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:

<br /> \frac{0.9 + 0.9}{1 + 0.9 \times 0.9} c = \frac{1.8}{1.81} c = \frac{180}{181} c<br />

 

[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:

<br /> \frac{0.9 + 0.9}{1 + 0.9 \times 0.9} c = \frac{1.8}{1.81} c = \frac{180}{181} c<br />

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

 

[DaveC426913]

vish, your questions are confused. Ask one at a time.

Two objects traveling towards each other track their motion with respect to a THIRD object (say, Earth) between them. They each calculate that they are moving wrt Earth at .9c.

When they each look beyond Earth, they see the other spaceship moving toward them at .994c (180/181).

This you must accept: addition of velocities at relativistic speeds does not occur as you expect. It happens as Dickfore explained in post 20. Remember, relativsitic motion involves time dilation; you cannot count on how time passes in other frames of reference.

As for the glass balls >20mm, I don't know what that has to do with anything.

 

[vish_al210]

As for the glass balls >20mm, I don't know what that has to do with anything.

They are not observing Earth or vice versa but observing each other as they travel towards each other, each with a velocity of .9C.
And the point of two glass spheres >20mm was, in response to the option of an elastic collision. I was aspiring to know if the impact of the two objects would be an equivalent of one of the sphere's moving towards the other sphere(stationary) at 1.8C. (I have been going bonkers about this assumption of mine for a few years now.)
And if relatively, the speed varies by only 0.09__ value(in the above spoken case..), the why do I experience a drastic change in velocity between me standing at a station and observing a train crossing me, and me traveling in a train at a speed equivalent to the train being observed and observing it.
if the speeds of the objects in much lesser than light then the relative speed seems to double but if the objects are moving at speeds closer to the speeds of light then the relative speed is only marginally more. I am not being sceptical of the great minds that worked a major chunk of their lives unlike me in understanding and relating this phenomenon, but the equation seems to me (as of now.. based on the nothing that I know) like a failsafe, emanating from the initial consideration of the notion about the elusive speed of light.. but yeah, .9C also would e very hard to achieve...

vish, your questions are confused. Ask one at a time.

;) sorry.. but my questions and rattlings are an extention of my present state.. you shld expect no less or more...

 

[Doc Al]

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.

That's only according to Earth observers. A and B themselves would measure different distances and times.

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

No, speeds don't add up like you think they do.

They are not observing Earth or vice versa but observing each other as they travel towards each other, each with a velocity of .9C.
And the point of two glass spheres >20mm was, in response to the option of an elastic collision. I was aspiring to know if the impact of the two objects would be an equivalent of one of the sphere's moving towards the other sphere(stationary) at 1.8C. (I have been going bonkers about this assumption of mine for a few years now.)
And if relatively, the speed varies by only 0.09__ value(in the above spoken case..), the why do I experience a drastic change in velocity between me standing at a station and observing a train crossing me, and me traveling in a train at a speed equivalent to the train being observed and observing it.

That's because the speeds of you and the train are much smaller than the speed of light.

if the speeds of the objects in much lesser than light then the relative speed seems to double but if the objects are moving at speeds closer to the speeds of light then the relative speed is only marginally more.

Exactly right. For small speeds, the speeds add up just like you'd think. For example, if you are on a train traveling at 50 mph with respect to the track and another train is traveling towards you at a speed of 50 mph with respect to the track, then you'd see the other train approaching you at very close to 100 mph. (Actually, very slightly less--but close enough.)

But as speeds get closer to the speed of light, they no longer add so simply. If you play around with the addition of velocity formula, you'll see that. As far as understanding how it all works, you'll have to study a bit of relativity.

 

[vish_al210]

Thanks Doc_Al. and also Dave and everyone else...

But forgive me... it might take some more time and understanding for it to sink in .. I guess u understand...

but could you tell me if this is possible as asked in my earlier post..

I was aspiring to know if the impact of the two objects (each moving at .9c) would be an equivalent of one of the sphere's moving towards the other sphere(stationary) at 1.8C. (I have been going bonkers about this assumption of mine for a few years now.)

 

[Doc Al]

but could you tell me if this is possible as asked in my earlier post..

I was aspiring to know if the impact of the two objects (each moving at .9c) would be an equivalent of one of the sphere's moving towards the other sphere(stationary) at 1.8C. (I have been going bonkers about this assumption of mine for a few years now.)

No. There's no such thing as one sphere moving towards the other at 1.8c--it's physically impossible. A collision between two spheres each moving toward each other at 0.9c (with respect to the earth) is equivalent to one being stationary and the other moving at 0.994c.

 

[vish_al210]

but what is the individual energies of the two sphere's (each moving @ 0.9C) and when they collide what would be their result, (let us consider some tangible mass).
Is that energy equation the same as that of one sphere standing and the other impacting it at 0.994C.. I am really puzzled.. I believe and hope u have the answers.. as what I am considering and thinking would have already been thought of and answered...
Thank you for your patience though..

 

[Doc Al]

but what is the individual energies of the two sphere's (each moving @ 0.9C) and when they collide what would be their result, (let us consider some tangible mass).
Is that energy equation the same as that of one sphere standing and the other impacting it at 0.994C.. I am really puzzled.. I believe and hope u have the answers.. as what I am considering and thinking would have already been thought of and answered...

Realize that whether you see two objects moving at 0.9c or one stationary and the other moving at 0.994c is just a matter of reference frame. It's the same collision! (The kinetic energy of each object depends on the reference frame, of course.)

 

[vish_al210]

So .. the kinetic energy of the object is different if the observer is at different points.. How does the observer alter the kinetic energy of a moving object ?
The kinetic energy/momentum of an object is relative to the energy consumed in moving forward right?? or am I wrong??
And I do not mean the kinetic energy alone but the total energy of A.. and separately the energy of B.
I mean throwing a ball on a wall with speed V1 is not the same as throwing the wall with speed V1 on the ball... It is different right..

 

[vish_al210]

So finally the general simplified equations neither work in infinitesimal conditions or in substantially large scale conditions...

 

[DaveC426913]

but what is the individual energies of the two sphere's (each moving @ 0.9C)

Kinetic energy is frame-dependent. Two objects moving wrt each other have kinetic energy as measured by their relative speeds. Two objects at rest wrt each other have zero kinetic energy.

Picture an apple whizzing past Earth at .9c. The kinetic energy of that apple from Earth's frame of reference is dependent on exactly two properties: the apple's mass and its velocity. Its kinetic energy would be quite high, yes?

Now picture an orange whizzing alongside the apple, same speed, same path. What is the apple's kinetic energy wrt the orange? In the orange's frame of reference, the apple is at rest, therefore its kinetic energy is zero.

Now, picture the apple and orange both whizzing toward Earth from opposite directions at .9c. Earth calculates each kinetic energy based on its mass and its velocity wrt Earth.

But what does the orange calculate about the apple's kinetic energy? Well, the apple is moving towards it at .994c, so that's what's used to calculate its kinetic energy.

You add kinetic energy using the same formula that use to add their velocities, i.e.: .9+.9 = .994

 

[vish_al210]

But then what about momentum, is that also frame of observation dependent??

 

[Doc Al]

But then what about momentum, is that also frame of observation dependent??

Of course. Anything that depends on velocity is frame dependent.

 

[Dickfore]

The op needs to enroll in an introductory course in Special Theory of Relativity.

 

[vish_al210]

@ Everyone..
Thanks,...
But I am still not clear.. but understand that u all must be stating the right thing. I tried reading on this topic, but understand that just skimming the articles won't help..
So I shall try a little more indepth reading on this subject.
Well ;) pray for me guys... That I have a clearer head...(by that I don't mean blank..)
Thank you all for your patience. though I am a lot more muddled//confused in the head right now, I know that my understanding is wrong.. Now! that's a start...
All of you folks, take care... Wishing you all a merry Christmas//Kwanzaa// or well to everyone a wonderful year ending..
Take care...

 

[DaveC426913]

@ Everyone..
Thanks,...
But I am still not clear.. but understand that u all must be stating the right thing. I tried reading on this topic, but understand that just skimming the articles won't help..
So I shall try a little more indepth reading on this subject.
Well ;) pray for me guys... That I have a clearer head...(by that I don't mean blank..)
Thank you all for your patience. though I am a lot more muddled//confused in the head right now, I know that my understanding is wrong.. Now! that's a start...
All of you folks, take care... Wishing you all a merry Christmas//Kwanzaa// or well to everyone a wonderful year ending..
Take care...

Do not be shy to ask questions, even the most basic ones. We will fall all over ourselves to answer questions. That's why we're here.

 

[vish_al210]

Thanks Dave. And well I am not ashamed or shy of asking questions... It is good that finally the question/condition boiling in my head for over a few years is at least broken down...
I am not all satisfied as it shattered a myth in my head(well that was expected!).
But better shatter the myth than enjoy its fancy..
But I suppose that if eventually, something does travel faster than light what would we be able to see.. I mean see of the object...as our perception is light. I know we got equations and inferences to showcase that it wld not be possible... But just suppose that the equation has a loop hole.. as with everything else there is an exception..
What would we see or observe(I mean seeing in plain simple terms)

 

[DaveC426913]

But I suppose that if eventually, something does travel faster than light what would we be able to see.. I mean see of the object...as our perception is light. I know we got equations and inferences to showcase that it wld not be possible... But just suppose that the equation has a loop hole.. as with everything else there is an exception..
What would we see or observe(I mean seeing in plain simple terms)

Well, what you're asking is "What would the laws of physics be like if the laws of physics were different than what they are?"

The problem with that is that the answer can be anything you want.


But I know that's never satisfactory, so I'll give you a teaser.

Relativity does not forbid massive particles moving faster than the speed of light; it only forbids them from reaching and transitioning the speed of light. This leads to the hypothetical possibility of particles existing that always travel faster than the speed of light. These hypothetical particles are collectively known as tachyons (fast ones). They normally travel much faster than c, but when they absorb energy, they actually slow down closer and closer to c. The closer they get to c, the more energy it takes to push them closer. No amount of energy can get them to slow down to c or slower. They are mirrors of our slower-than-light particles.

What is interesting about them is that, to us, they would be traveling backwards in time. In their interactions, effect would precede cause.

 

[vish_al210]

If I consider an object moving past me ..(not through me.. I want to live ;) ) with a speed greater than light...
then I could draw lines form each of its state towards me in the path to indicate the light traveling from the object @ C.. Now as the light would take different time intervals to reach me(slower than the passage of the object), is there a possibility of me seeing the object as an extended line or multiple objects...I mean if we do find an object moving at speed greater than C, how would we identify it?

 

[Dickfore]

Tachyons (from Greek takhus = swift) are particles considered from a purely theoretical point of view that can and always travel faster than c. They have the peculiar property that they accelerate when their kinetic energy decreases, reaching infinite speed when the kinetic energy is zero (or having zero kinetic energy when they accelerate to infinite speeds). Mathematically they should have imaginary rest mass.

 

[henryc09]

I think something you need to get your head around is that there is nothing fundamentally that you can measure velocities with respect to. On Earth we tend to measure velocities w.r.t the earth, but when you consider the universe as a whole there is nothing you can do this with. It therefore only really makes sense to consider measuring velocities w.r.t a given frame of reference. However, experimentally it was found that regardless of an observer's relative motion to a beam of light, the same speed of light is ALWAYS measured. Special relativity provides a way of knowing what people in different reference frames will observe, and leads to the velocity addition formula used earlier which prevents anything from traveling faster than the speed of light! It's strange, I know! But it all follows from the experimental fact that the speed of light is the same regardless of relative motion.