time slows down when you approach the speed of light?

[catia]

hi

I read a few other post about this but i'm an engineer, not a physicist, not a genius and didn't get much wiser. And if it's possible try not to use to many technical terms when you answer this, please.

As it says in the title "time slows down when you approach the speed of light", it's driving me crazy.

I watched a documentary on Discovery about the 100 greatest physics discoveries and it was a bit superficial.

They said that...

Time slows down when you approach the speed of light
If you have a satelite moving at 18000 miles/hour around earth you have to account for relativistic effects


Some literature says that it has been exsperimentally confirmed but not how.

I just don't understand, maybe because how i think of time... i see time as a man made thing, if humans cease to exist time has no meaning or also cease to exist. This doesn't mean that all the processes in the universe stops. Like lenght for exsample, what's the meaning of a cm or meter if there are no humans or marketing for that matter, etc.

So why would the processes in your body slow down (and you would age at a lower rate) just because you're moving extremely fast?

[mgb_phys]

Mass, length and time all depend on relative motion.
If you move fast your mass, size and measurement of time differ compared to a stationary observer.
It might not be obvious but it is true!

It has been experimentally verified in a number of ways, unstable particles created in upper atmoshpere reach the surface but don't live long enough to do that - unless time for them is slowed down.
The clocks in GPS satellites have to be adjusted to take this effect into account (actualy it's more complicated because they are slowed down by their speed = special relativity, and speeded up because of the lesser gravity = general relativity)

[HallsofIvy]

Remember this is "relativity"! You can only "near the speed of light" relative to some other frame of reference. If you speed up relative to another frame of reference, people in that frame, watching you would see your clocks, bodily functions, etc. slow down. You would not notice any change. In fact, since "speed" does not have a direction, you would see their clocks, bodily functions, etc. slow down. And, while it is true that such units of measure as "second" or "cm" or "meter" are 'manmade', time and distance are not. If humans ceased to exist, animals and plants would still be born and die. THAT'S what time is.

[sysreset]

Remember this is "relativity"! You can only "near the speed of light" relative to some other frame of reference. If you speed up relative to another frame of reference, people in that frame, watching you would see your clocks, bodily functions, etc. slow down. You would not notice any change. In fact, since "speed" does not have a direction, you would see their clocks, bodily functions, etc. slow down.

Help me out here, HallsofIvy. If I go on a near-light-speed trip and come back to earth, my clocks will have been running slow compared to earth clocks. There should be a clock discrepancy when I get back, with my clock being slow and the earth clock being fast. But you just said speed does not have a direction, which would cause the earth clock to slow down the same amount, and there would be no discrepancy. Something isn't right here.

[DaveC426913]

But you just said speed does not have a direction...No, but acceleration does. And the acceleration is experienced by you, not by Earth.

[lightarrow]

hi

I read a few other post about this but i'm an engineer, not a physicist, not a genius and didn't get much wiser. And if it's possible try not to use to many technical terms when you answer this, please.

As it says in the title "time slows down when you approach the speed of light", it's driving me crazy.

I watched a documentary on Discovery about the 100 greatest physics discoveries and it was a bit superficial.

They said that...

Time slows down when you approach the speed of light
If you have a satelite moving at 18000 miles/hour around earth you have to account for relativistic effects
They have already answered you, I only add that if you could accelerate (in a reasonable time) to near the speed of light with respect our planet, let's say to 0.999999999999999999999999995 c, then you would reach the present (visible) limit of the univere and back to earth, in one day of your clock, but 28 billions of years on earth (non considering the universe expansion), if it will still exist! Anyway, your biological life would be exactly one year older, not even a little less.

[catia]

thank you all for your replies... i'm still a bit confused thought, but i'm going to read some books once my exams are over.

i just have one last question... does special relativity hold true for an electron and other quantum mechanical systems?

just a note: all this started when i wanted to explain the electrical properties of carbon nanotubes in depth. I started at Bohr and Einstein and at that time relativity didn't seem to matter, but now i'm back at relativity.

[lightarrow]

thank you all for your replies... i'm still a bit confused thought, but i'm going to read some books once my exams are over.

i just have one last question... does special relativity hold true for an electron and other quantum mechanical systems?There are entire branches of physics about it (quantum electrodynamics and quantum field theory).
Anyway, just to say three things:

1. the magnetic field generated by an electric current is a relativistic effect of charges's movement;
2. electrons in an accelerator obeys special relativity laws;
3. the electron's spin (and all the consequences of it e.g. Pauli's principle, the way atoms are made; magnetization of matter) comes from Dirac Equation which is the relativistic generalization of the Schrodinger equation for the electron.

[catia]

thank you lightarrow.

i was hoping it wasn't, it would have made life easier.

I started with a book on quantum mechanics and as i incountered subjects i had little or no kwonlegde of i read about them. I know i still lag knowlegde about quite alot of physics subjects, so please bare with my stupid questions.

OH... and happy new year everyone

[lightarrow]

thank you lightarrow.

i was hoping it wasn't, it would have made life easier.

I started with a book on quantum mechanics and as i incountered subjects i had little or no kwonlegde of i read about them. I know i still lag knowlegde about quite alot of physics subjects, so please bare with my stupid questions.No, don't worry!

OH... and happy new year everyoneThe same to you!

[dbecker215]

For further reading on the subject of time and it's complexities I would suggest "About Time" by Paul Davies. It's an excellent book and Mr. Davies is very good at explaining the complexities of time in an easy to understand writing style.

[pervect]

Help me out here, HallsofIvy. If I go on a near-light-speed trip and come back to earth, my clocks will have been running slow compared to earth clocks. There should be a clock discrepancy when I get back, with my clock being slow and the earth clock being fast. But you just said speed does not have a direction, which would cause the earth clock to slow down the same amount, and there would be no discrepancy. Something isn't right here.

Have you seen a space-time diagram? It's just a plot of position vs time, basically. Usually the time axis goes up the paper though. Think of a huge piece of paper scrolling downwards, and the rockets being pens that move left and right. Then the rockets (pens) draw a space-time diagram on the sheet of paper as the pen marks their position as a function of time.

These are maps, of course, where time is represented by a spatial dimension. The map represents the territory, but the map is not the territory.

Now, in the twin paradox, the two twins will draw a triangle. One twin, who does not accelerate, draws a "straight line". The other twin, who does, draws two different straight lines - sides of the triangle.

On these diagrams, it is important to know that the time elapsed by a clock following a particular path through space-time between two events will be represeted by a "length" function related to the "length" of the path. The "length" is, however, not measured in quite the usual way, it is computed by the formula for the timelike Lorentz interval. This means that one takes dt^2 - ds^2 as the interval squared, where dt is the time interval, and ds is the space interval. This is very similar to the pythagorean theorem, but different because of a minus sign.

On the plane, there is an identity known as the "triangle identity" that says that the shortest distance between two points is a straight line. So if you have a straight line connecting two points, it will be shorter than a path that follows two sides of a triangle.

In space-time, the twin paradox says that the longest time interval between two points is a straight line. This has also been called the principle of maximal aging, or sometimes extremal aging - google for some recent posts about this. (I'll post the link if needed). This principle of maximal aging is analogus to the way the shortest distance between two points is a straight line.

So some of the details are involved, but the end result is simple. In *flat* spacetime (this works in SR, not in GR), the twin that follows a straight line on the space-time diagram is the one who experiences the most proper time. The twin who follows a different course takes a shorter amount of proper time.

[catia]

ok, so it wasn't my last question. I may have been taking the "relative to the observer" part a little to lightly.

but first thank you dbecker for telling me about that book.

Everything is relative, right?

If there was a space that is completely empty, no vacuum and no background radiation, just complete nothingness and you place one object in that space.

I first thought that it would not be possible to move at any speed because it would be relative to nothing, but wether or not that space is infinite or finite and no matter what its shape might be that object will always move relative to the center of that space, right?

And even if you remove that object and wether or not the space is constant or expanding, then the space would still be relative to its own center.

[lightarrow]

ok, so it wasn't my last question. I may have been taking the "relative to the observer" part a little to lightly.

but first thank you dbecker for telling me about that book.

Everything is relative, right?

If there was a space that is completely empty, no vacuum and no background radiation, just complete nothingness and you place one object in that space.

I first thought that it would not be possible to move at any speed because it would be relative to nothing, but wether or not that space is infinite or finite and no matter what its shape might be that object will always move relative to the center of that space, right?

And even if you remove that object and wether or not the space is constant or expanding, then the space would still be relative to its own center.

Ok, you have to remember that "space" must have a physical meaning (in physics). How would you establish the centre of the space? You could think to put a flag in some point and define that as the centre, but how do you know that flag won't move with respect to that "ideal" point? How can you measure its speed to that point? It's impossible. The fact is that ideal point doesn't exist in the physics realm and you are left with the flag (a physical object) with respect to which you can measure distances and speeds.

[HallsofIvy]

ok, so it wasn't my last question. I may have been taking the "relative to the observer" part a little to lightly.

but first thank you dbecker for telling me about that book.

Everything is relative, right?

If there was a space that is completely empty, no vacuum and no background radiation, just complete nothingness and you place one object in that space.

I first thought that it would not be possible to move at any speed because it would be relative to nothing, but wether or not that space is infinite or finite and no matter what its shape might be that object will always move relative to the center of that space, right?

And even if you remove that object and wether or not the space is constant or expanding, then the space would still be relative to its own center.
"If there was a space that is completely empty", then neither you nor the object would exist! If the space is "completely empty" except for you and the object, then the object would move relative to you. There is no reason to think that space, whether finite or infinite, has a "center".

[catia]

thank you Ivy... i think i got a bit too philosophical.

what i really wanted to ask was just "is there anything i the real/physical world that is not relative", cause i can't think of something that isn't.

Anyway none of you have to be more specific than yes or no, but if yes you could provide just one exsample.

And at least for my sake that will be the end of this thread. I have read about more stuff, the last couple of weeks, than my brain can contain in such sort time, i can hardly remeber a page at time right now.

Thanks to all who have helped in this thread.

[HallsofIvy]

Well, in a sense, while velocity is "relative" to some frame of reference, acceleration is not! If you feel a force, given by F= ma, then you will feel a force in any frame of reverence. Of course, if you are will to ascribe that force to some outside influence, such as "gravity", then acceleration is "relative" again!

[yuiop]

ok, so it wasn't my last question. I may have been taking the "relative to the observer" part a little to lightly.

but first thank you dbecker for telling me about that book.

Everything is relative, right?

If there was a space that is completely empty, no vacuum and no background radiation, just complete nothingness and you place one object in that space.

................................


Here is something to ponder. With just one object in an empty universe it would be impossible to tell if it had linear motion or not. However it would be possible to tell if that lonely object was spinning or not. If you had just 2 objects in our otherwise empty universe it would be impossible to tell which one was stationary and which was moving. However, if one of the objects was spinning, you would be able to tell which one is spinning. For practical purposes assume an object is a large body that holds an observer who has a light source, clocks, rulers, mirrors and a few other bits of lab equipment to make measurements with.

[DaveC426913]

Here is something to ponder. With just one object in an empty universe it would be impossible to tell if it had linear motion or not. However it would be possible to tell if that lonely object was spinning or not.That is because anything larger than fundamental subatomic particles are not single particles at all; they are composed of two or more particles, and you're back to comparing theit relative orientation.

But if you did have a single subatomic particle, no you would not be able to tell if it were spinning.

[yuiop]

That is because anything larger than fundamental subatomic particles are not single particles at all; they are composed of two or more particles, and you're back to comparing theit relative orientation.

But if you did have a single subatomic particle, no you would not be able to tell if it were spinning.

The point I was trying to make is that there appears to be something fundementally more absolute about rotation compared to linear motion. If the Earth was in an otherwise empty universe , you could launch rockets and carry out all sorts of experiments and measurement of "relative orientation" and still be unable to determine if the Earth had absolute linear motion, yet you could by assuming the simplest possible laws of physics infer that the Earth had absolute rotation relative to the vacuum of space.

[lightarrow]

The point I was trying to make is that there appears to be something fundementally more absolute about rotation compared to linear motion. If the Earth was in an otherwise empty universe , you could launch rockets and carry out all sorts of experiments and measurement of "relative orientation" and still be unable to determine if the Earth had absolute linear motion, yet you could by assuming the simplest possible laws of physics infer that the Earth had absolute rotation relative to the vacuum of space.But that is wrong according to Mach's principle (and so, I think, according to GR, but I'm not sure): in the absence of other objects which creates space-time itself, you couldn't say if that only object is spinning or not.

[lightarrow]

thank you Ivy... i think i got a bit too philosophical.
Your question was not philosophical at all. See post n. 21.

[lightarrow]

They have already answered you, I only add that if you could accelerate (in a reasonable time) to near the speed of light with respect our planet, let's say to 0.999999999999999999999999995 c, then you would reach the present (visible) limit of the universe and back to earth, in one day of your clock, but 28 billions of years on earth (non considering the universe expansion), if it will still exist! Anyway, your biological life would be exactly one year older, not even a little less.

Sorry, it should be one day of course.

[jtbell]

Everything is relative, right?

No, there are quantities that are the same in all inertial reference frames, and they are very important in relativity!

For example: measure the energy E and momentum p of an object. Different observers (moving relative to each other) will get different values of E and P. Nevertheless, they will all calculate the same result for the quantity

m = \frac{\sqrt {E^2 - (pc)^2}}{c^2}

which we call the invariant mass. It's also known as the "rest mass" of the object.

Another example: various observers measure the position and time of two different events. Event 1 occurs at position x_1 and time t_1. Event 2 occurs at position x_2 and time t_2. In general, each observer will measure different values for the x's and t's. Nevertheless, they will all calculate the same result for the quantity

s = \sqrt{c^2 (t_2 - t_1)^2 - (x_2 - x_1)^2}

which we call the invariant (spacetime) interval between the two events.

[catia]

to my own post #16
sorry i forgot to say thanks to lightarrow for post #14 i totally missed that.

thanks again everyone... i won't ask anymore questions for now.

[yuiop]

But that is wrong according to Mach's principle (and so, I think, according to GR, but I'm not sure): in the absence of other objects which creates space-time itself, you couldn't say if that only object is spinning or not.

Although Einstien showed an interest in Mach's ideas when he was formulating GR, ultimately he rejected Mach's principle and that principle is not an inherent part of GR or even in agreement with it. By Mach's principle the accelerations felt by an observer on a spinning body would be equivalent to the acceleration caused by the mass of all the distant stars spinning around the stationary body. The maths does not quite work out in exact agreement with GR (as far as I am aware). One difficulty is that even for quite low rates of rotation of the body, assuming the body was stationary would require the distant stars to have a tangential velocity exceeding the speed of light.

[Garth]

Although Einstien showed an interest in Mach's ideas when he was formulating GR, ultimately he rejected Mach's principle and that principle is not an inherent part of GR or even in agreement with it. By Mach's principle the accelerations felt by an observer on a spinning body would be equivalent to the acceleration caused by the mass of all the distant stars spinning around the stationary body. The maths does not quite work out in exact agreement with GR (as far as I am aware). One difficulty is that even for quite low rates of rotation of the body, assuming the body was stationary would require the distant stars to have a tangential velocity exceeding the speed of light.
That last point of course would not matter if the distant stars did not imparting information to the body at a rate exceeding the speed of light.

One modification of GR that does include Mach's Principle is the Brans Dicke (http://en.wikipedia.org/wiki/Brans-Dicke_theory) theory, which is fully covariant so the inertial information conveyed by the scalar field \phi travels at the speed of light.

A further modification of the Brans Dicke theory is Self Creation Cosmology (http://en.wikipedia.org/wiki/Self-creation_cosmology), which also fully includes Mach's Principle and which also does not violate the "light-speed" restriction on information flow.

Garth

[dbecker215]

The point I was trying to make is that there appears to be something fundementally more absolute about rotation compared to linear motion. If the Earth was in an otherwise empty universe , you could launch rockets and carry out all sorts of experiments and measurement of "relative orientation" and still be unable to determine if the Earth had absolute linear motion, yet you could by assuming the simplest possible laws of physics infer that the Earth had absolute rotation relative to the vacuum of space.
You wouldn't be able to tell whether the object is spinning or not unless you were on the outside looking in, but then it would be spinning relative to you. If you were on the object you would be rotating with in such a way that you wouldn't be able to tell. Take earth for an example: you throw a ball "straight" up in the air, it comes "straight" down to you. The main reason we can tell that the earth is spinning is due to the changes in our relationship with our sun and moon. This is a very simple point, it can become very complex if we let it, but nonetheless it means that all motion including, including rotational motion or spin, is relative to an external object.

[lightarrow]

You wouldn't be able to tell whether the object is spinning or not unless you were on the outside looking in
[...]
The main reason we can tell that the earth is spinning is due to the changes in our relationship with our sun and moon.
That's not true, you have forgot centrifugal force.

[DaveC426913]

That's not true, you have forgot centrifugal force.

If your frame of reference is a point, then you won't experience angular momentumm which means there's no way to tell you're rotating (or more to the point, you cannot rotate).

If your frame of reference is not a point, then you are talking about a non-zero radius, which means your angular momentum can just as easily be treated as translational movement over short distances.

[yuiop]

You wouldn't be able to tell whether the object is spinning or not unless you were on the outside looking in, but then it would be spinning relative to you. If you were on the object you would be rotating with in such a way that you wouldn't be able to tell. Take earth for an example: you throw a ball "straight" up in the air, it comes "straight" down to you. The main reason we can tell that the earth is spinning is due to the changes in our relationship with our sun and moon. This is a very simple point, it can become very complex if we let it, but nonetheless it means that all motion including, including rotational motion or spin, is relative to an external object.

If you used a (very long) tape measure you could detect a bulge at the equator (even the sea bulges) due to centripetal force. If you sent sent two directional radio signals East and West you would find that the West going signal would circumnavigate the Earth faster and return first. You could also transport atomic clocks East and West and find the East going clock loses more time than the West going clock. A gyroscope on your desk would precess every 24 hours. You would observe a similar 24 hour precession in a Foucault pendulum. When you weigh an object at sea level on the equator and then weigh it at sea level at the North pole you would expect a difference due to the difference in radius at those points, but you would find there is very little difference due to centripetal force. If a tunnel was drilled through the centre of the Earth from one point on the equator to another, an object dropped into the tunnel would collide with the East side of the tunnel rather than drop straight through. None of these methods require a sun, moon or stars to detect the spin of the Earth.

[dbecker215]

You're right that you would be able to figure out whether it was spinning or not but the thing missing in all this is that your spin is still relative. The spin of the object is relative to the coordinate 0 at the axis of the object. So all the measurements that could possibly be done would conclude that the object was spinning relative to the axis, but none would conclude that the object was spinning in relation to the vacuum of space. You still would not be able to conclude that space itself was not spinning.

[yuiop]

If your frame of reference is a point, then you won't experience angular momentumm which means there's no way to tell you're rotating (or more to the point, you cannot rotate)..

It's true that a point particle cannot rotate. However, if we define a point as something with a radius of less than or equal to the Planck length then no elementry particle is a point. Protons and classical electrons are about 10^20 Planck lengths and quark is about 10^17 Planck lengths.



If your frame of reference is not a point, then you are talking about a non-zero radius, which means your angular momentum can just as easily be treated as translational movement over short distances.

If by translational movement you mean linear movement then that kind of motion cannot be detected and can only be expressed relative to an observer. AS HallsOfIvy pointed out, acceleration is absolute in the sense that it can be detected without reference to another body. Rotational or angular motion of a macro object is a form of acceleration and is absolute. I brought up the subject of rotation as the original poster asked if there was anything that was not relative.

[lightarrow]

If your frame of reference is a point, then you won't experience angular momentumm which means there's no way to tell you're rotating (or more to the point, you cannot rotate).

If your frame of reference is not a point, then you are talking about a non-zero radius, which means your angular momentum can just as easily be treated as translational movement over short distances.In this way you could say accelerations don't exist at all...

[DaveC426913]

It's true that a point particle cannot rotate. However, if we define a point as something with a radius of less than or equal to the Planck length then no elementry particle is a point. Protons and classical electrons are about 10^20 Planck lengths and quark is about 10^17 Planck lengths.
Correct me if I'm wrong but: subatomic particles act as points, have no inner structure nor anything that could be ascribed to them as "rotating" in the classical sense.

In fact there is no way to tell if a electron (or proton) is rotating, even in principle - in the same way that two electrons (or protons) cannot be distinguished from one another.

[yuiop]

Correct me if I'm wrong but: subatomic particles act as points, have no inner structure nor anything that could be ascribed to them as "rotating" in the classical sense.

In fact there is no way to tell if a electron (or proton) is rotating, even in principle - in the same way that two electrons (or protons) cannot be distinguished from one another.

I guess your right. At that level quatum physics prevail and ascribing a exact location or orbit to an electron is pretty much impossible. I am talking about rotation at macro scales but at quatum scales classical physics and even general relativity (as far as I know) break down.

[DaveC426913]

I guess your right. At that level quatum physics prevail and ascribing a exact location or orbit to an electron is pretty much impossible.
I wasn't even talking about uncertainty or whether it's measurable. I was simply pointing out that the particles themselves do not have anything that is akin to orientation and thus rotation.

In fact, I'm not even sure a single atom does, though I'm open to education on this. I'm not sure that the orbitals of a single atom can be considred to have an orientation and thus a rotation until they hook up with other atoms.


I am talking about rotation at macro scales but at quatum scales classical physics and even general relativity (as far as I know) break down.
Yeah, but as soon as you talk about anything macro, you involve more than one atom, and as soon as that happens, you can assign both rotation and inertial motion to them.

What I'm getting at is that I think the very idea that "rotational motion can be determined without an external reference" is an illusion. I'm not positive though.

[yuiop]

I wasn't even talking about uncertainty or whether it's measurable. I was simply pointing out that the particles themselves do not have anything that is akin to orientation and thus rotation.

In fact, I'm not even sure a single atom does, though I'm open to education on this. I'm not sure that the orbitals of a single atom can be considred to have an orientation and thus a rotation until they hook up with other atoms.

Upon reflection, I tend to agree that you cannot detect rotation in a single atom. The nearest I can find is detection of rotation in a molecule of CF4 in a superfluid.

http://www.sciencewatch.com/may-june2002/sw_may-june2002_page7.htm


Yeah, but as soon as you talk about anything macro, you involve more than one atom, and as soon as that happens, you can assign both rotation and inertial motion to them.

What I'm getting at is that I think the very idea that "rotational motion can be determined without an external reference" is an illusion. I'm not positive though.

A lot depends on what you count as an external reference. Would light count as an external reference? In the case of linear motion, light is not very helpful in detecting your motion as light moves at c with respect to any inertial observer. In the case of rotational motion, the motion can be measured with light by basically using the Sagnac effect. So if you were standing on the inside a hollow cylinder that had a vaccuum inside (wearing a space suit) you could work out your rotation rate using light, but without being able to reference anything outside the cylinder. You could assume you are rotating with respect to the vacuum, or you could say that the cylinder is stationary and the vaccuum is rotating. You would then have to explain the fact you can comfortably stand anywhere on the curved inner face of the stationary cylinder due to an outward gravitational force caused by the rotating vacuum. The anisotropic behavior of light you observe would be accounted for by a helical spacetime. You would note that when you walk in certain direction and at certain speed that the gravity disappears and you can orbit inside the cylinder. You would put this down to your speed speed matching that of the rotating vacuum so that you are now stationary with respect to the vacuum. Either way you would be assigning special properties to the vacuum.

The problem with rotating vacuum idea is that if you got a cigar tube out your pocket and spun it on an axis tangential to the larger cylinder you would need to superimpose another spinning vacuum tangential to the original one to account for the behavior of the cigar tube. Assuming light travels at a constant c in all directions with respect to the vacuum and that both the cylinders are spinning with respect to the stationary vacuum results in a much simpler model.

[DaveC426913]

...the motion can be measured with light by basically using the Sagnac effect.
?

I guess I'll have to look that up.

[ΔxΔp≥ћ/2]

catia,

Here is the most intuitive way I have found to understand why one says that time slows down in special relativity.

Special relativity has two very simple, very intuitive postulates and all the consequences of the theory can be understood on a basic level with very simple math.

The first postulate is:
1.The laws of physics are the same in all inertial frames of reference.

This means that if I did an experiment on the earth, for example, and then did the exact same experiment in a car moving at a uniform speed, in a straight line, the results would be the same.

We experience this all the time. If in your car, you decide to throw a ball upwards it will feel exacly the same as if you where at rest. It has been proven experimentally by Michelson, Moreley and others that there is even no way to prove that you are moving if you a going at a constant speed in a straight line.

You may think that you are at rest, but the earth is now orbiting the sun, that movement we can prove. Ask yourself whether the sun is moving, that we cannot.

2.The speed of light in a vacuum is a universal constant, independent of the motion of the light source.

Imagine that you where to throw a ball at 2m/s. The ball is moving at a speed of 2m/s. Now, if you where to run 2m/s and then throw the ball again, directly in front of you, at the same speed as before, 2m/s, the ball would move at a speed of 2m/s + 2m/s = 4m/s for a stationary observer and 2m/s for you (because you are running towards it). This is how we can move from one reference system to another.

Light is a totally different issue. You can run toward or away from a beam of light and it will not matter. No matter how you are moving with respect to the light, you will measure its speed to be the same.

The two previous postulates seem totally irreconcilable, one is practically the opposite of the other. But do not forget what may be the first physics equation that you ever learned:

Speed = distance travelled / time

It is possible for all observers to agree on the speed of light if their conception of space and time are different.

Since your question was about time, I will show you how it works for time.

Imagine a clock, but this is not your ordinary clock. This clock is quite simple. Take two parallel mirrors and make them face each other, one on the ceiling and one the floor. Now we will place a photon (light particle) so that it may be reflected back and forth between the two mirrors. When it hits the top mirror, we shall hear a "tic" and when it hits the bottom mirror, we shall hear a "toc". We shall now define a unit of time as what must lapse for the photon to travel from one mirror to the other:

Time = distance between the mirrors / the speed of light (constant)

Here is where it gets freaky...

Lets put this clock in a space ship. Lets also make the ship fly by us at a constant speed, in a straight line. The first postulate stated that the pilot of the ship is perfectly valid in stating the he is not moving. Therefore the pilot will experience time flow as defined just above. We however experience something quite different when looking at the pilot:

I shall start with Pythagorean theorem

(Horizontal line)^{}2 +(vertical line)^{}2 = (hypotenuse)^{}2

(I wish I could draw this for you)

Now, using the equation:

Distance = speed x time

We shall replace the horizontal line with a distance (speed of the space ship x time observer). This is the distance the observer sees the ship travel.

We shall replace the vertical line with a distance (speed of light x time of space ship). This is the distance that the light travels from one mirror to the other viewed from an observer inside the ship. This observer can rightfully say he is at rest.

We also replace the hypotenuse with a distance (speed of light x time of observer). This is the distance that the observer sees the photon make between a tic and a toc.

Simple algebra reveals the proper transformation, but just looking at the triangle, it becomes evident why time cannot be the same.

The same operation can be taken out with the mirrors on the walls parallel to the movement and it becomes evident why people also perceive distances to be different.

Does time really go slower?

Not in this case. The space ship 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.

I know this was not very visual, but it is the best I can do here.

[||spoon||]

About length contraction and time dilation...?

I was thinking the other day, say you were able to travel somewhere at 99% of the speed of light then the factor of length/time dilation would be something like 7. (cant actually remember this may be incorrect but please ignore its not really important!).

So say you wished to travel to a distant planet which from the reference point of someone on earth was 7 lightyears away. Once you actually got up to 99% of the speed of light the length has "contracted" to 1 lightyear away.

While thinking about this i was wondering "has the length really contracted?? Wasn't the length from earth to this distant planet ALWAYS 1 lightyear away in the frame of refrence of moving at 99% the speed of light?"

Similarly could you not argue the point that time has not actually dilated, it would always take you 1 year (approximately) to travel to that planet... in that refrence frame.

Im not sure if that makes sense or its just stupid haha, i only just finished high school physics and we didnt do relativity :( If it does make sense is it right or completely off the mark?

[dbecker215]

So say you wished to travel to a distant planet which from the reference point of someone on earth was 7 lightyears away. Once you actually got up to 99% of the speed of light the length has "contracted" to 1 lightyear away.


I don't believe what you're saying is correct simply because if it is 7 light years away then 7 years is the quickest you can get there. For something to be 7 light years away that means that if you were traveling at the speed of light it will take you 7 years to get there. If you are traveling at 99% the speed of light then it will actually take you longer than 7 years to get there because you're traveling slower than the speed of light.

I'm not sure what you're looking for exactly based on this question... sorry.

[ΔxΔp≥ћ/2]

Nope, you would get there faster. In your conception of time, time would pass normally. However, because you will see everything else contract, the distance will not be the same.

Light experiences no time.

All this being said though, things would get a lot more complicated than that though, because acceleration would have to be involved.

If I am wrong please correct me.

[mihais18]

hello. i have just posted a new thread on time dilation but i would also like to discuss this aspect with you... sorry if i repeat what i have just posted:

I have read a lot of information about time dilation, the twin paradox, the doppler effect and the lorentz transform, but, because I am not a physicist (as a matter of fact i teach french), I have to confess that I understand time dilation only partially.

On the internet there are lots of examples that go with the theoretical explanations. (eg. http://www.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm or http://www.walter-fendt.de/ph11e/timedilation.htm). There is this example of the 2 clocks that are synchronized. One of them stays on the earth, the other is placed on a spaceship that travels at near-lightspeed. Both of them work with a light beam that bounces off a mirror. The basic idea is that the clock on the spaceship ticks slower, because it takes the lightbeam more time to bounce off the mirror. There is also the case of the twin paradox that is brought into the discussion.

Now you’ll have to excuse my childish ignorarace: for me these examples only demonstrate that at relativistic speeds a light beam clock ticks slower, not that time itself goes slower. As for the twin paradox, why does the twin brother who travels on the spaceship age slower than the one on earth? In what way are the biological processes slowed down? Is that because the particles of the atoms that make up the human body are also slowed down, just like the light beam?

it is known that for an object that travels at a certain speed time goes slower than for an object that stays still. So I would like someone to explain to me the relationship between speed and time dilation. If this has also to do with light or the speed of light, then I would like to get an explanation about the relation between time dilation and speed of light…

thank you in advance!

[||spoon||]

I don't believe what you're saying is correct simply because if it is 7 light years away then 7 years is the quickest you can get there. For something to be 7 light years away that means that if you were traveling at the speed of light it will take you 7 years to get there. If you are traveling at 99% the speed of light then it will actually take you longer than 7 years to get there because you're traveling slower than the speed of light.

I'm not sure what you're looking for exactly based on this question... sorry.

i was under the impression that there was a general agreeance throughout the scientific community that time and length changes according to SR... It seems to me you are neglecting this and are thinking only form a reference frame on earth

[yuiop]

[QUOTE=mihais18;1560573]
.
.
Is that because the particles of the atoms that make up the human body are also slowed down, just like the light beam?
QUOTE]


Basically...yes.

If all biological, physical and chemical processes did not slow down in exactly the same way that a light clock does, then relativity and most of modern physics would be completely wrong. The only thing that does not slow down in an inertial frame is light itself.

But time is relative. Imagine you are a twin that likes reading a lot. You decide your ambition is to read all the books in large library. You work out if you spend every waking hour reading it will take 2000 years to read them all. You jump on a very fast spaceship (which has all the books stored on a computer.) and set about trying to read them all.2000 years later (Earthtime) you return to discover your twin died hundreds of years ago and not only did you not read all the books but that you read no more books than your twin did.

[ΔxΔp≥ћ/2]

mihais18,

I would like to refer you to post #40 that I wrote, I know that it is a long read.

Please note that because the observer in the space ship is valid in stating that he is at rest, he observes the light to travel like someone with the same clock that is at "rest" (on the ground).

The person who observes the space ship fly by notes that the light travels a longer distance than the one observed by the pilot in the space ship.

If both these people agree on the speed of light, they must disagree on distances and time.

Once you let it sink in that they are both perfectly valid in their statements, it will become obvious why time seems to slow down.

Also note that when the space ship pilot looks out his window, he will see the observer in slow motion. This is symmetry between the two observers. Both are right.

If you go beyond special relativity and look at general relativity, you will see that you actually can travel to someone’s past or future. (Not really my area of expertise though.

If you have difficulty understanding this in English, I can also explain this to you in French.

[yuiop]

?

I guess I'll have to look that up.

This is an informal description of the sagnac effect:

Imagine you have a ring of mirrors so that light can be sent in either direction around the ring and return to its starting point. Imagine that light moves at a constant speed relative to the vacuum. Now if the ring rotates clockwise relative to the vacuum, a light pulse going clockwise around the ring will take slightly longer to return to the start than a pulse going anticlockwise. An interferometer can detect very small differences in the pulse arrival time and is sensitive enough to detect rotations of less than 1 rpm without refence to any external source. (The light source and interfermoter are mounted on the ring.) Real solid state (no moving parts) Sagnac gyroscopes are commercially available and work on this principle. In fact they are replacing traditional high precision, "flywheel" type inertial gyroscopes that are very expensive to make.

[dbecker215]

i was under the impression that there was a general agreeance throughout the scientific community that time and length changes according to SR... It seems to me you are neglecting this and are thinking only form a reference frame on earth

It's a bit more complicated than that. Distance, or length, doesn't change with relativity. Distance itself is defined as being a scalar quantity. Scalar quantities do not require direction therefore do not change in a coordinate system. (see Wikipedia for distance, scalar, and magnitude) This means that it is space that warps, b/c space and time are viewed as being inseparable therefore if time warps space must also warp. When you add in space distortion this changes your displacement and your vector, but not distance.

If you can picture some astrological being with a cosmic ruler measuring your scenario of a planet 7 light yrs away, from his perspective the distance you travelled did not change with your speed. Your vector probably will have b/c of the space distortion, but the measuable distance, from point A to point B, did not.

As far as agreeance goes I wouldn't say that loosely. I have read several books on the subject of relativity and time dilation, ranging from textbooks to personal writings from well known physicists, and I wouldn't call it agreeance. There seems to be many disagreements still. Some of this comes from the fact that what experiments we have tend to not line up with the same formula for calculating time dilation. I just read an article the other day about a physicist saying that Einstein's original formulas don't explain the results of different time dilation experiments. I'll have to find his article again and post a thread to get some response. Regardless I am wary of saying that the science community is in agreement.

[Doc Al]

I was thinking the other day, say you were able to travel somewhere at 99% of the speed of light then the factor of length/time dilation would be something like 7.
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.

I don't believe what you're saying is correct simply because if it is 7 light years away then 7 years is the quickest you can get there. For something to be 7 light years away that means that if you were traveling at the speed of light it will take you 7 years to get there. If you are traveling at 99% the speed of light then it will actually take you longer than 7 years to get there because you're traveling slower than the speed of light.
You are missing the point. As measured by the space traveler, the distance has contracted to about 1 light year so the time required would only be about 1 year of spaceship time.
It's a bit more complicated than that. Distance, or length, doesn't change with relativity. Distance itself is defined as being a scalar quantity. Scalar quantities do not require direction therefore do not change in a coordinate system. (see Wikipedia for distance, scalar, and magnitude) This means that it is space that warps, b/c space and time are viewed as being inseparable therefore if time warps space must also warp. When you add in space distortion this changes your displacement and your vector, but not distance.
Huh? This is incorrect. Distance is not invariant--it depends on who's doing the measuring.

As far as agreeance goes I wouldn't say that loosely. I have read several books on the subject of relativity and time dilation, ranging from textbooks to personal writings from well known physicists, and I wouldn't call it agreeance. There seems to be many disagreements still. Some of this comes from the fact that what experiments we have tend to not line up with the same formula for calculating time dilation. I just read an article the other day about a physicist saying that Einstein's original formulas don't explain the results of different time dilation experiments. I'll have to find his article again and post a thread to get some response. Regardless I am wary of saying that the science community is in agreement.
Nonsense. There is widespread agreement--and experimental evidence--that special relativity is correct.

[||spoon||]

Doc al: is the way i was thinking about the dilation factors wrong?

As in how i think about it is that the distance was ALWAYS 1 light year, but only in the reference frame of 99% the speed of light... and the time taken was ALWAYS 1 year aswell...

So i guess i think of it that time and distance only contract with refernce to some inertial refernce frame... does this make sense?

also if this is true arent there numerous "versions" of the universe, all with different lengths and times dependant on your speed? (im not saying parallel universes but that if we were all moving at 99% of c then everything would seem 7 times closer and this would be the norm)

Thanks

[Doc Al]

Doc al: is the way i was thinking about the dilation factors wrong?

As in how i think about it is that the distance was ALWAYS 1 light year, but only in the reference frame of 99% the speed of light... and the time taken was ALWAYS 1 year aswell...
Since distance is always defined with respect to some observer's frame, as long as that frame existed the distance measured by them was the same shorter length (compared to that other frame).

So i guess i think of it that time and distance only contract with refernce to some inertial refernce frame... does this make sense?
Special relativistic effects are always due to the relative motion of reference frames.

also if this is true arent there numerous "versions" of the universe, all with different lengths and times dependant on your speed? (im not saying parallel universes but that if we were all moving at 99% of c then everything would seem 7 times closer and this would be the norm)
I would say there's only one universe, but that the distances and times between various events within that universe depend on who (what frame) is doing the measuring. Since these measurements are frame-dependent, they are less fundamental than we had first thought. To use a weak analogy, just like speed is frame dependent so are distance and time measurements. (But there are quantities which are invariant--the same for all observers; these can be said to represent the more fundamental structure of the universe.)

[yuiop]

It's a bit more complicated than that. Distance, or length, doesn't change with relativity. Distance itself is defined as being a scalar quantity. Scalar quantities do not require direction therefore do not change in a coordinate system. (see Wikipedia for distance, scalar, and magnitude) This means that it is space that warps, b/c space and time are viewed as being inseparable therefore if time warps space must also warp. When you add in space distortion this changes your displacement and your vector, but not distance.

The poster was talking about distances as measured by observers in different reference frames, and Doc Al is right in saying they are not invarient. In the example a person at rest with the Earth would say the distance to the planet if 7 light years and the rocket traveler would measure the distance as 1 light year. Neither can prove the other is wrong, as neither can prove they are at rest with some absolute reference frame.

Whether distances are classed as scalar or vector quantities is not really relevent in this context but for the record the distance refered to by x in the Lorentz transformation t = y(t' +vx') and x = y(x' +vt') is a vector quantiy as it can take a + or - sign according to which direction from the origin of the reference frame that the distance x is measured. Perhaps you meant that proper distances (distances that are measured by an observer at rest with the endpoints of the distance being measured) are invarient?

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!

http://en.wikipedia.org/wiki/Bell's_spaceship_paradox

[YellowTaxi]

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!

http://en.wikipedia.org/wiki/Bell's_spaceship_paradox

well if you think the string breaks then you must believe the 2 spaceships actually physically stretch (viewed from their own frame) for precisely the same reason...
So does this actual physical stretching precisley cancel out the observed length contraction seen from the launch-pad ?

[phyti]

Here is a simple analogy. I fly from home to a city 200 miles distant at 100 mph in 2 hr.
Later I fly from home to the same city at 200 mph in 1 hr. The distance didn't change, the time did. The space travelers time changes with his speed, he gets there quicker according to his clock. That's because his clock is parsing time into longer units.
Reasoning if his units are longer, there will be more events recorded in them, so he will observe events happening at a faster rate outside his ship in the direction of motion and slower in the opposite direction. Consider the doppler shift, faster ahead, slower behind.
Reasoning tells you launching into space does not shrink the universe! What physical process would accomplish this?

[yuiop]

well if you think the string breaks then you must believe the 2 spaceships actually physically stretch (viewed from their own frame) for precisely the same reason...
So does this actual physical stretching precisley cancel out the observed length contraction seen from the launch-pad ?


In the paradox the 2 spaceships accelerate at the same rate. From the launch frame the distance between the 2 spaceships remains the same, but the spaceships themselves appear to be length contracting. If the distance between the spaceships is large compared to the lengths of the spaceships we can ignore the length contraction of the spaceships themselves as far as the string is concerned. Imagine the string is connected from the centre of one ship to the centre of the other, so that the string is unaffected by any change in length of the ships themselves.

From the point of view of the observers on the spaceships the lengths of their ships remains unchanged, but the distance between the 2 ships is increasing. (They can measure that distance by sending light signals from one ship to the other and measuring the round trips times of the signals.) From their point of view, the increasing distance causes the string to stretch and eventually snap. From the launch frame the distance between the two ships remains the same but the string is trying to length contract and snaps because it is shorter than the seperation distance.

[YellowTaxi]

From the launch frame the distance between the two ships remains the same but the string is trying to length contract and snaps because it is shorter than the separation distance.
Then kev, you're saying that the distance from the front tip to the rear end of each ship will actually increase for the same reason. The ships will eventually tear themselves apart by that logic. On top of that there should be no observable length contraction from the ground.

[yuiop]

Then kev, you're saying that the distance from the front tip to the rear end of each ship will actually increase for the same reason. The ships will eventually tear themselves apart by that logic. On top of that there should be no observable length contraction from the ground.

Nope, I said from the launch frame the ships would length contract (tip to rear length). The ships are solid objects made of atoms bound together so they length contract. The distance between the two ships is empty space so it does not pull the ships together. The only thing between the ships is the string, and for the purpose of the paradox it is assumed the string is not strong enough to pull the ships together and snaps.

If the string was replaced by a very strong cable then the ships would be pulled together and the distance between the ships would appear constant to the observers onboard the ships and length contracted to the observers in the launch frame.

Hope that makes sense :P

[YellowTaxi]

Nope, I said from the launch frame the ships would length contract (tip to rear length). The ships are solid objects made of atoms bound together so they length contract. The distance between the two ships is empty space so it does not pull the ships together.

??
So if you fly to a distant planet close to c, the space you travel doesn't look length contracted just because it's empty space? That can't be true.

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.

[yuiop]

??
So if you fly to a distant planet close to c, the space you travel doesn't look length contracted just because it's empty space? That can't be true.

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.

As you accelerate towards the distant planet, you could imagine that your rocket engines are holding you stationary against a gravitational field that is pulling the planet and the Earth towards some huge black hole behind you. The planet would appear to be accelerating faster towards you than the Earth is receding from you so the distance between the Earth and the planet would appear to be contracting. This is a different situation from the rockets in Bell's paradox because they are both accelerating at the same rate with respect to an observer on the Earth. If the 2 ships accelerated at different rates with the rear ship accelerating faster (so that the gap between them remained constant from their point of view), then the distance between them would be length contracting from the Earth point of view. One way they could maintain constant separation would be to maintain constant tension on the string and then of course the string would not break.


The ships themselves are also length contracting from the POV of the Earth observer, but time dilation of the spaceship clocks and the way they syncronise their clocks make it seem to them that the length of their spaceships remain unchanged.

[||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 earths 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 earths 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 lenght 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 lenght 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. :wink:)

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. :wink:)


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 couldnt really make much sense of it. Hadnt thought of the simultaneity aspect, quite silly i suppose... but again i havent learnt it before so i figure i get some slack :approve:

[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 earths 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 any one 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 refering 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 refering 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 space ship 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 travelling 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 travelling 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 travelling 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 travelling 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 therfore 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 travelling 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 travelling 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.