B Does Time Dilation Occur in a Perfectly Circular Orbit at Near-Light Speed?

[jbriggs444]

The farther the ship is the longer it takes for it to complete an orbit doing the same speed 0.5c.
In one case it is 12 sec and in the other 90 min.

Right. So you can arrange for the orbital period of a satellite moving at 0.5c to be anything you want by simply selecting the right altitude. And arranging for the requisite centripetal acceleration.

It would help if, instead of mentioning "Marcela" you had referenced the question you were trying to answer: One common technique in these forums is to mention the post number where the referenced passage exists. In this case, that would be #27:

A spaceship named "Marcela" circle kilometers (maybe about 129e6 km) away at 0.5c.
It has synchronized its time of orbit with IS that it sees it all the time at the same position and distance.
What is the speed of Marcela relative to IS and how the optic clocks explain this time dilation ?

Here you have postulated a spacecraft which is still moving at 0.5c relative to earth. You have varied its orbital radius so that it remains directly above the ISS at all times (this seems to assume that the ISS is in an equatorial orbit -- which it is not). You proceed to ask about its speed relative to the ISS.

The answer is obviously 0.5c. The velocity of the ISS is negligible compared to 0.5c. So the velocity of the 0.5c orbiter is still 0.5c in any inertial frame where the ISS is momentarily at rest, just as it is in an Earth-centered inertial frame.

However, there is a potential ambiguity. You have said that "it sees it all the time at the same position and distance". This suggests that you are adopting a frame of reference that rotates with the orbit of the ISS. In this reference frame, you have stipulated that Marcela is motionless.

The answer now is obviously 0.

But changing the reference frame does not change anything about the situation. The time dilation formula based on an object moving in an inertial frame still applies. The fast orbiter is still moving at 0.5c and its time dilation is the same as before.

 

[Janus]

This is the speed to calculate time dilation. see Marcela question to understand my point.
There compare to the ISS 0.5c Marcela got 0 relative speed and no time dilation.
If you understand my point and think that i am wrong please explain.
It is definitely out of my league I knew it before looking back into it ☺

Just because Marcela and the ISS do not have a relative motion as measured from their rotating frame, does not mean that there is not going to be no time dilation between them. This is due to the fact that they are viewing each other from a non-inertial rotating frame.

Earlier I advised you to forgo these types of non-inertial scenarios until you had come to grips with inertial motion scenarios. The above is an example of what I was talking about. You came to an incorrect conclusion by trying to analyze a situation you weren't ready to deal with.

The easiest way to analyze the Marcela-ISS scenario is to first look at it from the inertial frame which does not rotate with the pair, and then transform that into what the pair will see. First we will consider a signal transmitted from ISS to to Marcela. For ease of explanation, we will assume a tight beam transmission.
The following image will help in illustrating the scenario.

The small circle is the ISS path and the larger one is for Marcela. The blue line joins the ISS and Marcela. If the ISS wants to transmit a signal to Marcela, it can't just point along the blue line. Because in the time it takes for the Signal to travel from the inner circle to the outer one, Marcela (the solid M) will have maoved to the Position shown by the Outline M, and the signal would miss the target. Instead the ISS has to "lead" Marcella and aim for where Marcella will be when the signal arrives.
The other thing we have to note that the signal. traveling outward at c meets Marcela at a right angle to Marcela's own 0.5c. This means that the Closing speed between Marcela and the signal will, in this frame be greater than c. Thus the momentum involved in their meeting will be greater than if the closing speed had been just c.
If we jump to what Marcela detects, the signal will appear to come from some where a bit ahead of it in the direction of its motion. Since it will measure the signal arriving at c, the extra momentum will be seen as an increase in frequency, or Doppler shift, of the incoming signal at a rate of ~1.155
Thus if the ISS sent a ten second message at 100 khz, this represents 1 million waves transmitted. Marcella will receive the signal at 115.5 khz and thus get those same 1 million waves in just 8.66 seconds. He will see things happening on the ISS as happening 1.155 times faster.

Next we consider a signal sent from Marcela to the ISS:

Again, if Marcela trys to send a signal to the ISS by pointing it antenna towards the ISS, the signal will actually follow the path shown by the dotted line*. This is similar to what is shown in the earlier light clock example where the light travels at a diagonal as seen by someone watching the clock pass by. In order to get the signal to travel along the solid yellow arrow, Marcela actually has to aim its antenna in the direction indicated by the orange arrow. Since this points somewhat in the opposite direction of Marcella's direction of motion, this subtracts momentum from the signal.
Again the ISS measures the signal as coming in at c, and thus measures this as a decrease in frequency of 0.866. If Marcella sends a 10 sec message at 100 khz, the ISS will receive those 1000,000 waves making up the signal over 11.55 sec. The ISS sees everything occurring on Marcella as happening 0.866 as fast as it does for itself.

Now since neither the ISS or Marcella measure the distance between themselves as changing, they can not attribute any of the Doppler shift they see as being due to a change in propagation delay and can only put it down to events actually running faster for the ISS than they do on Marcella.

Changing the distance at which Marcela circles at 0.5c will complicate the signal analysis some, but will not change the degree of time dilation between the two.

*This also happens with the ISS, but since the speed at which it travels is so small compared to the speed of light, the difference between the direction of aim and the actual light path direction will be extremely small. It does have a small effect of the measured time dilation, but it is only the difference between the time dilation factor at 0.5c and at 4.99974c.

 

[Steeve Leaf]

Thanks,
Frequency of wave doesn't change its speed.
Light speed is the same for every observer.
Think instant of shooting light at a target or tennis ball at AO (Australia Open), each transmitting a tv signal of the clock .( and you adjust the frequency on the receiver.).
How you factor in if an object traveling at 0.5c toward you (+) and away from you (-) in the formula ?
Where 4.99974c # come from ?
I still think that in Marcella case at least another point of view will be good.
You are right this subject is difficult , at least for me , so the part of acceleration effect on gravity is the issue for me now, at least its got direction symbol in it.:)

 

[Janus]

Thanks,
Frequency of wave doesn't change its speed.
Light speed is the same for every observer.
Think instant of shooting light at a target or tennis ball at AO (Australia Open), each transmitting a tv signal of the clock .( and you adjust the frequency on the receiver.).
How you factor in if an object traveling at 0.5c toward you (+) and away from you (-) in the formula ?

Relativistic Doppler shift.
f_o = f_s \sqrt{\frac{1+\frac{v}{c}}{1-\frac{v}{c}}}
This incorporates both time dilation and the changing propagation delay caused by increasing or decreasing distance.
fo is the frequency observed and fs is the frequency at the source

Where 4.99974c # come from ?

it is the difference between the ISS orbital speed and Marcela's speed of 0.5c

I still think that in Marcella case at least another point of view will be good.
You are right this subject is difficult , at least for me , so the part of acceleration effect on gravity is the issue for me now, at least its got direction symbol in it.:)

 

[Steeve Leaf]

4.99974c is a little over the speed limit .
In the doppler effect the frequency comes up when the signal emitter come closer and the other way when it is traveling away , they don't cancel out I agree.

 

[Janus]

4.99974c is a little over the speed limit .
In the doppler effect the frequency comes up when the signal emitter come closer and the other way when it is traveling away , they don't cancel out I agree.

Sorry, that should be .499974c
Sometimes when I type too fast, I'll transpose characters. With words, I usually catch it. In this case since I knew the number I meant, It actually took me a while for me to even notice the misplaced decimal.

 

[Steeve Leaf]

Yes, ISS speed is 27,576 - 28080 km/h.
I admit that I don't completely understand this, but let's leave it for now.
( I can't tell how fast M is moving if it is broadcasting tv signal of the clock and you don't know the original broadcast frequency ).

Acceleration and gravity affect on visualized clocks, that need now your appreciated explanation.

 

[Bartolomeo]

In fact this is very interesting question, because it shows the ambiguity of the notion of relative motion of two point objects.

I think that @Janus gives correct and very spectacular answer.

I also like Bertrand Russell’s quote in his post.

Yes, distance between the Earth and rotating spaceship doesn’t change. However, the orbiter moves not relatively to the Earth but relatively to the reference frame. Imagine grid, which covers painting with the Earth and spaceship. Spaceship moves and changes its spatial position in this frame.

Amount of time dilation depends solely on linear velocity. According to time dilation tests based on Mossbauer Effect, rotating absorber (source is at rest) measures increase of source’s frequency at gamma times, i.e. clock in the center actually runs faster. (Kholmetskii et all 2008). There were some related polemical articles in Physica Scripta recently – Zanchini, Chen, Kholmetskii. Everything is very simple though.

If absorber is placed in the center and source rotates, absorber will measure decrease of source’s frequency gamma times. I.e. rotating clock actually dilates.

Why rotating clock “sees” acceleration of clock in the center? Because it ACTUALLY dilates. If you would become much slower than usual, you would see that everything around you runs faster.

All that can be analyzed through the Transverse Doppler effect. Transverse Doppler Effect (deviation of measured frequency from proper one) is in accordance with dilation of moving clock. If observer on the Earth releases beam of light, he keeps laser pointer at right angle to direction of motion of the orbiter. Photon approaches orbiter at oblique angle from the front, thus for orbiter the source on Earth appears to be in the front (aberration of light). Photon blue shifts, this appears to be in the shorter range of wave spectrum.

Angles of emission and reception tied with relativistic aberration formula.

Motion in arbitrary direction: https://en.wikipedia.org/wiki/Relativistic_Doppler_effect

If the orbiter holds a mirror, he has to keep this mirror at tangential to circumference “as he moves”. Photon bounces back from the mirror and travels back to the Earth. Orbiter will emit the beam at oblique angle again and photon will come back to the Earth and will enter right into laser pointer’s mouth.

Obviously, mirror was a moving source and moving clock. Moving clock appears to be slower at gamma, thus received frequency was also gamma times lower than it was at the orbiter.

If the ISS releases a photon and wants to hit an orbiter at a higher orbit, it has to tilt the laser pointer a bit backward, as @Janus correctly explains.

Animation in the internet, rotational motion episode:

Therefore it is interesting to note that if two orbiters rotate on the same orbit but wish to send a beam from certain point of circumference to opposite point of circumference, one of them (source) has to keep his laser pointer backward and another (target) has to keep his telescope forward at equal angles. Good to know, that in this case they will not see any frequency change, neither dilation nor acceleration of opposite clock. This was confirmed by Champeney and Moon time dilation test. There are two interesting articles in Nature by I. Essen (1964, 1965) in regard of this time dilation test. Also related articles are: “Ray path in a rotating system”, R.C. Jennison, Nature (No. 4898, 1963, p. 739) and “Reflection from a transversely moving mirror” , R.C. Jennison, Nature (Vol. 248, 1974, p. 660).

Thus, orbiters at different orbits, but rotating with the same angular velocity will actually accumulate different amount of time dilation and their clocks will show different time at the meeting on the Earth.

Also the last paragraph: http://www.mathpages.com/home/kmath587/kmath587.htm

If we will place a photo camera on the Earth and will make a picture of the orbiter, the orbiter will appear Lorentz – contracted in direction of its motion. If we will place a photo camera on the orbiter and will take a picture of a measuring rod on the Earth, the rod will appear not contracted, but stretched. Why? Because film in the photo – camera Lorentz contracts itself.

In these reflections we omit gravitational time dilation.

 

[Steeve Leaf]

Thanks,
ISS got a clock on it , it send a tv signal of this clock to M which broadcast it with no delay back to ISS monitor.
Now at ISS we can see the actual watch and the tv image after the round journey.
Why the difference between the actual clock and the monitor clock will change and not stay the same if
1. the distance stays the same.
2. The change in frequency don't change the time the signal travel.
3.This is ISS own clock.
4. No looking through telescope.
The difference is the time it takes to cover the unchanged distance.
Where the complication occur ?.☺

 

[jbriggs444]

Thanks,
ISS got a clock on it , it send a tv signal of this clock to M which broadcast it with no delay back to ISS monitor.
Now at ISS we can see the actual watch and the tv image after the round journey.
Why the difference between the actual clock and the monitor clock will change and not stay the same if

A hypothetical technician on M would see a high frequency carrier wave encoding a high speed video. As I understand the scenario, the high speed video is rebroadcast on a normal (according to M) frequency carrier wave.

Back on ISS, we would receive a low frequency carrier wave encoding a normal speed video. What would lead you to expect otherwise?

 

[Steeve Leaf]

Ahaha , That is the big question. you got it , that is my problem Doctor !

The man on M will he see the tv clock moving at faster rate than his own because of time dilation on M , but because the signal doesn't delay at M at all this dilation don't appear on ISS TV.
Any delay on M will affect the tv on iss with dilation.
Am I correct ?

 

[Janus]

Thanks,
ISS got a clock on it , it send a tv signal of this clock to M which broadcast it with no delay back to ISS monitor.
Now at ISS we can see the actual watch and the tv image after the round journey.
Why the difference between the actual clock and the monitor clock will change and not stay the same if
1. the distance stays the same.
2. The change in frequency don't change the time the signal travel.
3.This is ISS own clock.
4. No looking through telescope.
The difference is the time it takes to cover the unchanged distance.
Where the complication occur ?.☺

There won't be a difference between The ISS local clock and the TV image on the ISS monitor.
To illustrate why this is, we will consider a 10 second message again. The ISS send a message that is 10 seconds long. Remember from my last discussion, Marcella will receive this message over a period of 8.66 sec. If it immediately rebroadcasts this message just as it received it, it will transmit a 8.66 sec long message. You will also remember that the ISS would receive a signal from Marcela over a time period 1.155 times longer that of the length of the message sent by Marcela. Thus is Marcela sends a 8.66 sec message at its end, the ISS receives a 10 second message at its end.
The ISS sends out a 10 sec message and gets a 10 sec message echoed back.

 

[jbriggs444]

Any delay on M will affect the tv on iss with dilation

A fixed delay on M will result in a corresponding, but longer fixed delay on ISS. If, for instance, the incoming signal were fed through a 10 second delay loop on M before being rebroadcast, the result would be an 11.55 second delay added to the round trip time measured at the ISS.

That's not dilation. That's an offset.

 

[Janus]

Yes, ISS speed is 27,576 - 28080 km/h.
I admit that I don't completely understand this, but let's leave it for now.
( I can't tell how fast M is moving if it is broadcasting tv signal of the clock and you don't know the original broadcast frequency ).

Acceleration and gravity affect on visualized clocks, that need now your appreciated explanation.

We can consider acceleration in the following way:

Consider a rocket ship, with a clock at the nose and one at the tail. It is undergoing a constant acceleration. Imagine signals traveling between the two clocks, each keeping the other appraised of the others tick rate.

The signal leaves the nose clock at some moment t₀ and arrives at the nose clock at t₁.
Since the rocket is accelerating, The velocity of both clocks changes between t₀ and t₂
This is important, because, in the Relativistic Doppler shift formula I gave in an earlier post, v stands for the difference in velocity of the transmitter at the moment of transmission and the velocity of the receiver at the moment of reception.

Since the tail clock has been accelerating for a duration of t₁-t₀, it will have a different velocity at reception than the nose clock had at transmission. This results in a positive non-zero value of v, and the tail clock will see a higher frequency coming from the Nose clock than that emitted by the nose clock. If the Nose clock sent a 100 hz signal, at 100 waves per one sec tick of the clock, the tail clock receives those 100 waves at a higher frequency in less than one second, and thus measures the nose clock at ticking fast.

Conversely, a signal sent from the tail clock to the nose clock also takes some non-zero time to cross between them, meaning that the nose clock will have a different velocity when it receives the signal than the tail clock and this results in a negative non-zero value for v (in the first example, the tail clock's direction of acceleration is towards the nose clock, But now we are considering the nose clock, and its direction of acceleration is away from the tail clock. The result is that the tail clock sees a lower frequency coming from the tail clock and measures it as ticking slower.

Again, like in the ISS-Marcela scenario, the actual distance between the two clocks does not change, and they can not attribute any of the frequency shift they measure as being due to changing propagation time, and can only be attributed to the two clocks actually keeping different time.

Also note that if we increase the distance between the tail and nose, while maintaining the same acceleration, we increase the length of time between t₀ and t₁. Thus the receiving clock have a longer time to accelerate between transmission and reception, and this results in a larger difference in velocities. Ergo, increasing the distance between the clocks increases the difference in tick rates even if the acceleration is the same.

Of course, it should go without saying that increasing the acceleration and maintaining the same distance will also cause a larger difference in tick rate.

Now consider the viewpoint of someone in the rocket. If you let's go of an object at the nose of the rocket, to them it will "fall" towards the tail. the speed at which it arrives at the tail relative to the rocket will be the same as if it were accelerated by a constant force for the distance it fell, much like it would in a gravity field.
It will also have gained kinetic energy relative to the rocket.

You can also treat the light traveling from nose to tail the same way, It can't gain speed relative to the rocket as measured by the rocket, but it will gain momentum/energy, and that will be measured as an increase in frequency. Conversely, light moving from tail to nose loses energy with respect to the the rocket and decreases frequency. The combination of the distance of the fall and the acceleration during the fall determines the frequency change.

The same happens with light in a gravity field, light traveling upwards against gravity loses energy/frequency as it does so, and light moving downwards gains energy/frequency. Again this occurs even if the distance between transmitter and receiver is constant and none of the frequency shift can be the result of changing distance. Thus a clock higher in a gravity field runs faster than than one lower. Note that really doesn't have anything to do with gravity being weaker at the higher clock. In fact, the drop off in gravity actually decreases the difference in time rates vs no difference in gravity.

 

[Steeve Leaf]

A fixed delay on M will result in a corresponding, but longer fixed delay on ISS. If, for instance, the incoming signal were fed through a 10 second delay loop on M before being rebroadcast, the result would be an 11.55 second delay added to the round trip time measured at the ISS.

That's not dilation. That's an offset.

10 sec delay cause 1.55 sec time dilation. that is 11.55 offset ?

There won't be a difference between The ISS local clock and the TV image on the ISS monitor.
To illustrate why this is, we will consider a 10 second message again. The ISS send a message that is 10 seconds long. Remember from my last discussion, Marcella will receive this message over a period of 8.66 sec. If it immediately rebroadcasts this message just as it received it, it will transmit a 8.66 sec long message. You will also remember that the ISS would receive a signal from Marcela over a time period 1.155 times longer that of the length of the message sent by Marcela. Thus is Marcela sends a 8.66 sec message at its end, the ISS receives a 10 second message at its end.
The ISS sends out a 10 sec message and gets a 10 sec message echoed back.

The first wave in the 10 sec broadcast will take the same time as the last wave to reach M.
Because M is moving toward ISS it receives the transmission in higher frequency and in less time but the clock on M ticks slower in a manner that even for it the time it took the first wave to reach it is equal to the time it takes the last wave to reach it ?

The same happens with light in a gravity field, light traveling upwards against gravity loses energy/frequency as it does so, and light moving downwards gains energy/frequency. Again this occurs even if the distance between transmitter and receiver is constant and none of the frequency shift can be the result of changing distance. Thus a clock higher in a gravity field runs faster than than one lower. Note that really doesn't have anything to do with gravity being weaker at the higher clock. In fact, the drop off in gravity actually decreases the difference in time rates vs no difference in gravity.

Gravity is measured in the same units as acceleration,but with gravity the distance and the speed are not changing it is a "potential acceleration" only still the same affect on the clocks, Can you detect the effect of small change in gravity such as on ISS on signal broadcast frequency from Earth surface ?
How gravity has the same affect as acceleration when the distance and speed stay constant , is it the force behind the two which make the affect ?

 

[Janus]

Gravity is measured in the same units as acceleration,but with gravity the distance is not changing it is a "potential acceleration" only still the same affect on the clocks, Can you detect the effect of small change in gravity such as on ISS on signal broadcast frequency from Earth surface ?

Neither is the distance changing with the accelerating rocket. The tail and nose clock maintain a constant distance, yet still measure each other as ticking at different rates from each other.

With the ISS, You have two factors effecting its time dilation: Its orbital altitude and its orbital velocity.

A far distant observer would see its clock time dilated by a factor of
\sqrt{1- \frac{3GM}{rc^2}}
Where M is the mass of the Earth and r is the radius of the ISS orbit.

The same observer would see a clock time dilated by a factor of
\sqrt{1- \frac{2GM}{r_ec^2}- v^2/c}
where r_e is the equatorial radius of the Earth and v is the tangential velocity of the Earth's surface at the Equator measured relative to the Earth centered inertial frame.
I'll leave it to you to work what difference this would result in between Earth surface and ISS.

 

[Steeve Leaf]

It is very small, still we can measure it.
Edit : ",but with gravity the distance and the speed are not changing "
Can we say that when light is blueshift at A the clock at A will run slower and when light redshift at B the clock at B will run faster than at the source?
What if M lost its visual connection with ISS would it be possible to stay synchronized only from Instrumental data monitoring the changes in frequency and time and delay and they have the same software ?

 

[Steeve Leaf]

I can't figure it out, if the clock on M running 86.6 % the rate of the one on ISS than wouldn't it measure the time a light beam travels from ISS to it 115% faster ?
The two observers they can't agree on time , distance etc, what they can agree on ?

 

[_PJ_]

I can't figure it out, if the clock on M running 86.6 % the rate of the one on ISS than wouldn't it measure the time a light beam travels from ISS to it 115% faster ?
The two observers they can't agree on time , distance etc, what they can agree on ?

Observers agree on:

(Square) Spacetime interval between events.
Speed of light.

 

[Steeve Leaf]

Can you give an example of faster than light signaling that will create reference frame in which the signal arrived before it was sent ?

 

[Ibix]

Can you give an example of faster than light signaling that will create reference frame in which the signal arrived before it was sent ?

You are headin away from me at any speed. I send a signal that reaches you instantaneously in my frame. You reply with a signal that reaches me instantaneously in your frame, which arrives before I sent my signal.

You can do it with non-instantaneous communication too, but the circumstances where things go acausal are a bit more restrictive and I'd need to do some maths.

You could Google for "tachyonic anti-telephone".

 

[Steeve Leaf]

You are headin away from me at any speed. I send a signal that reaches you instantaneously in my frame. You reply with a signal that reaches me instantaneously in your frame, which arrives before I sent my signal.

You can do it with non-instantaneous communication too, but the circumstances where things go acausal are a bit more restrictive and I'd need to do some maths.

You could Google for "tachyonic anti-telephone".

What is heading away from me at any speed, got to do with it, is it just appear to be because you know (assume in this case) that FTL is not possible ?.

 

[PeroK]

Can you give an example of faster than light signaling that will create reference frame in which the signal arrived before it was sent ?

In a single reference frame, FTL signals do not do anything strange. They simply get from one place to the other faster than light.

To derive a problem with FTL signals, you need to consider the relationship between two reference frames moving with respect to each other. And, you need to assume as a postulate of relativity (or an experimentally proven fact) that the speed of a light signal is the same for both frames. It's when you put this together with a FTL signal between the two that you get a problem.

 

[Nugatory]

What is heading away from me at any speed, got to do with it, is it just appear to be because you know (assume in this case) that FTL is not possible ?.

As Ibix said... Google for "tachyonic anti-telephone".

 

[Ibix]

What is heading away from me at any speed, got to do with it

Because that's the condition for the simultaneity conventions of your frame and mine to clash in a way that is paradoxical if faster than light communication is possible within relativity.

 

[Steeve Leaf]

What Einstein had to say about this ?
I don't see any observer that will say that the reply was received before the message was send.
Math must be interesting .

 

[PeroK]

What Einstein had to say about this ?
I don't see any observer that will say that the reply was received before the message was send.
Math must be interesting .

You should read post #73. You need two frames of reference and a Lorentz Transformation between them. Otherwise, there isn't going to be a problem in anyone frame.

 

[Steeve Leaf]

2807 Ok.
It was mention as " any speed " , in order for Lorentz Transformation to play a role they have to move apart close to the speed of light, then time dilation might cause the reply to be received before... but I THINK it will work only with instantaneous info transfer .

Am I 'm even getting closer

 

[PeroK]

2807 Ok.
In was mention as any speed , in order for Lorentz Transformation to play a role they have to move apart close to the speed of light,than time dilation will cause the reply to be received before... but I THINK it will work only with instantaneous info transfer .

Am I 'm even close

FTL causing a problem is not that easy to demonstrate since you first need to really understand how events (time and place) in one frame relate to another frame. It's a thing called the relativity of simultaneity (not time dilation) that is really the issue.

If you have an absolute notion of time (as in classical physics), then FTL is not a problem at all. But, time is not absolute, simultaneity is relative and when you have a FTL signal, that causes a problem.

Using an instantaneous signal is easier than using a signal at, say, twice the speed of light; but both can lead to a problem.

Did you find anything on line?

 

[Steeve Leaf]

To use the tachyonic antitelephone, the user must aim it at the position in space occupied by the Earth in 1930. The user then must dial the telephone number a2807a and the device will transmit a beam of modulated tachyons back into time. When the beam arrives in 1930 the telephone will ring at the University of Berlin and none other than Albert Einstein will answer to help the caller with his or her physics questions.
http://www.physics.buffalo.edu/ubexpo/posters/tachyonic.pdf
You see that you got the answer before the question.
You absolutely right the relativity of simultaneity was recommended by the Prof but I thought I can skip it, I guess I was wrong ( again ).

 

[nitsuj]

To derive a problem with FTL signals, you need to consider the relationship between two reference frames moving with respect to each other. And, you need to assume as a postulate of relativity (or an experimentally proven fact) that the speed of a light signal is the same for both frames. It's when you put this together with a FTL signal between the two that you get a problem.

Maybe oxymoron is a better term? One hand says you can do instantaneous physics and the other says you're limited to c.

 

[Nugatory]

Maybe oxymoron is a better term? One hand says you can do instantaneous physics and the other says you're limited to c.

It's more a proof by contradiction, because the argument that causal effects cannot propagate faster than $c$ is that if you assume it is possible, contradictions appear.

 

[Nugatory]

http://www.physics.buffalo.edu/ubexpo/posters/tachyonic.pdf
You see that you got the answer before the question.

You would be better served by the wikipedia article: https://en.wikipedia.org/wiki/Tachyonic_antitelephone

You absolutely right the relativity of simultaneity was recommended by the Prof but I thought I can skip it, I guess I was wrong

There is indeed no way of making it through special relativity unless you have thoroughly understood the relativity of simultaneity.

 

[Ibix]

It was mention as " any speed " , in order for Lorentz Transformation to play a role they have to move apart close to the speed of light

No. The only thing moving faster does is let you detect the effects more easily. For the case where you pass me at time zero and I send you a tachyon pulse at time Tlater, the "reply" arrives $v^2T/c^2$ seconds before the original signal. Either T needs to be really huge or v needs to be very close to c for this kind of gap to be detectable. But it's there for any speed.

Faster than light communication is impossible in relativity, but it is a general truth that relativistic effects are present at all speeds, just undetectably small for every day speeds.

 

[PeroK]

Maybe oxymoron is a better term? One hand says you can do instantaneous physics and the other says you're limited to c.

An oxymoron is a linguistic device, where you combine two words that ought not to go together. Perhaps "thought experiment" is a relevant example.

 

[nitsuj]

An oxymoron is a linguistic device, where you combine two words that ought not to go together. Perhaps "thought experiment" is a relevant example.

? in other words speed is limited and unlimited...an oxymoron.

 

[jbriggs444]

? in other words speed is limited and unlimited...an oxymoron.

A claim that speed is both limited and unlimited would be a contradiction, not an oxymoron. But no such claim has been advanced in this thread.

 

[Dale]

You absolutely right the relativity of simultaneity was recommended by the Prof but I thought I can skip it, I guess I was wrong ( again ).

The relativity of simultaneity is the single most difficult concept in relativity. Students struggle more with that than anything else.

 

[Steeve Leaf]

Maybe oxymoron is a better term? One hand says you can do instantaneous physics and the other says you're limited to c.

http://www.theculture.org/rich/sharpblue/archives/000089.html
If we need others point of view I recommend the above link.
I'm ready for the first lesson about relativity. ( I Think ).

 

[Ibix]

http://www.theculture.org/rich/sharpblue/archives/000089.html
If we need others point of view I recommend the above link.
I'm ready for the first lesson about relativity. ( I Think ).

Decent article. The comments might be subtitled "fifty ways to miss the point", though.

 

[Arkalius]

Can you give an example of faster than light signaling that will create reference frame in which the signal arrived before it was sent ?

Let's say I'm here on Earth you're on your ship about a light year away traveling away from Earth with some speed. I send you an instantaneous signal when my clock says it's 1100. At this instant for me, your clock says, say 1340. You get this signal at 1340 your time. However, due to the fact that you're moving relative to me, and the relativity of simultaneity, for you at the instant your clock says 1340, my clock says it's 0300. So, you send an instantaneous signal back to me, and it arrives to me before I ever sent one to you. Of course, for me, when the time was 0300, your time was something like 0600, so I can respond back asking about this bizarre message I receive from you, and then you're confused because that comes before you ever replied to my first one, and this keeps going.

Keep in mind here the numbers are just arbitrary, but the effect they're pointing to is real. The concept of "now" is relative. There is no universal instant that everyone shares.

This effect can still happen with non-instantaneous signals that travel faster than light, but this puts a lower bound on the speed at which the target of the signal must be traveling away from you for it to happen. The faster the signal, the less this lower bound is. That bound basically uses the velocity addition formula for doubling the velocity. So the lower bound for velocity $v$ a frame must be traveling away from you for a signal that moves at faster than light velocity $k$ such that it could repeat that signal back to you so that it arrives before it was sent is given by $v = \frac {2k} {1 + k^2}$

So, if you could send signals at twice the speed of light, the target has to be moving away from you faster than 0.8c to send it back to you before you sent it. If you can send it at 10x the speed of light, the target only has to be moving away faster than about 0.198c. You'll see that as $k$ approaches infinity (instant communication), $v$ approaches 0.

How much further in the past the return signal will go depends both on how much faster than the lower bound the target is moving and how far from you they are.

 

[Steeve Leaf]

Decent article. The comments might be subtitled "fifty ways to miss the point", though.

Some of the comments are from Rich the writer himself and some others are interesting and informative I didn't go through all of them( did you ? ) so maybe fifty is the right figure , ☺

 

[Steeve Leaf]

Let's say I'm here on Earth you're on your ship about a light year away traveling away from Earth with some speed. I send you an instantaneous signal when my clock says it's 1100. At this instant for me, your clock says, say 1340. You get this signal at 1340 your time. However, due to the fact that you're moving relative to me, and the relativity of simultaneity, for you at the instant your clock says 1340, my clock says it's 0300. So, you send an instantaneous signal back to me, and it arrives to me before I ever sent one to you. Of course, for me, when the time was 0300, your time was something like 0600, so I can respond back asking about this bizarre message I receive from you, and then you're confused because that comes before you ever replied to my first one, and this keeps going.

Keep in mind here the numbers are just arbitrary, but the effect they're pointing to is real. The concept of "now" is relative. There is no universal instant that everyone shares.

This effect can still happen with non-instantaneous signals that travel faster than light, but this puts a lower bound on the speed at which the target of the signal must be traveling away from you for it to happen. The faster the signal, the less this lower bound is. That bound basically uses the velocity addition formula for doubling the velocity. So the lower bound for velocity $v$ a frame must be traveling away from you for a signal that moves at faster than light velocity $k$ such that it could repeat that signal back to you so that it arrives before it was sent is given by $v = \frac {2k} {1 + k^2}$

So, if you could send signals at twice the speed of light, the target has to be moving away from you faster than 0.8c to send it back to you before you sent it. If you can send it at 10x the speed of light, the target only has to be moving away faster than about 0.198c. You'll see that as $k$ approaches infinity (instant communication), $v$ approaches 0.

How much further in the past the return signal will go depends both on how much faster than the lower bound the target is moving and how far from you they are.

I don't have to be there physically do I ? We can replace me with a device that just return the information back to us at Earth, don't tell nobody about it and make a lot of money. We tried to get patent on it but every time at appears that it already patented .
I think that the problem here is first to receive the signal of FTL radiation/interaction than work on not breaking causality if it is in danger. Suddenly causality becomes a FORCE a fifth one.

 

[Ibix]

Some of the comments are from Rich the writer himself and some others are interesting and informative I didn't go through all of them( did you ? ) so maybe fifty is the right figure , ☺

I didn't go through them all. The author's were good, but a lot of the time he was replying to say "no, you've missed the point". And note the comment he added at the top of the article. Maybe there are insightful comments, but they seem to be a bit lost in the noise.