If Two Spaceships Close at 1.4c...?

[Janus]

if you can clarify the wiki statement, please do.

 

Okay, here goes. Say you are in a space ship out in darkest space. You fire a laser from the back of the ship to the front, and time how long it takes and measure its speed to be c relative to the ship. You drop off a buoy and then fire your engines and accelerate in the same direction as you fired the laser. You turn off your engines and coast. You now have a different velocity than you did before ( as evidenced by the relative velocity difference between you and the buoy)

You fire the laser again and measure its speed relative to the ship. You again get an answer of c.

 

Now you signal the buoy and have it fire a laser in your direction. You measure the laser light's speed as it passes the ship, you again get an answer of c relative to the ship (If you fired the laser at the tail of your ship at the same instant the laser from the buoy past it, both lasers would reach the nose of the ship at the same instant. )

 

Thus no matter what you do, you cannot measure your own motion using light, because you always get the same answer.

 

 

Now, what exactly about this do you think contradicts my animation, keeping mind that it was made from the inertial frame of the Earth observer.

[LightHeavyW8]

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.

 

I like Janus' response MUCH better. Please note that the topic description of MY thread is Superluminality, so I hereby declare TON 202 intimately related!

 

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?

 

Umm, please go to http://www.scienceforums.net/topic/54179-if-two-spaceships-close-at-14c/page__view__findpost__p__585420

[Klaynos]

!

Moderator Note

This thread is about spaceships closing at 1.4c relative to some third party reference frame.

I think it's best to keep it to that until you understand the mathematics involved in switching frames.

[LightHeavyW8]

As far as I can tell, the bottom line is that just as there is no problem with EO observing A & B close at 1.4 c, there is no problem with EO observing A & B exchanging info at 1.7 c by using light, although A & B will not see it that way. Anyone who wants to explore the implications of particle collisions at nearly 2 c may want to look at http://en.wikipedia.org/wiki/Proper_velocity. And if I want to discuss Superluminalityas it pertains to QSOs, I'd better open a new thread in the Cosmology Area. Peace, y'all!

[Spyman]

As far as I can tell, the bottom line is that just as there is no problem with EO observing A & B close at 1.4 c, there is no problem with EO observing A & B exchanging info at 1.7 c by using light, although A & B will not see it that way.

There is no problem with an Earth observer viewing the spaceships closing at 1.4c and there is no problem with signals from the spaceships to close at speed up to nearly 2c if made of matter and exactly 2c if made of light, from Earth view. But neither the spaceships and the signals are moving through space faster than light from either view, which includes the Earth view also. So expressing it like "A & B exchanging info at 1.7 c" could be interpreted as slightly missleading.

 

But I don't know if I am going to be able to explain it better than what the other posters already have said.

(I especially liked Janus animation in post #34.)

 

 

...an observer in either one of the spaceships will NOT measure the closing speed to be 1.4 c since they are in another frame of reference and both space and time appear different for them relative observers on Earth, in their view the spaceships will be closing at speeds less than c.

I do not dispute this - I only claim they will be totally surprised by their collision at 1.4 c as observed from Earth, if they rely upon light for their information.

First I would like to say that there is an important difference between closing speed and speed through space, spaceships or particles with mass can NOT move through space individually at speeds of light or greater but together they can close or separate at speeds greater than light from a third party view. That doesn't mean that they are moving faster than light through space in any of the three views, a photon will always outrace mass in vacuum.

 

Secondly, since you agree that the pilots in the spaceships will measure their closing speeds to be less than c and yet you claim that they will be surprised by their collision at 1.4 c as measured from Earth, I need to point out that all three observers hold equal right to claim what they see as real. Everything that happens in their respective individual view will follow the laws of physics as they see it, thus the pilots will measure light to close their distance faster than the spaceships.

 

 

The observer using photons will have slightly newer information than the one using matter but both will be able to observe when and where the collision will take place before it happens.

Imho, this statement is inconsistent with the behavior observed by particles which can be made to close at > c in accelerator/colliders. An answer that requires a separate space-time continuum for every particle is somehow less than satisfying for me, stuck as I am in the same one as you. I still appreciate and thank you for your response!

It is not inconsistent with the accelerator/collider experiment when you take into account that the particles individually don't move through space faster than c. That might not be satisfying but that is the reality as understood by the science of today, we are stuck in the same Universe but we don't have to share frames of relativity. Different observers in different frames, measures different values for distances and durations.

 

 

Special relativity

 

Special relativity is a theory of the structure of spacetime. It was introduced in Albert Einstein's 1905 paper "On the Electrodynamics of Moving Bodies" (for the contributions of many other physicists see History of special relativity). Special relativity is based on two postulates which are contradictory in classical mechanics:

 

1. The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity),
   
2. The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.
   

The resultant theory agrees with experiment better than classical mechanics, e.g. in the Michelson-Morley experiment that supports postulate 2, but also has many surprising consequences. Some of these are:

 

• Relativity of simultaneity: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion.
  
• Time dilation: Moving clocks are measured to tick more slowly than an observer's "stationary" clock.
  
• Length contraction: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
  
• Mass-energy equivalence: E = mc², energy and mass are equivalent and transmutable.
  
• Maximum speed is finite: No physical object or message or field line can travel faster than light.
  

The defining feature of special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations. (See Maxwell's equations of electromagnetism and introduction to special relativity).

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

Edited January 25, 2011 by Spyman

[LightHeavyW8]

But neither the spaceships and the signals are moving through space faster than light from either view, which includes the Earth view also. So expressing it like "A & B exchanging info at 1.7 c" could be interpreted as slightly missleading.

 

Now this is where you start to sound dogmatic. As we see in Janus' animation, B is clearly approaching the light from A at 1.7c from our perspective, and A & B will collide at 1.4c from our perspective, and ALL parties may stop their watches at the same instant. This is consistent with the known behavior of particles (from our perspective) in accelerator/colliders. If you claim otherwise, will we not have a paradox?

[D H]

and ALL parties may stop their watches at the same instant.

This is a big part of your problem. What "same" instant? Simultaneity is relative. There is not such thing as "the same instant".

 

 

 

[LightHeavyW8]

This is a big part of your problem. What "same" instant? Simultaneity is relative. There is not such thing as "the same instant".

 

Back in my original post, one of the questions I posed was - would A & B somehow merrily continue to exist in their own spacetime continuum, or reference frame, even AFTER EO sees them collide? I was hoping for a simple "yes" or "no", not "it depends".

[Spyman]

Now this is where you start to sound dogmatic.

IOW you are disagreeing with the theory of Relativity, this is where I abandon this thread.

[Sisyphus]

Back in my original post, one of the questions I posed was - would A & B somehow merrily continue to exist in their own spacetime continuum, or reference frame, even AFTER EO sees them collide? I was hoping for a simple "yes" or "no", not "it depends".

 

If they collide in one reference frame, they will collide in all reference frames. It's the "at the same instant" that is the problem, because there is no such thing.

[LightHeavyW8]

IOW you are disagreeing with the theory of Relativity, this is where I abandon this thread.

 

My apologies for my poor choice of wording. I am enjoying all the responses, and am trying to respond to them all thoughtfully. Please clarify what you mean by "So expressing it like "A & B exchanging info at 1.7 c" could be interpreted as slightly misleading".

 

If they collide in one reference frame, they will collide in all reference frames. It's the "at the same instant"; that is the problem, because there is no such thing.

 

It seems to me that the collision is an event that all parties can set their watches by - I understand that EO's knowledge of the event will be delayed by the time it takes the light to reach him.

[Sisyphus]

My apologies for my poor choice of wording. I am enjoying all the responses, and am trying to respond to them all thoughtfully. Please clarify what you mean by "So expressing it like "A & B exchanging info at 1.7 c" could be interpreted as slightly misleading".

 

It is misleading because nowhere is information traveling at greater than C, relative to any observer.

 

It seems to me that the collision is an event that all parties can set their watches by

 

Nevertheless, it is impossible to "set their watches" between reference frames, and have them agree on the time that other events occur.

 

- I understand that EO's knowledge of the event will be delayed by the time it takes the light to reach him.

 

That doesn't really matter. The lack of simultaneity is not the result of just travel time for the information. That can be compensated for.

[LightHeavyW8]

It is misleading because nowhere is information traveling at greater than C, relative to any observer.

Umm, didn't EO see it that way, though?

 

Nevertheless, it is impossible to set their watches between reference frames, and have them agree on the time that other events occur.

Playing Devil's Advocate here, If A & B set their watches when they cross the reference beams that are sent from Earth, and they look at their watches when they collide, they may disagree with EO about how many seconds elapsed, but that's all, isnt it?

[Janus]

Umm, didn't EO see it that way, though?

 

 

Playing Devil's Advocate here, If A & B set their watches when they cross the reference beams that are sent from Earth, and they look at their watches when they collide, they may disagree with EO about how many seconds elapsed, but that's all, isnt it?

 

No, they will also disagree as to how much time elasped for the EO , and What time the EO's watch read when they crossed the beams. I went over this in a earlier post.

 

For instance, if Both A and B start their watches from 0 the moment they cross the reference beam and the EO sets his watch to zero the instant A and B cross the reference beams according to him( so that according to the EO, all three watches start from zero at the same instant), then according to A or B the EO starts his watch before the respective ship crosses its reference beam. Not only that, but according to A, B crosses its reference beam and starts its watch before A or the EO start their watches, and according to B, A crosses its reference beam and starts its watch before B or the EO start their watches.

[Spyman]

My apologies for my poor choice of wording. I am enjoying all the responses, and am trying to respond to them all thoughtfully.

Apology accepted, although I am still suspicious since you did neither deny or confirm your stance of Realtivity.

 

 

Please clarify what you mean by "So expressing it like "A & B exchanging info at 1.7 c" could be interpreted as slightly misleading".

I thought I did that im my post #55, but lets look at your statement from post#56 to emphasize what I ment.

 

You said: "B is clearly approaching the light from A at 1.7c from our perspective, and A & B will collide at 1.4c from our perspective".

 

But from our Earth perspective we can clearly see that B is NOT approaching the light from A at 1.7c THROUGH space, from our view B is moving THROUGH space with the speed of 0.7c and the light from A is meeting up with B THROUGH space with the speed c, they are approaching each other at 1.7c TOGETHER. Likewise neither A or B are moving towards the collision point through space with the speed of 1.4c, they are both moving through space with the speed of 0.7c and are only closing with 1.4c togheter.

 

As Sisyphus already explained in post #62, nothing is actually moving through space at speeds greater than c and expressing it like "A & B exchanging info at 1.7 c" would likely be interpreted by others as information is actually moving through space faster than light.

Edited January 26, 2011 by Spyman

[LightHeavyW8]

Apology accepted, although I am still suspicious since you did neither deny or confirm your stance of Relativity.

Science, unlike religion, should be able to handle all the scrutiny and hard questions anyone can put to it, imho.

 

But from our Earth perspective we can clearly see that B is NOT approaching the light from A at 1.7c THROUGH space, from our view B is moving THROUGH space with the speed of 0.7c and the light from A is meeting up with B THROUGH space with the speed c, they are approaching each other at 1.7c TOGETHER. Likewise neither A or B are moving towards the collision point through space with the speed of 1.4c, they are both moving through space with the speed of 0.7c and are only closing with 1.4c together. As <b>Sisyphus</b> already explained in post #62, nothing is actually moving through space at speeds greater than c and expressing it like "A & B exchanging info at 1.7 c" would likely be interpreted by others as information is actually moving through space faster than light.

We do agree, however, that EO observes "A & B exchanging info at 1.7 c", do we not? I agree that no observer will observe light travelling at > c. To me, this is NOT the same as saying "nothing can go faster than light". The EO has the same perspective as the operator of a particle collider - "closing speed" tells him WHEN a collision will occur. If "nothing can go faster than light" is truly a dictum of the Universe, then c must be the maximum expansion rate of the Universe. All those remote galaxies, bound together by the force of Lorentz covariance - it boggles the mind...

[IM Egdall]

 

If "nothing can go faster than light" is truly a dictum of the Universe, then c must be the maximum expansion rate of the Universe. All those remote galaxies, bound together by the force of Lorentz covariance - it boggles the mind...

 

I don't think that's right. I believe it goes something like this. Nothing can go faster than the speed of light through space, per special relativity. But the expansion of the universe is the expansion of space itself. And space can and does expand faster than the speed of light (per general relativity).

[LightHeavyW8]

I don't think that's right. I believe it goes something like this. Nothing can go faster than the speed of light <i>through space</i>, per special relativity. But the expansion of the universe is the expansion of <i>space itself</i>. And space can and does expand faster than the speed of light (per general relativity).

 

Umm, not to bring the entire forum down upon my head, but do you have any proof that Space, i.e., Nothing, can expand - other than Hubble's "Law"? FYI, I posted some info about a curiously superluminal QSO, TON 202 over in the BigBang thread.

[Spyman]

Science, unlike religion, should be able to handle all the scrutiny and hard questions anyone can put to it, imho.

IMHO both science and religion should be able to handle all the scrutiny and hard questions anyone can put to it.

 

But that is completely irrelevant to what I said.

 

 

We do agree, however, that EO observes "A & B exchanging info at 1.7 c", do we not? I agree that no observer will observe light travelling at > c. To me, this is NOT the same as saying "nothing can go faster than light". The EO has the same perspective as the operator of a particle collider - "closing speed" tells him WHEN a collision will occur. If "nothing can go faster than light" is truly a dictum of the Universe, then c must be the maximum expansion rate of the Universe. All those remote galaxies, bound together by the force of Lorentz covariance - it boggles the mind...

Do you have problems with the English language or are you being obstinate?

 

Nothing is going faster than the speed of light through space in your examples!

 

 

Secondly and minor off topic, the Big Bang theory accepted as mainstream science is not about a central explosion throwing out pieces into surrounding space, there is no superluminal speeds in it and in fact it doesn't involve any outward speeds at all. In mainstream cosmology the Universe is not expanding by movement but instead the geometry of space expands increasing metric distances to very remote objects.

 

 

Umm, not to bring the entire forum down upon my head, but do you have any proof that Space, i.e., Nothing, can expand - other than Hubble's "Law"?

Space is NOT nothing, event if it should be completely empty and flat it would still have distances through it.

 

FYI, metric expansion of space without outward movement into preexisting space is validated by scientific consensus and if you want to argue against accepted science the burden of proof is on you.

[36grit]

I don't want to interupt or anything but I just have a quick question that I was wondering about when I found this thread.

The subject scene:

So, we have these things traveling at each other both faster than half the speed of light in this universe, and an outside observer watching the distance close at a velocity faster than the speed of light. Each thing has it's own frame of time reference.

 

It seems to me that all three experience a time dialation. The two objects traveling towards each other will observe the closing distance at a speed equal to the speed of light (or perhaps, slightly slower). Could it be that, time dialation is excess velocity being converted to waves of distance radiating out from the event?

The third party observer witnessing the event would only experience the event as a very slight expansion of the universe relative to the time/space dimension of his own standing.

If this is, than could time dialation be the dark energy that we've all been searching for?

Edited January 27, 2011 by 36grit

[LightHeavyW8]

 

Did you have something to say, 36grit? If so, it seems to have gotten lost in cyberspace...

 

It seems to me that all three experience a time dialation.

I cannot recall ever hearing that the operators of particle colliders, with the same reference frame as EO, have this experience - do you know of any?

[swansont]

Could it be that, time dialation is excess velocity being converted to waves of distance radiating out from the event?

 

This makes no sense at all.

[LightHeavyW8]

This makes no sense at all.

Maybe not, but kindly allow me, as the thread author, to be the one to say so.

[losfomot]

Maybe not, but kindly allow me, as the thread author, to be the one to say so.

LOL.... that's funny... just gr8.

[swansont]

Maybe not, but kindly allow me, as the thread author, to be the one to say so.

 

I am disinclined to acquiesce to your request.

 

Being the thread author gives you input on what is not on-topic and in the areas of the forums where you have a position (be it opinion or a proposal), the right to defend your position. i.e. you have the right to respond to questions asked of you or about a position you hold that can only be known to you. It does not, however, afford you a monopoly on questions regarding objective information, in this case established physics — you are free to answer, of course, but nobody else is required to wait around for you to do so. Nor does it prevent others from weighing in with their own opinions, when that's appropriate. I would think that having a physicist rebut an erroneous statement would be preferable, as it should carry more weight, but even of you don't agree, I don't see where your proposal is within the spirit or letter of any rules or guidelines of the site.