Time slows down when you approach the speed of light?

[||spoon||]

Reasoning tells you launching into space does not shrink the universe!

I know this, what i am saying is that would it not APPEAR to have shrunk to someone begins moving near c?

for example, if you measured the distance to a planet whilst in Earth's frame of reference and found it to be say 7 lightyears away... then flew in a spaceship at 99% of c and undertook the same measurement to the planet, wouldn't it appear to be only one light year away now??

I know if i did this it would seem to APPEAR to me that space had contracted.

 

[lightarrow]

I know this, what i am saying is that would it not APPEAR to have shrunk to someone begins moving near c?

for example, if you measured the distance to a planet whilst in Earth's frame of reference and found it to be say 7 lightyears away... then flew in a spaceship at 99% of c and undertook the same measurement to the planet, wouldn't it appear to be only one light year away now??

I know if i did this it would seem to APPEAR to me that space had contracted.

It's not only that it appears contracted to you, it *is* contracted because you can *measure* that contraction in you ref. frame.

But if you meant something else, that is that things are *mechanical* compressed, then that is not true (at least in special relativity, with constant velocities); the contraction is due to relativity of simultaneity, that is that to measure a ruler's length you have, by definition of lenght's measure, to simultaneously find the positions of its initial and final points, and if two spatially separated events are simultaneous in a ref. frame, they are not in another which is moving with respect to the first, so the ruler's length that you measure is different.

 

[Doc Al]

Bell's spaceship paradox provides a good insight into the nature of length contraction and distances in special relativity. I think Bell once said that if you do not understand that the string between the rockets will snap then you do not really undersand relativity. Interestingly, in a straw poll of scientists at CERN theory division, most of the scientists got the paradox wrong!

Bell's spaceship paradox is a great example of just how subtle these things can be. (And just how weak a grasp many folks who should know better have on this material. )

The separation between the 2 spaceships must look length contracted from the ground. If it doesn't then relativity is flawed.

From the ship's point of view, the distance is constant otherwise they would themselves have to stretch (regardless of their size kev ;-) )

ps I understood your argument, it's just a badly flawed one as far as I can tell.

The "paradox" hinges on just how the spaceships are being accelerated. By stipulation, they are being uniformly accelerated according to Earth observers, thus their distance apart will remain constant according to Earth observers.

Think of the accelerations happening in bursts. At some point the ships are a distance L apart and move at speed V (according to Earth observers). They fire their rockets to add a burst of speed \Delta V. Ah, but those rockets fire simultaneously according to Earth observers, but not according to the rocket observers. According to the rocket frame, the lead rocket fired first--thus stretching out the distance between the rockets in the rocket frame. (The string breaks, of course.)

 

[||spoon||]

Bell's spaceship paradox is a great example of just how subtle these things can be. (And just how weak a grasp many folks who should know better have on this material. )


The "paradox" hinges on just how the spaceships are being accelerated. By stipulation, they are being uniformly accelerated according to Earth observers, thus their distance apart will remain constant according to Earth observers.

Think of the accelerations happening in bursts. At some point the ships are a distance L apart and move at speed V (according to Earth observers). They fire their rockets to add a burst of speed \Delta V. Ah, but those rockets fire simultaneously according to Earth observers, but not according to the rocket observers. According to the rocket frame, the lead rocket fired first--thus stretching out the distance between the rockets in the rocket frame. (The string breaks, of course.)

hahaha i hadn't really read much into this paradox but when looking at the linked wiki site i couldn't really make much sense of it. Hadnt thought of the simultaneity aspect, quite silly i suppose... but again i haven't learned it before so i figure i get some slack

 

[YellowTaxi]

The "paradox" hinges on just how the spaceships are being accelerated. By stipulation, they are being uniformly accelerated according to Earth observers, thus their distance apart will remain constant according to Earth observers.

Think of the accelerations happening in bursts. At some point the ships are a distance L apart and move at speed V (according to Earth observers). They fire their rockets to add a burst of speed \Delta V. Ah, but those rockets fire simultaneously according to Earth observers, but not according to the rocket observers. According to the rocket frame, the lead rocket fired first--thus stretching out the distance between the rockets in the rocket frame. (The string breaks, of course.)

OK I'll think about that.
kev's explanation was simply that the spaceships cannot possibly stretch because they were made out of some absolutely rigid, solid material (- whatever that is !), And that the string wasn't manufactured from the same amazing stuff...

 

[HallsofIvy]

In relativity, there is not and cannot be any "absolutely rigid, solid material".

If there were, imagine a thing rod made of that material extending from the Earth to the moon. Give a hard rap on your end that moves the rod, say, .001 cm. If the material were really "absolutely rigid" the other end would move at exactly the same time giving you a way to communicate with the moon instantaneously. Since a message cannot be sent faster than the speed of light, any material must be sufficiently elastic that the "rap" moves up the rod in a wave with speed less than the speed of light.

 

[YellowTaxi]

In relativity, there is not and cannot be any "absolutely rigid, solid material".

If there were, imagine a thing rod made of that material extending from the Earth to the moon. Give a hard rap on your end that moves the rod, say, .001 cm. If the material were really "absolutely rigid" the other end would move at exactly the same time giving you a way to communicate with the moon instantaneously. Since a message cannot be sent faster than the speed of light, any material must be sufficiently elastic that the "rap" moves up the rod in a wave with speed less than the speed of light.

Yes, that's exactly the basis of my reply to kev.
The spaceships are not rigid - there's way too many gaps (relatively huge empty spaces) between the atoms/nuclei in a real spaceship.

 

[phyti]

I know this, what i am saying is that would it not APPEAR to have shrunk to someone begins moving near c?

for example, if you measured the distance to a planet whilst in Earth's frame of reference and found it to be say 7 lightyears away... then flew in a spaceship at 99% of c and undertook the same measurement to the planet, wouldn't it appear to be only one light year away now??

I know if i did this it would seem to APPEAR to me that space had contracted.

If they appear contracted, they would also appear deformed (contracted in direction of motion but not perpendicular). If that were true, what would accomplish that?
If not true then the objects are just moving by you faster! The 1st choice leaves you with another question, the 2nd does not.

 

[yuiop]

OK I'll think about that.
kev's explanation was simply that the spaceships cannot possibly stretch because they were made out of some absolutely rigid, solid material (- whatever that is !), And that the string wasn't manufactured from the same amazing stuff...

I never said the the spaceships were absolutely rigid and I am fully aware that Relativity rules out any material being absolutely rigid. I just said the the spaceships were tougher than the string and also said the they length contract.

If you tie a string between two cars and get the front car to accelerate harder the string will initially strech then snap. This does not require the cars to made of infinitely rigid material and can easily to be demostrated. In the spaceship example the front spaceship has to thrust harder in order to maintain a constant gap (as measured in the Earth frame) while the two ships are connected by the string, and the additional energy being expended by the front ship results in stress on the string and eventually snaps it.

Yes, that's exactly the basis of my reply to kev.
The spaceships are not rigid - there's way too many gaps (relatively huge empty spaces) between the atoms/nuclei in a real spaceship.

Sure there are gaps between the atoms that make up anyone spaceship but they are bound together by electromagnetic forces (in a none rigid way). There is a (quite large) gap between the Earth and the Sun but we are still bound to the Sun by gravity and if the Sun were to move we would tend to follow it. I say "tend" because we are bound in a non rigid way like atoms.

 

[YellowTaxi]

Regardless kev, the point I was making was that your argument doesn't hold any water on purely logical grounds. And if anything it only reinforces my view that the space inside the spaceships (or trains or whatever) cannot be treated any differently from the space outside - regardless of which frame you are referring to.

I still have to think over DocAl's referal to the usual problem/explanation of simultaneity.
If anything that is more likely to lead to a loigical explanation. - If there is one. ;-)

 

[Doc Al]

Regardless kev, the point I was making was that your argument doesn't hold any water on purely logical grounds. And if anything it only reinforces my view that the space inside the spaceships (or trains or whatever) cannot be treated any differently from the space outside - regardless of which frame you are referring to.

By focusing on the details of how the rockets contract (despite there being insufficient information given) I think you distract yourself from the point of this thought experiment. The reason why a flimsy string is used is to make the answer to the question "does it break?" unambiguously yes or no--if there's any stress at all, it will break. While the ends of the string (attached to the rockets) are stipulated to be accelerated uniformly according to Earth observers (this is key!), no details are given about exactly how the rockets are being accelerated.

If, by some strange process that I can't imagine, all the pieces of the rocket are also uniformly accelerated with respect to Earth observers, then you're right--the rocket, unless extremely tough, will be torn apart just like the string. Measured from earth, its length will not contract at all. (Of course, that's because it's being stretched apart by whatever forces are accelerating it!) But there's no reason to assume such a strange arrangement.

In my mental picture of this "paradox" the string is miles long while the rockets are tiny. We care about the string breaking, not about what happens to the rockets--who cares how they contract?

I still have to think over DocAl's referal to the usual problem/explanation of simultaneity.
If anything that is more likely to lead to a loigical explanation. - If there is one. ;-)

I think what I pointed out earlier holds the key to understanding the spaceship paradox and length contraction in general.

Whether an accelerating body contracts or not depends on the details of how it is accelerated. Usually we just assume that somehow the rocket accelerates to some final speed and managed to do that without destroying itself. The idealized case is that the rocket pieces accelerate uniformly with respect to a reference frame co-moving with the rocket (actually a continually changing inertial frame). In that case, the rocket is never under any stress at all--it never changes length with respect to that co-moving frame. Of course, earth observers measure the contraction as the rocket speeds up. (To Earth observers, the pieces do not accelerate uniformly--it's the converse of the Bell spaceship scenario.)

 

[Xeinstein]

catia,

Does time really go slower?

Not in this case. The spaceship pilot will feel normal as stated by the first principle. He will observe that the observer is in slow motion. Neither is really, it only seems that way. Add acceleration to the mix though and you can actually travel to other people’s future.

Are you sure time only "seems" to go slower, Not really goes slower?
If that's the case, then can you tell us why the traveling twin is younger than the one stays at home?

 

[woodmeister]

I saw a show on tv where some understanably skeptical scientists synched 2 very accurate clocks and then put 1 on an airplane flying fast while the other clock was not on the plane. The 2 clocks told different time that was exactly as predicted by Es theory.

 

[Magic Man]

That's about right. If you are going fast enough, you can travel a 7 light year distance (as measured from the Earth frame) in about 1 year of your time. Of course, Earth observers would say it takes about 7 years of Earth time.

See, now that is why I can't understand the general scientific acceptance of this area.

If the destination is seven light years away from Earth, i.e. it takes light severn years to reach us from there, the speed of light is constant etc. then we can calculate the physical distance between the Earth and this destination.

If a spaceship is traveling to this destination it still has to travel that same physical distance irrespective of its speed. How can it do anything other? If the distance is the same and the speed of light is constant, then how can the time taken be anything other than seven years...?

 

[DaveC426913]

Yes, that's exactly the basis of my reply to kev.
The spaceships are not rigid - there's way too many gaps (relatively huge empty spaces) between the atoms/nuclei in a real spaceship.

That's bit misleading, since it implies that, if there were fewer or smaller gaps, it could be rigid enough to transmit a signal ftl.

Without getting too picky about the technical deets, there is one theoretical substance where there are almost no gaps - neutronium - and I'll bet dollars to donuts you can't send a signal through a neutron star ftl either.

 

[Doc Al]

See, now that is why I can't understand the general scientific acceptance of this area.

If the destination is seven light years away from Earth, i.e. it takes light severn years to reach us from there, the speed of light is constant etc. then we can calculate the physical distance between the Earth and this destination.

If a spaceship is traveling to this destination it still has to travel that same physical distance irrespective of its speed. How can it do anything other? If the distance is the same and the speed of light is constant, then how can the time taken be anything other than seven years...?

You miss the point that the measured time and distance for the voyage of the spaceship depends on what frame is doing the measuring. In the Earth frame, of course the distance is 7 light years and the travel time is a bit greater than 7 years. But in the spaceship frame, the time and the distance are quite different.

 

[pervect]

See, now that is why I can't understand the general scientific acceptance of this area.

If the destination is seven light years away from Earth, i.e. it takes light severn years to reach us from there, the speed of light is constant etc. then we can calculate the physical distance between the Earth and this destination.

If a spaceship is traveling to this destination it still has to travel that same physical distance irrespective of its speed. How can it do anything other? If the distance is the same and the speed of light is constant, then how can the time taken be anything other than seven years...?

The problem you have is that you are incorrectly assuming that distance is "physical" and therefore the same for all observers.

The point of relativity is that distance depends on the observer. There is a quantity which involves both space and time defined between two events which is independent of the observer doing the measuring, but this quantity is not distance, but rather the Lorentz interval.

Philosophically, this means that distance is not very fundamental (because it depends on who measures it). The Loretnz interval is a more fundamental and physically significant quantity, just because it is independent of the observer.

 

[Magic Man]

Of course distance is physical, the distance between the two points is a physical constant irrespective of who observes the event or what the observers measure the distance to be.

The spaceship has to travel that same distance between the two points that light travels. If it it traveling at light speed or just below then it can only take seven years or a little more as observed from the Earth. What the guy in the spaceship observers may be different but it is still seven years irrespective of what he interprets the time to be - don't go with this time slows as you approach light speed theory I'm afraid. It's just a speed, why should biological processes slow just because you increase the speed at which the body is traveling let along time itself - what is observed is just that, an observation.

 

[lightarrow]

Of course distance is physical, the distance between the two points is a physical constant irrespective of who observes the event or what the observers measure the distance to be.

"Observes" in this case means "measures". Distance is not invariant, unless with "distance" you mean "distance in the ref. frame in which the two points are stationary".

 

[Magic Man]

Distance in the ref. frame - yes, the actual, real distance, which doesn't change.

Forget all these different frames of reference since they produce incorrect 'observed' measurements.

 

[Doc Al]

Distance in the ref. frame - yes, the actual, real distance, which doesn't change.

Forget all these different frames of reference since they produce incorrect 'observed' measurements.

Enough already. You're behind the times by over a century.