J.C.MacSwell

'']Also, if it was shot out below the water's surface the water would provide thrust directly forward. If it was shooting out of the water some of the thrust would be directed downwards.

Good point. As a minimum you would lose the "head" height of the jet trajectory (or exit) above the waterline. This would also add to the displacement (effective weight) of the craft unless the jet was redirected downward which would of course lose even more forward thrust.

its a particle, a physical thing, but it isnt matter? O.o

I think that's right.

In another thread I claimed a photon absorbed by an atom added/created matter to the atom but was convinced that strictly speaking (particle physics definition) that was incorrect. That portion of the energy did not represent matter even though you have a mass increase.

Whay do you think it matters?

Assuming all other things being more or less equal (diameter of jet etc.):

Below the waterline the surrounding water would provide more back pressure than the air above and therefore more thrust for the same power.

Because you would be effectively propelling more mass aftward (relative to the body of water or freestream) you would be imparting more momentum for the same power. (the exit velocity would actually be slower). So more of the energy would go to the craft and less to the jet.

Hope this makes sense.

Here's a possible method for faster than light communication:

 

Imagine two star systems (A and B) separated by a distance of 10 light years' date=' with a laser beam splitter exactly half-way between them. The laser beam splitter creates two entangled beams of laser light (A and B), each headed toward one of the two star systems.

 

After 5 years, the entangled laser beams reach the two star systems. At each star system is a receiver for the entangled laser beams consisting of a double-slit detector. If the two entangled beams arrive at each star system undisturbed, they will both create an interference pattern on their respective double-slit detector. This is because the probabilistic wave function for the entangled beams has not collapsed.

 

If laser beam A is disturbed (i.e. observed) just before reaching its double-slit detector, then the wave function for that beam collapses, as does the wave function for laser beam B just before it reaches its detector. This happens instantaneously across the 10 light year distance. The consequence of this is that laser beam B will NOT create an interference pattern at its double-slit detector.

 

It’s not hard to imagine that one can create a pattern of disturbance on laser beam A that can be translated instantaneously to the detector of laser beam B simply by observing whether there is an interference pattern or not. If the beam is continuous you can send a message just by encoding binary over short time period "frames". Interference pattern during the frame = 0. No interference = 1. From there all the complexity of digital communication protocols can be layered on top of this simple transport mechanism. In this sense, faster-than-light communication can be achieved.

 

Follow this link for a more in-depth discussion and implications for space exploration:

 

http://www.seti.org.au/spacecom/quantumcom.html[/quote']

 

The way I understand it:

 

If you observe a photon before it reaches the two slits, so that it could then enter either slit an interference pattern should still result.

In 3 slit experiments with a detector at one slit you should see a combination of no interference detections at that slit and interference otherwise between the remaining 2 slits.

 

So I don't see the collapse of the wave function with respect to interference as you have just described.

 

Please correct me if I am wrong.

 

I am not convinced that a message could not be sent FTL on a statistical basis. ( By altering the pattern from a random distribution over a number of tests on polarization for example) but it would sure change our perception of SR (or we could send messages into the past by prior arrangements of Code)

 

Sorry if that was vague.

I have always heard light discribed as being both energy and matter. if its matter' date=' it has to have mass. but according to E=MC^2, nothing with mass can reach the speed of light. so how can light reach light speed?

 

my understanding of this stuff is limited, so sorry if im missing something important.[/quote']

 

It can be considered a massless particle, but I don't think it technically can be considered matter.

I have a question about jet boats or personal water craft or jetskis or what ever you like to call them. It concerns the water jet propulsion unit in the back of the boat from which water exits as a jet stream and moves the boat forward. One of Newton’s laws at work here I believe. My question is: With an equal amount of power supplied to the system, forget about drag etc, is it better to have the water jet propulsion unit above the surface of the water or below it? Another way to put this is, do you get more measurable thrust, with the same amount of power applied, with the jet above the water surface or below the water surface?

Below.

an electrical current is a flow of charge or flow of electrons (same thing)

A stream of Beta radiation would fit that description. Would that be considered electricity?

How about a stream of alpha particles? That would also be a flow of charge although obviously not electrons.

If the streams above were deflected what differences would there be in the resulting Electromagnetic radiations?

Any "theoretical" upper limit on EM radiation. (seriously)

Could a "Rogue Wave" single photon come and wipe us out while we are carefully scanning the heavens for threatening asteroids?

I think this belongs in one of the mechanics threads' date=' and I'm pretty sure it fits here, but feel free to move it.

 

Anyways, I know that things like radio waves, light, and microwaves all fit under the Electromagnetism category. I'm curious to know what fits under the same category, so anyone interested in making/contributing to a list would be welcomed. Also, if someone could give me a basic reason why these fit under the same category, that'd be nice. I'll try my best to understand it.

 

Plus, are there ways to convert between these forms? Can you, for example, convert radio waves to visible light? Or could you possibly piggyback these things, like by (ignore practical uses, just can this be done) putting light waves on with the radio waves in order to make the waves visible?

 

I don't know where I'm headed with this, and I don't know that anyone wants to find out, but it always is nice to know [/quote']

 

If you get in a rocket and accelerate away from x-rays at some point they become visible light. Similarly for radio waves if you accelerate toward them.

Try this thought experiment.[/b']?

No question in my mind that in this thought experiment the clock that went further will be "younger", primarily due to the due to the SR effect. The GR effects could be symmetrical.

I do wonder: What is the "BEST" experimental evidence to support this. All evidence I have seen has a major GR component chomping into any margin of error.

It is impossible to get any results on SR without being exposed to some GR effects. The best we can hope for is to make them symmetrical (with respect to what though, time or distance etc.) or to factor them out based on theory.

It is also impossible to make them perfectly symmetrical or you would be duplicating the same test. No comparison could be made.

Apologies if any of that does not make sense.

You are adding mass in these scenarios' date=' but not rest mass. AFAIK rest mass of an atom assumes ground state. Excitaton at some level includes KE of particles, and as the bouncing rubber balls show, that's becoming absurd by the time you reach macroscopic systems.

 

Mass is a property of matter, but is not a synonym for matter. Matter is something that has mass and takes up space. Severian gave a more technical definition here.

 

Whether mass represents how much matter you have, I think is more to do with semantics. Is it energy or is it number of particles? One needs to define the problem more clearly, and declare which metric one is to use to measure it.

Thank-you. I was surprised that Sevarians particle physics description of matter excluded the gluon.

The point about not adding to the constituents' rest mass is why I would argue that you haven't created any matter. To make an energy argument, you would also have to say that a particle in motion constitutes more matter than a particle at rest.

But doesn't exciting an atom add rest mass to the atom, though (I think, not sure- I see it as kinetic?) not the electron?

By this definition (which I now suspect is wrong but was my original thought) I would have created additional matter.

Using the broadest possible definition if you had a big hollow sphere in outer space, filled with rubber balls bouncing around would their kinetic energy add to the "rest mass" of the big hollow sphere/system? I think it would be equivalent (add to it's inertia etc.) would it not? Obviously a stretch to call "that" additional matter.

What definition are you using? I don't think anyone actually mentioned a definition in this thread.

I was using, "energy of rest mass" as constituting matter, but I now suspect this is incorrect. An excitation of an atom would add to it's rest mass, would it not? Any thoughts?

On the same subject would the heat energy of a body add to it's rest mass? I would say yes (even though it does not add to the rest mass of the constituent atoms) but I would not think of this as matter so I am obviously unsure of where the line is drawn.

I think (now) matter is supposed to have some degree of "permanence" under normal conditions, so this may be too broad a definition that I was using. (even though matter created in some accelerator experiments is far from "somewhat permanent under normal conditions").

I would like to know where the lines are drawn (and more interestingly "why")

What if you had two cats in the box, one male and one female. After, say, 10 minutes a decay device will trigger one of them dead in a way that won't effect the other. (Say it triggers a cyanide capsule to rupture in the stomach).

So you have a live/dead cat and another live/dead cat. Can they interfere with each other?

Assume there is enough food and water in the box/system and half the box was isolated/compartmented so the cats can't get to it because of a trap door than wasn't (quite) big enough.

After a year the trap door is sealed and the isolation/compartment is saved and the remaining half is jettisoned into a black hole.

When the isolation/compartment is opened could it contain any kittens?

What matter exists that didn't exist before?

More energy is bound (somewhat) in the atom. The rest mass of the atom has increased.(?)

Does this not represent an increase in matter? Or am I getting the definition wrong? (I suspect I may be))

If a photon is absorbed by an atom bumping up an electron isn't that an increase of matter.(and therefore creation/transformation of energy into matter)

T

 

So that begs the question then' date=' [i']"What is the real reason why a spaceship can't accelerate to light speed?"[/i] The reason is because of the limitations imposed by relativity's Addition of Velocities formula. Let's say that you, a stationary observer, see the craft going 1000 MPH slower than lightspeed, and then it accelerates. The pilot of the craft might discern that his recent engine thrust boosted his speed by 5000 MPH (confirmed by his observation of something he left behind, eg. a sister ship). But to you, when the speed 5000 is added to the speed (c-1000), the sum does not surpass c. You have to use the velocity addition formula, (u+v)/(1+(uv/c^2)).

So it's Einstein's fault? Nobody blamed Newton when they fell down.

What do you mean. What is a rest[/b'] frame? Please don't say that it's a frame at rest!

Isn't it a non-accelerating frame?

But how can you distinguish them? You can't - every physical observable that both Bob and Bill can measure will give the same result (assuming they are in a box' date=' or a lift or somesuch) so you cannot distinguish which one is in a gravitational field and which one is being 'slowed down'. Therefore there can be no difference in age.

 

[/quote']

 

 

Let's have the twins meet in outer space, Bob at rest and Bill 1/2 light speed with respect to Bob and they synchronize their watches on the "fly by". Both are then blindfolded. Bob "stays put" and feels nothing. Bill gets deflected around a series of gravitational objects without getting captured in their orbits until he again meets Bob again and they again check their watches on the second "fly by". Bill has felt nothing except possibly some slight tidal effects.

 

Would you say their watches still agree?

surely then the conversion of the entire ship into Energy would be required then?

and it would have to be a VERY efficient conversion leaving even the smallest of particle(s) massless' date=' and thus rendering the excersize futile, and Jakari point Valid?[/quote']

 

I think this is basically correct if you mean in order to "get to" light speed.

 

Lightsword was correct, I think, for the local frame of reference. He could still accelerate "normally". From your reference point this "normal" acceleration would seem futile as he would be using an awful lot of energy for a miniscule acceleration. But from his point of view this miniscule acceleration and the energy expended would be business as usual.

 

Remember that he changed his reference frame as he approached light speed so that he would be unaware of any mass (whatever mass is) increase. Having changed his reference frame he is also no closer to the speed of light. So he can happily accelerate away in his local reference frame even though it's perfectly obvious to you that he's just "spinning his wheels"

I agree with pretty much everything that Martin (and JaKiri) said. I only have one clarifying comment:

 

The twin paradox is normally formulated assuming Bob is at rest in a frame with no gravity - so he is feeling no force' date=' while Bill feels the acceleration 'force' when he turns round. If Bob were on Earth the whole time and Bill maintained an acceleration/decelaration of 1g the entire time of his travel, their clocks at the end would show the same time - they would have aged the same amount because one could not tell their frames apart from the forces they feel.[/quote']

 

Is this correct?

 

Bob is in a constant situation. Let's say you doubled the duration of the test (Bob's time) he would be twice as old. Bill would be closer to light speed (Bob's perspective) through the "middle half" of the longer test than in the first and last quarters (the "other half" which is identical to the original test) of the longer test. So for Bill the "middle half" would be of less duration than the "other half" whereas for Bob it would be the same duration.

 

Bob would think the test doubled where Bill would think it increased somewhat but not doubled.

 

Thoughts?