A problem with time dilation help?

[Rishavutkarsh]

No, I didn't say that. I was talking about comparing the measured round trip speed of light for two flashes approaching you from opposite directions. I said that even without time dilation and length contraction, you will measure the same value for the two flashes. I did not say that you would get a value of c, which you will not, if there weren't any time dilation and length contraction. But with time dilation and length contraction, which is what happens in our real world if you actually made the measurement, you would get the value of c for both measured round trips.

oh.. can you please elaborate so a 13 year old can understand it?
if time dilation and length contraction is still what you insist on things get cocky. i

 

[ghwellsjr]

OK, you were claiming that when you are traveling at a high speed (99%C) and two light rays approach you from opposite directions, one from in front of you and one from behind you, that you will be able to detect something different about the speed of these two light rays because time dilation couldn't cause one to slow down and the other one to speed up at the same time, correct?

So I showed you in post #21 that even without time dilation (or length contraction) you wouldn't be able to detect any difference in the speed of those two light rays because you can only measure the round trip speed, not the one-way speed, and you understood that, correct?

And then I pointed out that the speed that you would measure if there were no time dilation (or length contraction) would not be equal to C but since we don't live in a world like that, you would, in fact, measure the round trip speed of both rays as traveling at C because we do live in a world that has time dilation and length contraction. I'm not trying to explain why you measure C in our real world, only that you cannot detect any difference in the round trip speed of light approaching you from opposite directions, no matter how fast you are traveling.

This, by the way, has nothing to do with the Theory of Special Relativity. This concept was known by all scientists prior to Einstein and understood by them, it's just logical reasoning: you cannot measure the one-way speed of light, you can only measure the round-trip speed of light and you will always get the same answer for light approaching you from opposite directions, no matter what your own speed is.

Do you understand all of this so far?

 

[Rishavutkarsh]

OK, you were claiming that when you are traveling at a high speed (99%C) and two light rays approach you from opposite directions, one from in front of you and one from behind you, that you will be able to detect something different about the speed of these two light rays because time dilation couldn't cause one to slow down and the other one to speed up at the same time, correct?

So I showed you in post #21 that even without time dilation (or length contraction) you wouldn't be able to detect any difference in the speed of those two light rays because you can only measure the round trip speed, not the one-way speed, and you understood that, correct?

And then I pointed out that the speed that you would measure if there were no time dilation (or length contraction) would not be equal to C but since we don't live in a world like that, you would, in fact, measure the round trip speed of both rays as traveling at C because we do live in a world that has time dilation and length contraction. I'm not trying to explain why you measure C in our real world, only that you cannot detect any difference in the round trip speed of light approaching you from opposite directions, no matter how fast you are traveling.

This, by the way, has nothing to do with the Theory of Special Relativity. This concept was known by all scientists prior to Einstein and understood by them, it's just logical reasoning: you cannot measure the one-way speed of light, you can only measure the round-trip speed of light and you will always get the same answer for light approaching you from opposite directions, no matter what your own speed is.

Do you understand all of this so far?

can't say i don't! but what if a high tech device is made which can measure speed of light when it falls on it ?

 

[ghwellsjr]

can't say i don't! but what if a high tech device is made which can measure speed of light when it falls on it ?

In order to measure the speed of anything, you have to know how long it took for that something to get from its starting point to its ending point and how far away those two points are, correct?

 

[Rishavutkarsh]

In order to measure the speed of anything, you have to know how long it took for that something to get from its starting point to its ending point and how far away those two points are, correct?

yes you correct. but technology can evolve anything correct?

 

[ghwellsjr]

yes you correct. but technology can evolve anything correct?

No, technology cannot measure the one-way speed of light. It can only measure the round-trip speed of light by the technique I described earlier: one timing device and a mirror a fixed, measured distance away from it, or something equivalent.

If you had all the technology at your disposal, what would you do to measure the one-way speed of light?

 

[rede96]

No, technology cannot measure the one-way speed of light. It can only measure the round-trip speed of light by the technique I described earlier: one timing device and a mirror a fixed, measured distance away from it, or something equivalent.

If you had all the technology at your disposal, what would you do to measure the one-way speed of light?

Couldn't you fire a laser beam down a long, dark tube, which had a number of sensors spaced equidistant, then just measure the time between the sensors detecting the light source as it moved through the tube?

 

[ghwellsjr]

Couldn't you fire a laser beam down a long, dark tube, which had a number of sensors spaced equidistant, then just measure the time between the sensors detecting the light source as it moved through the tube?

How do you propose measuring the time between the sensors?

 

[Ryan_m_b]

Couldn't you fire a laser beam down a long, dark tube, which had a number of sensors spaced equidistant, then just measure the time between the sensors detecting the light source as it moved through the tube?

But you wouldn't be measuring the speed of light hitting you though. The first sensor would absorb and re-admit a new photon.

 

[rede96]

How do you propose measuring the time between the sensors?

Each sensor is attached to cables that are of equal length, which in turn are attached to one central, very accurate clock. The clock just measures the time between signals.

 

[ghwellsjr]

Each sensor is attached to cables that are of equal length, which in turn are attached to one central, very accurate clock. The clock just measures the time between signals.

Cables introduce a time delay just like the return light signal from a reflector. You haven't eliminated the problem in that you are still measuring a round-trip time that is less accurate than simply using a reflected light.

 

[rede96]

Cables introduce a time delay just like the return light signal from a reflector. You haven't eliminated the problem in that you are still measuring a round-trip time that is less accurate than simply using a reflected light.

But the time delay for each cable will be the same, so the signal duration will still be representative of the time between the sensors being triggered.

I can't see where the round trip is? Light is always moving in one direction.

 

[Ryan_m_b]

But the time delay for each cable will be the same, so the signal duration will still be representative of the time between the sensors being triggered.

I can't see where the round trip is? Light is always moving in one direction.

How exactly are you sensing this light? If it hits a sensor it is absorbed by the sensor. It's not like a photon can travel through a tube and be measured without interference.

 

[rede96]

How exactly are you sensing this light? If it hits a sensor it is absorbed by the sensor.

Ok, good point, and this is where my lack of knowledge in physics doesn't help.

However, I imagined the beam will have millions of photons, so if one gets absorbed, there will be another in close proximity at the front of the beam. So the sensors are thus measuring the first photon they absorb.

Less accurate, but the objective wasn't to be accurate, just to measure light in a one way direction.

 

[ghwellsjr]

But the time delay for each cable will be the same, so the signal duration will still be representative of the time between the sensors being triggered.

I can't see where the round trip is? Light is always moving in one direction.

You're measuring the time it takes for the light to travel in one direction plus the time it takes for an electrical signal to get back to you in the other direction. The electrical signal travels at some percentage of the speed of light. It's not instantaneous. How is that better than just using light in both directions?

 

[rede96]

You're measuring the time it takes for the light to travel in one direction plus the time it takes for an electrical signal to get back to you in the other direction.

Just to clarify my understanding of that, I am measuring the time for an electrical signal to pass through a cable sure. But as the time taken for each signal to pass down it's respective cable is the same, I can subtract that time for all measurements and I am left with the time between each sensor.

Also, I don't see what the direction of the cables has to do with it? The cables could be laid in any direction, or even coiled up in circles. I don't understand how that is relevant?

The electrical signal travels at some percentage of the speed of light. It's not instantaneous. How is that better than just using light in both directions?

Yes, but as mentioned above, all the times will be the same and can thus be subtracted.

Also, I am not comparing systems, although I am sure the accuracy would compare if set up right; the test was to see if I could measure the speed of light traveling in one direction.

Which as far as I understand it, I have achieved. (EDIT: As the claim from ghwellsjr was "Technology cannot measure the one-way speed of light", which I didn't understand how that could be so.)

 

[ghwellsjr]

Just to clarify my understanding of that, I am measuring the time for an electrical signal to pass through a cable sure. But as the time taken for each signal to pass down it's respective cable is the same, I can subtract that time for all measurements and I am left with the time between each sensor.

Also, I don't see what the direction of the cables has to do with it? The cables could be laid in any direction, or even coiled up in circles. I don't understand how that is relevant?



Yes, but as mentioned above, all the times will be the same and can thus be subtracted.

Also, I am not comparing systems, although I am sure the accuracy would compare if set up right; the test was to see if I could measure the speed of light traveling in one direction.

Which as far as I understand it, I have achieved. (EDIT: As the claim from ghwellsjr was "Technology cannot measure the one-way speed of light", which I didn't understand how that could be so.)

But what time will you subtract?

Just like you can't measure the one-way speed of light, you can't measure the one-way speed of an electrical signal down a cable. If you have too much cable so that it is coiled and the signal takes longer, it's only going to make matters worse. You'd like for there to be no delay in your cables, then you could actually measure the one-way speed of light. If you could instantly communicate the time that the light arrived at your distant target, some fixed, measured distance away, then you could measure the one-way speed of light. But light is the fastest thing we have, so, again, why do you want to use a sensor and a cable to communicate back to your timing device when to stop the measurement of the time interval when the reflected light will do the job better than anything else?

 

[rede96]

But what time will you subtract?

Just like you can't measure the one-way speed of light, you can't measure the one-way speed of an electrical signal down a cable. If you have too much cable so that it is coiled and the signal takes longer, it's only going to make matters worse. You'd like for there to be no delay in your cables, then you could actually measure the one-way speed of light. If you could instantly communicate the time that the light arrived at your distant target, some fixed, measured distance away, then you could measure the one-way speed of light. But light is the fastest thing we have, so, again, why do you want to use a sensor and a cable to communicate back to your timing device when to stop the measurement of the time interval when the reflected light will do the job better than anything else?

I think where I am coming from is, as long as the system was set up so the delay is exactly the same in each cable, then the time it takes the signal to travel down the cable is irrelevant as far as measuring the intervals.

E.G If the delay in each cable is 5 nano seconds then I am simply getting each signal 5 nano seconds later. However the interval between each signal is exactly the same as the interval between each sensor as it detects the light. So in this respect, it is no less accurate than if I could measure the signals instantaneously.

Also, I am not saying that this system is better or worse than using a mirror, just that the results for measuring c would be the same.

The reason for this thought experiment was because I understood from a previous post of yours, that it was not possible to measure the speed of light in one direction, which I obviously thought it could be.

 

[ghwellsjr]

I think where I am coming from is, as long as the system was set up so the delay is exactly the same in each cable, then the time it takes the signal to travel down the cable is irrelevant as far as measuring the intervals.

E.G If the delay in each cable is 5 nano seconds then I am simply getting each signal 5 nano seconds later. However the interval between each signal is exactly the same as the interval between each sensor as it detects the light. So in this respect, it is no less accurate than if I could measure the signals instantaneously.

Also, I am not saying that this system is better or worse than using a mirror, just that the results for measuring c would be the same.

The reason for this thought experiment was because I understood from a previous post of yours, that it was not possible to measure the speed of light in one direction, which I obviously thought it could be.

How does setting up a sequence of sensors with matched sets of cables make any difference than having just one sensor with one cable?

I agree that the delay in each cable section is the same as all the others but so what? You're just multiplying the problem over and over again without addressing the problem.

I agree if the delay in a cable segment is 5 nano seconds then the problem is solved, but where'd you get that number from?

You can get reflections of signals down cables just as easily as you can get reflections of light off of mirrors, there's no difference in principle. So let's say you have a straight piece of cable that's 1000 feet long and you leave it unterminated (open circuit). Then you apply a current to one end of the cable at the same time that you start your timing device. Three microseconds later, you measure the reflected signal and stop your timer. You have measured the round-trip signal speed through this cable and can calculate the average speed of the signal as being 2000 feet divided by 3000 nanoseconds or 2/3 feet per nanosecond. But if you think that the signal took 1.5 microseconds to get to the far end of the cable and another 1.5 microsecond to return, then you are jumping to a conclusion, because you haven't measured that. You can't tell if it took 1 microsecond to go down the cable and 2 microseconds to come back to you or the other way around or any other pair of numbers that add up to 3 microseconds.

In the same way, although you can know the average speed of a signal through a cable if the signal reflects back to you or if you have a loop of cable that starts and ends at one location, but this won't tell you when the signal reaches various points in the cable.

So if you agree that cables are no better or worse than just using light, why'd you introduce cables?

 

[rede96]

How does setting up a sequence of sensors with matched sets of cables make any difference than having just one sensor with one cable?

As you've said, to measure the speed of light we need a start point, and end point, a known distance between them and a time interval. So I guess just two sensors would be ok. (I would have done a number of them in series to help measure the error that’s all.)

I agree if the delay in a cable segment is 5 nano seconds then the problem is solved, but where'd you get that number from?

I have to admit, this was plucked from thin air, just to demonstrate the point.

I agree that the delay in each cable section is the same as all the others but so what? You're just multiplying the problem over and over again without addressing the problem.

You can get reflections of signals down cables just as easily as you can get reflections of light off of mirrors, there's no difference in principle. So let's say you have a straight piece of cable that's 1000 feet long and you leave it unterminated (open circuit). Then you apply a current to one end of the cable at the same time that you start your timing device. Three microseconds later, you measure the reflected signal and stop your timer. You have measured the round-trip signal speed through this cable and can calculate the average speed of the signal as being 2000 feet divided by 3000 nanoseconds or 2/3 feet per nanosecond. But if you think that the signal took 1.5 microseconds to get to the far end of the cable and another 1.5 microsecond to return, then you are jumping to a conclusion, because you haven't measured that. You can't tell if it took 1 microsecond to go down the cable and 2 microseconds to come back to you or the other way around or any other pair of numbers that add up to 3 microseconds.

Ok, first of all I think I need to clarify my experiment, as I am not measuring the signal from the sensor to the clock. The signal from the sensor to the clock is simply carrying a wave of information from the sensor to the clock to say that the sensor has detected a light soruce. I have no interest or no need to know how long it takes for that information to get from the sensor to the clock.

All that is important is that the time taken for that information to get from the sensor to the clock is the same for both detections.

So in effect, I fire a laser beam down a tube which has two sensors A and B which are placed at a distance x, When A detects a light source, it sends a wave of information, in a one way direction, to the clock. The clock registers that signal and makes a note of the time t1. The light source continues down the tube until it registers at B, which sends a one-way wave of information to the clock and registers a time t2.

I now have an elapsed time between two events (t2-t1) and a known distance x, hence I now know the speed the light source was traveling through the tube.

That seems fairly straight forward to me, I don't understand why that causes a problem.

So if you agree that cables are no better or worse than just using light, why'd you introduce cables?

To test your hypothesis that we cannot measure the speed of light traveling in one-way direction.

EDIT: Just to add to that, the reason for cables is that I am only using one clock as I didn't know if using two clocks at each sensor would cause problems, as they would be separated by a distance.

If this is not an issue, I could do away with the cables and have two sensors with clocks in that were synchronised. Then just take the difference in readings from the clocks to establish elapsed time.

 

[ghwellsjr]

All that is important is that the time taken for that information to get from the sensor to the clock is the same for both detections.

Yes, that is the all-important requirement. But why should the time be the same for both detections? Isn't sensor B farther away from the clock than sensor A?

EDIT: Just to add to that, the reason for cables is that I am only using one clock as I didn't know if using two clocks at each sensor would cause problems, as they would be separated by a distance.

Yes, having two clocks separated by a distance would cause problems. I'm sure you're aware of the so-called Twin Paradox where two clocks are colocated and have the same time on them. One of them is moved to another location and then brought back and the two clocks have different times on them. Moving clocks around is known to make them have different times on them.

If this is not an issue, I could do away with the cables and have two sensors with clocks in that were synchronised. Then just take the difference in readings from the clocks to establish elapsed time.

Well now you just nailed down the problem. How do you synchronize two clocks that are at different locations? That's exactly the same problem as measuring the one-way speed of light. Don't forget how this thread started out: someone traveling at 99%C. Two observers traveling with respect to one another will not agree on how to synchronize a pair of clocks.

 

[rede96]

Yes, that is the all-important requirement. But why should the time be the same for both detections?

Because I have designed the system that way.

The physical position in space of the clock relative to Sensor A and Sensor B can help, but is not critical. Although it makes sense to have the clock equidistant.

As you’ve said, what is important is the process for sending information from A to the clock takes exactly the same time as the process to send information from B to the clock. So I have made the assumption that both processes have been calibrated and will take the same time, which is straight forward enough to do.

Just to add, this type of error exists inherently in ANY type of measuring system, as it is not possible to detect anything 'instantaneously'.

There is a process of detection, then sending information from a detector through something that will transform the detection into a result we can interpret.

No information can travel greater than c, so there will always be a delay and hence a potential error in any system of two or more detections.

The process of detecting a light source reflected from a mirror is no different in that respect.

Yes, having two clocks separated by a distance would cause problems. I'm sure you're aware of the so-called Twin Paradox where two clocks are colocated and have the same time on them. One of them is moved to another location and then brought back and the two clocks have different times on them. Moving clocks around is known to make them have different times on them.

Yes, this was the problem I was thinking of so I left it out. Although I am sure that done correctly, the value of error could made insignificant compared the value of the measurement.

Well now you just nailed down the problem. How do you synchronize two clocks that are at different locations?

As I am using just one clock, this is no longer an issue. Problem Solved :)

 

[Rishavutkarsh]

hey i just thought a thing as of twin paradox first twin's (traveller) has his same twin get older when he returns,correct?
so that means clock of first twin is running slow correct?
now then the first twin must see his twin faster in time when he was traveling so second twin sees him slow does it mean that if we get close to speed of light outer world seems to go faster and for the outer world we seem to get slower ?

 

[ghwellsjr]

hey i just thought a thing as of twin paradox first twin's (traveller) has his same twin get older when he returns,correct?
so that means clock of first twin is running slow correct?
now then the first twin must see his twin faster in time when he was traveling so second twin sees him slow does it mean that if we get close to speed of light outer world seems to go faster and for the outer world we seem to get slower ?

I only brought up the Twin Paradox to illustrate that for two clocks that start out at the same place with the same time on them and one of them is moved to another place, there is no guarantee that the two clocks will still have the same time on them.

When any two clocks are in relative motion, they will keep different time. Each one will see, measure, and conclude that the other clock is ticking slower than itself. It's reciprocal, just like relative speed is. Whatever speed I see you traveling at is exactly the same speed you see me traveling at. Whatever slowed down rate I see your clock ticking at is exactly the same slowed down rate you see my clock ticking at. In Special Relativity, nobody moving at a constant speed ever sees any other clock ticking faster than their own, they're all always ticking slower.

 

[harrylin]

Because I have designed the system that way.

The physical position in space of the clock relative to Sensor A and Sensor B can help, but is not critical. Although it makes sense to have the clock equidistant.

As you’ve said, what is important is the process for sending information from A to the clock takes exactly the same time as the process to send information from B to the clock. So I have made the assumption that both processes have been calibrated and will take the same time, which is straight forward enough to do.

Just to add, this type of error exists inherently in ANY type of measuring system, as it is not possible to detect anything 'instantaneously'.
[..]

No information can travel greater than c, so there will always be a delay and hence a potential error in any system of two or more detections.

The process of detecting a light source reflected from a mirror is no different in that respect.

[..]
As I am using just one clock, this is no longer an issue. Problem Solved :)

Indeed, there is no problem to obtain a pure measurement of a round trip speed (using one reference clock and one reference ruler), either with light signals or cables.

Note that these are both electromagnetic signals, but according to relativity theory it doesn't matter what you use: you can always pretend that the one-way speeds wrt *your* choice of inertial reference system are the same in both directions.

And with equal success, you can choose another inertial system that is moving wrt the first, and pretend that the one-way speeds are the same in all directions wrt that system.

There have been almost endless discussions about exactly the same in other threads that also did not have that topic - for example from post 189 of:
https://www.physicsforums.com/showthread.php?t=461266&page=12

Cheers,
Harald

 

[rede96]

There have been almost endless discussions about exactly the same in other threads that also did not have that topic

Hi harrylin, thanks for the link and point taken. :)

 

[ghwellsjr]

Hi harrylin, thanks for the link and point taken. :)

Yes, harrylin, all your points are well taken and exactly right.

Let's go over them one by one.

Indeed, there is no problem to obtain a pure measurement of a round trip speed (using one reference clock and one reference ruler), either with light signals or cables.

This is what I have been saying. You can measure the round trip speed of light and it's no better or worse than cables.

Note that these are both electromagnetic signals, but according to relativity theory it doesn't matter what you use: you can always pretend that the one-way speeds wrt *your* choice of inertial reference system are the same in both directions.

Yes, note that it's an arbitrary choice to define the time intervals for each direction of the measurement to be equal.

And with equal success, you can choose another inertial system that is moving wrt the first, and pretend that the one-way speeds are the same in all directions wrt that system.

And if you make that arbitrary choice, the two time intervals for each direction of the measurement will not be equal.

Whichever choice you make, it is purely arbitrary, and it is not a measurement.

There have been almost endless discussions about exactly the same in other threads that also did not have that topic - for example from post 189 of:
https://www.physicsforums.com/showthread.php?t=461266&page=12

Yes, some people never understand this simple concept, no matter how hard we try to explain it to them.

Cheers,
Harald

Rede96, do you understand what harrylin is saying?

 

[rede96]

Yes, some people never understand this simple concept, no matter how hard we try to explain it to them.

Rede96, do you understand what harrylin is saying?

To be honest not fully no.(Sorry!) The one bit that is confusing me is where the 'round trip' comes from in my experiment.

If I measure a light source between two points, where is the round trip? Maybe if I get that, the rest will follow.

 

[harrylin]

To be honest not fully no.(Sorry!) The one bit that is confusing me is where the 'round trip' comes from in my experiment.

If I measure a light source between two points, where is the round trip? Maybe if I get that, the rest will follow.

I think that it has been said before, but Einstein answered it rather well in section I of his first 1905 paper, http://www.fourmilab.ch/etexts/einstein/specrel/www/ .

In summary, you need to synchronise your distant clock in order to do a one-way measurement; and according to SR, whatever method you use to do that, you'll always obtain the same result, based on your assumptions.

The common method is to use radio signals, and the standard assumption is to declare your system to be "in rest". Effectively what you do then, is to read the distant clock time instead of reflecting the signal with a mirror at that point. But first you have to set it to the "correct" time. If you try to be as precise as possible, then you will set that clock at <distance> times <roundtrip speed> later than the time of emission. That doesn't add any new information. It's merely you defining half of the round trip time as "one-way time", and then you "measure" the one-way speed of light value that you had defined yourself.

As you appeared to have already explained that yourself, so it's unclear to me what is not clear to you. Perhaps what is needed, is that you do a little exercise: calculate what you will obtain if you assume that your whole system is in motion. You should then verify that with that assumption, everything also works out although the one-way speed wrt you is different in different directions.

Regards,
Harald

 

[rede96]

In summary, you need to synchronise your distant clock in order to do a one-way measurement; and according to SR, whatever method you use to do that, you'll always obtain the same result, based on your assumptions.

But I am not using a 'distant clock'. I only have one clock. So I do not need to do any synchronisation.

What I have are two sensors that will detect a light source as it passes them. Each time a sensor is triggered, it sends a signal to my one clock, which acts as a lap timer, starting with the first signal and stopping with the second.

As long as I know how long it would take for the signal for each sensor to reach my clock, I can work out the elapsed time for the light beam to pass between my two sensors. No return trip.

Timing the duration of a light beam passing my two sensors is no different to me than timing a spaceship passing them for example.

The only difference with a spaceship is that I can get a different time depending on what direction the same ship is moving relative to me.

However, that cannot happen with light, as the speed of light is the same for all observers.

As you appeared to have already explained that yourself, so it's unclear to me what is not clear to you. Perhaps what is needed, is that you do a little exercise: calculate what you will obtain if you assume that your whole system is in motion. You should then verify that with that assumption, everything also works out although the one-way speed wrt you is different in different directions.

As the speed of light is the same for all inertial frames, I would say that I have to measure the speed of light to be c, no matter what direction I was moving in.