# A few basic questions

1. May 7, 2005

### daytripper

Hello everyone. I'm new to the boards so please excuse my ignorance. While I originally came here to ask one question, while I was here I came across some things that gave rise to new questions. If someone could explain these questions to me or give me a link that I could find my answer at, it would be greatly appreciated.
1) From what I understand, the theory of relativity states that motion can't exist because it needs a reference point before it can be defined as motion (i.e. The boy was moving at 4 miles per hour relative to the earth). How far off am I? And if I'm correct, where does E=mc^2 come in to play?
2) I read something that suggested that the "mirror world" isn't EXACTLY as it is in this world. How can that be? If everything in a mirror isn't really a world of it's own but just a reflection of this world, how can it be different?
3) What is a virtual particle? Is it matter or is it just something to help explain forces that we can't yet explain? And if it isn't matter, how does it give momentum to the particles that use them for propulsion?
4) What's the difference between Quantum Physics, Classical Physics, and General Physics?
Ok, that's all of them. Sorry if they're too elementary to waste time on but even these simple questions are at the top of my understanding and I'm hoping to add to that. Thank you for your time.

Last edited: May 7, 2005
2. May 7, 2005

### JamesU

To get you started:

3. May 7, 2005

### Steven Douglas

...and if you Google the term "yomamma" it will ask you, Did you mean yo mamma? after which it will take you to some old, outdated jokes.

4. May 7, 2005

5. May 7, 2005

### Steven Douglas

A high school sophomore wrote that. How many here were that eloquent or well-centered as a teen?

Absolutely magnificent.

::: bowing :::

Don't ever lose that spirit about you.

"Excuse me, Your Majesty, call me unworthy or whatever, but I can see your nasty little tally-whacker, and it's pretty ugly from here." ROFL

6. May 7, 2005

### daytripper

I'll try not to misplace my spirit. haha. While I couldn't quite tell from the text, i'll assume you weren't being sarcastic. And while I only know you from what I've read of yours, I'll follow your answer because I know you will do what is in the best intrest of knowlege instead of in the intrest of laziness (which might be assumed of yomamma, there). Do you know the answers to my questions or do you think I should google it

7. May 7, 2005

### Steven Douglas

Firstly, no sarcasm whatsoever, meant every word. Secondly, I'm sorry to say that I found your questions as interesting as they were beyond my ability to answer - I was waiting for someone else to come along and enlighten us both. I have a background in electronics, but I'm an artist and a writer. I'm actually here as part of some research I'm doing for a screenplay (I'm an unpublished nobody, so no big deal there either).

8. May 7, 2005

### daytripper

I see. I decided that I would go to wikipedia becuase they usually have understandable information but as I suspected, gibberish. I understand a little more though. Apparently if you're traveling at 2 miles per hour relative the earth and I'm traveling just under the speed of light, we will both still see the same light. I guess timespace will warp just for us. Don't you feel special?

9. May 7, 2005

### Steven Douglas

Time and space are constantly warping for me. I thought it was just me, so now I feel less special.

10. May 7, 2005

### Rocky143

42 or Life, the Universe, and Everything

I may be asking for trouble, but I'll take a stab at this. (Just saw House of Wax--which has a thermodynamic climax.)

I'm not a physicist, but I play one on the Internet. My answers should be considered skeptically until a real physicist verifys them.

1. The Theory of Relativity says that motion can only be understood RELATIVE to a frame of reference. It's the ol' on the train vs. standing by the tracks or twin on the spaceship vs. twin on the earth trick. If you know enough math and spend enough time studying relativity, the relativity of motion has something to do with the equivalence of matter and energy. For us mortals, it suffices to know that the experts have this under control.

2. Never heard of "mirror worlds." It's probably best to forget them. You'll sleep better.

3. At Google, define:virtual particle gave this: In the description of the interaction between elementary particles in quantum field theory, a virtual particle is a temporary elementary particle, used to describe an intermediate stage in the interaction. A virtual particle is never the end result of such a process. Using the language of Feynman diagrams, a virtual particle is associated with the internal lines of the diagrams, the so-called propagators. These virtual particles are responsible for the interactions of the external particles (ext en.wikipedia.org/wiki/Virtual_particle

In plain english, that says that virtual particles are wiggly lines.

When two particles interact, they must exchange energy. As I understand it, virtual particles are bookkeeping devices used to keep track of the exchanged energy. Read up on Feynman diagrams. I don't know about virtual particles as a means of "propulsion," however, momentum can be transferred as energy. It isn't necessary to hit something with matter to make it move.

This is visible in evacuated radiometers. The toy kind of radiometer you can buy cheaply works by hitting with matter. The vanes are painted white on one side and black on the other. The gas in the interior is at low pressure to reduce aerodynamic drag on the moving vanes. When gas molecules hit the black side of the vane, they bounce off faster because the black side is hotter--heated by incoming light. (Just accept this. It is true. Don't make me get into the relative temperatures of the gas, the white side, and the back side. If you would like to know about these things, please treat this as a homework assignment.)

Evacuated radiometers are delicate and expensive. Their vanes are silver on one side, black on the other. Their interior contains as little gas as possible--almost none. When photons hit the black side, they are absorbed and the black side heats up--the momentum of the photons is converted to heat. This has no effect on the motion of the vanes. When photons hit the silvered side, they are reflected (or maybe they are absorbed-emitted to the same effect). Some momentum is transferred to the silver and the vane is pushed from the silvered side. Notice that toy radiometers rotate away from the black side of the vane, while in expensive radiometers the vanes rotate toward the black side.

[Do I understand radiometers? I'm not so sure about my explanation of the behavior of the photons in the evacuated kind.]

4. Quantum mechanics is the theory that explains matter and energy at the scale of molecules and at smaller scales. To read about this, search "standard model" at Google.

Relativity is the theory that explains matter and energy at scales larger than molecules. You may want to find the book about relativity written by Einstein for nonscientists. Check at http://www.bookfinder.com There are also lots of other good books, encyclopedia articles, etc. on relativity.

"Classical physics" refers to the physical theories developed by Newton and elaborated upon by subsequent physicists. Classical or Newtoninan physics is a special case of relativity, when distances, velocities, and forces are on the small end of the range over which relativity applies.

Most physicists suspect that the universe is unified, not dichotomously divided into overlayed quantum and relativistic universes. They seek a single "Theory of Everything" or "Grand Unified Theory" or "Unified Field Theory" that will explain, well, life, the universe, and everything from the smallest particle to the largest galactic cluster. "String theory" is an attempt to do this. Search at Google for more on string theory.

"General Physics" is the name often given to a high school or college course that surveys all of the fields of physics.

Rocky

Last edited: May 7, 2005
11. May 7, 2005

### daytripper

That was nice, man. I didn't quite understand the radiometers but it's ok because I get what a virtual particle is. I'll check out that book. Thanks for the tips. I heard a lot about the string theory so I'll look into that too. I just read tons of stuff on wikipedia about entanglement and I have one question about that if someone dares to answer it:

What makes the entangled particles react to one another? Is information transmitted in the form of a wave or does it do something funky like going through a 5th dimension?

Thank you, rocky for taking the time to answer my questions. Haha, I just the joke behind your name, you're a geologist, rocky.

12. May 8, 2005

### da_willem

Motion exist in the theory of relativity, but as you said only relative to something else.

E=mc^2 comes in upon noticing that it becomes increasingly difficult to accelerate an object with mass, as it approaches the speed of light (one of the major consequences of the theory of special relativity is that nothing with mass can reach this speed). This can be though of as an increase in the objects momentum (as Newtons second law states that the effort (force) needed to increase an objects velocity is proportional to the increase not in it's velocity but it's momentum). This has as a consequence that momentum in special relativity is not simply the mass of an object times it's velocity but a this is multiplied by a factor that increases dramatically when approaching the speed of light (this is called the gamma factor and goes to infinity when the velocity goes to the speed of light):

$$p=m \gamma v$$ with $$\gamma= \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$$

[with p, m and v an objects momentum, mass and velocity, and c the speed of light]
[nonrelativistically p=mv, and by filling in some values for v much smaller than c, you will notice gamma is almost equal to one, wich is why we never notice this new ceoncept of momentum]

Now you could also incoorporate this extra factor in the mass and still say momentum is mass times velocty, but now mass varies with velocity:

$$p=mv$$ with $$m=m_0 \gamma= \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}$$

with $m_0$ the mass of the object at zero velocity (it's rest mass).

Now for small velocities this can be approximated by:

$$m \gamma \approx m + \frac{1}{c^2} (\frac{1}{2} m v^2)$$. Where you recognise $\frac{1}{2}mv^2$ as the objects kinetic energy. So the correction to an objects mass is proportional to it's energy with the conversion factor [itex] c^2[/tex]. This shows the relation between mass and energy.

By the mirror world I take it you mean a world wich is reflected in space, left and right are reversed. It turns out that almost all the laws are indifferent to such a change (they are 'invariant under a 'parity' transformation'). Gravity, electromagnetism, nuclear forces all don't care about left and right.

There is no fundamental reason (aside from it's aesthetic appearence) why the laws of nature should be the same in a world where left and right are reversed. Note that this is not the same as our world as seen from a mirror!

And indeed phenomena can be found in nature wich are not the same in the 'mirror world'. For a very nice account please get a hold on the Feynman lectures on physics part I, chapter 52. He explains it very clear and accessible.

It is matter and has mass, momentum and every other property associated with normal matter. The distictive property of a virtual particle is that it only exists for a limited period of time (depending on its energy). The laws of nature allow the conservation of energy to be violated momentarily! If the involved energy is very large, the energy can exist ('borowed') only for a very short period of time. If the energy is very small, nature allows you to borrow it for some longer time. So despite of the violation of conservation of energy out of nothing particles can spontaneously arise. As these particles have to be gone after some time they are called virtual.

And as you remarked, virtual particles are also believed to consitute fundamental forces. A particle borrows some energy form nature to create a few virtual particles. These are fired at another nearby particle, and the impact of these particles constitute a force. A force, in this description is called an 'exchange force'. This mechanism explains a lot of features about elementary forces. Like, why does a force usually decrease with increasing distance. Now, for a virtual particle has to exist for a certain amount of time to reach these distances. But the longer the particles exist, the smaller their energy, and thus the smaller their impact, and the smaller the force.

Quantum physicis usually deals with the small (not per definition), and the rules of the game are pretty different than we usually experience them. (virtual particles are a consequence of the fact that nature allows a temporarily violation of the conservation of energy, this is a quantum rule). Classical physics uses the laws as they appear to us in everyday life (in our daily life experience energy is conserved e.g.).

You're welcome. I hope this helps...

13. May 8, 2005

### daytripper

Wow, thank you for the very thorough explanation. This has cleared everything up quite well.