In 1894, the president of the Royal Society, Lord Kelvin, predicted that radio had no future. The first radio factory was opened five years later. He also predicted that heavier-than-air flying machines were impossible.

A physical law, scientific law, or a law of nature is a scientific generalization based on empirical observations of physical behavior. Empirical laws are typically conclusions based on repeated scientific experiments over many years, and which have become accepted universally within the scientific community. The production of a summary description of nature in the form of such laws is a fundamental aim of science.

Several general properties of physical laws have been identified, although each of the characterizations is not necessarily original to them). Physical laws are:

Often those who understand the mathematics and concepts well enough to understand the essence of the physical laws also feel that they possess an inherent intellectual beauty. Many scientists state that they use intuition as a guide in developing hypotheses, since laws are reflection of symmetries and there is a connection between beauty and symmetry. However, this has not always been the case; Newton himself justified his belief in the asymmetry of the universe because his laws appeared to imply it.

Physical laws are distinguished from scientific theories by their simplicity. Scientific theories are generally more complex than laws; they have many component parts, and are more likely to be changed as the body of available experimental data and analysis develops. This is because a physical law is a summary observation of strictly empirical matters, whereas a theory is a model that accounts for the observation, explains it, relates it to other observations, and makes testable predictions based upon it. Simply stated, while a law notes that something happens, a theory explains why and how something happens.

Well-established laws have indeed been invalidated in some special cases, but the new formulations created to explain the discrepancies can be said to generalize upon, rather than overthrow, the originals. That is, the invalidated laws have been found to be only close approximations (see below), to which other terms or factors must be added to cover previously unaccounted-for conditions, e.g., very large or very small scales of time or space, enormous speeds or masses, etc. Thus, rather than unchanging knowledge, physical laws are actually better viewed as a series of improving and more precise generalisations.

Some laws only are approximations of other more general laws, and are good approximations with a restricted domain of applicability. For example, Newtonian dynamics (which is based on Galilean transformations) is the low speed limit of special relativity (since the Galilean transformation is the low-speed approximation to the Lorentz transformation). Similarly, the Newtonian gravitation law is a low-mass approximation of general relativity, and Coulomb's law is an approximation to Quantum Electrodynamics at large distances (compared to the range of weak interactions). In such cases it is common to use the simpler, approximate versions of the laws, instead of the more accurate general laws.

Some extremely important laws are simply definitions. For example, the central law of mechanics F = dp/dt (Newton's second "law" of mechanics) is often treated as a mathematical definition of force. Although the concept of force predates Newton's law, there was no mathematical definition of force before Newton. The principle of least action (or principle of stationary action), Schroedinger equation, Heisenberg uncertainty principle, causality and a few other laws also fall into this category (of mathematical definitions).

Most of the other fundamental physical laws are mathematical consequences of various mathematical symmetries. Specifically, Noether's theorem connects the fundamental conservation laws to symmetries. For example, conservation of energy is a consequence of the shift symmetry of time (no moment of time is different from any other), while conservation of momentum is a consequence of the symmetry (homogeneity) of space (no place in space is special, or different than any other). The indistinguishability of all particles of each fundamental type (say, electrons, or photons) results in the Dirac and Bose statistics which in turn result in the Pauli exclusion principle for fermions and in Bose-Einstein condensation for bosons. The partial symmetry between time and space coordinate axes results in Lorentz transformations which in turn results in special relativity theory. Symmetry between inertial and gravitational mass results in general relativity, and so on.

The inverse square law of interactions mediated by massless bosons is the mathematical consequence of the 3-dimensionality of space.

So to large extent laws of nature are not laws of nature per se, but mathematical expressions of certain simplicities (symmetries) of space, time, etc. In other words, there are quantities (e.g. the origin of the coordinates for time and space, the identity of a specific electron) upon which nothing depends. Currently the search for the most fundamental law(s) and most fundamental object(s) of nature is synonymous with the search for the most general mathematical symmetry group that can be applied to the fundamental interactions.

Conclusions

The application of these laws to our needs has resulted in spectacular efficacy of science – its power to solve otherwise intractable problems, and made increasingly accurate predictions. This in turn resulted in design and implementation of variety of reliable transportation and communication means, in building more quality and affordable shelters, in creating variety of drugs, in finding new energy sources, in developing variety of entertainments, etc.

An intrusion detection system is used to detect several types of malicious behaviors that can compromise the security and trust of a computer system. This includes network attacks against vulnerable services, data driven attacks on applications, host based attacks such as privilege escalation, unauthorized logins and access to sensitive files, and malware (viruses, trojan horses, and worms).

An intrusion prevention system is a computer security device that monitors network and/or system activities for malicious or unwanted behavior and can react, in real-time, to block or prevent those activities. Network-based IPS, for example, will operate in-line to monitor all network traffic for malicious code or attacks. When an attack is detected, it can drop the offending packets while still allowing all other traffic to pass. Intrusion prevention technology is considered by some to be an extension of intrusion detection (IDS) technology. The term "Intrusion Prevention System" was coined by Andrew Plato who was a technical writer and consultant in IT Security.

Intrusion prevention systems (IPS) were invented in the late 1990s to resolve ambiguities in passive network monitoring by placing detection systems in-line. A considerable improvement upon firewall technologies, IPS make access control decisions based on application content, rather than IP address or ports as traditional firewalls had done. As IPS systems were originally a literal extension of intrusion detection systems, they continue to be related.

Intrusion prevention systems may also serve secondarily at the host level to deny potentially malicious activity. There are advantages and disadvantages to host-based IPS compared with network-based IPS. In many cases, the technologies are thought to be complementary.

An Intrusion Prevention system must also be a very good Intrusion Detection system to enable a low rate of false positives. Some IPS systems can also prevent yet to be discovered attacks, such as those caused by a Buffer overflow.