# relative speed vs time passage rate

Status
Not open for further replies.
K

#### killium

##### Guest
It is said that the fastest something goes, the slower the time passes. fine! but then,... imagine some object somewhere in space, which had never had any contact with Earth in the past. How does this object "knows" what should be the rate of passage of time relative to Earth, since it obviously have some "speed" relative to Earth ?

S

#### SpeedFreek

##### Guest
The faster A goes, relative to B, the slower time passes for A, when calculated by B.

It doesn't matter where you are or what you are doing, whether you are A or B, 1 second always lasts 1 second for you. It is when you compare how time passes for you with how time passes somewhere else, that you see time passing differently elsewhere. This is because we cannot separate space and time in any absolute way.

If an object (B) in deep space can somehow work out how fast the Earth (A) is moving relative to them, then B calculates time passing at a slower rate for Earth (A), when compared to their own time, which always passes at 1 second per second.

If an object has never had any contact with Earth in the past, how is it obvious that they have some speed relative to Earth? Why can they not consider themselves at rest and that the Earth has some speed relative to them?

O

#### origin

##### Guest
killium":19fyf8av said:
It is said that the fastest something goes, the slower the time passes. fine! but then,... imagine some object somewhere in space, which had never had any contact with Earth in the past. How does this object "knows" what should be the rate of passage of time relative to Earth, since it obviously have some "speed" relative to Earth ?
I think your confusion comes from thinking that the earth would have some special designation which is not true. The object will have a slower time relative to earth, BUT, the earth will also have a slower time relative to the object. It only depends on the observers point of reference. This at first seems like a paradox but that is because you are not taking general relativity into account.

If the object and earth are to agree on the passage of time then the object must decelerate or the earth must accelerate so that they are in the same inertial frame. In either case general relativity indicates that the passage of time will slow down for the accelerating (or decelerating) body relative to the constant velocity body. So in this case the accelerating body will see the constant velocity bodies actions going in 'fast motion' and the constant velocity body will see the accelerating bodies actions going in 'slow motion'. As the acceleration decreases and the velocity of the body approaches the other bodies velocity the 2 observers will notice that the others actions are changing in rate to match the speed of their actions. When the velocities exactly match, the 2 observers actions will have exactly the same rate of change and they both will agree on the rate of the passage of time.

edited for clarity (hopefully)

K

#### killium

##### Guest
origin":27r63o2d said:
killium":27r63o2d said:
It is said that the fastest something goes, the slower the time passes. fine! but then,... imagine some object somewhere in space, which had never had any contact with Earth in the past. How does this object "knows" what should be the rate of passage of time relative to Earth, since it obviously have some "speed" relative to Earth ?
I think your confusion comes from thinking that the earth would have some special designation which is not true. The object will have a slower time relative to earth, BUT, the earth will also have a slower time relative to the object. It only depends on the observers point of reference. This at first seems like a paradox but that is because you are not taking general relativity into account.

If the object and earth are to agree on the passage of time then the object must decelerate or the earth must accelerate so that they are in the same inertial frame. In either case general relativity indicates that the passage of time will slow down for the accelerating (or decelerating) body relative to the constant velocity body. So in this case the accelerating body will see the constant velocity bodies actions going in 'fast motion' and the constant velocity body will see the accelerating bodies actions going in 'slow motion'. As the acceleration decreases and the velocity of the body approaches the other bodies velocity the 2 observers will notice that the others actions are changing in rate to match the speed of their actions. When the velocities exactly match, the 2 observers actions will have exactly the same rate of change and they both will agree on the rate of the passage of time.

edited for clarity (hopefully)
so acceleration is the key... not relative speed. Two objects that have a constant relative speed will agree on passage of time, right ?

Relativity states that two objects moving at a constant high speed relative to each other would be affected by time dilatation. Is that wrong ?

K

#### kelvinzero

##### Guest
killium":93c3mpal said:
so acceleration is the key... not relative speed. Two objects that have a constant relative speed will agree on passage of time, right ?
Nup, they will both consider each other's time to be moving more slowly, by the same amount. This would seem to cause contradictions and is called the twin paradox. There appears to be a paradox if you only consider special relativity and do not consider the acceleration, which requires general relativity.

(I might be wrong about requiring general relativity according to that link)

J

#### Jerromy

##### Guest
It seems to me that the relative motion through space causing time dilation would be relative to the speed of light. If you look at two velocities as departing from each other at the same rate relative to the speed of light then they would be truly at the same velocity. If one point was stationary and the second point was receding near the speed of light then time would flow slower for the point at high velocity as compared to the stationary point. It matters nothing what is at that point, it could be a human, mechanical clock or radioactive isotope... all would age slower in motion than they would at rest. Gravitational dilation which includes acceleration/deceleration is a seperate effect on the passage of time.

O

#### origin

##### Guest
Jerromy":16taor5e said:
It seems to me that the relative motion through space causing time dilation would be relative to the speed of light.
Well since everybody measures the speed of light at exactly the same speed how could that be?

If you look at two velocities as departing from each other at the same rate relative to the speed of light then they would be truly at the same velocity. If one point was stationary and the second point was receding near the speed of light then time would flow slower for the point at high velocity as compared to the stationary point
.

Seems like thats the way it should be but it isn't. The reason is that the speed of light is measured exactly the same for every observer. There is no special point that is stationary so there is only relative motion.

It matters nothing what is at that point, it could be a human, mechanical clock or radioactive isotope... all would age slower in motion than they would at rest. Gravitational dilation which includes acceleration/deceleration is a seperate effect on the passage of time.
The stationary person would observe the person flying away age slower, the person flying away would watch the stationary person and the rest of the universe for that matter age more slowly. You are getting hung up on the person or position at rest - there is no position at rest, there is only relative motion.

S

#### Shpaget

##### Guest
origin":1ka7gdcw said:
The stationary person would observe the person flying away age slower, the person flying away would watch the stationary person and the rest of the universe for that matter age more slowly.You are getting hung up on the person or position at rest - there is no position at rest, there is only relative motion.
If person A goes in a fast ship out towards the stars and leaves person B on a planet. Person B observes that A is aging slower than him, right?
At the same time A observes that B and the planet he is going away from also slowed down in aging.

But what happens if person A went back to the planet and meets up with B?
Person B would be looking at the person A coming closer and see that once again A ages slower than B.
Same with A. He would see B age slower than A.

Who would age more?
Both the traveler and the one that stayed on the planet would be certain that they are the one who aged more, but they can't both be right.

Experiments do show that the clocks in satellites are slower than their grounded sisters, so where does that leave us?

O

#### origin

##### Guest
If person A goes in a fast ship out towards the stars and leaves person B on a planet. Person B observes that A is aging slower than him, right?
right.

At the same time A observes that B and the planet he is going away from also slowed down in aging.
right.

But what happens if person A went back to the planet and meets up with B?
Person B would be looking at the person A coming closer and see that once again A ages slower than B.
Same with A. He would see B age slower than A.

Who would age more?
B would age more. The reason for this paradox is you are neglecting the affects of general relativity. There is no difference on time dialation between gravity and accleration. So when A acclerated to his high velocity B would see him age slower and A would see B age faster. As A returned and decelerated B would see B age in slower and A would see B age faster.

Experiments do show that the clocks in satellites are slower than their grounded sisters, so where does that leave us?
The clocks in satelites are slower as far as special relativity because we are the observers, however the bigger affect is the difference in the acceleration of gravity.

J

#### Jerromy

##### Guest
origin":2ud141ex said:
You are getting hung up on the person or position at rest - there is no position at rest, there is only relative motion.
Then I think using that logic as a basis someone could be travelling at (for argument's sake) half of the speed of light and shine a flashlight in the direction of travel, to find that the light emitted from the source still travels at c relative to their "presumed stationary velocity". Perhaps the observer shining the light might calculate that it appears the light is travelling away from them at 300,000 km/s but since they are moving at 150,000 km/s the true velocity of the light would equal 450,000 km/s. I'm quite certain that a stationary observer would see the light radiating at 300,000 km/s away from the craft travelling at 150,000 km/s and the sole reason the traveler would think the relative speed of light is the constant 300,000 km/s and not .5c relative to their motion is because of length contraction. I'm not sure of the verified ratios but for example a traveller moving at .5c would experience time more slowly so the seconds would be double. Since we are comparing apples (one Earthbound second) to oranges (one relativistic second), the distance electromagnetic radiation would propagate can only be accurately defined by a free-falling clock at zero velocity.

O

#### origin

##### Guest
Jerromy":3ha7jhby said:
origin":3ha7jhby said:
You are getting hung up on the person or position at rest - there is no position at rest, there is only relative motion.
Then I think using that logic as a basis someone could be travelling at (for argument's sake) half of the speed of light and shine a flashlight in the direction of travel, to find that the light emitted from the source still travels at c relative to their "presumed stationary velocity". Perhaps the observer shining the light might calculate that it appears the light is travelling away from them at 300,000 km/s but since they are moving at 150,000 km/s the true velocity of the light would equal 450,000 km/s.
No the true velocity of the light would be c or about 300,000 km/s. Again you are assuming that you are moving at 150,000 km/s relative to a fixed point. There are no fixed points you are simply traveling at 150,000 km/s relative to an arbitrary point. You might be going -50,000 km/sec to a different arbitrary point.

I'm quite certain that a stationary observer would see the light radiating at 300,000 km/s away from the craft travelling at 150,000 km/s and the sole reason the traveler would think the relative speed of light is the constant 300,000 km/s and not .5c relative to their motion is because of length contraction.
The stationary observer relative to the space ship would see the light moving at 300,000 km/s which means he would see the light moving at 150,000 km/s relative to the ship. In other words the 'stationary' observer would see the light and the ship are 150,000 km apart after 1 sec.

I'm not sure of the verified ratios but for example a traveller moving at .5c would experience time more slowly so the seconds would be double. Since we are comparing apples (one Earthbound second) to oranges (one relativistic second), the distance electromagnetic radiation would propagate can only be accurately defined by a free-falling clock at zero velocity.
Time dialation is not linear so going 1/2 c would not double the time it would be much less than that. The distance that em radiation propegates is always the same as measured by any individual in the universe. Your thought about a free falling clock at zero velocity is not possible becasue there is absolutely NO POSSIBLE WAY to determine zero velocity in an absolute sense.

J

#### Jerromy

##### Guest
I'm sorry to keep arguing about this and I hope there is no hard feelings but I still don't understand how there could be a discrepancy about the relative speed of light. If light travels at a fixed speed as all other wavelengths of EMR it should be obvious to measure the speed of light in opposite directions and determine ones true velocity in those vectors by the difference of relative motion. As far as my understanding goes the light in the direction you are travelling would be blue shifted and the light behind you would be red shifted, the rate at which the photons travel through empty space should be identical and the observer could measure ones absolute velocity in space relative to the beams of light. If the observer were to transmit beams of light in opposite directions at literally zero velocity with no gravitational differential the light would go the same distance in either direction for the same elapsed time, am I right?

O

#### origin

##### Guest
Jerromy":27r407yu said:
I'm sorry to keep arguing about this and I hope there is no hard feelings but I still don't understand how there could be a discrepancy about the relative speed of light.
There are absolutely no hard feelings and I think this is a discussion and not an argument! I consider this 'stuff' very difficult to grasp because it is counter to our everday experiences.

If light travels at a fixed speed as all other wavelengths of EMR it should be obvious to measure the speed of light in opposite directions and determine ones true velocity in those vectors by the difference of relative motion.
I'm not sure what you mean. If you are traveling at 50% of the speed of light and a laser is shined from behind you and a laser is also shined in front of you, you would measure both beams of light as traveling at c or about 30,000 km/s. The speed for the 2 beams would be independent of your speed.

As far as my understanding goes the light in the direction you are travelling would be blue shifted and the light behind you would be red shifted, the rate at which the photons travel through empty space should be identical and the observer could measure ones absolute velocity in space relative to the beams of light.
The speed would be the same for the 2 beams but as you said the beams would be red or blue shifted. You could deduce your velocity relative to the light sources from the wavelength shift but this is only your velocity relative to another point it is still not an absolute veleocity.

If the observer were to transmit beams of light in opposite directions at literally zero velocity with no gravitational differential the light would go the same distance in either direction for the same elapsed time, am I right?
Yes, but if you were to take off in a rocket and go 95% the speed of light and you transmited 2 beams of light in opposite directions they would also go the same distance in either direction for the same elapsed time. All observers will measure the speed of light as c

J

#### Jerromy

##### Guest
The misunderstanding all comes down to this... If you are travelling AT the speed of light and turned on your "headlights", you could not see your hand in front of your face... so to speak. The light would travel exactly the same distance as the observer has, looking to see with it. If you were moving half the speed of light with your headlights on the light would travel half as far "ahead of you" per (insert omnipotent timeframe here) as it would if you were "sitting still" for the same duration. If you were travelling at .5c with a light source behind you the light would be 450,000 km away from you after 1 second.

O

#### origin

##### Guest
Jerromy":8gnubupn said:
The misunderstanding all comes down to this... If you are travelling AT the speed of light and turned on your "headlights", you could not see your hand in front of your face... so to speak.
That may be right but luckly we don't have to answer that particular question because it is not possible to travel at the speed of light. That is a dodge - but an acceptable dodge.

If you were moving half the speed of light with your headlights on the light would travel half as far "ahead of you" per (insert omnipotent timeframe here) as it would if you were "sitting still" for the same duration. If you were travelling at .5c with a light source behind you the light would be 450,000 km away from you after 1 second.
You are really going to hate this response! All I can say is every observer will measure the speed of light at c. Every observer without exception. So what does this mean in this case:

For the person on the ship traveling at .5c - the speed of light would be 300,000 km/s, so the photons from the headlights would travel 300,000 km after 1 sec from your perspective, and the photons from your tail lights would travel behind you 300,000 km after 1 sec from your perspective.

An observer that was stationary relative to your space ship would observe the light emitted from the front of the ship would only be 150,000 km from the ship after 1 sec and the light emitted behind the ship would be 450,000 km from the ship after 1 sec. The reason for this is because the 'stationary' observer would see the ship moving and he would also measure the speed of light at c.

Look at it this way if a space ship flew by at 99.99999999999999%c and you could somehow see the ship and a photon moving past the ship it would look like the photon would just barely catchup to the space ship and then pass it at like 25 kph and then ever so slowly move ahead of the ship. If you were on the ship watching that photon pass you it would be moving (all together now) at 300,000 km/s.

Now the reason this is important is because there is no 'omnipotent timeframe'. That is to say there is no possible way to say this is 'really' the rate of time passing. That is because there is no way to say what is stationary or moving. Motion and therefore the passage of time is relative.

That was the whole point actually of the Michelson–Morley experiment, they wanted to see how fast we were moving through the aether by measuring the differences in the speed of light as we went towards and way from the sun. Einstein correctly realized that since this experiment and others always returned the same speed of light that it was constant. This invariant speed led to realization that there is no way to define absolute motion or even absolute time.

It is all relative.

S

#### SpeedFreek

##### Guest
Light is always measured as travelling 300,000 km/s faster than whoever is measuring it, regardless of any motion of the person doing the measuring.

K

#### killium

##### Guest
SpeedFreek":po6r5dhn said:
Light is always measured as travelling 300,000 km/s faster than whoever is measuring it, regardless of any motion of the person doing the measuring.
I understand that. I also understand this is a postulate. Was it ever proven ? Has there ever been an experiment where the light source is moving at an appreciable fraction of the speed of light relative to the observer and the speed of the passing signal measured (not calculated) to be c ?

Michelson-Morley only showed there was no difference in c according to orientation (about an hypothetical aether). They did not physically measured the speed of light.

O

#### origin

##### Guest
killium":1razidex said:
SpeedFreek":1razidex said:
Light is always measured as travelling 300,000 km/s faster than whoever is measuring it, regardless of any motion of the person doing the measuring.
I understand that. I also understand this is a postulate. Was it ever proven ?
yes
Has there ever been an experiment where the light source is moving at an appreciable fraction of the speed of light relative to the observer and the speed of the passing signal measured (not calculated) to be c ?
yes

Michelson-Morley only showed there was no difference in c according to orientation (about an hypothetical aether). They did not physically measured the speed of light.
Wrong, they did infact meaure the speed of light and found that the speed was constant independent of the speed of the earth relative to the light source (the sun).

Status
Not open for further replies.