# Are relativistic effects real ?

stillwonder
Are relativistic effects "real"?

At this point i'd like to detour the topic and ask why time is so special that it doesn't come back to its original measurement. After all, once all moving observers are stopped wrt to each other, and they compare their measuring rods, they measure equal. so why not the clocks?

If one goes by saying they do measure similarly afterwards, it makes sense to assume all the intermediate slowdown was only "virtual".

since clock is a "cumulative" device, if one were cycling on the spaceship, a stationary observer might calculate the cumulative distance traveled as different than what the astronaut believes. but when they actually come down, shake hands, and compare the logs, both will read same

[edit: I split off this topic from the original https://www.physicsforums.com/showthread.php?t=236885" thread--Doc Al]

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Mentor
At this point i'd like to detour the topic and ask why time is so special that it doesn't come back to its original measurement. After all, once all moving observers are stopped wrt to each other, and they compare their measuring rods, they measure equal. so why not the clocks?
I don't understand your point. Clocks do "come back" to their original measurement. Once two observers are stopped wrt each other if they compare their clocks they find that each measures 1s to be the same duration.

stillwonder
I don't understand your point. Clocks do "come back" to their original measurement. Once two observers are stopped wrt each other if they compare their clocks they find that each measures 1s to be the same duration.

the point being, the clock appearing slower is exactly that, "it appears".
a person walking from front to back of the spaceship that's 100m long, would say "i walked 100m today". a stationary observer would be incorrect in asserting "no you walked 50m, since the spaceship was traveling at so-and-so speed" and your meter shrunk to half of mine. The stationary observer may have illusion that his meter is the bigger one, but if he puts the two rods side by side (hence moving it at same speed as rocket), he can assure himself both are same length. this uncanny ability given to stationary observer to "eyeball" the moving observers instruments is highly suspicious

matheinste
Hello stillwonder.

First of all remember there is no observer in the privileged position of being stationary. All movement is relative so when two objects or observers move relative to each other, each can consider himself still and the other moving or vice versa.

As regards the contraction being an illusion, it is not. Once again remember that each sees the others measuring stick contract.

If when at rest relative to each other, in an inertial ( non-accelerating ) system, two rods of identical length are placed side by side they will, by direct comparison, be of the same length. If we could engineer a situation whereby these same two rods could pass each other in relative, inertial motion, as close as we please in space so as to remove any optical effects, and thus compare their lengths as before, by direct comparison, then Relativity predicts that each would be shorter than the other from the other observer’s frame. As far as I am aware the technical problems in doing such an experiment have not yet been overcome and so it is still a prediction but it is all part and parcel of the theory.

This does not fit our common sense ideas and is hard to imagine,but that is how it is.

Matheinste.

Mentor
the point being, the clock appearing slower is exactly that, "it appears".
And would you say that relativistically moving muons just "appear" to reach the earth?

neh4pres
Ok this is not an answer but a new question. Who is moving? if two observers are 10 lightyears apart, and i will not say how but their clocks both start at 0... Then observer B blasts of at .5c toward observer A. whos clock slows?? Do they both see the the others clock run slow?? Who knows which frame is stationary?? maybe they both began at .5c and when observer B blasted off toward observer A closing the gap at .5c he really stopped and became the stationary frame. if so then observer A,s clock would seem to be the one that suffers time dialation... Or maybe when they started they were both moving at .25c at which point when Observer B blasts of toward observer A closing the gap at .5c they are indeed both moving at .25c toward each other . and when they meet they will have the same reading on their clocks.

matheinste
Hello ne4pres

Quote:-

---Do they both see the the others clock run slow?? ----

Yes under all conditions of relative inertial motion.

Matheinste.

matheinste
Hello ne4pres

Yes under all conditions of relative inertial motion

!I should have added after allowing for any optical effects of their relative motion.

Matheinste.

stillwonder
Hello stillwonder.

First of all remember there is no observer in the privileged position of being stationary. All movement is relative so when two objects or observers move relative to each other, each can consider himself still and the other moving or vice versa.

As regards the contraction being an illusion, it is not. Once again remember that each sees the others measuring stick contract.

If when at rest relative to each other, in an inertial ( non-accelerating ) system, two rods of identical length are placed side by side they will, by direct comparison, be of the same length. If we could engineer a situation whereby these same two rods could pass each other in relative, inertial motion, as close as we please in space so as to remove any optical effects, and thus compare their lengths as before, by direct comparison, then Relativity predicts that each would be shorter than the other from the other observer’s frame. As far as I am aware the technical problems in doing such an experiment have not yet been overcome and so it is still a prediction but it is all part and parcel of the theory.

This does not fit our common sense ideas and is hard to imagine,but that is how it is.

Matheinste.

a basketball looking like a football instead of a baseball to a moving observer shows whether there is some wool being pulled. taking "instantaneous" snapshot while two rods are arbitrarily close is again suspicious. how about allowing arbitrarily large amount of time to make the measurement instead of "hurrying it up".

I have no problems with the predictions and measurements under relativity. But they are just that, measurements. To attribute these measurements to underlying reality, if any, is a jump of faith.

matheinste
Hello stillwonder.

I'm afraid i can't help you any more. I have given you the facts and any textbook will confirm what i have said. If you cannot accept that then that is your choice.

Matheinste.

how about allowing arbitrarily large amount of time to make the measurement instead of "hurrying it up".
How about telling us how this could be done, given two rods in relative motion?
I have no problems with the predictions and measurements under relativity. But they are just that, measurements. To attribute these measurements to underlying reality, if any, is a jump of faith.
I think that we get to the point now.
You seem to assume that an (idealized) measuring rod really is some 1-dimensional entity, which has a given length, and that it is impossible that this length really changes depending on the observer.
So far so good, but relativity tells us that we must not ignore the rod's extension in time. It existed some time before and will exist some time in the future.
Hence the rod really is a 2-dimensional entity, extended in one space- and one time dimension.
The 1-dimensional entity that you regard as the rod is merely a hypothetic slice throug the real rod, what remains if you observe it at a definite time t and forget about its extension in time.
The point is, every observer in relative motion to another will regard a different slice through the real rod as "the rod". Different slices of course have different lengths, but that does not change the real, 2-dimensional rod.
It's up to you whether you call length contraction real or not, but it misses the point.

It's also not uncommon that people reject this point of view as "absurd" or something like that. If you do so, that's ok with me, but I can assure you that then you will never understand relativty.

stillwonder
How about telling us how this could be done, given two rods in relative motion?

I think that we get to the point now.
You seem to assume that an (idealized) measuring rod really is some 1-dimensional entity, which has a given length, and that it is impossible that this length really changes depending on the observer.
So far so good, but relativity tells us that we must not ignore the rod's extension in time. It existed some time before and will exist some time in the future.
Hence the rod really is a 2-dimensional entity, extended in one space- and one time dimension.
The 1-dimensional entity that you regard as the rod is merely a hypothetic slice throug the real rod, what remains if you observe it at a definite time t and forget about its extension in time.
The point is, every observer in relative motion to another will regard a different slice through the real rod as "the rod". Different slices of course have different lengths, but that does not change the real, 2-dimensional rod.
It's up to you whether you call length contraction real or not, but it misses the point.

It's also not uncommon that people reject this point of view as "absurd" or something like that. If you do so, that's ok with me, but I can assure you that then you will never understand relativty.

if i read you correct, you are assuming two things

1. The rod was formed by streching a ball of iron
2. and the various stages of streching are what the observers observe when they move

i'll have to disagree with both
1. the ball was compressed from a larger rod
2. there is no "time travel" involved. otehrwise the moving observer should see the other rod increasing in length

stillwonder
Hello stillwonder.

I'm afraid i can't help you any more. I have given you the facts and any textbook will confirm what i have said. If you cannot accept that then that is your choice.

Matheinste.

thanks Matheinste.
As i said i have no dispute with what the textbook says.
Infact, all definitions in physics are "operational". ie what you measure is what it IS.
and the measurement is consistent with the theory.

if i read you correct, you are assuming two things

1. The rod was formed by streching a ball of iron
2. and the various stages of streching are what the observers observe when they move
No, I assume that the rod was there before you start the experiment and ist still there when you finish.
In a standard spacetime diagram (where the rod is at rest), the rod's endpoints are drawn as two vertical lines, and the rod itself is drawn as the area between these lines. The intersection of that "real" rod with the horizontal t=0 line is what you would call "the rod at time t=0".
Every relatively moving observer will have a different opinion about the t'=0 line. All those t'=0 lines intersect the real (2-dimensional) rod at different angles, therefore every observer will see a different "rod at time t'=0".

stillwonder
No, I assume that the rod was there before you start the experiment and ist still there when you finish.
In a standard spacetime diagram (where the rod is at rest), the rod's endpoints are drawn as two vertical lines, and the rod itself is drawn as the area between these lines. The intersection of that "real" rod with the horizontal t=0 line is what you would call "the rod at time t=0".
Every relatively moving observer will have a different opinion about the t'=0 line. All those t'=0 lines intersect the real (2-dimensional) rod at different angles, therefore every observer will see a different "rod at time t'=0".

maybe i am being paranoid, but i feel being categorized along with the relativity buster gang!
i have already stated i have no issues with the theory per se but only the personal baggage that people are loading it with.

two rods (one stationary another moving) appearing to be same length doesn't mean they are similar. the fact that one is moving is riveted to its identity in that frame. even if it appears to be same length, it cannot be same. that is what i meant by "appears".

how?

lets say a 1m (at rest) rod looks like a 0.5m rod to a moving (speed v) observer. he places a 0.5m rod besides it and sees they appear to be same length (assuming plenty of these rods are encountered on his path). he tells another person sitting at the corner of the ship, "hey these are exactly the same length so they must be similar" (he already did spectral analysis to find they are made up of same material). so the corner guy says, why don't you jump over the rod at speed v in the same direction its moving and see if they appear similar. he jumps and boom one contracts another expands.

a rod is measured longest in the rest frame. that's sort of special frame. directly or indirectly all calculations in intertial frames are wrt to this at rest frame. if you agree so far, there is something to be seen there, and i don't want to spoil it for anyone (assuming you haven't already come upon it), but the key word is "measurement".

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Mentor
At this point i'd like to detour the topic and ask why time is so special that it doesn't come back to its original measurement. After all, once all moving observers are stopped wrt to each other, and they compare their measuring rods, they measure equal. so why not the clocks?

since clock is a "cumulative" device, if one were cycling on the spaceship, a stationary observer might calculate the cumulative distance traveled as different than what the astronaut believes. but when they actually come down, shake hands, and compare the logs, both will read same
Yes, clocks are cumulative devices. And they function like a written log. But if a clock is keeping a record that is different throughout the flight, the clock won't suddenly spin to synchronize when he decelerates. Nor will the numbers written in a log magically change when you land to show the same elapsed time/distance. It will still have a different cumulative time elapsed. What becomes synchronized again when they land is the clock rate.

GPS satellites show exactly this effect (though we've never brought one back that I know if - we just record and adjust to the cumulative difference).

Similarly, if you happen to get hired to take a quick lap around Alpha Centuari at near light speed, you'll want to make sure you get paid by Earth hour or mile, not what's recorded in your log. You'll find your log shows much less than the 8.8 years/light years that Earth-based observers record.

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granpa
Hello stillwonder.

First of all remember there is no observer in the privileged position of being stationary. All movement is relative so when two objects or observers move relative to each other, each can consider himself still and the other moving or vice versa.

As regards the contraction being an illusion, it is not. Once again remember that each sees the others measuring stick contract.

If when at rest relative to each other, in an inertial ( non-accelerating ) system, two rods of identical length are placed side by side they will, by direct comparison, be of the same length. If we could engineer a situation whereby these same two rods could pass each other in relative, inertial motion, as close as we please in space so as to remove any optical effects, and thus compare their lengths as before, by direct comparison, then Relativity predicts that each would be shorter than the other from the other observer’s frame. As far as I am aware the technical problems in doing such an experiment have not yet been overcome and so it is still a prediction but it is all part and parcel of the theory.

This does not fit our common sense ideas and is hard to imagine,but that is how it is.

Matheinste.

stillwonder
You basically answered your own question here: Yes, clocks are cumulative devices. And they function like a written log. But if a clock is keeping a record that is different throughout the flight, the clock won't suddenly spin to synchronize when he decelerates. Nor will the numbers written in a log magically change when you land to show the same elapsed time/distance. It will still have a different cumulative time elapsed. What becomes synchronized again when they land is the clock rate.

GPS satellites show exactly this effect (though we've never brought one back that I know if - we just record and adjust to the cumulative difference).

Similarly, if you happen to get hired to take a quick lap around Alpha Centuari at near light speed, you'll want to make sure you get paid by Earth hour or mile, not what's recorded in your log. You'll find your log shows much less than the 8.8 years/light years that Earth-based observers record.

allow me to rephrase my last line "both will read same" to "both will agree on the results since the ground observer understands relativity and will make concessions accordingly".

although introducing accelaration changes the game, but in principle it shouldn't affect the point being made.

Assumption: Ground crew g can make two identical spaceships take off from opposite directions equidistant such that they pass each other with equal but opposite velocity at point g. Obviously the acceleration phase is taken care of so they achieve the velocity v by the time they are at their posts '|' and there will not be acceleration after that. This can be done by using all the physics they have learned in college :-)

S syncs up with O when he passes him and stops the clock when he reaches the other post '|'. Similarly O stops the clock when S1 reaches him. Now if S is brought to O, whose clock shows less time? throughout the experiment they thought the other was slower.

I understand O and S will not agree they stopped the clocks at the same time and therein lies the illusion. The ground crew g has calculated everything to arbitrary precision. The spaceships can be made identical. The distances |g and g| are precise. The light used to signal S and S1 to start is precise. So what gives?

--|----------g----------|--
O------------------<-S1
S->-------------------
---------<-S1---------
-----------S->--------
<-S1-------------------S->

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I understand O and S will not agree they stopped the clocks at the same time and therein lies the illusion.
What illusion? Therein lies the solution.

Staff Emeritus
Gold Member
allow me to rephrase my last line "both will read same" to "both will agree on the results since the ground observer understands relativity and will make concessions accordingly".

although introducing accelaration changes the game, but in principle it shouldn't affect the point being made.

Assumption: Ground crew g can make two identical spaceships take off from opposite directions equidistant such that they pass each other with equal but opposite velocity at point g. Obviously the acceleration phase is taken care of so they achieve the velocity v by the time they are at their posts '|' and there will not be acceleration after that. This can be done by using all the physics they have learned in college :-)

S syncs up with O when he passes him and stops the clock when he reaches the other post '|'. Similarly O stops the clock when S1 reaches him. Now if S is brought to O, whose clock shows less time?
Neither, both will show the same time.
throughout the experiment they thought the other was slower.
Actually, no. Because as noted below, there will be a period where their clock has stopped and the other has continued running.
I understand O and S will not agree they stopped the clocks at the same time and therein lies the illusion.
No illusion. for O, S really does stop after O. and for S, O stops after S.
Thus for O, when it stops at time T, clock S reads something less than T, but then continues to run until it reads T then stops. The reverse happens from S's perspective.
The ground crew g has calculated everything to arbitrary precision. The spaceships can be made identical. The distances |g and g| are precise. The light used to signal S and S1 to start is precise. So what gives?

--|----------g----------|--
O------------------<-S1
S->-------------------
---------<-S1---------
-----------S->--------
<-S1-------------------S->

Mentor
since clock is a "cumulative" device
by the way, this is an error that I noticed earlier and ignored until it had been repeated several times.

A clock is not a cumulative device, it simply is a device which "ticks" at a regular rate. The clock is the pendulum or quartz crystal or maser, not the display device. And by design one "tick" is just like another with no historical cumulative effects. In exactly the same way that a rod measures a spatial interval (dx), a clock measures a temporal interval (dt).

You can construct a cumulative readout for distances (eg odometer) just as for a clock, it is just a display.

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ThomasT
At this point i'd like to detour the topic and ask why time is so special that it doesn't come back to its original measurement. After all, once all moving observers are stopped wrt to each other, and they compare their measuring rods, they measure equal. so why not the clocks?

If one goes by saying they do measure similarly afterwards, it makes sense to assume all the intermediate slowdown was only "virtual".

The accelerated clock accumulates fewer seconds. There's no physical understanding of this behavior, but it would seem to suggest that empty space isn't empty -- eg. acceleration involves increasingly complex and intense wave interactions between the accelerated object and its environment.

Though an experiment involving the side by side comparison of two clocks, acceleration of one clock for a certain interval, then another side by side comparison of the clocks hasn't been done afaik, nevertheless the results of many analogous experiments do seem to suggest that this is a real physical phenomenon. That is, there's a lengthening of the periods of oscillators in accelerated bodies.

stillwonder
The accelerated clock accumulates fewer seconds. There's no physical understanding of this behavior, but it would seem to suggest that empty space isn't empty -- eg. acceleration involves increasingly complex and intense wave interactions between the accelerated object and its environment.

Though an experiment involving the side by side comparison of two clocks, acceleration of one clock for a certain interval, then another side by side comparison of the clocks hasn't been done afaik, nevertheless the results of many analogous experiments do seem to suggest that this is a real physical phenomenon. That is, there's a lengthening of the periods of oscillators in accelerated bodies.

i am for now limiting to clocks synced in intertial frames, and started/stopped after acceleration/deceleration. ie whether the time dilation seen in intertial clocks is "virtual" or not.

Mentor
i am for now limiting to clocks synced in intertial frames, and started/stopped after acceleration/deceleration. ie whether the time dilation seen in intertial clocks is "virtual" or not.
You didn't answer this before when I posed it, but do you consider the muons which reach the surface of the Earth to be "virtual", do they just "appear" to reach the earth, or do they really reach the earth?

neh4pres
Ok i would like to get something straight. I read in this thread that neither frame has the privilege of being the stationary frame. Then i read that both frames would see the others clock running slow. If i apply a little thought to this, you are saying that if i speed of to a star 10 lightyears away at a speed so as i get there in 5 years and then comeback at the same speed, stop and then compare the clocks they will both read like 10 years has passed? i believe something is wrong with this picture. This mean i could wait another 5 years , look at the star and see myself there. ALSO even better... If either frame could be the stationary one, and both will see the others clock as running slow and they don't read the same upon returning... then observer A will see a younger observer B come back from the trip... And observer B will see younger observer A when he gets home... This would make for multiple time lines... maybe in the Earth observer frame much time passes and when the young traveler comesback there's a war and they shoot the traveler from the sky... in the travelers frame he returns to Earth before the war , but as an old man...

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matheinste
Hello neh4pres.

The whole point is that the traveller does something that the non-traveller does not. Accelerates. This causes an asymmetry in the spactime paths taken. On return the clocks will not read the same. You should re-read the many recent ( and past ) threads on the twins (non)paradox.

All problems of this type have such an asymmetry.

Matheinste.

stillwonder
Neither, both will show the same time.Actually, no. Because as noted below, there will be a period where their clock has stopped and the other has continued running.No illusion. for O, S really does stop after O. and for S, O stops after S.
Thus for O, when it stops at time T, clock S reads something less than T, but then continues to run until it reads T then stops. The reverse happens from S's perspective.

Let me point out that following calculations are made in rest frame of O.
1. The distance beween the posts '||' = d
2. The velocity v of the spaceships S
Hence the time needed to cross distance d is d/v. These can be computed to arbitrary precision.
3. S's clock is stopped when it reaches '|'. This can be automated via having S fly arbitrarily close to post '|'. Infact, O can actually observe this himself that S stops the clock when he reaches the post '|'.

Assertions:

1. O's clock shows d/v
2. If there is time dilation, S's clock must show less than d/v
3. If there is no time dilation, S's clock also shows d/v

There is no option of S stopping his clock after crossing the post, since he would have crashed into mountain range M (not shown).

So, will the clocks read same value or not?

The choice for O is :
1. Are his scientific calculations wrong?
2. Is his "eyeball" observation of time dilation misleading because of delays in receiving signals from far.

neh4pres
OK new question if i accelerate to a point where i can travel ten lightyears in 5 years. then stop accelerating and just float under my inertia. how can a light ray sent from my start point ever catch me?

stillwonder
There is no asymmetry. The clocks are synced after attaining constant velocity v.
And stopped before deceleration is started. The entire experiment of synced clocks
is in inertial frames.

Now if one asserts the clock accelerating or decelerating changes something fundamentally and arbitrarily in the clock from that point onwards, then one has to face the fact that all mutually at motion inertial frames have undergone acceleration. Hence there IS not such thing as "purely inertial but at motion wrt each other" set of frames. This in turn would mean SR addresses a hypothetical situation that can never exist. What good would such a theory be?

matheinste
Hello stillwonder

Quote:-

--- Hence there IS not such thing as "purely inertial but at motion wrt each other" set of frames.-----

Are you saying that two objects, each moving inertially, cannot be in relative motion. Or do you want to reword your statement

Matheinste.

stillwonder
You didn't answer this before when I posed it, but do you consider the muons which reach the surface of the Earth to be "virtual", do they just "appear" to reach the earth, or do they really reach the earth?

the muon is a particle and hence accelerates to its speed v. that definitely isn't in the same category as current discussion where the time dilation and length contraction of inertial frames is discussed.

stillwonder
What illusion? Therein lies the solution.

But he would stop his clock at the same time S reaches his post. Thats how O setup his posts and rockets. So either his computations are wrong, or his bare eye observation is tricking him.

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stillwonder
Hello stillwonder

Quote:-

--- Hence there IS not such thing as "purely inertial but at motion wrt each other" set of frames.-----

Are you saying that two objects, each moving inertially, cannot be in relative motion. Or do you want to reword your statement

Matheinste.

Let me put it this way: how can there be two mutually in motion inertial frames, neither of which ever accelerated?
If one frame must accelerate at some point or the other to reach speed v, then your assertion that "that acceleration spoils the game, SR cannot be applied" implies "SR cannot be applied in any situation". This is logical conclusion of your assertion that no acceleration be used at no point. My assertion is "SR should be applicable when clocks are synced in inertial frames, and rest of the experiment happens in the inertial frames. How the inertial frame got to their respective velocities earlier to syncing, or what happens to them after the experiment are irrelevant"

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stillwonder
OK new question if i accelerate to a point where i can travel ten lightyears in 5 years. then stop accelerating and just float under my inertia. how can a light ray sent from my start point ever catch me?

light will always be faster than anything in vacuum by definition. so it will eventually catch you.

Mentor
the muon is a particle and hence accelerates to its speed v. that definitely isn't in the same category as current discussion where the time dilation and length contraction of inertial frames is discussed.
No, it doesn't. The muon is created in the upper atmosphere already moving at relativistic speed. It does not accelerate, but moves inertially its whole life. It is a perfect and exact example of precisely this discussion.

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