Does physics explain why the laws and behaviors observed in biology are as they are? I feel like biology and physics are completely separate and although physics determine what's possible in biology, we have no idea how physics determine every facets of biology. We know roughly how forces in physics may impact biological systems, but not every little connections and relations that exist between physics and biology. Am I wrong?
To get more insight into a question like this, you might like to ponder the relationship between logic gates and programming languages in the case of computers. This is a lot simpler than the physics—biology question, but begins to open up some of the issues. When a computer runs a program, certainly lots of logic gates and memory elements etc. are enacting the process described by the program. But the logic gates do not themselves tell you much about the structure and nature of a high-level programming language such as Java or Python. In a similar way, further study of atoms and molecules will not in itself reveal much about the immune system in mammals, or the social structure of an ant colony, and things like that.
This "answer" is really a brief comment on what is, in the end, quite a deep issue concerning the whole nature and structure of scientific knowledge. Another useful thing to ponder is the relationship between the concepts involved when one moves from the equations of particle physics to many-body physics. There is every reason to consider that the motions of a non-linear many-body system are all consistent with the description offered by the Standard Model of particle physics for all the various fields and interactions. However, the low-level description does not in itself tell us how to formulate a field theory which correctly captures the main elements of the collective behaviour.
It is a bit like the difference between knowing the rules of chess and knowing how the game is played to a high standard. For the latter one needs to appreciate some higher-level issues such as the importance of the central squares, pawn structure, open files and things like that. It is not that these are somehow operating without regard to the laws constraining the movement of the pieces, but rather the low-level laws (about how individual pieces may move) simply do not frame a language adequate to describe the higher-level issues. This analogy with chess is not perfect, of course, but it is apt nonetheless, and it illustrates why it is quite misleading to claim, as many do, that "physics explains chemistry". The situation is more subtle than that.
For example, the behaviour of many chemical reaction networks has features which do not depend much if at all on the individual reactions, but on the global structure of the network. It is not that such networks fail to respect any law of physics, but the description at the level of individual components cannot frame a language adequate to express and thus grapple with the higher-level issues such as whether the network is stable overall, and things like that. And what is really telling is that it is very common for these higher-level languages to have internal consistency and a certain robustness, such that they can be supported by more than one underlying hardware. This is similar to the way a given computer program can run on different types of hardware as long as the operating system is in common.
Physics does not determine every facet of biology! Biology is a great example of what is called emergence - the phenomena where the composite of many simple entities has properties that the entities themselves do not possess (and which are not always predictable on the basis of the properties of the entities).
I have already given an example in my comment (also given in the answer by @AndrewSteane) of computer programs: existence and properties of an operating system, such as Linux or Windows, is certainly dependent on the existence of the silicon atoms constituting the chip, but cannot be predicted from the properties of these atoms alone. There are many layers of complexity in between:
- the atoms make a crystal
- the logic gates in a chip are fabricated from this crystal
- the architecture of the logic gates on a chip is not directly determined by the properties of the gates themselves
- the assembler language, addressing the gates on the basic level could exist in many different versions
- the same assembelr language allows for many operating systems with wildly different properties
Claiming that everything is just physics simply because physics underlies the basic processes is a logically faulty argument.
There is a big part of biology, dealing with the phenomena that can be directly described using physics equations. Just to give a few examples:
- the molecular bonds and basic chemical kinetics
- folding of macromolecules
- mechanical properties of body tissues
- flow of blood and other liquids
- skeletal and muscular dynamics
For a deeper overview you may consult these books:
I also recommend the Lectures on statistical physics and protein folding by Kerson Huang, the author of the widely used basic textbook on statistical physics. This one is a short and very readable introduction to the complexity of the phenomena involved.
Physically-inspired biological models
In some cases one uses models inspired by physics to model the biological phenomena that are not really the same as those underlying the original physics model. E.g., one uses Ising spin chains for statistical analysis of RNA structures - where the probabilities of configurations reflect not thermodynamical properties, but the frequency of these configurations in organisms, determined evolutionary rather than physical constraints. In a related example, Henry Orland has used large N expansion of the quantum field theory for classifying possible RNA structures - this is again an author of the well-known text on many-particle physics. See also this question.
Another such application is the use of the equations for drift-diffusion in non-linear potential to model decision-making by mice.
Many problems in modern biology are not directly related to physics, but often require the mathematical, statistical, and research skills that physicists possess - it is not therefore surprizing that many computational biologists have physics background (I am among them). One could give as examples:
- modeling of non-linear phenomena in cells (e.g., a virus replication cycle can be described as switching between two distinct phases, controlled by the concentration of chemicals)
- Population biology (modeling gene propagation with generations, see the Gellespie's little textbook for the serious but short introduction or the Neher and Shraiman review, which is the crash-course for physicists.)
- Non-linear epidemiological equations (by now we have all see the curves that result from them, see Compartamental models in epidemiology)
- Bioinformatics, i.e., analysis of biological sequences (though this requires more computational than physics skills, there is a lot of matrix algebra in it)
Still, there are a lot of biological processes that are not described in terms of just physics. E.g., the processes of transcription and translation, which form the Central dogma of molecular biology are hardly reducible to physics or even chemistry. If you come from a physics background, the biochemistry describing them leaves a indeed taste of a fairy-tale (no Hamiltonian, no equations), but it is a result of systematic observations and statistical analysis.
@taciteloquence suggested in the comments a relevant quote from PW Anderson's 1972 article in Science More is Different.
The ability to reduce everything to simple fundamental laws does not imply the ability to start from those laws and reconstruct the universe.... At each stage entirely new laws, concepts, and generalizations are necessary, requiring inspiration and creativity to just as great a degree as in the previous one. Psychology is not applied biology, nor is biology applied chemistry.
As it was pointed out in connection to another question by @AndersSandbreg and @DvijDC, even if we could calculate behavior of complex biological systems from micrcoscopic physics laws (aka ab-initio simulation), this would not necessarily provide us with deeper insights into the behavior of these complex systems. Rather to the contrary, it is reducing the complex behavior to simple biological statements that gives us deeper udnerstanding (on a higher level). In this sens eit could be even claime that biology explains physics rather than the other way around.
Is biology an application of the laws of physics ? If you believe the laws of physics ultimately explain all physical phenomena then yes, it must be.
Are there grounds for this belief ? Well, so far we have not found any physical phenomena that definitely require something other than the laws of physics to explain them i.e. there is no empirical evidence for supernatural causes, for miracles, or for magic. And we know that biological phenomena follow regular patterns and can be investigated using the scientific method. Medicines do not come with warnings such as “will only work if you say the magic words and the stars are aligned properly”.
Do we know how some biological phenomena can be explained in terms of the laws of physics ? Yes, there is a whole field of study called biophysics that covers exactly this.
Do we know exactly and in detail how everything in biology can be shown to be an application of the laws of physics. No, not yet.
Is it a problem that we don’t know how exactly everything in biology is an application of physics ? No, not at all. Trying to reduce problems in biology to problems in physics is in many cases not a useful approach, even though we think it could be done in principle. By analogy, we know everything in cookery is an application of physics (since there is no magic pixie dust involved) but approaching every recipe as a physics problem will not get us very far.
To clarify some of the answers talking about emergence:
- Everything in biology is explained by physics, and physics predicts all of biology.
- A human who knows all of physics won't be able to use that knowledge to learn much about biology.
Some top answers right now claim that "physics does not explain biology" because biology involves "emergent" phenomena. This may be misleading. "Emergence" is a property of the person using a set of laws, not a property of the laws themselves.
When we say "a composite of parts has properties that the parts themselves do not," we mean that a human person looking at the (lowest-level) laws governing the behavior of the parts will have a hard time noticing that when you put the parts together, they follow some pattern that might be easy to spot if you watched the parts behaving together. We do not mean that the composite contradicts the low-level laws of the individual components, or that some additional rule is required to explain the composite's behavior.
Biology has a lot of "emergence," because living systems are so large and complex that no human could reasonably use the underlying laws of physics to predict their behavior. In principle, you could use particle physics to explain all of biology; in practice, that would be impossible, so virtually all useful biology is figured out top-down.
Physics does not explain all biological phenomena.
The key point other answers are making excellently is that any kind of problem (even in physics) is "layered". When worrying about a gas of Helium we don't worry about the standard model underneath, and we certainly don't worry about whether there is an as-yet undiscovered theory underneath that. No one knows for sure where the "bottom layer" actually is.
Many features of a system don't depend much on the layers underneath. For example consider the theory of evolution. Is it even possible to conceive of a set of laws of physics describing some kind of universe in which imperfect replicators (life) exist that are not subject to the theory of evolution? How could they not be? 
The theory of evolution is saying something very strong and something essentially independent of the layers underneath. Some other example emergent laws that seem very "robust" to the actual underlying rules might be thermodynamics, and some theories in economics (if more consumers want to buy a product the price goes up: that probably is true in any universe with capitalist consumers buying products, even if the physics of those universes were completely unlike our own).
: Unlimited resources might be a way to do this. If nothing ever needed to die then evolution would at least look quite different.
Every time this kind of discussions comes up I'm trying to remind people about the Mind projection fallacy.
It occurs when someone thinks that the way they see the world reflects the way the world really is.
We are using simplifications, abstractions and probabilistic reasoning because we are finite - our minds can only operate with a comparatively small amount of information about the world. Take for example the programming language analogy -- If we had the mental capacity to keep track of all the logic gates inside the computer, then we wouldn't need programming languages. We, humans, need operating systems and programming languages because we have to break the complexity of billions of transistors into manageable blocks.
So every time someone brings up concepts like "emergence" to explain some things, my clarification is that this "emergence" doesn't happen in the real world - it happens in our heads. (People usually don't like this clarification, though.)
Now, to answer your question. The difference between "physics" and "biology" is introduced by humans - their minds are too feeble to keep track of decillions of atoms moving around, so they abstracted some blobs of atoms into "organisms" and other such stuff. And then some (most) humans tricked themselves into believing that these abstractions are how the world really is.
Physics explains chemistry, and chemistry explains biology.
Physics explains what chemicals can be made, not what will be made. Likewise, chemistry explains what biology will work, not what will be made.
Chance explains some of biology. Some mutations happen, others do not. Some mutations are advantageous, some are not. It isn't true that every advantageous mutation spreads.
There are some aspects of biology for which we have no theory. Some of this is because the chemistry is very complex. Come back in a century, and we may know more.
The theory of the mind is only at the barest beginning. Personally, I don't have the faintest idea how sensation, emotion, and conciousness will be explained. The best I can say is "Whatever they are, they arise from the brain and I have no idea how." Religions provide answers, but the answers do not satisfy the standards of science. Many of the answers amount to "Whatever they are, they arise from God."
Edit - Comments on several answers are getting lengthy, and likely will be moved to chat soon. I have made several comments. I am moving some of them here. These are from comments on gandalf61's answer, where he says
... there is no empirical evidence for supernatural causes, for miracles, or for magic.
My comments address limits of what physics can say about biology, or perhaps what science can say about reality.
+1, but be careful about saying there is no evidence for miracles, etc. Science is all about laws of nature - repeatable patterns of behavior of the universe. A miracle is by definition not a repeatable effect of any physical cause. One could happen right there in the lab, and a good scientist would throw it out because it can't be verified by repeatable experiment. Not to say that miracles happen in the lab or elsewhere. But science has no evidence because by design science does not consider that kind of evidence or include that kind of artifact in any theory.
Science has no evidence for or against miracles. The lack of evidence is to be expected, and is not evidence for or against the existence of miracles. Science can't take a position on the question because it doesn't look for evidence.
However, science can take a position on other religious questions, such as the effectiveness of faith healing. Here a healer asks God for healing. We can measure if healing takes place and compare to similar cases where nothing is asked. I have heard of serious consideration about such a clinical trial. There are difficulties. How does one arrange a double blind trial, where neither healer nor patient knows if genuine or placebo prayer is being offered?
There are places we could look for things that have no natural cause. Messages encoded in the digits of pi. Patterns in quantum randomness. There are also areas where we have found strong patterns - natural laws - without being able to anything about their cause. I am not arguing for or against anything supernatural. Just trying to clarify the limits of science.
Any valid long English text encoded as ascii or whatever you specify in advance found early in pi can be your possible message. You can limit acceptable messages so the odds are you will not find one. You might find a message in French and throw it out, and thus fail to discover something valid. Or you might accept it, and maybe discover nonsense because you have violated the confirmation bias limits. Or an English message might be valid or invalid. Or there might be nothing to discover. You can arrange so that if you find something, the odds are it isn't chance.
If you do find something, the process doesn't tell you if it is unusual luck, great wisdom, a great lie, or something else.
Graham raises some points.
... it is absolutely correct to say that we have no evidence for miracles. The main issue is the "any physical cause" part. In all experiments, the biggest problem is isolating the many other potential physical causes from the one(s) you're investigating. Every experiment already has a repeatable source of "miracles", which is the rest of the world injecting noise which gets past your experiment design.
However science can take a position on "we know there are these things which would cause exactly this" or at least "we may not know the cause but we have repeatedly seen this happen occasionally". So science certainly can say that something is not a miracle, only a low-probability event. More than that, it must, because if you attribute anything at all, ever, to divine intervention then you destroy any possibility to investigate a physical source. And that is directly opposed to rational thinking. We have historical proof of why this is wrong.
Your problem there is confirmation bias. If you have a sufficiently large random sample (such as, hypothetically, the digits of pi), then any encoding you care to pick (maybe ASCII) will eventually find a string of digits which produce the message "I'm God and I'm a black woman. Fix the Sistine Chapel ceiling.". Just for an example. It's equally likely as the encoding producing "zadfafhtyuweljhbgjkfyfqemgeghrhdf", because that's how probability works. The risk for miracle hunting is that you find something because you're looking for something without knowing exactly what.
My point is that you have to say beforehand what message you're looking for. If you can't do that, then you can't calculate odds, because that would be asking "what's the probability of finding something I've already found?" to which the answer is "exactly 1, because you've already found it". But if you can predict it then by your definition it's not a miracle, so you'rea bit stuck there.
I feel like biology and physics are completely separate
For where biophysics is heading, and due to the distinction that is arising between "biophysics" and "physics of living systems", I think the idea that it goes physics $\to$ chemistry $\to$ biology is no longer the case (if it ever was). People are actually using biological systems to learn more about physics now. So these fields are actually deeply connected: many biological problems now require physics (either directly, or at least in application of its principles, techniques, concepts, etc.), and many biological systems can be/are used to learn about physics.
although physics determine what's possible in biology, we have no idea how physics determine every facets of biology.
It is true we do not have a complete knowledge of the physics of living systems. But we still have ideas as to how they are connected and applied.
Onto the title:
Does physics explain why the laws and behaviors observed in biology are as they are?
While my answer has claimed a strong link between physics and biology, there are of course distinctions. The statement "laws and behaviors observed in biology" is very broad. In physics, we all know the same principles; undergraduate physics education is essentially, "here are all of the principles of physics", and then if you go further into research you are essentially learning what those principles do in the universe. But if, say, a particle physicist wanted to talk to a solid-state physicist, they could still assume knowledge of the same base ideas in order to further understand each other.
In biology, it is somewhat the opposite. The field of biology is so vast and covers so many systems that there isn't really a shared set of underlying principles all biologists use. Someone working in developmental biology in the fruit fly will not be able to use their principles to determine what is going on with a biologist who is studying the interactions of species within some ecosystem. Just look across the answers to this question: there are so many areas of biology to consider, and they each have their own principles.
So while at the surface one can take the easy way out and say, "well, everything in the universe has to ultimately follow physics", if we dig deeper then we really need to be careful about what we mean by "laws and behaviors observed in biology". I might argue that there isn't a well-defined set of biological principles to even start exploring this question. (Although attempts are being made, e.g. William Bialek's book Biophysics: Searching for Principles)
What would a physics explanation of the mating behaviour of a bowerbird would look like, and who it would be useful to?
Biology is, in principle, reducible to fundamental physics but even if you were to solve any meaningful biological question in those terms - which is way beyond current abilities - the answer would get would have the wrong framing and explain at the wrong level. The meaningful answer for the bowerbird's wonderful mating ritual is one grounded in evolutionary theory not one grounded in particle physics.
Every biological system is comprised out of zillions of elementary particles. Each of these particles interacts with a fraction of the others. Every interaction can be accounted for with the help of elementary particle physics. So yes, every biological system can be accounted for in a physical way. You and I are huge collections of elementary particles having this conversation.
What physics, or chemistry, or biology (evolution conforming to the gospel according to "Sir" Richard Dawkins), nor psychology, or whatever -y, can't explain is the Nature of the soul, or the colors you see, the sounds you hear, the feelings you feel, the shivers you experience, the itch that itches, the pain that hurts, the aggression that haunts you, the love that moves you, the cravings that come along, the hate inside, the fear that freezes, the longing to win, the disappointment of losing, the mourning, the screaming that stops at your mouth, etc. How will you explain the feeling you feel when listening to your favorite music? This can't be explained in terms of physics. For, who knows the Nature of elementary particles as described by particle physics? Let alone of zillions of them interacting.
We are very good at describing small quantum mechanical systems, because today we have QM, and we do know that the world is ultimately quantum mechanical in nature. That being said, when it comes to predicting bigger biological systems (just like our own human nature), our capabilities are very limited. We are all made up of QM systems, elementary particles, who are reading this question, but if we would ask a question like, "can you explain why you are asking this question at all", then explaining it based just on QM is not possible. There are two main reasons for that:
- though we are very good at describing small QM systems, the task becomes extremely difficult for larger systems
- biology adds something extra, you can call it instinct, consciousness, life, nature, whatever you want, but it is governed by a biological program, and described by a programming language, the DNA (this about the programming language is nicely described in @andrewsteane answer). We are in babyshoes at describing biological system's behavior based on DNA, but in the future we might be able to do so with much more efficiency.
So the ultimate answer to your question is that biological systems are qualitatively more then just a "bunch" of elementary particles.
@Andrew Steane provided a really good answer. For further insight into his line of reasoning as I understand it, I would recommend reading Chapter 1 of Information Theory, Evolution, and the Origin of Life (2009) by Hubert Yockey.
The laws of physics and chemistry are much like the rules of a game such as football. The referees see to it that these laws are obeyed but that does not predict the winner of the Super Bowl. There is not enough information in the rules of the game to make that prediction. That is why we play the game. Chaitin (1985, 1987a) has examined the information content of the laws of physics by actually programming them. He finds the information content amazing small.
The reason that there are principles of biology that cannot be derived from the laws of physics and chemistry lies simply in the fact that the genetic information content of the genome for constructing even the simplest organisms is much larger than the information content of these laws (Yockey, 1992).
Bolded emphasis at the end is mine. While I have not read significantly further to really justify all of Yockey's claims, I think the spirit of his ideas is in the right place, and aligns with what Andrew was saying: just like you can't use the concept of a universal computer to specifically predict what programs will be written on it, you can't use physics to predict what the specific rules of biology will be. At best, you can only constrain the rules of biology to physically-possible scenarios.
Edit: Let me clarify what I was saying in the above answer. (tl;dr: Everything depends on how you define life, and, in my opinion, the question of how best to define life is not a physics question, although it can be guided by a physics understanding.)
I'm going to use the following definitions for "physics", "biology", and "life".
- Physics: A question can be answered by physics, if it is of the form: "If X occurs, will Y happen?" In other words, physics discusses possible physical scenarios and processes, contingent on a set of physical constraints. Unless we're being speculative, the physical constraints match those of the real world.
- Biology: The field of biology answers questions, sometimes quite high-level ones, about the physical processes involving living things.
- Life: Now, this definition is up for discussion! (Which is my entire point.)
We can choose a physics-dependent definition of life -- e.g. "A living thing is a localized collection of carbon and other atoms with X additional properties" -- or we can attempt to produce a relatively physics-agnostic definition of life (which Yockey attempted to do -- the question of whether his definition is a good one is a related, but different, matter).
If you choose a physics-dependent definition, then, yes, you can generally determine several biological rules -- but even then, not all of them, due to emergence, which others have mentioned. These physics-dependent definitions of life often run the risk of being too specific (e.g. too carbon chauvinist) -- we might have to expand the definition of life when we encounter new forms that we decide we want to be considered as alive.
However, I was merely noting that some people, including Yockey, have tried to produce relatively physics-agnostic definitions of life, which are of at least some epistemological interest (even if of little to no biological interest). I worry that Yockey's definition, which is based on information processing principles, runs the risk of being too general -- it is hard to see how computers, for example, would not be lumped into his definition of life. More generally, if several very different physical systems match a physics-agnostic definition of life, it is hard to argue that physics alone "predicts" how life ought to function: it simply constrains how life does/can function in our universe.
But, in my view, exactly how you define "life" is not a physics question (it's a biology question), and because the set of allowed biological processes is strongly dependent on how you define life, I would hesitate to say that physics predicts "why the laws and behaviors observed in biology are as they are". As others have mentioned, even if you a priori choose a physics-dependent definition of life, as we already know on Earth, living systems adopt a wide variety of behaviors, none particularly dependent a priori on the laws of physics, but just as much as on circumstances, history, hysteresis, and emergence.