If Two Spaceships Close at 1.4c...?

[losfomot]

Perhaps another way to put it is, can nothing truly travel faster than c, or are we just unable to tell?

 

On a universal scale, there are many things moving away from us faster than c.

 

Also, tachyons are hypothetical particles that may exist.

 

But locally, when discussing physical things that we can see and interact with (like light, or particles in (or out of) an accelerator, or spaceships), all experimental evidence shows that nothing can go faster than c... therefore no information can be conveyed faster than c

 

... but you already knew that was the answer

 

... are you satisfied with the explanations to your scenario?

[LightHeavyW8]

On a universal scale, there are many things moving away from us faster than c.

I assume you are referring to remote galaxies receding due to the rapid expansion of Space, i.e., Nothing.

 

But locally,...

Would you care to define just where locally ends?

 

all experimental evidence shows that nothing can go faster than c...

Except, and speaking strictly from the viewpoint of the reference frame we both share, remote galaxies and colliding particles, apparently.

 

are you satisfied with the explanations to your scenario?

Umm, albeit with the greatest respect to all of you, no.

 

I can't find the bolded part on the wikipedia page

Sorry, the bolded part was my own contention - I did not mean to make it look like it was in wiki.

[swansont]

Sorry, the bolded part was my own contention - I did not mean to make it look like it was in wiki.

 

Well, it's wrong.

[Klaynos]

Locally in terms of universal expansion refers to things where the gravitational attraction overcomes the universal expansion.

 

It appears your confusion here is due to you thinking (even if you do not realise it) that there is some preferred reference frame in which velocity is measured. This is not so.

 

You can pick your reference frame in SR arbitrarily, the only condition is that it is inertial.

 

You are also making the mistake of frame mixing. This is when you move from measuring a velocity (say, it could also be a distance, time etc...) in frame I and apply that measurement in frame II. When doing this you need to apply a transformation, in this case you need to use the velocity summation formula for adding a speed measured in a frame that is moving (the earth frame) relatively to the frame you are interested in (one of the space ship frames). When you apply this transformation which has been done for you above with reference links for you to read you find that in no frame do you ever measure something to be moving faster than c.

[zapatos]

Except, and speaking strictly from the viewpoint of the reference frame we both share, remote galaxies and colliding particles, apparently.

If I understand correctly the explanations given, no one ever said that colliding particles are moving faster than c. If they appear to be closing faster than c, that just means the distance between them is decreasing faster than c. No thing is actually moving faster than c.

[LightHeavyW8]

If I understand correctly the explanations given, no one ever said that colliding particles are moving faster than c. If they appear to be closing faster than c, that just means the distance between them is decreasing faster than c. No thing is actually moving faster than c.

WE see them close AND COLLIDE at almost 2c. A Hypothetical observer riding alongside one of the particles may only be able to see the other particle approach at c, but will he be able to anticipate his collision? Some responses insist that light must precede the particle, but Einstein's Postulate states that c is a constant for all observers. Particle accelerators make particles close, and "open" if they miss, at almost 2c, according to our reference frame, yet responders here claim that a proton stream fired at .99c ahead of B will not simply add to the .7c he already has, it must instead be transformed ala Lorenz so it does not exceed c in our reference frame - this seems inconsistent with what we see happen in accelerators, imho.

[Sisyphus]

WE see them close AND COLLIDE at almost 2c. A Hypothetical observer riding alongside one of the particles may only be able to see the other particle approach at c, but will he be able to anticipate his collision? Some responses insist that light must precede the particle, but Einstein's Postulate states that c is a constant for all observers.

 

Exactly. So the particle approaches at less than C, and the light approaches at C and therefore precedes it.

 

Particle accelerators make particles close, and "open" if they miss, at almost 2c, according to our reference frame, yet responders here claim that a proton stream fired at .99c ahead of B will not simply add to the .7c he already has, it must instead be transformed ala Lorenz so it does not exceed c in our reference frame - this seems inconsistent with what we see happen in accelerators, imho.

 

There is no inconsistency. The situation is exactly the same. Nothing in the particle accelerator is moving faster than C, either.

[LightHeavyW8]

Exactly. So the particle approaches at less than C, and the light approaches at C and therefore precedes it. There is no inconsistency. The situation is exactly the same. Nothing in the particle accelerator is moving faster than C, either.

 

Let's say that A and B are each 210,000 Km from the collision target, so their separation is 420,000 Km at that instant. Traveling at .7c, they will collide in 1 sec. If A fires a laser pulse to B at that instant, it will travel only 300,000 Km in 1 sec regardless of B's velocity, so both A and B will collide before light can inform B. I claim that this is by itself information at >c (for everyone's information, they collided). My second claim is that a proton stream fired from B should add to B's velocity just as A's velocity is added to B, even though it exceeds c in both cases. Keeping all observations in Earth's reference frame, we would see the proton stream from B reach A (and signal us with a laser pulse aimed at Earth). Next, we would see the collision and finally we would actually never see any laser pulse aimed at Earth from B, because the light pulse from A did not reach B before the collision. And when all the participants meet in the Hereafter, we will find out if A got to know about his imminent collision, just as the Earthlings saw that he did...

Edited January 23, 2011 by LightHeavyW8

[Janus]

Let's say that A and B are each 210,000 Km from the collision target, so their separation is 420,000 Km at that instant. Traveling at .7c, they will collide in 1 sec. If A fires a laser pulse to B at that instant, it will travel only 300,000 Km in 1 sec regardless of B's velocity, so both A and B will collide before light can inform B.

 

 

No, because in that same second, B will have traveled to the midpoint. Since the laser was intially fired when A was 210,000 km from the midpoint, and travels 300,000 km in one sec, it will reach the midpoint in less than 1 sec and before B does. This can only mean that the laser will intercept B at some point between where B started and the midpoint (the point of collision). The laser will reach B before it collides with A.

 

Here's quick animation showing how events transpire according to the Earth observer. The white circle represents a flash of light that radiates in all directions.

 

Edited January 24, 2011 by Janus

[swansont]

The photon will always beat the proton (in a vacuum).

[losfomot]

Let's say that A and B are each 210,000 Km from the collision target, so their separation is 420,000 Km at that instant. Traveling at .7c, they will collide in 1 sec. If A fires a laser pulse to B at that instant, it will travel only 300,000 Km in 1 sec regardless of B's velocity, so both A and B will collide before light can inform B.

Janus has explained how the EO will see it... the laser will reach B before the two ships collide... and, in fact, before a second has passed.

 

I think your problem is still in mixing frames. You see 420,000 km between the ships, and light only goes 300,000 km/sec... so, if what Janus says is true, light must have travelled faster than 300,000km/sec in order to reach ship B in less than one second.

 

That would only be true if ship A (or B) also measured 420,000km between themselves and the other ship. They do not. There is 420,000km between the ships in the EO's frame only.

 

In either ship's frame the distance is not 210,000km to the collision point. Either ship will instead measure 150,000km to the collision point.

 

Even weirder, In either ship's frame, the distance to the other ship is actually less than the distance to the collision point. (can someone else verify that I have this one right?)

 

You can describe the scenario all the way through correctly only if you stick to one frame at a time.

 

And each frame will describe the situation very differently... they will each measure different distances, and different times for everything that happens.

 

And, in all of this, the only way you can get speeds exceeding c, is not by direct measurement (because nothing can exceed c), only by deduction ie: closing speeds.

Edited January 23, 2011 by losfomot

[LightHeavyW8]

Janus has explained how the EO will see it... the laser will reach B before the two ships collide... and, in fact, before a second has passed. I think your problem is still in mixing frames. You see 420,000 km between the ships, and light only goes 300,000 km/sec... so, if what Janus says is true, light must have travelled faster than 300,000km/sec in order to reach ship B in less than one second. That would only be true if ship A (or B) also measured 420,000km between themselves and the other ship. They do not. There is 420,000km between the ships in the EO's frame only. In either ship's frame the distance is not 210,000km to the collision point. Either ship will instead measure 150,000km to the collision point.

I assume this is due to length contraction. Won't his measuring stick contract as well?

Even weirder, In either ship's frame, the distance to the other ship is actually less than the distance to the collision point. (can someone else verify that I have this one right?) You can describe the scenario all the way through correctly only if you stick to one frame at a time. And each frame will describe the situation very differently... they will each measure different distances, and different times for everything that happens.

Weirder, I will agree with! The problem with almost every scenario I have read is that it involves clocks and rods and lanterns and mirrors, and everyone in a different reference frame tells a different story. Believe it or not, I tried very hard to stay away from ambiguities. This is why I ended my experiment with a collision - there is no difference for anyone. Perhaps we could work backwards from there...

And, in all of this, the only way you can get speeds exceeding c, is not by direct measurement (because nothing can exceed c), only by deduction ie: closing speeds.

But closing speeds are not merely deduction from the EO's viewpoint - they are what he observes. And for the operators of particle accelerators/colliders, closing speeds tell them WHEN the collision will occur. And not to rain on the Relativity Parade, but there are numerous QSOs (Quasi-Stellar Objects) where superluminality is observed from Earth (assuming their red shifts are indicative of their velocities) - including one known as TON 202, with a proper velocity of 1100 c. As ever, I do thank you and appreciate all your responses!

 

On one thing Professor Dingle's critics are all agreed, that he is wrong. They do not all agree, however, on the nature of his error. - E. G. Cullwick

 

No, because in that same second, B will have traveled to the midpoint. Since the laser was intially fired when A was 210,000 km from the midpoint, and travels 300,000 km in one sec, it will reach the midpoint in less than 1 sec and before B does. This can only mean that the laser will intercept B at some point between where B started and the midpoint (the point of collision). The laser will reach B before it collides with A. Here's quick animation showing how events transpire according to the Earth observer. The white circle represents a flash of light that radiates in all directions. <img src="http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/ships.gif" />

 

It's a great animation - but doesn't it violate Einstein's postulate? Shouldn't B's motion change the frequency of the light B receives instead of arriving at B at a speed of 1.7c? I can see why some dogged stalwarts are still looking for the aether...

Edited January 24, 2011 by LightHeavyW8

[Janus]

I assume this is due to length contraction. Won't his measuring stick contract as well?

 

With Relativity, it is always the measuring sticks that move with respect to you that contract, never your own.

 

 

Weirder, I will agree with! The problem with almost every scenario I have read is that it involves clocks and rods and lanterns and mirrors, and everyone in a different reference frame tells a different story. Believe it or not, I tried very hard to stay away from ambiguities. This is why I ended my experiment with a collision - there is no difference for anyone. Perhaps we could work backwards from there...

 

In Relativity, different frames will disagree as to times and distances, but they will always agree as to what happened.

 

For example, let's replace your space ships with cars driving along a road. The cars carry clocks and the road is lined by km posts, each with its own clock. According to someone stationary with respect to the road, the km post clocks are synchronized.

 

The km post where the cars collide is 0, and the rest are numbered outward from there.

 

Thus each car is at a km post marked 210,000 when they emit a light towards the other. Both cars and the roadside observer will agree to that. They will also agree that the clocks on each car and the 210,000 km post it passes read zero at the moment the car emits the light.

 

Everyone will also agree that the cars collide at km post zero and that the cars' clocks read 0.714 sec when this happens and that the clock at km post 0 reads 1 sec.

 

Everyone will agree that the light emitted by the cars will reach km post 0 when the clock at km post 0 reads 0.7 secs.

 

However, means that according to each car, the distance between km markers is only 0.714 km and the distance between them and km post 0 is 149970 km at the time they emit the light, due to length contraction. Also, according to each car, the clocks on the km posts run 0.714 as fast as their own due to time dilation. This means that only 0.51 sec pass on the 0 km clock while each car travels between it and the 210,000 km post, according to each car. Since the 0 Km clock reads 1 sec when the cars collide at it,this means that according to each car, when it passes the 210,000 km post and the clock this post reads 0, the clock on the 0 km post already reads 0.49 sec. This is called the Relativity of Simultaneity. According the the roadside observer all the clocks on the mile posts read the same time, but according to the cars, they do not. He will also conclude that the other car passed its 210,000 km post and emitted its light before they did.

 

The upshot is that while everyone will agree what clocks at a given location read when events occur at that location, They will not agree as to what clocks at other locations, including their own, read when those events occur.

 

 

It's a great animation - but doesn't it violate Einstein's postulate? Shouldn't B's motion change the frequency of the light B receives instead of arriving at B at a speed of 1.7c? I can see why some dogged stalwarts are still looking for the aether...

Where in the world do you get the light traveling at 1.7c in this animation?

 

Einstein's postulate says that light travels at c with respect to any inertial reference frame when measured from that frame.

 

In this animation, the inertial reference frame is that of the Earth. Since the left ship is traveling at 0.7 c to the right in this frame and the light is moving at c to the right in this frame, the light will travel ahead of the ship. There is no traveling at 1.7c involved.

[LightHeavyW8]

Janus et al - Wiki entries such as this led me to my question about your animation:

http://en.wikipedia.org/wiki/Propagation_of_light_in_non-inertial_reference_frames'

 

"In an inertial frame an observer cannot detect their motion via light signals as the speed of light in a vacuum is constant."

 

But that entry also pointed to Proper Velocity which has clarified things greatly:

http://en.wikipedia.org/wiki/Proper_velocity

 

Thanks again to all responders!

Edited January 24, 2011 by LightHeavyW8

[swansont]

Janus et al - Wiki entries such as this led me to my question about your animation: <a href='http://en.wikipedia.org/wiki/Propagation_of_light_in_non-inertial_reference_frames''>http://en.wikipedia.org/wiki/Propagation_of_light_in_non-inertial_reference_frames' class='bbc_url' title='External link' rel='nofollow external'>http://en.wikipedia....eference_frames</a> <i>"In an inertial frame an observer cannot detect their motion via light signals as the speed of light in a vacuum is constant."</i> But that entry also pointed to <i>Proper Velocity</i>, which has clarified things greatly: <a href='http://en.wikipedia.org/wiki/Proper_velocity''>http://en.wikipedia.org/wiki/Proper_velocity' class='bbc_url' title='External link' rel='nofollow external'>http://en.wikipedia....Proper_velocity</a> Thanks again to all responders!

 

Just copy-paste the url. What's with all the formatting?

 

http://en.wikipedia.org/wiki/Propagation_of_light_in_non-inertial_reference_frames

http://en.wikipedia.org/wiki/Proper_velocity

 

The first link doesn't apply, since we are in inertial reference frames.

[LightHeavyW8]

Swansont -

 

The formatting is certainly not something I am intentionally adding - is there any problem with using Firefox for a browser? I see the formatting appear in everyone's posts when I try to reply, so I laboriously remove it all and preview before I post.

 

To your point, I am having trouble correlating the wiki statement:

 

"In an inertial frame an observer cannot detect their motion via light signals as the speed of light in a vacuum is constant."

 

with your statement:

 

"The first link doesn't apply, since we are in inertial reference frames."

Edited January 24, 2011 by LightHeavyW8

[Janus]

Janus et al - Wiki entries such as this led me to my question about your animation:

http://en.wikipedia.org/wiki/Propagation_of_light_in_non-inertial_reference_frames'

 

"In an inertial frame an observer cannot detect their motion via light signals as the speed of light in a vacuum is constant."

 

 

 

My animation adheres to this.

 

If I made an animation showing the exact same situation according to the left ship, it would show the left ship stationary with the light expanding outward from it at c (the ship always remaining at the center of the circle) and the Right ship traveling to the Left at 0.94c to meet the light.

 

An animation for this situation according to the Right ship would show the light expanding in a circle from the point where the left ship emittied it and the left ship moving to the right at 0.94, following closely behind the right edge of the circle of light.

 

In all three cases the light travels at c relative to the observer and the light intercepts the Right ship before the ships collide.

 

Closing speed is the difference in speed between two objects as measured by someone who is not at rest with repect to either object.

Realtive speed is the difference in speed between the objects as measured by either of the objects.

These two values will not be the same. If they were, then the whole concept of closing speed would be superfluous, and it would be relative speed in both cases.

[swansont]

Swansont -

 

The formatting is certainly not something I am intentionally adding - is there any problem with using Firefox for a browser? I see the formatting appear in everyone's posts when I try to reply, so I laboriously remove it all and preview before I post.

 

Could be a Firefiox settings issue, I don't know. Thanks for the data point.

 

To your point, I am having trouble correlating the wiki statement:

 

"In an inertial frame an observer cannot detect their motion via light signals as the speed of light in a vacuum is constant."

 

with your statement:

 

"The first link doesn't apply, since we are in inertial reference frames."

 

The article is called "Propagation of light in non-inertial reference frames" How does that indicate a problem with the animation?

[LightHeavyW8]

The article is called "Propagation of light in non-inertial reference frames". How does that indicate a problem with the animation?

 

Regardless of the title, the wiki article contains the statement "In an inertial frame an observer cannot detect their motion via light signals as the speed of light in a vacuum is constant." This led me to think there may be something here which is inconsistent with Janus' animation, AND indeed DOES apply to the scenario I posed - if you can clarify the wiki statement, please do. And if you can explain why there are numerous QSOs (Quasi-Stellar Objects) where superluminality is observed from Earth (assuming their red shifts are indicative of their velocities) - including one known as TON 202, with a proper velocity of 1100 c, I will give you extra credit!

Edited January 24, 2011 by LightHeavyW8

[Sisyphus]

Regardless of the title, the wiki article contains the statement "In an inertial frame an observer cannot detect their motion via light signals as the speed of light in a vacuum is constant." This led me to think there may be something here which is inconsistent with Janus' animation, AND indeed DOES apply to the scenario I posed - if you can clarify the wiki statement, please do.

 

What is it that you think seems inconsistent with the animation?

[LightHeavyW8]

Sisyphus - Please see http://www.scienceforums.net/topic/54179-if-two-spaceships-close-at-14c/page__view__findpost__p__585400

[swansont]

Regardless of the title, the wiki article contains the statement "In an inertial frame an observer cannot detect their motion via light signals as the speed of light in a vacuum is constant." This led me to think there may be something here which is inconsistent with Janus' animation, AND indeed DOES apply to the scenario I posed - if you can clarify the wiki statement, please do. And if you can explain why there are numerous QSOs (Quasi-Stellar Objects) where superluminality is observed from Earth (assuming their red shifts are indicative of their velocities) - including one known as TON 202, with a proper velocity of 1100 c, I will give you extra credit!

 

The situation you describe and the statement about inertial frames are all consistent. The examples you describe above are not examples of inertial frames.

[LightHeavyW8]

The situation you describe and the statement about inertial frames are all consistent. The examples you describe above are not examples of inertial frames.

 

In http://www.scienceforums.net/topic/54179-if-two-spaceships-close-at-14c/page__view__findpost__p__585351 you said "The first link doesn't apply, since we are in inertial reference frames."

 

Please clarify who is and who is not in an inertial reference frame - You? Me? A and B? TON 202?

[Janus]

- including one known as TON 202, with a proper velocity of 1100 c, I will give you extra credit!

 

As far as TON 202 goes, the calculated proper velocity comes from its proper motion( its side to side motion against the background of the sky), and assuming that its redshift is indicative of it being at a great distance. That is, if TON202 is as far away as its redshift seems to predict, and the measured proper motion is accurate, then it would be moving at 1100c. However, I don't think that there is any serious consideration that this is the case. In fact, this calculated proper velocity is taken to mean that one or both of these assumptions are flawed.

[Sisyphus]

I'm guessing you found "TON 202" by googling "faster than light motion" or something similar, since it has nothing to do with the thought experiment. I don't know anything about it myself, but a little googling of my own seems to show that it's an optical illusion caused by very high velocity component away from the observer. Since it's unrelated to this thread, I don't think a longer explanation is necessary.

 

Anyway, the rest reference frames of the Earth, spaceship A, and spaceship B, can all be considered inertial frames for the purposes of the thought experiment.

 

Now, again, what seems like a contradiction?