# Is symmetry useful in physics?

1. Apr 27, 2007

### Rothiemurchus

If there were no symmetry theories in physics would our practical understanding of the world be worse - is there any everyday device/machine that would not have been thought up and invented without symmetry theories?

2. Apr 27, 2007

### G01

If nothing else, symmetry makes so much stuff......ALOT easier!!!!!!

3. Apr 27, 2007

### Parlyne

If there were no symmetries, I doubt we would be able to understand much of anything. In fact, the laws of physics themselves could well change from place to place and time to time.

There's an extremely powerful mathematical result, known as Noether's Theorem, which proves that there is an intimate connection between symmetries and conservation laws. For every symmetry of a system, there must be a corresponding conserved quantity. For example, translation symmetry - the fact that the laws of physics are the same at all places - is responsible for the conservation of momentum. Time translation symmetry - the fact that the laws of physics don't change over time - is responsible for the conservation of energy. Rotational symmetry causes conservation of angular momentum. And so on.

Without these sorts of principles, we would have nowhere to even start talking about the world.

4. Apr 27, 2007

### mjsd

symmetry is the cornerstone of physics.... and indeed in many sciences. "Pattern matching" is usually the most naive starting point in the development of any theories: if you saw two events being similar, you may conjecture that a same set of fundamental laws were at play. When you have seen more and more similar events, eventually you may be able to build up a theory to explain all these events and how they are "similar".... in essence, what you have done in this little exercise is in fact looking for some "symmetry" of nature.

5. Apr 28, 2007

### AlephZero

Of course I agree that symmetry is the cornerstone of physics.

But re the original question, I'll turn it round:

Would somebody like to explain how "symmetry" helped the native Australians to invent the boomerang, as a very effective everyday tool for hunting animals?

I'm just making the point that understanding phyisics (in the modern sense of the word) is not a necessary requirement for developing good technology.

6. Apr 28, 2007

### mjsd

able to develop good technology is the result of understanding science (in this case we are talking about physics).... I am sure a shotgun, grapping hook, speargun... are probably more effective than the boomerang in catching animals. I guess your point is that there are certain things which can be developed without the use of "symmetry", physics or science... sure, that's called "trial and error"... but note that when learning about your mistake, it is like a form of "pattern matching".. in that you realise why something always goes wrong..etc. Realising that there is a pattern is akin to realising a "symmetry" in the system. As to whether one can invent something rather elaborate out of nothing purely by one's creativity and absolutely no contribution from experience acculmulated by interacting with the surrounding environment, I doubt that very much. (besides it is probably not the way the brain works )

7. Apr 28, 2007

### ZapperZ

Staff Emeritus
Actually, it is, at least for some people.

Maybe the material scientists and electrical engineers do not need to understand the physics behind the NEW semiconductor material that they are using, but someone HAD to to be able to (i) identify that that material has the properties that is required in modern electronics (ii) come up with the necessary recipe to make such a thing (after all, you can't just throw things randomly together to make the infinite variety of material).

Most of your modern electronics came about DUE to the basic understanding of physics. Look at the invention of the transistor. It is not a coincidence that one of the co-inventor is a THEORIST. If you had read John Bardeen's biography, you would have realized how much quantum mechanics played a part in its invention. I can't think of any other device that has as much of an impact in modern electronics than this invention. So this is one clearly and direct example of how the basic knowledge in physics led to the invention of a practical application.

I could easily go on and on about MRI, PET scans, GPS, etc... etc..

Zz.

8. Apr 28, 2007

### AlephZero

Sure, but a long list of things that were invented USING a knowledge of physics is irrelevant to answering the OP's question: are there "everyday devices/machines" that were invented NOT USING a knowledge of physics.

The answer to the OP's question is self evidently yes, unless you can show that no devices invented before the concepts of "physics" and/or "symmetry" were formalized are still in common use.

If you don't like the boomerang as a counter example, the wheel will do just as well.

I like the boomerang better than the wheel as an example, because

(1) the physics of why it works is rather complicated, and there was NO theoretical understanding of how it worked by science before about 1850 (and possibly not before 1900).
(2) it was demonstrably invented by a group of people who weren't bothered about understanding the world in any ways that "western science" would recognize as valid.

9. Apr 28, 2007

### ZapperZ

Staff Emeritus
You don't consider a radio or computer an "everyday" machine?

Zz.

10. Apr 28, 2007

### AlephZero

Taking a global view of the development of technology by humans, no.

I won't be around to see if I'm right, but personally I expect the number of radios and computers in use on earth on 2100 will be about the same as in 1900. Look up "unsustainable" in the dictionary...

11. Apr 28, 2007

### ZapperZ

Staff Emeritus
Then we have a different idea of what "everyday" machines are.

You have a "narrow" idea of what tools are, both in terms of types and time frame. If you only want the most primitive of tools, then sure. You should also see if you can LIVE with just those and deprive yourself of all your modern conveniences. But don't come in here and start questioning of basic physics is completely useless and has no impact on what YOU use everyday. That would be completely absurd.

But not only that. The understanding of basic physics is the main impetus for getting rid of the ignorance of our world that was rife with supernatural explanations that enslaved millions of people. The high priest could use the flooding of the Nile, the eclipse of the sun, the sound from the forest, etc. to justify their "power" over the people. The existence of a rational explanation based on basic physics that make these things no longer mysterious and supernatural is worth MORE than any "tools" that is produced.

Zz.

12. Apr 29, 2007

### AlephZero

If you think I'm some sort of religious crackpot, you are completely mistaken. But I don't have an unthinking belief in the power of science and technology to solve all known problems either.

I've spent my working life as an engineer. You can look a random selection of my 650+ posts here to judge for yourself what level of technical knowledge I have, and whether I have contributed anything constructive to the forum.

There is one piece of hard scientific evidence at the bottom of my best guess at futurology: you can't have a stable ecosystem in which the population of the top predatory species has been growing exponentially for the last 70,000 years or so. Eventualy, that "transient blip" will correct itself one way or another. Whether the trigger is disease (H5N1, an AIDS mutation, whatever) , war, mass migration caused by loss of basic resources for survival like water or energy, or natural disaster, doesn't much matter. It's debatable what level of human population is sustainable in the medium term (say the next 1000 years), but I would estimate something between 1% and 10% of the current level.

Bertrand Russell defined "belief" as "that for which there is no evidence". IMO the idea that there is a technology fix for every problem is nothing more than "belief".

Sure, I use modern technology while it's available. I just don't share your optimistic belief that it will availble for ever, when all the evidence suggests otherwise.