# More relativiy thought experiments

Discussion in 'Physics' started by BillO, Apr 12, 2013.

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1. ### BillO Thread Starter Well-Known Member

Nov 24, 2008
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Okay, here is one for all you fans of special relativity.

We have two space spaceships, A and B.

A is moving west at .9999C and B is moving east at .9999C and they are moving toward each other rather than apart. They have always been moving at these speeds as they were launched from frames of reference that had close to those velocities.

Now, the beings on A are suffering from an infection of some weird fungus and are seeking a cure. They know from a previous communication that there is a doctor on B that can cure this fugal infection. So the beings on B plan to launch a shuttle with the doctor on it to rendezvous with A, however neither A or B can afford to slow down.

The initial plan suggests that the doctor leave B in westerly direction and accelerate to rendezvous with A at some later point to the west where they match speed. Obviously timing is critical. However, the captain of B complains that they will need to expend all of their energy resources in accelerating the doctors shuttle and in doing so would guarantee their own demise, so he refuses.

The beings on A are not willing to give up hope and put their best minds to work on the problem. One particularly bright physicist claims that the captain of B is wrong. He suggests that since B is approaching A at more than .9999C, they could just use their magnetic decelerator beam to decelerate the doctors shuttle and actually extract an enormous amount of energy for future use, as well get their cure.

Who is right?

2. ### russ_hensel Well-Known Member

Jan 11, 2009
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Too much effort for me to grind through this but remember momentum will also be conserved.

3. ### SplitInfinity Member

Mar 3, 2013
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Uhhh...they are all SCREWED!

First of all at .9999c...and on top of that they are moving toward each other...Time Dilation and Relativistic Effect would not allow any form of sensors or anything on the EM Spectrum to be used as readings would be jibberish.

Since at .9999c...both crafts mass would be getting close to Infinite...I don't think anything electronic would even be working on the damn things.

At .9999c...you can just FORGET about communications...so I can't see how any coordination of using some beam...nevermind a Magnetic one...how the hell is that going to work? LOL!

If you are just asking this as a strictly theoretical question then..."in a RoseAnna RoseAnna Danna voice".....Nevermind.

Split Infinity

4. ### Wendy Moderator

Mar 24, 2008
20,765
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I went through something very similar to this problem once. I even looked up the math and calculated it.

I'm not willing to go that far this time. The key is there is an assumed static point, where we are calling the speed zero. Such a thing doesn't really exist. It is all relative speeds to each other, so the math has to be done for each object from the others point of view. When you do this the answers vary, completely according to the point of view of the object you are picking as the reference object.

So, when you talk about 99.99% of the speed of light you generally assume from the point of view of earth, which really does not enter into the problem at all. Or you are talking about the starting point, which is equally meaningless in the context of a two body problem.

My example was two spacecraft approaching earth at 50% of C. Does this mean they are approaching each other at 100% C? Nope, the equations are not linear, the spacecraft are approaching each other at 7/8 C, and earth at 50%C, because each speed has to be calculated separately for each object.

5. ### THE_RB AAC Fanatic!

Feb 11, 2008
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The true question after the .9999C BS is stripped away is valid for any speed and is very simple.

1. The shuttle is a projectile.
2. For ship A to make their own shuttle *change* speed to match ship B will require adding addional energy.
3. If ship A just releases the shuttle (requires zero energy), and it smashes into ship B, energy will be released.

We no real understanding of the working of ship B's "magnetic decelerator beam" but the end result would be the same for a catcher's mitt. (see point 3 above).

6. ### russ_hensel Well-Known Member

Jan 11, 2009
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Bill: excellent points, maintaintaining distinctions like these are key to getting relativity right.

As to SplitI: "Since at .9999c...both crafts mass would be getting close to Infinite...I don't think anything electronic would even be working on the damn things." You are having trouble with your frames. In a moving frame all the laws of physics are exactly the same ( with acc = 0 ). This is a key assumption or postulate of SR. Thus the electronics would work as they normally do.

7. ### BillO Thread Starter Well-Known Member

Nov 24, 2008
985
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So far, you are all right in a way, but let's not over think this. The solution is fairly simple. Russ was the closest so far.

They are both right. The energy realized by the ship B will be taken out of their forward velocity in altering the velocity of the doctor's shuttle.

The point the exercise is supposed to make is that one man's acceleration is another man's deceleration. The difference as to whether energy is being expended or being released is dependent on your frame of reference. Conservation laws are not broken in relativistic systems.

One thing I would like to point out is, communication between two ships traveling WRT each other at near C is still quite possible.

8. ### Markd77 Senior Member

Sep 7, 2009
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They had better think fast, because even at 10g acceleration it is going to take over a month for the shuttle to slow down.

9. ### SplitInfinity Member

Mar 3, 2013
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Sorry I missed this one thus why I took so long to respond.

As far as SR is concerned...I don't think it cover's the specifics of what exactly is going to happen or how a piece of electronic machinery would work if it was traveling close to or at c.

First off all any electronics specific to a sensory capacity thus analyzing and detecting a variety of anything on the EM scale such as Radar, Radio signals, Light Detection...well perhaps...and that is a BIG IF...the electronics aboard the craft might be able to be turned on...but that doesn't mean their ability to detect anything EM outside the craft would be anywhere near 100% if anything at all.

And since this example is specific to the crafts accelerating and moving in normal Space/Time and not specific to Folding Space/Time...I don't see any way that ANY form of propulsion that would exit and force the craft into motion and thus be controlled by the electronics inside the craft is possible in obtaining such velocity.

Split Infinity

10. ### BillO Thread Starter Well-Known Member

Nov 24, 2008
985
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Traveling at or close to C with respect to what, exactly?

You seem to have missed the point of my post, and of what Albert E. was trying to say.

Einstein took great efforts to explain, over and over again, that there is nothing special about any frame of reference that is traveling at any constant velocity. In fact, that is the single most significant tenet of special relativity. Everything else, the fixed speed of light, the contractions and dilations, were all known and understood before SR.

11. ### SplitInfinity Member

Mar 3, 2013
369
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Perhaps I should have been more specific. What I am trying to say is...reguardless of SR...the reality of being able to use electronic equipment specific to sensors to enable the crew of one craft to properly calculate anything from distances to communications between crafts is something that won't work given the velocity.

Sure...say you flip a switch and your radar turn on as you need to calculate distances and speeds in order to perform an action. Now it's not like the EM broadcast that is radar is going to travel away from your craft at c + craft velocity. As well your ability to properly read any radar return is going to be an issue.

Split Infinity

12. ### russ_hensel Well-Known Member

Jan 11, 2009
818
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It is not a matter of being more specific, it is a matter of missing the fact that all inertial frames have the same laws of physics. SR shows how to translate measurements in one frame to another. What references on SR have you consulted?

13. ### SplitInfinity Member

Mar 3, 2013
369
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What I am saying is that my above explaination has nothing to do with SR at all. It is specific to the ability of any electronic sensor device being able to properly work due to the inability to get a proper reading due to both crafts velocity.

Split Infinity

14. ### WBahn Moderator

Mar 31, 2012
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So we look at galaxies that are moving away from us at near light spped and make quite accurate measurements regarding them. How is that possible?

15. ### SplitInfinity Member

Mar 3, 2013
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Bill...I am not disputing that.

What I am saying is that due to the extreme velocities involved here and the closing rate of speed and time specific to two craft headed toward each other at .9999c...which is impossible to begin with unless folding Space/Time...but reguardless...any electronics specific to sensory data would not be capable of working as advertised as the closing rate is too quick for any acurate or even close to reality reading to come from it.

I am not saying that the actual electrical power and circuit performance within the electronics on these craft won't work...I am saying thay won't work as far as providing any usable data.

Split Infinity

16. ### russ_hensel Well-Known Member

Jan 11, 2009
818
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I see no justification for the above, can you give a reference?

17. ### WBahn Moderator

Mar 31, 2012
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Then, once again, how is it that we are able to get useful data regarding distance galaxies that are receding away from us at these same speeds?

18. ### SplitInfinity Member

Mar 3, 2013
369
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Wooba...sorry...I just came up with that name...if you don't really like it tell me...as far as being on Earth, using a Radio Telescope to determine the distance of say...NGC 3115...the Spindle Galaxy in the Constellation of Sextans.

Sextans...once known as Sextans Uraniae...was the name given to this constellation by Johannes Hevelius. He chose this name to commemorate the loss of his Sextant which he had used to measure the position of many stars.

This very famous astronomer had his sextant and many other instruments destroyed in a fire in september of 1697...AND NO ONE ASK ME HOW I KNOW ALL THIS becaue I really don't know...I have a tendency to retain things...and this fire led to Hevelius' famous saying..."Vulcan overcame Urania." This refering to the Fire God having defeated ASTRONOMY'S MUSE.

There had better be a few Astronomy Majors on this board that know this saying or I am going to feel like the Biggest Astronomy Geek on the Planet Earth!

Anyway's...as we use a Radio Telescope to view NGC 3115 and say use other instruments to gather such data as Red/Blue Shift, any visual Gravitic Lensing that might appear, various Frequency of Light, IR-Light...etc..etc...we on Planet Earth as well as NGC 3115...are not each directly traveling at each other at .9999c...thus having a closing rate of velocity of 372,526.74 miles per second.

Since the velocity of c is 186,282 miles per second...the closing rate of the two craft is 372,526.74 mps....very close to twice the velocity of light. If say RADAR was used by one craft and the radar reflects off the craft and traveling at c both ways by the time it returns the distance between the two craft will be about TWICE the distance the Radar instrumentation will report the craft to be...even worse....

....if the two craft are close enough to each other that a radar broadcast took 50 seconds to reach the other craft...and thus would take a about 25 seconds to return to the craft using radar and at the time the radar signal was able to be read off the electronics on the sending craft....the two closing craft would have just a moment to alter course before THEY CRASHED INTO ONE ANOTHER!

THAT is the problem of using Electronic Sensors to determine important data specific to location and distance when such craft have a CLOSING RATE OF 372,526.74 MILES PER SECOND.

I hope that helped.

Split Infinity

19. ### WBahn Moderator

Mar 31, 2012
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4,788
Well, it certainly underscored the point that you don't understand relativity.

20. ### SplitInfinity Member

Mar 3, 2013
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Oh for GOD sakes...of course I understand GR and SR.

I think we are having a misunderstanding here so please tell me what you object to in my above example?

Split Infinity