So time not only runs slower when it is freed from the terrible oppression of gravity, it also runs slower when travelling fast.

So travelling fast through space would have a doubled effect.

Only thing is, i imagine there might be a spacecraft travelling very fast and there are space travellers on board. Only they aren't moving exactly... They're sitting around just doing their normal things. Everything seems normal to them except that when they look out the spacecraft window, everything outside the spacecraft appears to slow down.

Another way of saying this; when i drive my car, am i moving at 100km/h? Or am i sitting stationary in the driver's seat?

Kolch, time runs slower in a stronger gravitational field, otherwise your image of the spacecraft is correct.

When you are sitting in you car you are stationary in the drivers seat, but moving at 100km/h relative to the road. The Earth is spinning at aprox 1670km/h (near the equator), the add the speed you are going around the sun..and the speed the galaxy is rotating...and so on. It's all relative. There is no absolute speed of motion.

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Reality Internet Personality

When you are sitting in you car you are stationary in the drivers seat, but moving at 100km/h relative to the road. The Earth is spinning at aprox 1670km/h (near the equator), the add the speed you are going around the sun..and the speed the galaxy is rotating...and so on. It's all relative. There is no absolute speed of motion.

But there is... You just explained it yourself, in a way. You just have to look at it universally, or absolutely if you like. Starting point in the universe (A), finishing point in the universe (B), travel time (C), distance between A and B (Use a string if it's not a straight line ;-P and call this distance D). So Absolute speed = D/C.

100km/h is always 100km/h, its just not the same 100km/h that you deal with day in day out. That is because it is not necessary day to day to concern ourselves with how far through space we have moved.

With regards to Lorentz Contraction the page says:

Lorentz contraction is a relative effect and not an absolute one.

This means that you are measuring the length of the bus relative to the man watching it, but if you freeze time the actual, absolute length of the bus is still the same.

Isn't the universe also expanding? Wouldn't that mean that two points in space that are exactly 100km apart immediately start moving away from each other?

Points in space don't move. Objects in space however do. By measuring how far those objects move from one point to another we can measure the rate at which they are moving.

Here I must admit that we are short sighted as observers and measure everything's rate of movement relative to our point of observation. Until we are better able to map/co-ordinate the points of the wider universe it will be difficult to accurately determine absolute speed.

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Kolch - Crunching for the BOINC@Australia team since July 2004.
Search for your own intelligence...

Only thing is, i imagine there might be a spacecraft travelling very fast and there are space travellers on board. Only they aren't moving exactly... They're sitting around just doing their normal things. Everything seems normal to them except that when they look out the spacecraft window, everything outside the spacecraft appears to slow down.

Another way of saying this; when i drive my car, am i moving at 100km/h? Or am i sitting stationary in the driver's seat?

When you are sitting in you car you are stationary in the drivers seat, but moving at 100km/h relative to the road. The Earth is spinning at aprox 1670km/h (near the equator), the add the speed you are going around the sun..and the speed the galaxy is rotating...and so on. It's all relative. There is no absolute speed of motion.

OK, and how fast is my clock going? According to the car speed, or according to the Earth's around the Sun speed, or according to Sun's speed orbiting the Milky Way centre (, etc...)?

Let's take an example of what I'm meaning. Star S2 in the Milky Way centre is orbiting around Sagittarius A along an elliptical orbit with orbital period is 15.2 years. The largest speed is more than 5000 km/s, I assume that the smallest speed can be around 350 km/s. Let's assume a planet E2 with an obsever (sitting in a 100 km/h car :-) is orbitting the S2 star with approximately same speed as Earth around the Sun and spinning also the same. S2's speed in its perigeum is already so high, that the Relativity theory should kick in - if the observer on E2 is measuring one day interval (24 hours), there should be approx. 15 seconds difference when measured in S2's perigeum or in its apogeum.

I think, if we would like to comparte the travel speeds and time speeds of the two different observers, it is not enough to take their relative speed into account. Or am I wrong? I might stand corrected...

Peter

OK, and how fast is my clock going? According to the car speed, or according to the Earth's around the Sun speed, or according to Sun's speed orbiting the Milky Way centre (, etc...)?

Let's take an example of what I'm meaning. Star S2 in the Milky Way centre is orbiting around Sagittarius A along an elliptical orbit with orbital period is 15.2 years. The largest speed is more than 5000 km/s, I assume that the smallest speed can be around 350 km/s. Let's assume a planet E2 with an obsever (sitting in a 100 km/h car :-) is orbitting the S2 star with approximately same speed as Earth around the Sun and spinning also the same. S2's speed in its perigeum is already so high, that the Relativity theory should kick in - if the observer on E2 is measuring one day interval (24 hours), there should be approx. 15 seconds difference when measured in S2's perigeum or in its apogeum.

I think, if we would like to comparte the travel speeds and time speeds of the two different observers, it is not enough to take their relative speed into account. Or am I wrong? I might stand corrected...

Peter

You would only need to take into account how fast the hands rotate..is that what you meant?

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Reality Internet Personality

But there is... You just explained it yourself, in a way. You just have to look at it universally, or absolutely if you like. Starting point in the universe (A), finishing point in the universe (B), travel time (C), distance between A and B (Use a string if it's not a straight line ;-P and call this distance D). So Absolute speed = D/C.

No..because there is no absolute distance.

You were no doubt taught at school that line always travels in straight lines? So we could use a light beam in the same way we use the string. We find that the light curves around objects with a large enough gravitational field..the space itself is warped. So therefore D is no longer a constant.
..and again, the time (C) depends on how fast you are moving when you measure it.

100km/h is always 100km/h, its just not the same 100km/h that you deal with day in day out. That is because it is not necessary day to day to concern ourselves with how far through space we have moved.

In your frame of reference that is true...but in another person's that is not necessarily the case.

Imagine you are in a moving train..the windows have been blacked out so you can't see out. There is no way for you to even tell you are moving...let alone how fast you are moving. The only thing you can detect is an acceleration...you cannot absolutely measure speed.

With regards to Lorentz Contraction the page says:

Lorentz contraction is a relative effect and not an absolute one.

This means that you are measuring the length of the bus relative to the man watching it, but if you freeze time the actual, absolute length of the bus is still the same.

For that frame of reference...I'm not sure what you are getting at here. If you freeze time you couldn't measure the bus anyway.

Points in space don't move. Objects in space however do. By measuring how far those objects move from one point to another we can measure the rate at which they are moving.

The space between the objects is expanding, pushing the objects further apart.

Here I must admit that we are short sighted as observers and measure everything's rate of movement relative to our point of observation. Until we are better able to map/co-ordinate the points of the wider universe it will be difficult to accurately determine absolute speed.

There is no absolute speed. It doesn't matter how well you measure the universe.
It is all totally relative. There is physically no way to detect a difference between a stationary object and one moving at a constant speed.

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Reality Internet Personality

You would only need to take into account how fast the hands rotate..is that what you meant?

No. (Or am I already totally confuded?) I still mean whether the time-slowdown is caused by the object's absolute speed or its speed relative to something else.

But I've just got an idea. The speed of light is also claimed to be constant, absolute (except gravity etc, I'm taking just velocities into account now), so there have to be somethng like an "absolute" speed of an object (relative only to some imaginary origin), which should be used when calculating time slowdown for the observers in the traveling object.

Otherwise it could be possible, that two ships travel in same direction, one behind another, with v=c/2 and if the second ship would send some light ray towards the first ship, the light would go with (relative) speed equal to v+c = effectively 3/2 c.

Peter

You would only need to take into account how fast the hands rotate..is that what you meant?

No. (Or am I already totally confuded?) I still mean whether the time-slowdown is caused by the object's absolute speed or its speed relative to something else.

But I've just got an idea. The speed of light is also claimed to be constant, absolute (except gravity etc, I'm taking just velocities into account now), so there have to be somethng like an "absolute" speed of an object (relative only to some imaginary origin), which should be used when calculating time slowdown for the observers in the traveling object.

Otherwise it could be possible, that two ships travel in same direction, one behind another, with v=c/2 and if the second ship would send some light ray towards the first ship, the light would go with (relative) speed equal to v+c = effectively 3/2 c.

Peter

ok..Pepo, you are starting to get to the core of the problem.

If a train were travelling at the speed of light..and you walked to the front of the train..how fast would you be travelling?

Suppose you shone a torch down the train..how fast would the light be travelling?

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Reality Internet Personality

ok..Pepo, you are starting to get to the core of the problem.

If a train were travelling at the speed of light..and you walked to the front of the train..how fast would you be travelling?

Suppose you shone a torch down the train..how fast would the light be travelling?

Small language problems...

a) I'd travel some few m/s faster than light (IF I were allowed to, i.e. if physically possible for me),

b) actually it should be twice the speed of light, compared to train. Or? :-)

Peter

You would only need to take into account how fast the hands rotate..is that what you meant?

No. (Or am I already totally confuded?) I still mean whether the time-slowdown is caused by the object's absolute speed or its speed relative to something else.

But I've just got an idea. The speed of light is also claimed to be constant, absolute (except gravity etc, I'm taking just velocities into account now), so there have to be somethng like an "absolute" speed of an object (relative only to some imaginary origin), which should be used when calculating time slowdown for the observers in the traveling object.

Otherwise it could be possible, that two ships travel in same direction, one behind another, with v=c/2 and if the second ship would send some light ray towards the first ship, the light would go with (relative) speed equal to v+c = effectively 3/2 c.

Peter

ok..Pepo, you are starting to get to the core of the problem.

If a train were travelling at the speed of light..and you walked to the front of the train..how fast would you be travelling?

Suppose you shone a torch down the train..how fast would the light be travelling?

You would be walking about 3 MPH to another person on the train.
For someone outside the train you would be at the same speed as the train because he can only use light to see you and measure your speed.
The light in the train from outside would be just traveling at the speed of light in the oposite direction as the speeding train.
Measurements of speed are done with measuring the time delay of a radar pulse.

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Gas price by zip at Seti

The speed of light is also claimed to be constant, absolute (except gravity etc, I'm taking just velocities into account now), so there have to be somethng like an "absolute" speed of an object (relative only to some imaginary origin)...

Well said. Speed of light measured in m/s, or km/h, it doesn't matter it's always the same. So it must also be true that a metre is always a metre and a second is always a second. [edit]We just use these units of measurements because they are convenient to us. I might add here that there is evidence that some of our measures were originally devised by stone age astrologers when they counted the number of times a pendulum swung in the time it took venus to move a predetermined distance across the sky. However thihs is a discussion for another thread.[/edit]

Gravity bends light, i'm not sure it slows it down. I don't think its even fully understood why gravity bends light as it has no mass upon which gravity could act.

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Kolch - Crunching for the BOINC@Australia team since July 2004.
Search for your own intelligence...

Not sure if I'm any the wiser let alone you!

Nice Q&A example, thanks. OK that's fully clear.

(Maybe except the (controversal) "photons has no mass && photons has inertial mass and thus an equivalent gravitational mass". I have to absorb (digest) it first...)

Peter

My Head Hurts!

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Not sure if I'm any the wiser let alone you!

Well, the theory is clear, it has mass because it is moving. Still its about as easy to wrap your head around as is the concept of infinity.

The other point i wondered on was how they could tell the light of the star behind the sun apart from the light from the sun. I would have thought that would be pretty tough.

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Kolch - Crunching for the BOINC@Australia team since July 2004.
Search for your own intelligence...

Still its about as easy to wrap your head around as is the concept of infinity.

Yes, I'm not able to fully imagine the infinity. What's behind? :-)

The other point i wondered on was how they could tell the light of the star behind the sun apart from the light from the sun. I would have thought that would be pretty tough.

To my understanding, they have probably seen some star near the (hidden) Sun, although they knew the star should be placed behind the Sun at the moment. This observation on 1919 was possibly the first (known and) understood one? Such observations (effect of gravitational lensing) are still done now with various stars around. IIRC there was also an observation of such star on both sides of some heavy object? Maybe the collegues from the SETI@home Science section could step in to make it clear the understandable way.

Peter

Well said. Speed of light measured in m/s, or km/h, it doesn't matter it's always the same. So it must also be true that a metre is always a metre and a second is always a second. [edit]We just use these units of measurements because they are convenient to us. I might add here that there is evidence that some of our measures were originally devised by stone age astrologers when they counted the number of times a pendulum swung in the time it took venus to move a predetermined distance across the sky. However thihs is a discussion for another thread.[/edit]

Gravity bends light, i'm not sure it slows it down. I don't think its even fully understood why gravity bends light as it has no mass upon which gravity could act.

Kolch, think about what you are saying. If the speed of light is always the same then the distance or the time must change. Otherwise when you measure the speed of light on a moving train it would be different to the speed of light for a stationary observer. Think very hard about it.

As an aside..gravity bends light because the light is following a curved path around the 'massive' object eg. a star. When a star was viewed during the solar eclipse it appeared to be in a different position as the light coming from it was bent as it passed the sun..much like a fish viewed in a fish tank can appear in a different place because of refraction.

I'm obviously not doing a very good job at explaining things here..I shall go away and try to think of a better way of making it clear.

Es99 <- Physics BSc (Even though I'm a girl)

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Reality Internet Personality

To my understanding, they have probably seen some star near the (hidden) Sun, although they knew the star should be placed behind the Sun at the moment. This observation on 1919 was possibly the first (known and) understood one? Such observations (effect of gravitational lensing) are still done now with various stars around. IIRC there was also an observation of such star on both sides of some heavy object? Maybe the collegues from the SETI@home Science section could step in to make it clear the understandable way.

Peter

Well don't I feel like crap now. :-(

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Reality Internet Personality

My Head Hurts!

LOL!! It's your own fault ya know.

Mine hurts too actually. :-)

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Air Cold, the blade stops;
from silent stone,
Death is preordained

Calm Chaos Forums : Everyone Welcome

My Head Hurts!

LOL!! It's your own fault ya know.

Mine hurts too actually. :-)

That's because you are doing it right. If it were easy, it would be no fun!

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Reality Internet Personality

My Head Hurts!

LOL!! It's your own fault ya know.

Mine hurts too actually. :-)

That's because you are doing it right. If it were easy, it would be no fun!

Well...at least I know I am on the right track. Thanks Es! :-)

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Air Cold, the blade stops;
from silent stone,
Death is preordained

Calm Chaos Forums : Everyone Welcome

This subject matter seems to have moved a bit from the original Atomic clock title, but its certainly food for thought!!!!

http://www.newton.dep.anl.gov/askasci/phy99/phy99415.htm

That link has a bit more on it, and an interesting paragraph

From a classical point of view light refracts as it
moves from one medium to another that can slow the light down. The r
refraction is limited by the denser medium. as one approaches the critical
angle the angle of refraction approaches 90 degrees. After this total
internal reflection is achieved. This is easily demonstrated with Snell's
law from any physics text.

Refraction is not bending, and no idea what Snells law is, but I thought that the speed of light was constant at all times????

Refraction is bending. Light slows down in different media. Snells law is the law of refraction..oh never mind. I give up. None of you clearly believe a word I say. I'm only a physics teacher. What do I know.

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Reality Internet Personality