Ekpyrotic

As long as you're stationary the beams will always travel towards each other at c, so they'd always meet at [math]\frac{x+y}{2}[/math]. The sources are like expanding circles.

As I understand it, under those conditions the answer is no.

Did you mean yes?

Kinda' like that video but with only one observer.

First, let's say I'm stationary.

Just thinking here.

If two moving, and opposite, sources emitted a radio signal from distances x and y. Would you always see the light beams meet at [math]\frac{x+y}{2}[/math]?

If you were in-between the sources and nearer to one?

Hey guys. I'm interested in knowing how you get to:

[math]t = \gamma t'[/math]

...from:

[math]t' = \gamma (t - \frac{vx}{c^2})[/math]

Revising for Jan test; it's become apparent that my spec. rel. knowledge isn't watertight. I have a particular problem with questions in the following form.

Anna is in a ship traveling at 0.8c; Bob is at relative rest. (At rest both the ships measure 100m). So, Anna whizzes past at 0.8c when the two ship's tails are next to each other the tail's clocks both read 0. Note: There is also a clock in Anna's ship's nose; Bob's ship has a camera next to Anna's ship's nose.

(a) Where is Anna's nose relative to Bob's ship?

This stuff is no problem. Lorentz factor = [math]\frac{1}{\sqrt{1-0.8^2}} = \frac {5}{3}[/math]

So, [math]\frac{5}{3}*100=60m[/math]

(b) What time does the camera record for Anna's clock that is in the nose. (I have problems here)

I know that the clock should read an earlier time; but don't know how to calculate that time.

Thanks.

Ripiptickakee! Got it, ridiculous question on my part.

Here's where I am. The object has grav. potential at the start, then transfers that to kinetic energy.

[math]gpe = \frac{1}{2} I \omega^2 + \frac{1}{2}mv_x^2 + \frac{1}{2}mv_y^2[/math]

We can work out [math]I[/math], which is equal to [math]\frac{1}{3} mL^2[/math]

Any advice? Hints?

I have a rod with a fixed base, a hinge.

It starts to fall sideways (rotating). Using the conservation of energy, how is the angular speed related to the angle with the ground?

About "Black Swan"---I've seen it in the book store but haven't read it. Is it worth the money?

It depends on what you like, it's a pretty vigorous analysis of the world (particulary economics) from a mathmatical slant. Although it's very easy reading. Worth reading the blurb and seeing whether it piquets your interest, very well written.

And, again, it doesn't much make sense to talk about "densities" of fundamental particles.

It's a matter of habit, I'll try and stop.

http://en.wikipedia.org/wiki/Two-photon_physics

 

there you go, photons interacting with each other.

Thanks insane_alien.

It's actually really interesting:

A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion/ anti-fermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. In the latter case, we distinguish several cases:

 

* Direct or pointlike: The photon couples directly to a quark inside the target photon. At this small scale, alpha_s is small and this process can be calculated in perturbative QCD.

* Single resolved: The quark pair of the target photon form a vector meson. The probing photon couples to a constituent of this meson.

* Double resolbed: Both targed and probe photon have formed a vector meson. This results in an interactions between two hadrons. For the latter two cases alpha_s is large, and this Vector Meson Dominance (VDM) process has to be modelled in non-perturbative QCD.

Time to research.

Thanks,

Jordan (Ekpyrotic)

BenTheMan:

This is bordering on something called the anthropic principle. One can note that, without very specific values for fundamental constants in our universe, intelligent life would be impossible. (For example, if the cosmological constant were not small and positive, then the universe would expand too quickly for life to evolve, or it would collapse back on itself.) But because there is intelligent life, the fundamental constants have to be exactly as they are.

This makes a lot of sense, I've been subject to the narrative fallacy. (have you read 'Black Swans' by Nassim?) Thanks for the clarification.

Swansont:

If you questioned how it was somehow odd or contradictory that a chair could be more massive and dense than a person, then, yes. The implication (to me) was that there was some conundrum in having the top quark be as massive as a gold nucleus, but a gold nucleus contains no top quarks.

I must have worded my question badly. The problem was in having the top quark as massive as a gold nucleus, yet much smaller in size. However we couldn't say it was more dense.

Hope that has cleared up the issue, thanks,

Jordan (Ekpyrotic)

I'll respond to each commentor in turn.

insane_alien: Thanks again for your response.

did you read the article?

I very much did, and it took a good 15 minutes too. The quoted references in your response seem rather vague, let us remember we want evidence that photons interact with each other. I can't see that anywhere in these quotes:

In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. It is the carrier of electromagnetic radiation of all wavelengths, including gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. The photon differs from many other elementary particles, such as the electron and the quark, in that it has zero mass[3]; therefore, it travels (in vacuum) at the speed of light, c...

 

...The photon is massless[3], has no electric charge[12] and does not decay spontaneously in empty space.

This tells me the following: photons have 0 mass, they are the perpetrator of the electromagnetic force, they have no charge, and travel at c in a vacuum. Not that they interact with each other - which I don't doubt - I just like sturdy evidence.

BenTheMan: You really know the art of writing well informed, insightful posts - if you don't mind the flattery. This particularly took my interest:

But how does the higgs give things mass? Well, imagine every point in space as a small pendulum. You start the pendulum rocking, and it rocks back and forth and back again. But where is the average position of the pendulum? Its average is at the minimum---zero. This is how the higgs acts---except for one important caveat. The exception is that the higgs doesn't oscillate around zero, it oscillates around another place. The fact that it doesn't oscillate around zero MEANS that all of the particles that it couples to now have mass. (This statement can be mathematically motivated, but I don't know how useful a bunch of maths would be!)

And however geeky this might seem I'd love to see the maths behind it, it always seems clearer that way.

In fact, when everything is massless, the mass terms for fermions are interractions between two fermions and a higgs, with a dimensionless coupling constant. It is only when the higgs gains a vev that the effective coupling constant becomes dimensionful, and the terms become mass terms.

This makes it a lot clearer, I'll need to spent some time reading up on the subject as of yet - but thanks again.

Thanks,

Jordan (Ekpyrotic)

hehe, I got moved to the pseudoscience.

I read the Wikipedia entry through, and I couldn't find any implicit mention of massless particle interaction. The closest I get was the following:

as a particle, it can only interact with matter by transferring the amount of energy

I know I'm being demanding, but I like to be throughout.

Thanks,

Jordan (Ekpyrotic)

Thanks for the fast response insane,

I'd like to take the time to discuss your points, please excuse my ignorance.

1, photons have no rest mass, they interact.

Can you quote a source that I can read up on. I thought this was the case but I couldn't find any description online for this behavior. All I could think of was Young's double slit, which would mean the particles are interacting with a 'Higgs wave', is this the case?

5, being massless doesn't mean useless.

This refute is dependent on the integrity of your argument against the first point.

Thanks,

Jordan (Ekpyrotic)

Warning: I'm kinda' crossing physics and philosophy here.

I've constructed a logically proof for the inexistence of a Higgs particle, and I need some physicists to point out the logical fallacy in the argument because I can't see it myself, although I can see potential flaws.

(1) Massless particles can't interact. (this is a major flaw, but I can't seem to find information for this either way)

(2) In order for a massless particle to attain mass it must interact in a Higgs' field. Therefore the Higgs particle requires a mass.

(3) For a Higgs particle to aquire mass it must interact with another Higgs that already has mass.

(4) Step (3) repeat until we work back to the first Higgs particle.

(5) The first Higgs couldn't have come into existence because it would have been massless, which makes it useless.

I'm sure it will be shot down on the first point. Thanks for reading.

Jordan (Ekpyrotic)

Hi swansont,

Neutrons and protons that comprise nuclei are made up of up and down quarks, so the comparison with the top quark mass with a gold nucleus isn't really apt. I'm not sure in what mesons the top quark appears, if any.

I'd hate to question your scientific prowess, but isn't this equivalent to saying you can't compare the mass of a chair and a human because they are made of different materials.

Thanks everyone for your swift answers,

The thing is, the top quark is fundamental (as you pointed out), so---if you don't believe string theory---zero dimensional! But what is mass, really? In high energy physics, mass is just some arbitrary parameter---the mass comes from the coupling of the particle to the Higg's field. So, whereas a gold nucleus is made up of a bunch of up and down quarks which couple very weakly to the Higg's field, the top quark couples very strongly to the Higg's field.

 

Think of it like this---suppose you have two (metal) blocks and a huge magnet. Now you know that some materials are attracted more strongly to a magnet, so you wouldn't be surprised if you found that one of the blocks was very easy to move around in the magnetic field, and one of the blocks were very difficult to move around in the magnetic field. You would think "Oh, well, one of these blocks just has a bigger magnetization''.

 

The same is true for the top quark and the up/down quarks. The top quark couples more strongly to the Higg's field.

BenTheMan that is a great answer/explanation. Thanks for taking the time to write it. I suppose the next question that the topic is begging for is why does a top quark interact so much more in the Higg's field than an of the other quarks/leptons.

But I don't suppose particle physics has the answer to that yet.

Thanks for the answer w=f[z],

The mass I think you mean to be GeV/c^2. (Unless you defined c to be 1....)

Indeed that is what I meant, I'm prone to stupid-error-blindness when proof reading my posts.

For starters, where does this information/problem come from?

The problem came from my head, and the information quoted is accessible from a wide range of resources. [1].

By density, do you mean mass density (there is also number density, etc.)?

I used mass in the previous post because it is the most logical solution. But, in any form it breaks down because density essentially boils down to: amount of stuff in a particular stuff. When we're talking about a top quark the former amount of stuff doesn't exist because it is pointlike.

Hope I have articulated myself,

Jordan (Ekpyrotic)

I have a certain problem that has been bugging me. I'll try to set up the problem at straightforward as possible.

(1) It is accepted that a top quark has the mass of 174.3 +/- 5.1 GeV, which in analogical terms is about the mass of a gold nuclei.

(2) A top quark is much smaller (in size) than a gold nuclei.

(3) A top quark can't possibly be more dense than a gold nuclei because it is made of only itself. (disregarding any bizarre fringe physics)

(4) So a top quark is the same mass as a gold nuclei, but it smaller, yet not more dense. How is this possible?

Thanks,

Jordan (Ekpyrotic)