Superluminal motion. How's that possible?

[xander1984]

Homework Statement

A friend of mine taking physics course told me about this but I didn't understand it because she wasn't able to explain the ideas easy enough for me to understand. All I know is that nothing can travel faster than the speed of light and how's it even possible? The theory of special relativity suggested that nothing can travel faster than the speed of light, right?

Homework Equations

The Attempt at a Solution

 

[RocketSci5KN]

All our current knowledge says nothing can travel faster than light. End of story. Everyone hopes our current knowledge is wrong...

 

[xander1984]

I did some digging and found that it only appears to be faster (an optical illusion?).
I still don't understand how and how it relates to the theory of relativity.
http://www.answers.com/topic/superluminal-motion

 

[Tide]

Saying that nothing can travel faster than light means that matter, energy or information cannot travel faster than light. However, some "things" can and do move faster than light. Imagine standing at the center of a circle whose radius is 186,000 miles. There is a reflecting wall on the circumference of the circle. Now suppose you shine a bright (narrowly focused) light beam at the wall and you rotate full circle in 1 second. The spot on the wall travels $$\pi$$ times the speed of light!

 

[xander1984]

Thank you.
I supposed that 186000 miles/sec is the speed of the light. Anyway, could you distinguish the difference between movement and traveling? Because they sound the same to me. If one thing moves at a certain speed sounds the same as one thing travels at a certain speed. I don't have much background in science. I should taken some science courses in college. And the example you wrote. How can I conceive the idea more easily?
The direction of you spinning and the direction of light being reflected are different, aren't they?
Thank you for your help.

 

[Kevin_Axion]

He's kidding, it doesn't actually travel faster than the speed of light.

 

[DaveC426913]

Nothing can travel faser than the speed of light locally. But our universe is expanding at (and thus countless galaxies are receding at) speeds in excess of the speed of light.

The reason this is possible is because, while locally those galaxies are not moving anywhere particularly fast (thus not exceeding c), in fact, the intervening space between us and them is expanding. The larger the distance between two distant points, the faster space is expanding between them. Two points far enough apart are actually receding from each other faster than c. SR does not forbid this.

 

[DaveC426913]

Saying that nothing can travel faster than light means that matter, energy or information cannot travel faster than light. However, some "things" can and do move faster than light. Imagine standing at the center of a circle whose radius is 186,000 miles. There is a reflecting wall on the circumference of the circle. Now suppose you shine a bright (narrowly focused) light beam at the wall and you rotate full circle in 1 second. The spot on the wall travels $$\pi$$ times the speed of light!

This is a red herring, confusing the issue. Light beams are not "things"; they are comprised of individual non-interacting components, none of which affect each other and none of which exceed c. A common analogy is sweeping a machine gun 180 degrees. You can get the "beam" of bullets to travel arbitrarily fast, even though no bullet is exceeding its muzzle velocity.

But the "beam" of bullets, like the beam of light is only a "thing" inasmuch as humans conceptually group all the components together due to similar-looking (though actually independent) behaviour.

An even more obvious trick: I stand on a mountain 10 miles to the north of you. I drop a tennis ball in the grass. One brazillionth of a second later, my friend, on a mountain 10 miles to the south of you, drops another tennis ball in the grass. Huuzah! The tennis ball "beam" has "moved" 20 miles in a brazillionth of a second! This is identical to the machine gun and the laser beam.



Again, as explained in my previoius post, things actually can exceed the speed of light relative to each other, if you account for expansion of the universe.

 

[xander1984]

Thanks Dave,
I'm re-reading what you wrote but frankly have hard time understanding. I don't seem to get the picture. I'd be among the most noob-kind-of-guys in this forum. Would you mind rephrasing it? The universe is expanding at a greater speed of the light. I guess it's possible because the universe is so massive that the speed of its motion expanding it is greater than the speed of the light? Am I on the right page?

 

[DaveC426913]

Thanks Dave,
I'm re-reading what you wrote but frankly have hard time understanding. I don't seem to get the picture. I'd be among the most noob-kind-of-guys in this forum. Would you mind rephrasing it? The universe is expanding at a greater speed of the light. I guess it's possible because the universe is so massive that the speed of its motion expanding it is greater than the speed of the light? Am I on the right page?

The universe is expanding; its edge, and everything else in it (that is not gravitationally bound**) is receding from everything else. The rate at which two things are receding from each other is directly proportional to their distance. i.e. a galaxy 10 billion light years away from us is receding twice as fast as a galaxy that is only 5 billion light years away, which is, in turn, receding from us five times as fast as a galaxy only one billion light years away. The galaxies at the edge of our observable universe are so far away that thre are actually receding at the speed of light. Beyond that, galaxies are recding such that their light will never reach us. They are forever lost to us. This is the edge of the observable universe.

** things that are gravitationally bound do not get pulled apart. The expansion of space is so small and weak that even large structures like galaxies and even galaxy clusters stick together. Expansion only occurs in the vast gaps between clusters of galaxies where it can overcome the puny gravitational effects.

Imagine pennies glued all over an expanding balloon. The pennies get farther apart as the balloon expands, but the pennies themselves do not get torn apart - the physical cohesion of metal far exceeds the balloon's ability to pull it apart.

Note as well, that any two pennies recede from each other at a rate directly proportional to their distance (2 pennies an inch apart will separate twice as fast as two pennies only a half inch apart), just like the universe.

 

[Delphi51]

Maybe there actually are particles that move faster than the speed of light locally, without violating special relativity. Check out
http://en.wikipedia.org/wiki/Tachyon

As for information moving much faster than light, how about quantum entanglement?

 

[DaveC426913]

Maybe there actually are particles that move faster than the speed of light locally, without violating special relativity. Check out
http://en.wikipedia.org/wiki/Tachyon

Please note in the description the word hypothetical. As in: hypothetical unicorns.

As for information moving much faster than light, how about quantum entanglement?

How about it? Quantum entanglement does not permit information traveling faster than light.

 

[Dickfore]

But our universe is expanding at (and thus countless galaxies are receding at) speeds in excess of the speed of light.

Can you back your claim by any observational data?

 

[xander1984]

I've got this assignment but have missed a few classes due to a health reasons.
To be honest I have no idea for how to approach this problem. I don't ask you guys to answer it for me but I'd like to know what I can do to approach this problem. For example, what should I read and think about in order to be able to answer this question.
I have to write a 2-page report on this question. I'd really appreciate any help.

Homework Statement

Some collections of stars called galaxies release jets of gas. Some of these jets have been measured to travel faster than the speed of light in a vacuum. Write a two page report on these measurements and explain why relativity theory is not violated.

Interesting. I just found this question in the forum.
Galaxies release jets of gas? What type of gas? and it's measured to travel faster than the speed of light in a vacuum? Is it also an optical illusion?

 

[NobodySpecial]

Interesting. I just found this question in the forum.
Galaxies release jets of gas? What type of gas?

Probably hydrogen but that doesn't matter.

and it's measured to travel faster than the speed of light in a vacuum?

No

Is it also an optical illusion?

Yes - it's exactly the same 'lighthouse effect' as described above.
The gas moves out of the gas in a stream at high speed but not >c
The galaxy is also rotating
So one bit of gas left the source at time 't', as the galaxy rotated another bit of gas left a little bit later but in a slightly different direction.
A million years later they have both gone say a 1/2 million light years, the first one hits a gas cloud and lights it up, then a year later the second gas beam hits another cloud of gas 1000 light years away from the first (because it was going in a different direction)
Viewed from Earth it seems that a bright point of light suddenly moved from left -> right 1000lyr in 1 year!

This is also the same effect you get with a 'moving light' on a light rope of christmas decorations - nothing is really moving, things are just turning on and off with a delay between them.

 

[xander1984]

Thanks for all the contributions.

 

[xander1984]

A million years later they have both gone say a 1/2 million light years, the first one hits a gas cloud and lights it up, then a year later the second gas beam hits another cloud of gas 1000 light years away from the first (because it was going in a different direction)

In the outer space, how does gas like this get ignited?

 

[DaveC426913]

In the outer space, how does gas like this get ignited?

It's not ignited. It just shines by reflected light. It's called a reflection nebula.

 

[vish_al210]

What happens when an object of mass k traveling at 0.9 C has a headon collision with another object of mass k traveling at 0.9C??
Effectively both objects would have observed the other traveling at 1.8 C, what would they observe.. and what energy would be dissipated on collision, at what speed would the fragments travel... What would happen...
If you want to change the mass and dim of the two objects relative to each other be my guest... tc... Merry Christmas and Kwanzaa and well to everyone a wonderful year ending.

 

[Dickfore]

What happens when an object of mass k traveling at 0.9 C has a headon collision with another object of mass k traveling at 0.9C??
Effectively both objects would have observed the other traveling at 1.8 C,

No, they would be traveling at:

$$\frac{0.9 + 0.9}{1 + 0.9 \times 0.9} c = \frac{1.8}{1.81} c = \frac{180}{181} c$$

 

[vish_al210]

If A travels at .9C and B travels at .9C in the opposite direction, then u mean to say that A would find B traveling at 180/181C. Well regardless, what happens when these two objects collide?

 

[sigmaro]

depends on what kind of collision it will be, if it is an inelastic collision, they will stop, if it is an elastic collision they will exchange their velocities, fortunately just like in the classical mechanics for this spesific case

 

[vish_al210]

How convenient.. well consider two glass sphere's of diameter greater than 20mm.

 

[vish_al210]

No, they would be traveling at:

$$\frac{0.9 + 0.9}{1 + 0.9 \times 0.9} c = \frac{1.8}{1.81} c = \frac{180}{181} c$$

Could you please tell me wrt what//who is this equation and who is it that sees//experiences this? The observer from a distance or one of the two bodies//objects moving toward each other.

 

[Doc Al]

Could you please tell me wrt what//who is this equation and who is it that sees//experiences this? The observer from a distance or one of the two bodies//objects moving toward each other.

It's an example of the relativistic "addition of velocities" formula. It gives the relative velocity of each object as viewed by the other.

See: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html#c1"

 

[sigmaro]

i don't think i understood your question quite well, if they are glass they will break i think.
and with all due respect, i will answer for dickfore;
when you are at A's reference frame you see B moving to right with a speed of (180/181)c.
when you are at B's reference frame you see A moving to left with a speed of (180/181)c so 180c/181 is the relevant SPEED (not velocity it is a vector).
i advise you to study "lorrentz transformations"

 

[vish_al210]

Imagine standing at the center of a circle whose radius is 186,000 miles. There is a reflecting wall on the circumference of the circle. Now suppose you shine a bright (narrowly focused) light beam at the wall and you rotate full circle in 1 second. The spot on the wall travels $$\pi$$ times the speed of light!

It is not light that has travelled, but u have turned around, the source has moved away and as the propagating speed of light is higher than the speed of the source turning, the observer sees the incidence of light along the circular wall as a continuous movement of the light, but is actually the light is still traveling as a narrow beam from the source towards the wall at its own speed (I guess u wld like to call it C).
So I cannot understand what you are trying to imply.

Also with each subsequent reflection the actual beam of light reduces in intensity (.5 - 3% atleast)

 

[vish_al210]

i don't think i understood your question quite well, if they are glass they will break i think.
and with all due respect, i will answer for dickfore;
when you are at A's reference frame you see B moving to right with a speed of (180/181)c.
when you are at B's reference frame you see A moving to left with a speed of (180/181)c so 180c/181 is the relevant SPEED (not velocity it is a vector).
i advise you to study "lorrentz transformations"

But I was referring to what is the observation by A or B with regards to the other. ;) well.. if glass sphere's could observe and decipher. Or consider it as a bubble which can withstand the 0.9C speed of travel. now if I were to sit in A. and you were to sit in B, which are moving towards each other in the bubble. I hope we are clear that the moving objects are A and B. As both a and B are moving they would find the other moving towards them at 1.8C, but how would they observe each other faster than light...

 

[Doc Al]

But I was referring to what is the observation by A or B with regards to the other. ;) well.. if glass sphere's could observe and decipher. Or consider it as a bubble which can withstand the 0.9C speed of travel. now if I were to sit in A. and you were to sit in B, which are moving towards each other in the bubble. I hope we are clear that the moving objects are A and B. As both a and B are moving they would find the other moving towards them at 1.8C, but how would they observe each other faster than light...

Again, this is incorrect. They would find the other moving towards them at 0.994c, not 1.8c.

 

[vish_al210]

Again, this is incorrect. They would find the other moving towards them at 0.994c, not 1.8c.

I still don't get it.. I am not questioning what you all have earlier stated but am only trying to understand.
Given data:
a. distance between the two objects is 100 light seconds (effective length traveled by light in 100 sec)
effectively A and B both would contact each other at 50 light seconds from their respective start points, considering both experience similar conditions and friction etc..
travelling at 0.9C each they meet after 50/.9, i.e., after 55.556 seconds.
Shouldn't the observer sitting in A feel that B was moving at 1.8C. As the light from B to A is traveling at C, and A is traveling at .9C towards the light, while the source of the light from B (B itself) is also moving towards A with speed .9C