Measuring velocity w/o a frame of reference

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Measuring velocity w/o a frame of reference

#1  Postby LjSpike » Feb 09, 2017 9:13 am

I was wondering about ways of determining the actual velocity of an object, or at least, which object is moving less... Now, its fairly commonly accepted that movement can only be determined relative to another object, but if you were to factor in the effect movement had on spacetime, or the effects that approaching the speed of light presents, couldn't you at least determine what direction your moving in...

I'll explain the experiment I thought up, as I see it being very unclear in that first paragraph. Lets say you can ignore the influence of gravity on spacetime, and can ignore interference caused by an observer. If you fire, lets say many muons, and you fired them in different directions, the muon which decays first is moving the slowest. The muons moving faster experience time slower, therefore last for "longer", which is an observable property. So, then you have the direction your whole system is moving in, if you then fire many muons in that direction, at varying speeds, the muons that again, decay first, could be concluded to being closest to being stationary. You wouldn't be able to know the exact speed of them, but could fairly accurately determine the direction and speed your experiment is moving in, and the accuracy would improve with the more muons you fire.

I suspect the increase of mass of a particle once it approaches the speed of light could also be used to measure speed, in a similar type of setup.

Obviously, its fairly hard to actually carry out this experiment, as you'd have to accurately fire muons, ensure you could accurately measure when they decayed, and would somehow have to determine the effect gravity is having on the experiment. Potentially it could be carried out on a larger scale, with atomic clocks fired into space? That setup would be quite costly though, and again, tricky to control the experiment.
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Re: Measuring velocity w/o a frame of reference

#2  Postby Thommo » Feb 09, 2017 9:21 am

That wouldn't work, the results of what you'd see are not those you suggest you would see.

If you fired muons in different directions you would not see them decay faster in one direction as opposed to another.
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Re: Measuring velocity w/o a frame of reference

#3  Postby DavidMcC » Feb 09, 2017 12:12 pm

LjSpike wrote:
I'll explain the experiment I thought up, as I see it being very unclear in that first paragraph. Lets say you can ignore the influence of gravity on spacetime, and can ignore interference caused by an observer. If you fire, lets say many muons, and you fired them in different directions, the muon which decays first is moving the slowest. The muons moving faster experience time slower, therefore last for "longer", which is an observable property. So, then you have the direction your whole system is moving in, if you then fire many muons in that direction, at varying speeds, the muons that again, decay first, could be concluded to being closest to being stationary. You wouldn't be able to know the exact speed of them, but could fairly accurately determine the direction and speed your experiment is moving in, and the accuracy would improve with the more muons you fire.

I suspect the increase of mass of a particle once it approaches the speed of light could also be used to measure speed, in a similar type of setup.

Obviously, its fairly hard to actually carry out this experiment, as you'd have to accurately fire muons, ensure you could accurately measure when they decayed, and would somehow have to determine the effect gravity is having on the experiment. Potentially it could be carried out on a larger scale, with atomic clocks fired into space? That setup would be quite costly though, and again, tricky to control the experiment.

The experiment is defective, partly because no particle (including muons) decays after an exact time in any frame of reference.
Also, particle masses cannot be used to determine absolute speed. You need to study special relativity better than you apparently have.
Furthemore, the very existence of an absolute frame of reference implies that there is a centre to the universe, which is not the case.
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Re: Measuring velocity w/o a frame of reference

#4  Postby LjSpike » Feb 09, 2017 6:13 pm

DavidMcC wrote:
LjSpike wrote:
I'll explain the experiment I thought up, as I see it being very unclear in that first paragraph. Lets say you can ignore the influence of gravity on spacetime, and can ignore interference caused by an observer. If you fire, lets say many muons, and you fired them in different directions, the muon which decays first is moving the slowest. The muons moving faster experience time slower, therefore last for "longer", which is an observable property. So, then you have the direction your whole system is moving in, if you then fire many muons in that direction, at varying speeds, the muons that again, decay first, could be concluded to being closest to being stationary. You wouldn't be able to know the exact speed of them, but could fairly accurately determine the direction and speed your experiment is moving in, and the accuracy would improve with the more muons you fire.

I suspect the increase of mass of a particle once it approaches the speed of light could also be used to measure speed, in a similar type of setup.

Obviously, its fairly hard to actually carry out this experiment, as you'd have to accurately fire muons, ensure you could accurately measure when they decayed, and would somehow have to determine the effect gravity is having on the experiment. Potentially it could be carried out on a larger scale, with atomic clocks fired into space? That setup would be quite costly though, and again, tricky to control the experiment.

The experiment is defective, partly because no particle (including muons) decays after an exact time in any frame of reference.
Also, particle masses cannot be used to determine absolute speed. You need to study special relativity better than you apparently have.
Furthemore, the very existence of an absolute frame of reference implies that there is a centre to the universe, which is not the case.


I admit, my special relativity knowledge isn't spectacular, however your final point of there not being a centre of the universe I would dispute. The big bang was a single infinitesimally small point, from which the universe expanded out of. That starting point would therefore be the centre of the universe?

Perhaps an alternative way of measuring, rather than the lifetime of muons, would be to launch atomic clocks in different directions? It would still be measuring the dilation of time.
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Re: Measuring velocity w/o a frame of reference

#5  Postby Thommo » Feb 09, 2017 7:19 pm

LjSpike wrote:I admit, my special relativity knowledge isn't spectacular, however your final point of there not being a centre of the universe I would dispute. The big bang was a single infinitesimally small point, from which the universe expanded out of. That starting point would therefore be the centre of the universe?


The big bang is not an expansion of material into space, it's an expansion of space. That "centre" starting point of a singularity expanded to be everywhere. Every point in spacetime has equal claim to be "that point" you refer to. Not that this actually affects whether your experiment works or not.

LjSpike wrote:Perhaps an alternative way of measuring, rather than the lifetime of muons, would be to launch atomic clocks in different directions? It would still be measuring the dilation of time.


The problem is not that the lifetime of muons is not fixed (you could produce a distribution for a large sample of muons to get extremely accurate averages). The problem is that there would be no differential time dilation in different directions. This actual problem (that what you predict is not what you'd observe) is equally true whatever method you try to use to measure it. Sending atomic clocks would produce the same result - no difference in dilation depending on angle.

This is one of the things that has been established by experiments such as the Michelson-Morley experiment. In short, what appears to be happening is that you are assuming that time dilation only works from a central "rest frame", but this is not the case, it is a phenomenon predicted by special relativity, which falls out of the assumption that there is no such rest frame. The amount of the dilation is given by the Lorentz factor:
Image
Where v is the velocity relative to any inertial frame, not the velocity relative to a specific, preferred inertial frame.

In terms of the muons (or clocks, or photons, or anything else you try and measure time with in different directions) you will always be measuring their velocity relative to your own to get v, and there are a number of ways you might not be thinking about that correctly, of which I would suspect that you are reasoning that if you are traveling at some appreciable fraction of the speed of light, c and send a muon off at a very high fraction of the speed of light, say 0.99c then there's a problem because it will be traveling faster than c in some other inertial frames. This is not the case, because vector addition of velocity in SR is not linear (i.e. u = v + u'), but instead uses the formula:-
Image

From the point of view of the muons (clocks, etc.) in various frames, they are not time dilated, in their own frame, but instead they have traveled less far than you as an observer in another frame think they have. Since space is also contracted by a factor of γ.
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Re: Measuring velocity w/o a frame of reference

#6  Postby DavidMcC » Feb 09, 2017 7:35 pm

LjSpike wrote:
DavidMcC wrote:
LjSpike wrote:
I'll explain the experiment I thought up, as I see it being very unclear in that first paragraph. Lets say you can ignore the influence of gravity on spacetime, and can ignore interference caused by an observer. If you fire, lets say many muons, and you fired them in different directions, the muon which decays first is moving the slowest. The muons moving faster experience time slower, therefore last for "longer", which is an observable property. So, then you have the direction your whole system is moving in, if you then fire many muons in that direction, at varying speeds, the muons that again, decay first, could be concluded to being closest to being stationary. You wouldn't be able to know the exact speed of them, but could fairly accurately determine the direction and speed your experiment is moving in, and the accuracy would improve with the more muons you fire.

I suspect the increase of mass of a particle once it approaches the speed of light could also be used to measure speed, in a similar type of setup.

Obviously, its fairly hard to actually carry out this experiment, as you'd have to accurately fire muons, ensure you could accurately measure when they decayed, and would somehow have to determine the effect gravity is having on the experiment. Potentially it could be carried out on a larger scale, with atomic clocks fired into space? That setup would be quite costly though, and again, tricky to control the experiment.

The experiment is defective, partly because no particle (including muons) decays after an exact time in any frame of reference.
Also, particle masses cannot be used to determine absolute speed. You need to study special relativity better than you apparently have.
Furthemore, the very existence of an absolute frame of reference implies that there is a centre to the universe, which is not the case.


I admit, my special relativity knowledge isn't spectacular, however your final point of there not being a centre of the universe I would dispute. The big bang was a single infinitesimally small point, from which the universe expanded out of. That starting point would therefore be the centre of the universe?

Perhaps an alternative way of measuring, rather than the lifetime of muons, would be to launch atomic clocks in different directions? It would still be measuring the dilation of time.

Thommo has said it all above.
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Re: Measuring velocity w/o a frame of reference

#7  Postby Calilasseia » Feb 09, 2017 8:45 pm

What you would be observing, is the time-dilated decay of muons relative to you, not relative to any external frame of reference. Which would remain unchanged regardless of your motion.
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Re: Measuring velocity w/o a frame of reference

#8  Postby crank » Feb 09, 2017 9:25 pm

What we need is the best Keith Muon style speedometer we can find. One frame of reference that is always on hand is the cosmic background radiation. You would then just check it's spectrum to give you your velocity relative to it. What that all means in any fundamental sense is beyond me.
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Re: Measuring velocity w/o a frame of reference

#9  Postby LjSpike » Feb 10, 2017 7:27 am

Thommo wrote:
LjSpike wrote:I admit, my special relativity knowledge isn't spectacular, however your final point of there not being a centre of the universe I would dispute. The big bang was a single infinitesimally small point, from which the universe expanded out of. That starting point would therefore be the centre of the universe?


The big bang is not an expansion of material into space, it's an expansion of space. That "centre" starting point of a singularity expanded to be everywhere. Every point in spacetime has equal claim to be "that point" you refer to. Not that this actually affects whether your experiment works or not.

LjSpike wrote:Perhaps an alternative way of measuring, rather than the lifetime of muons, would be to launch atomic clocks in different directions? It would still be measuring the dilation of time.


The problem is not that the lifetime of muons is not fixed (you could produce a distribution for a large sample of muons to get extremely accurate averages). The problem is that there would be no differential time dilation in different directions. This actual problem (that what you predict is not what you'd observe) is equally true whatever method you try to use to measure it. Sending atomic clocks would produce the same result - no difference in dilation depending on angle.

This is one of the things that has been established by experiments such as the Michelson-Morley experiment. In short, what appears to be happening is that you are assuming that time dilation only works from a central "rest frame", but this is not the case, it is a phenomenon predicted by special relativity, which falls out of the assumption that there is no such rest frame. The amount of the dilation is given by the Lorentz factor:
Image
Where v is the velocity relative to any inertial frame, not the velocity relative to a specific, preferred inertial frame.

In terms of the muons (or clocks, or photons, or anything else you try and measure time with in different directions) you will always be measuring their velocity relative to your own to get v, and there are a number of ways you might not be thinking about that correctly, of which I would suspect that you are reasoning that if you are traveling at some appreciable fraction of the speed of light, c and send a muon off at a very high fraction of the speed of light, say 0.99c then there's a problem because it will be traveling faster than c in some other inertial frames. This is not the case, because vector addition of velocity in SR is not linear (i.e. u = v + u'), but instead uses the formula:-
Image

From the point of view of the muons (clocks, etc.) in various frames, they are not time dilated, in their own frame, but instead they have traveled less far than you as an observer in another frame think they have. Since space is also contracted by a factor of γ.

Well, I'll have to do a bit of reading up on those principles, however... Let me use a simple example:
There are two people, one is moving in a racecar incredibly fast, the other is stationary.
Both people view their own time as being unchanged. The person standing still, see's the race drivers watch as running slowly. The race driver see's the watch of the person standing still as being fast.

If that is true, which would concur with many examples and explanations of time dilation due to velocity, then the dilation couldn't be the persons v relative to your v. To the race driver, the person is moving past them incredibly quickly, and to the person, the race driver is moving past them equally quickly. They're time therefore would dilate equally, therefore the dilation would be unobservable, yet it has been observed by atomic clocks.
Another example, if my time dilates relative to other objects, there are other objects moving at all sorts of speeds (relative to me), therefore my time should dilate to every different amount, simultaneously. It does not though? I have a time, it appears unchanging to me, but it for sure is dilating by one specific amount. Just having anything as a frame of reference seems to bring up so many problems. Perhaps later today or tomorrow I'll draw up a diagram.

I cannot say that the Michelson-Morley experiment is unrelated, however I cannot say that it is necessarily totally related. Superficially it does seem incredibly similar, but it was working off presumptions of Aether theory. Additionally they were trying to measure speed, not time dilation. The velocity of whatever the object is being fired on has an effect on the final velocity, as seen on a larger scale: Mythbusters Ball Experiment
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Re: Measuring velocity w/o a frame of reference

#10  Postby Calilasseia » Feb 10, 2017 7:46 am

Wrong.

In your example, the racing car driver would see the stationary person's clock as running slow as well. That on its own calls into serious question your understanding of even the elementary concepts involved.
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Re: Measuring velocity w/o a frame of reference

#11  Postby Thommo » Feb 10, 2017 8:01 am

LjSpike wrote:Well, I'll have to do a bit of reading up on those principles, however... Let me use a simple example:
There are two people, one is moving in a racecar incredibly fast, the other is stationary.
Both people view their own time as being unchanged. The person standing still, see's the race drivers watch as running slowly. The race driver see's the watch of the person standing still as being fast.


The prediction of special relativity, which is where time dilation comes from is that each sees the other's clock running slow by a factor of γ. The race driver sees the watch of the spectator as running slow and the spectator sees the watch of the race driver as running slow.

LjSpike wrote:If that is true, which would concur with many examples and explanations of time dilation due to velocity, then the dilation couldn't be the persons v relative to your v. To the race driver, the person is moving past them incredibly quickly, and to the person, the race driver is moving past them equally quickly. They're time therefore would dilate equally, therefore the dilation would be unobservable, yet it has been observed by atomic clocks.


This isn't right, if you look at a standard discussion of time dilation in special relativity, such as the twin paradox, you find:-
https://en.wikipedia.org/wiki/Twin_paradox
In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect[1][2][3] and naive[4][5] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged more slowly.


Both clocks run slow, but if you change inertial frames (e.g. by acceleration) then there's an offset as well. I should be clear here, that perhaps it is this sort of thing that is giving you a misimpression - what they're referring to as "incorrect and naive" is the failure to account for a change of inertial frame. The understanding that the theory predicts that as each travels away from the other without accelerating means both clocks run slow is correct.

For experimental confirmation of time dilation you could look at something like the Hafele-Keating experiment.

I'm not sure what examples and explanations you have in mind that seem to say otherwise, perhaps you could link one?

LjSpike wrote:Another example, if my time dilates relative to other objects, there are other objects moving at all sorts of speeds (relative to me), therefore my time should dilate to every different amount, simultaneously. It does not though? I have a time, it appears unchanging to me, but it for sure is dilating by one specific amount.


You're always in the same frame of reference as yourself. The theory predicts that each of those other inertial frames will see your time as dilated by a different factor. It does not predict that you ever see your own time dilated.

LjSpike wrote:Just having anything as a frame of reference seems to bring up so many problems. Perhaps later today or tomorrow I'll draw up a diagram.


Sure, that might help clear this up.

LjSpike wrote:I cannot say that the Michelson-Morley experiment is unrelated, however I cannot say that it is necessarily totally related. Superficially it does seem incredibly similar, but it was working off presumptions of Aether theory. Additionally they were trying to measure speed, not time dilation. The velocity of whatever the object is being fired on has an effect on the final velocity, as seen on a larger scale: Mythbusters Ball Experiment


I'm sorry, I have no idea what you're trying to say here. Speed terms for both objects appear in both the Newtonian version of addition of velocities and in the Relativistic version of that formula. For that mythbusters clip where u' = -v, the answer to what you'll observe will be u = 0 in both cases anyway.
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Re: Measuring velocity w/o a frame of reference

#12  Postby DavidMcC » Feb 10, 2017 2:03 pm

LjSpike wrote:...
I cannot say that the Michelson-Morley experiment is unrelated, however I cannot say that it is necessarily totally related. Superficially it does seem incredibly similar, but it was working off presumptions of Aether theory. Additionally they were trying to measure speed, not time dilation. The velocity of whatever the object is being fired on has an effect on the final velocity, as seen on a larger scale: Mythbusters Ball Experiment

(My bold.)
Ironically, what the Michelson-Morley experiment showed is that no aether can be detected in such an experiment. Indeed, that is why Einstein proposed his special relativity - to explain the non-detectability of an aether.
Also, the Mythbusters ball experiment has nothing at all to do with relativity, if only because the ball velocities involved are extremely low relative to the speed of light.
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Re: Measuring velocity w/o a frame of reference

#13  Postby LjSpike » Feb 10, 2017 5:18 pm

Thommo wrote:
LjSpike wrote:Well, I'll have to do a bit of reading up on those principles, however... Let me use a simple example:
There are two people, one is moving in a racecar incredibly fast, the other is stationary.
Both people view their own time as being unchanged. The person standing still, see's the race drivers watch as running slowly. The race driver see's the watch of the person standing still as being fast.


The prediction of special relativity, which is where time dilation comes from is that each sees the other's clock running slow by a factor of γ. The race driver sees the watch of the spectator as running slow and the spectator sees the watch of the race driver as running slow.

LjSpike wrote:If that is true, which would concur with many examples and explanations of time dilation due to velocity, then the dilation couldn't be the persons v relative to your v. To the race driver, the person is moving past them incredibly quickly, and to the person, the race driver is moving past them equally quickly. They're time therefore would dilate equally, therefore the dilation would be unobservable, yet it has been observed by atomic clocks.


This isn't right, if you look at a standard discussion of time dilation in special relativity, such as the twin paradox, you find:-
https://en.wikipedia.org/wiki/Twin_paradox
In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect[1][2][3] and naive[4][5] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged more slowly.


Both clocks run slow, but if you change inertial frames (e.g. by acceleration) then there's an offset as well. I should be clear here, that perhaps it is this sort of thing that is giving you a misimpression - what they're referring to as "incorrect and naive" is the failure to account for a change of inertial frame. The understanding that the theory predicts that as each travels away from the other without accelerating means both clocks run slow is correct.

For experimental confirmation of time dilation you could look at something like the Hafele-Keating experiment.

I'm not sure what examples and explanations you have in mind that seem to say otherwise, perhaps you could link one?

LjSpike wrote:Another example, if my time dilates relative to other objects, there are other objects moving at all sorts of speeds (relative to me), therefore my time should dilate to every different amount, simultaneously. It does not though? I have a time, it appears unchanging to me, but it for sure is dilating by one specific amount.


You're always in the same frame of reference as yourself. The theory predicts that each of those other inertial frames will see your time as dilated by a different factor. It does not predict that you ever see your own time dilated.

LjSpike wrote:Just having anything as a frame of reference seems to bring up so many problems. Perhaps later today or tomorrow I'll draw up a diagram.


Sure, that might help clear this up.

LjSpike wrote:I cannot say that the Michelson-Morley experiment is unrelated, however I cannot say that it is necessarily totally related. Superficially it does seem incredibly similar, but it was working off presumptions of Aether theory. Additionally they were trying to measure speed, not time dilation. The velocity of whatever the object is being fired on has an effect on the final velocity, as seen on a larger scale: Mythbusters Ball Experiment


I'm sorry, I have no idea what you're trying to say here. Speed terms for both objects appear in both the Newtonian version of addition of velocities and in the Relativistic version of that formula. For that mythbusters clip where u' = -v, the answer to what you'll observe will be u = 0 in both cases anyway.



Thank you for useful correction and explanation. That makes it far more clear, that inertial frames (and acceleration?) causes said effect with the twin paradox, rather than speed/velocity.
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Re: Measuring velocity w/o a frame of reference

#14  Postby Thommo » Feb 10, 2017 9:27 pm

Yep! :thumbup:

The twin paradox is a paradox precisely because the time dilation for comoving (inertial) frames is symmetrical. Both frames will "see" the clocks of the other as running slow by a factor of γ.However, if one person travels in multiple frames, by undergoing acceleration then that symmetry is broken.

This effect isn't strictly dependent on acceleration. You can imagine instead scenarios in which one spaceship traveling away from Earth synchronises a clock with another spaceship traveling towards Earth at an equal and opposite speed as they pass to also get a "twin paradox" that doesn't involve acceleration, but still involves a change of inertial frame that breaks symmetry, which is the essential part.
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