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[Virex]

In theory, it IS possible to stop a beam of light with enough matter.

At the very least you could catch up to it with a car: http://www.hno.harvard.edu/gazette/1999/02.18/light.html

As for the nature of light, as far as we know, light exhibits the so-called wave/particle duality. We know this because light exhibits interference, which only works if it's a wave. But to explain the Photo-electric effect, one needs to assume that light is made up of particles (I'm citing wikipedia here, I know. However, I have seen both effects before, in experiments done in high school. So I assume I don't have to cite more reliable sources here)

A common misinterpretation of this is that light is a vibrating photon. In fact, it most certainly isn't. As far as we know, the wave-state of light consists of an electrical field and an electromagnetic field that's perpendicular to the electrical field. Both fields are perpendicular to the direction in which light travels, and they oscillate at the light's frequency. The polarisation of light is directly related to the orientation of these two fields, and as such a magnetic field can change the polarisation of light. A simple experiment regarding this effect, called the Faraday Rotation, is described here.
At the same time, light can be a particle. For the system between this effect, there are several proposed theories, but as far as I know, there isn't a universally accepted solution. Then again, I'm not a theoretical physicist.

Do note that the effects described aren't exclusive to light. Matter can exhibit these properties as well, in which case the associated wave is called a de Broglie wave, and these have been reported for electrons in de broglie's original experiment, described here. Also, electron microscopes use this property. You can read about the most common kind, the TEM, here. This paper gives an example of the practical use of a de broglie beam of sodium atoms, and this site describes an experiment that verifies that even blackballs, large football-like structures of carbon atoms, can act as a wave. And in the world of particles, those things are huge.

This causes me to think. Light DOES experience drag. But some force is making it go forward. If that were true, then it would manage to accelerate far more than the speed of light. Maybe the drag at that speed, even in space, cause it to not be able to accelerate any more?

Light does experience drag indeed, which is why the speed of light in for example glass is about 2/3 of the speed of light in a perfect vacuum. this page gives a basic introduction to this concept, and explains how it is related to the refractive index of a substance. It also gives values for the refractive indices for several materials, which can be used to calculate the speed of light in these materials.

But you are indeed right, since space isn't a perfect vacuum, the speed of light is slightly lower then one'd expect. But I'd dare to say that this difference is to small to be measured.
Another interesting effect related to this is the existence of Cherenkov radiation.

I'm not quite sure, but I think that light can't be slowed down by gravity. Instead, it experiences gravitational redshift, which is explained here. I'm to tired to dig up a scientific article backing that up though...

[Il Palazzo]

Il P's school...etc
Oh no, it's a wall of text.
Wait, I'll put these into spoilers.

Spoiler: Sean Mirrsen (click to show/hide)

Question on photons: if light indeed has an associatable "particle", wouldn't that mean that if we take an object far enough away (think individual stars in other galaxies), then it will become invisible? (or rather begin to become invisible, blinking in and out of view as fewer and fewer photons hit the observer in any given time frame; would also mean the visibility of the object you're looking at is proportional to the diameter of the outer lens of your observing device) Wouldn't that then mean it is not a wave, which would only change in intensity with distance?

ok, all questions are answered with yes, apart from the last one, which I'm not sure I understand. Wave's intensity(I'm sure you mean amplitude) if emited from a point-like source(e.g.star) in three dimensions falls with distance in the same fashion as a photon flux(amount of photons passing a surface - e.g.your retina, telescope's aperture) does. That is, it's inversely proportional to the square of distance. It's a characteristic of geometry of 3D space, more than wave/particle question.

Light (and other things) is the vibration of the other medium, the "energy" one. Which would explain the differences in behavior when being passed through various substances. It also allows to understand the reason why certain materials allow light to pass while others block it.

the medium in question is not some "energy"(or aether, as some proposed), but electromagnetic field.
The property of light and other forms of EM radiation to pass/not pass through objects is IIRC related to said material's molecular structure and light's wave-like behaviour. It was something about a wave having to be of a similar (wave)lenght to the atoms/molecules in the material in question, in order to be absorbed/reflected. I'm not entirely sure if I'm not pulling this out of my hat, though.
(Virex ninja'd me here, if you'll check this link, which he provided, it explains it well enough)

There are the questions then about the effects observed, specifically light being subject to gravity

It's a geometry question again. If you'll try and read the article on wikipedia(even the introductory one) on General Relativity, then you'll see that non-euclidean geometry of space-time is what makes light be affected by gravity. Basicly, when gravity bends space-time, light travels the least-time(sort of like shortest) route.
Einstein himself wrote a popular(sort of) article on the subject of his theory, in a non-crazy-scientific way. It's still not an easy read, of course.

What if it is possible for gravity, as one of the most basic forces in the Universe, to affect the energy medium as well? Would kinda begin to explain the thing that's been bugging me - gravity exerts a force on everything around it without expending the energy of the object in any obvious way. Of course, it throws up other questions, like what happens to all the energy being absorbed, does the object gain mass from energy, etc, etc.

1.four basic forces of the universe(strong, weak, electromagnetic, gravity) operate on their own fields and use their own gauge particles(e.g. photon, W boson, graviton-still just theoricaly). As far as I remember, these do not affect each other directly.
2.mass generates gravitational field in the same fashion that electron generates an electric field, it's not that exceptional at all.
3.there's no need to expend any energy, as long as there's no energy gain. By moving away from the field source you gain potential energy, that you can later exchange for some other kind(kinetic, most likely). However when you are stationary(like on Earth's surface), you do not gain or loss any energy.

Spoiler: Yanlin (click to show/hide)

Virex ninja'd me here. heh.

Light DOES experience drag. But some force is making it go forward. If that were true, then it would manage to accelerate far more than the speed of light. Maybe the drag at that speed, even in space, cause it to not be able to accelerate any more?

You really shouldn't use "drag" there. What I believe that you're trying to describe, is the fact that something makes the light go slower in various media. "Drag" is used to describe a force - which acts to decelerate something, not keep the velocity at certain level.

Anyway, getting back on topic:
About light sail form of propulsion, with a laser as a propellant - just imagine that, you build this thing somewhere in space and expect it to operate continously for 100 years(yes it's just a made up number), which is hardly possible, considering the amount of energy that it has to convert and transfer. So you'd have to send maintenance and/or fuel(if it's not solar-powered) to keep it going for all this time, or else the precious spaceship loses thrust - which might be especially bad if it's decelerating.
Now look at the human political and economical history, and imagine that you tell those colonists to get on the ship and count on people back home to spend resources to keep them alive. There might be more important things to do, you know. Like wars, or economic crisises, or disasters or whatnot.
My point is, as much as it's a feasible idea from the physics point of view, it seems too vulnerable to social situation on Earth.
Anyone has anything to say in it's defense, or should we cross it out of our "realistic space travel" options?

[Virex]

Anyway, getting back on topic:
About light sail form of propulsion, with a laser as a propellant - just imagine that, you build this thing somewhere in space and expect it to operate continously for 100 years(yes it's just a made up number), which is hardly possible, considering the amount of energy that it has to convert and transfer. So you'd have to send maintenance and/or fuel(if it's not solar-powered)

That's why your ship should be solar-powerd, though a nuclear battery could also run for long enough. Any other supplies could be made by efficient recycling, probably involving tricks like geneticly modified yeast and bacteria. Also, brining a large part of the population in stasis or a similar state might help.

As for the propulsion, I'd only use the laser in the early stages, since after a decade, the ship will be too far away for the laser to work realy efficiently.

to keep it going for all this time, or else the precious spaceship loses thrust - which might be especially bad if it's decelerating.

In interstellar space, a solar sail would only decelerate if it's closer to a star in front of it then any star behind it. At first it won't be noticalbe, untill you get close, at which point you want to slow down, or else you'll overshoot the star system. So this can be fixed by propper planning.

Also, you could use a radiation coating to supply thrust. Basicly, it's radiating in all directions, but because you're catching the radiation going in one direction, the radiation pressure isn't canceling itself out anymore and you get a net thrust.

If it'd be slowing down due to another reason, then you've missed something. And that'd be a pretty big miss for a well-prepared mission to a relatively close planet. Also, if the sail's causing drag, you could always fold it up again

Now look at the human political and economical history, and imagine that you tell those colonists to get on the ship and count on people back home to spend resources to keep them alive. There might be more important things to do, you know. Like wars, or economic crisises, or disasters or whatnot.

Only if you're assuming the people have to stay ready for the full 100 years. It'd be more realistic to pack everything up in the 30 years prior to the mission, and launch everything in a short window of time. Because when the ship's gone, there's probably no way to resupply it, unless you'd like to wonder off to wormholes or something.

My point is, as much as it's a feasible idea from the physics point of view, it seems too vulnerable to social situation on Earth.
Anyone has anything to say in it's defense, or should we cross it out of our "realistic space travel" options?

I wouldn't cross it out yet, since I think that the problems are smaller then the problems of a nuclear or chemical rocket. It has all the same problems, but it needs many times more fuell.

[Sean Mirrsen]

Isn't a gravitational redshift essentially the same as being slowed down? If you were to imagine a soundwave going through water past a pipe that sucks water in, the part of the wave going against the current would become dragged-out and increase in length. I'm not sure if such experiments were ever made (with pipe and sound), but it seems logical at first glance.

Unrelated notes:
Cherenkov radiation is similar in more than one way to sonic shockwaves.
I wonder if it's possible to attain supersonic speeds underwater without supercavitation?

edit: IP, yes I am aware of differences in molecular structure being the deciding factor in whether or not a material absorbs or reflects light. The difference in speed isn't due to light bouncing around the molecules though, as the wiki article on Cherenkov radiation seems to state (likening it to a train that travels with many stops).
Also gravity does seem to affect EM fields. What with black holes being strong enough to pull apart most of the waveforms that pass near, including and not limited to, light. And what does gravity have to do with space-time? While I can understand speed, motion through the space part of space-time, being able to affect the time flow, how can what essentially amounts to a force field affect space-time? It surely exerts a force on anything around it, including the energy medium (that can go by any name it pleases), but I don't think there was ever enough research going on to determine the space-time effects of gravity.

note: don't take offense if I'm being overly "dumb" about things you consider true facts. 1st, this forum is for "creative projects", and I'm being creative. 2nd, a little practice in disproving theories can never hurt a scientist.

[Sergius]

This causes me to think. Light DOES experience drag. But some force is making it go forward.

How do you know that there's a force making it go forward? Maybe you need to have mass to be able to stay still (well relatively still compared to light).

[mainiac]

This causes me to think. Light DOES experience drag. But some force is making it go forward.

Congratulations, we've reached pre-Kepler physics.

IIRC it was vogue to argue that angels were responsible.

[Sean Mirrsen]

No, I think it's rather the same as with sound. It'll go as fast as the medium allows it to go, because it's not something separated from its medium. Which is why it cannot be slowed down in a conventional way.

[Servant Corps]

IIRC it was vogue to argue that angels were responsible.

In about 24 hours, some angel (aka me) is going to lock this thread.

I tolerated talk about how to do realistic space travel, but seeing an entire argument devolve into non-scientists arguing over how they know more science than the other non-scientists mean that this thread is going off-topic, and thereby not helping me out in my main goal. In fact, the people on IRC chatroom was more useful in telling me what to do ("It would be far better to colonize the oceans." "Consult the space colonization project by scientists". "What will happen once they forget they are even on said ship?" Etc.)

EDIT: THe project I was told to look at is "Millenium project". Saving it for postierty.

[Sean Mirrsen]

Well, what do you want us to do, build prototypes?

Interstellar travel is currently possible, but unfeasible.

The best way to traverse the stars is with a planetship.

The best reason to make a planetship is to avoid the Sun's explosion.

The best way to make a planetship is to build a lot of underground bunkers, a deployable artificial sun, and some damn huge non-newtonian thrusters attached to the planet. Accelerate the planet until it leaves the orbit, hide in bunkers until the planet clears the Oorte cloud, deploy artificial sun, go wherever you please with your mangled planet (from the asteroid impacts, the stopping of the planet's rotation that will cause it to resume being a sphere rather than a planetoid, the gravitational stress from leaving the orbit and passing other major bodies).

[Virex]

Isn't a gravitational redshift essentially the same as being slowed down? If you were to imagine a soundwave going through water past a pipe that sucks water in, the part of the wave going against the current would become dragged-out and increase in length. I'm not sure if such experiments were ever made (with pipe and sound), but it seems logical at first glance.

The analogy to sound is usualy flawed, mainly because the two aren't the same kind of wave. Sound is an oscilating medium, vibrations in a carrier substance. Light consists of two oscilating fields. They behave in similar ways when things like interference are concerned, since that has litle to do with the carier substance. But when it comes to interactions with the surrounding media thigns start to differ.

In this case I thought that a part of the potential energy that light has is converted into "light energy", which means a change of colour, and thus you get a blueshift when light's comming from an area with high gravity towards an area with low gravity. The other way around means "light energy" is converted into potential energy, and thus you get a redshift. Or at least I'd like to think it works like that, since the potential energy light has due to gravity has to go somewhere...

I wonder if it's possible to attain supersonic speeds underwater without supercavitation?

Maybe a near-frictionless surface would work? A polarisable liquid, like watter, could also be magnetised and consequently aligned to reduce friction, or at least I think it's possible in theory. You'd need an ungodly powerfull magnet though.

edit: IP, yes I am aware of differences in molecular structure being the deciding factor in whether or not a material absorbs or reflects light. The difference in speed isn't due to light bouncing around the molecules though, as the wiki article on Cherenkov radiation seems to state (likening it to a train that travels with many stops).

Well, as far as I know, the reason light's slower has to do with it interacting with electrons, which absorb the light and consequently re-emit it. There's a small delay between the absorbion and the emmiting though, causing light to "lag"  a bit and thus it's macroscopical speed is lower then c.

Also gravity does seem to affect EM fields. What with black holes being strong enough to pull apart most of the waveforms that pass near, including and not limited to, light.

Are you certain that this isn't a side-effect of light (And all other waveforms) having impulse? Also, remember that black holes can create very powerfull magnetic fields, due to the acceleration of charged particles around it.

And what does gravity have to do with space-time? While I can understand speed, motion through the space part of space-time, being able to affect the time flow, how can what essentially amounts to a force field affect space-time?

As far as I know, gravity causes a deviation from euclidian geometry to everything it can affect. Since the geometry of space contains the 3 dimenstions of motion and time, time also starts to behave in a non-euclidian way. And don't ask me exactly what happens, because I don't know.
What I'm interested in is if magnetic forces can cause a similar deviation from euclidian geometry for charged particles.

[Sean Mirrsen]

There exists a [/quote] tag on the forum.

Has anyone tried to polarize sound? For example, in water?

I did not observe light in any meaningful way, but why exactly two ocillating fields? Can you point me to something that sheds some, uh, light on this? I doubt anyone actually observed the light's structure, only its effects. What effects require the twin-field structure to be possible?

Also, maybe we should move this highly educational discussion to another thread? As the OP noted, we've not only derailed, we've laid fifty miles of track through an underground tunnel with this thread.

[Il Palazzo]

Servant Corps: don't. Just rename the thread to "My physics iz moar awesome than yours"

[Virex]

There exists a [ /quote ] tag on the forum.

Dully noted

Has anyone tried to polarize sound? For example, in water?

How would you go about measuring the polarity of a sound wave? And what would you try to polarise in the first place? as far as we know, sound waves are formed by causing a medium to oscilate, and I have a hard time imagining how an oscilating medium could be polarised.

I did not observe light in any meaningful way, but why exactly two ocillating fields? Can you point me to something that sheds some, uh, light on this? I doubt anyone actually observed the light's structure, only its effects. What effects require the twin-field structure to be possible?

I'm not quite sure, but I think it follows from Maxwell's equations. As in, the equations don't make sense if light isn't an electromagnetic radiation and the electric and magnetic field arn't perpendicular. But as to why it is this way, I don't know. You might want to study quantumelectrodynamics, or maybe even string theory to understand it.

[mainiac]

The best way to traverse the stars is with a planetship.

Dude... just... just no.  No.  So very much no.  You don't even have a basic notion of scale, let alone physics.

[Yanlin]

Right. I'm going to leave this thread. This is too much to process and separate from opinion.