texts.