PDA

View Full Version : Absolute Velocity Measurement



rodin
2010-Mar-03, 12:46 PM
According to mainstream science is it possible to measure absolute velocity in a vacuum? (I think it is)

grant hutchison
2010-Mar-03, 01:03 PM
It isn't. Velocity is always relative to something else.

Grant Hutchison

tusenfem
2010-Mar-03, 01:04 PM
There is no "absolute" velocity, it is always measured with respect to something.
However, you may be referring to the fact that for all observers the speed of light is the same.

rodin
2010-Mar-03, 01:07 PM
Would this not work?

http://i664.photobucket.com/albums/vv9/ContrarianThinker/Absolutespeedmeasurement.png?t=1267619551

hhEb09'1
2010-Mar-03, 01:14 PM
Would this not work?

http://i664.photobucket.com/albums/vv9/ContrarianThinker/Absolutespeedmeasurement.png?t=1267619551It would not work, that's the point of special/general relativity.

The diagram is a monochromatic light source between two frequency detectors.

Ken G
2010-Mar-03, 01:23 PM
Yes, the inability for that setup to work was essentially the outcome of the "Michelson-Morley experiment."

rodin
2010-Mar-03, 01:25 PM
It would not work, that's the point of special/general relativity.

The diagram is a monochromatic light source between two frequency detectors.

yes

And if the source is moving should not the frequency of emitted radiation be Doppler shifted? Blue in the direction of travel and red opposite? And should not the frequency detectors detect this difference? And should not the difference enable us to calculate the absolute velocity?

Grumbel
2010-Mar-03, 01:48 PM
According to mainstream science is it possible to measure absolute velocity in a vacuum? (I think it is)
You could measure the average velocity of all the stuff around you (stars, galaxies, etc.) and compare your speed to that average and thus get an "absolute" velocity. If that absolute velocity has any special meaning or is just a pointless number, no idea.

Ken G
2010-Mar-03, 02:03 PM
And if the source is moving should not the frequency of emitted radiation be Doppler shifted? Blue in the direction of travel and red opposite? And should not the frequency detectors detect this difference? And should not the difference enable us to calculate the absolute velocity?No to all questions. What is not clear is whether you are imagining these things would happen at the second order in v/c, or first order. If you think it would happen at the level of the "first order Doppler shift", like a cop uses to detect your car's speed, then you are simply overlooking the fact that the source is not moving relative to the detectors, compensating for any first-order Doppler shift (a cop moving at the same speed as your car gets a zero reading). If you think it would happen at second order in v/c, then you are imagining an aether (like air for sound waves), and then you could tell the movement relative to the aether (as you could for sound in air). That was also the expectation of the Michelson-Morley experiment-- when it didn't turn out that way, the Nobel prize was earned, and relativity was born.

rodin
2010-Mar-03, 03:21 PM
No to all questions. What is not clear is whether you are imagining these things would happen at the second order in v/c, or first order. If you think it would happen at the level of the "first order Doppler shift", like a cop uses to detect your car's speed, then you are simply overlooking the fact that the source is not moving relative to the detectors, compensating for any first-order Doppler shift (a cop moving at the same speed as your car gets a zero reading). If you think it would happen at second order in v/c, then you are imagining an aether (like air for sound waves), and then you could tell the movement relative to the aether (as you could for sound in air). That was also the expectation of the Michelson-Morley experiment-- when it didn't turn out that way, the Nobel prize was earned, and relativity was born.

Michelson Morley involved sending light out then back at right angles using a 50% splitting mirror. No difference is to be expected in a vacuum because all Doppler effects cancel out ie if the light was blueshifted in one direction it would be redshifted by a similar amount coming back.

The experiment described here would measure light frequency directly both fore and aft of the source.

Has this been tried?

hhEb09'1
2010-Mar-03, 03:26 PM
Michelson Morley involved sending light out then back at right angles using a 50% splitting mirror. No difference is to be expected in a vacuum because all Doppler effects cancel out ie if the light was blueshifted in one direction it would be redshifted by a similar amount coming back.

The experiment described here would measure light frequency directly both fore and aft of the source.Well, that does sound like you are talking about first order effects. The shift due to the movement of the source is cancelled out by the shift due to the movement of the detector, at that level.

rodin
2010-Mar-03, 06:01 PM
Well, that does sound like you are talking about first order effects. The shift due to the movement of the source is cancelled out by the shift due to the movement of the detector, at that level.

A frequency detector measures waves per second. It takes t for the light to travel from the source to either detector in this diagram if c is constant irrespective of the motion of the source and the number of wavelengths per t is different forward and back. Simply count them.

http://i664.photobucket.com/albums/vv9/ContrarianThinker/mono.png?t=1267638942

Alternatively lightspeed is not constant in a vacuum

astromark
2010-Mar-03, 06:35 PM
As to the OP... As you know... Light speed is a constant. In the vacuum of interstellar space the only method of defining the velocity of any 'target' is by reference to 'other' things. Which is fraught with error, because everything has motion. After all. What is still... ? nothing.
This experiment you show us has some merit. How would you propose to measure any objects velocity ? Text book theory is not real science, is it ?

Strange
2010-Mar-03, 07:07 PM
The experiment described here would measure light frequency directly both fore and aft of the source.

Has this been tried?

Interesting question. The reason the M-M experiment (and similar) are able to measure very small changes to a high degree of accuracy is because they rely on interferometry. This makes it possible to detect changes of a fraction of a wavelength. I suspect (I don't know) it might be difficult to measure to similar accuracy with independent detectors.

Ken G
2010-Mar-03, 08:31 PM
The experiment described here would measure light frequency directly both fore and aft of the source.

Has this been tried?Are you seriously asking if a much simpler experiment than Michelson-Morley was tried? Michelson-Morley does all the same things, only with much higher precision, by using the interference of light instead of just a frequency measurement. It's a far more sensitive measurement of all the same physics as your apparatus, and it got the same null result that you would get, but to higher precision.

Ken G
2010-Mar-03, 08:34 PM
A frequency detector measures waves per second. It takes t for the light to travel from the source to either detector in this diagram if c is constant irrespective of the motion of the source and the number of wavelengths per t is different forward and back. Simply count them.
It sounds to me like you are mixing two frames, one in which there is a frequency difference, and another in which it takes t for the light to travel in either direction. There is no frame where both of those statements are true. Most importantly, in the frame of the apparatus, where the statement about t is correct, there is no frequency shift at all, and in the frame where the apparatus is moving, there is both a frequency shift and a different time for the two pulses to travel (because the detectors are moving in that frame), and these compensate to agree with what you get in the apparatus frame (i.e., the same number of detected pulses either way). Interestingly, if you just use that statement to try and calculate the frequency shift you observe, given that you see the light move at c, it only works to first order in v/c-- to get it to all orders, you also have to throw in time dilation, and we're into the heart of relativity.

ShinAce
2010-Mar-03, 08:43 PM
Was the Michelson-Morley interferometer really that precise? On the order of a fringe? Or was it not an experimented repeated many times to get the accurate results?

Just a tag-a-long question, for those wondering.

Ken G
2010-Mar-03, 08:48 PM
Because it was interferometry, it was automatically precise to the scale of a fringe. That is pretty much the defining characteristic of interferometry-- you let nature do its interfering before you even measure anything, so you are letting nature do the precise work for you.

ShinAce
2010-Mar-03, 09:28 PM
Thanks Ken. Might I add "cool!".

Jens
2010-Mar-04, 01:43 AM
yes

And if the source is moving should not the frequency of emitted radiation be Doppler shifted? Blue in the direction of travel and red opposite? And should not the frequency detectors detect this difference? And should not the difference enable us to calculate the absolute velocity?

Well you would get a shift, but you wouldn't know if it's because the object is moving or whether the detectors are moving. How can you know whether the detectors are moving or not?

forrest noble
2010-Mar-04, 02:51 AM
Rodin,

(edit out this paragraph since it misinterprets the model) The problem with your model, I think, is that you are assuming your detectors are stationary with a body moving between them. Change your perspective by making the body in the middle "stationary" and move the detectors (Jens et. al.) It would seem that you would get the same results that you propose but also no "absolute velocity" determination based upon their relative motions can be made.

If you are fond of absolute velocities I would suggest putting many clocks on bodies with relative motions to each other in a given field. By using Lorentz transforms, which are based upon a background field, the clock with the fastest passing of time (the least time dilation) could be made the preferred reference frame(s) and the others the dilated moving objects in a background-field scenario, by defining them so.

The present paradigm might shift someday if a physical background field such as dark matter, gravitons, Higgs particles, Planck particles, quantum foam, etc. is found.

If there was a background field which included any of the above examples or new discovered entity(s), as in Lorentz's background-field (known as aether at that time) model, the fastest passing of time would accordingly occur in any location within the field volume where the objects motion at that location would be the same as the background field's motion at that point.

Until or if such a paradigm shift ever occurs both SR and GR assert that there is no preferred reference frame for judging absolute motion/velocity. However some might argue that this is only a matter of perspective and definition. Accordingly one could define a preferred reference frame, within a given field, as the one with the least time dilation within the considered volume. In practice we use a "preferred reference frame" of the Earth's surface all the time even though it is not the frame of least time dilation in our local but it is a perspective that all can relate to. No absolute frames is a difficult concept for many to fathom.

AstroRockHunter
2010-Mar-04, 02:51 AM
Well you would get a shift, but you wouldn't know if it's because the object is moving or whether the detectors are moving. How can you know whether the detectors are moving or not?
But remember, in rodin's diagram, the light source and the detectors are moving in lock-step. There is no relative movement between them, thus, no frequency shift.

Unless, of course, I'm horribly mistaken. It's been known to happen.:lol:

ETA: Wasn't this one of Einstein's thought experiments that helped him come up with SR in the first place?

forrest noble
2010-Mar-04, 03:24 AM
AstroRockHunter,


But remember, in rodin's diagram, the light source and the detectors are moving in lock-step. There is no relative movement between them, thus, no frequency shift........

I see what you mean and edited my last comment accordingly. I misinterpreted the model similar to Jen's vision. I believe you are right. If there is no relative motion there could by no Doppler shifting to detect.

Rodin,

In a strong gravitational field you might be able to detect a gravitational redshift, in the model, when moving away from the gravitational body at a fast-enough speed. This effect has sometimes been called an Einstein redshift. But still, based upon SR and GR perspectives, the meaning of such a shift if it existed would still not necessarily imply any preferred/ absolute frame as a velocity reference point.

StupendousMan
2010-Mar-04, 02:48 PM
Was the Michelson-Morley interferometer really that precise? On the order of a fringe? Or was it not an experimented repeated many times to get the accurate results?

Just a tag-a-long question, for those wondering.

You can read the entire paper for yourself at

http://spiff.rit.edu/classes/phys314/lectures/mm/mm.html

and see my simplified explanation of it at

http://spiff.rit.edu/classes/phys314/lectures/mm_result/mm_result.html

At the end of that page is a figure from the MM paper
showing the precision of the measurements. The authors
estimated that they were able to measure the difference
in path length to about 0.01*lambda, where lambda was
around 550 nm.

Not too shabby.

rodin
2010-Mar-06, 05:44 PM
But remember, in rodin's diagram, the light source and the detectors are moving in lock-step. There is no relative movement between them, thus, no frequency shift.

The situation you describe is similar to someone bursting a paper bag in the middle of a moving aircraft. Quite apart from what effect this would have on the passengers the microphones at each end of the fuselage would record the same waveform ie zero Doppler shift. However lightspeed is supposed to be invariant. Therefor it should exhibit Doppler shift and a frequency detector should show this. If however it did not show Doppler shift the conclusion in the first instance would be that lightspeed relative to a source in a vacuum may well be constant, but that absolute lightspeed varies.

Are we sure some light is not in fact moving through space at c + or - v?

neilzero
2010-Mar-06, 05:51 PM
In original Star Trek, Captain Kirk ordered "all ahead stop" when the Enterprise was close to a nebula. I wonder what Scotty thought that meant, as the Enterprise would accelerate toward the nebula at zero velocity with respect to the nebula? Reducing thrust to zero would cause the Enterprise to continue on the orbit it established at the instant power was cut.
Still it does seem stationary, is possible if we travel rapidly in a path than negates the motion of the nebula, the galaxy and the galactic group? This would be very fast with respect to most near by objects. Neil

rodin
2010-Mar-06, 05:51 PM
You can read the entire paper for yourself at

http://spiff.rit.edu/classes/phys314/lectures/mm/mm.html

and see my simplified explanation of it at

http://spiff.rit.edu/classes/phys314/lectures/mm_result/mm_result.html

At the end of that page is a figure from the MM paper
showing the precision of the measurements. The authors
estimated that they were able to measure the difference
in path length to about 0.01*lambda, where lambda was
around 550 nm.

Not too shabby.

It was an excellent experiment but it could never detect red/blue Doppler shift nor (as I understand it) was it expected to. They were looking for ether drag ie a velocity displacement. The reason no Doppler shift could be observed is because it is a 'there and back again' setup where any shift outgoing would be exactly balanced by a counter shift returning.

Unless I am mistaken, not unheard of.

rodin
2010-Mar-06, 06:00 PM
It sounds to me like you are mixing two frames, one in which there is a frequency difference, and another in which it takes t for the light to travel in either direction.

Ah I see your point - I think...

In the time it takes for light to leave the source and hit the detector the light origin has moved towards (or away from) the source?

Jeff Root
2010-Mar-06, 07:57 PM
The detectors in post #4 would detect accelerations to the left or right
as Doppler shifts, but the Doppler shifts would return to zero when the
acceleration ended. It's an acceleration measurer, not a speed measurer.

-- Jeff, in Minneapolis

Ken G
2010-Mar-06, 10:59 PM
The detectors in post #4 would detect accelerations to the left or right
as Doppler shifts, but the Doppler shifts would return to zero when the
acceleration ended. It's an acceleration measurer, not a speed measurer.

That's a better way to frame it, it's a "proper acceleration" detector.

rodin
2010-Mar-06, 11:23 PM
That's a better way to frame it, it's a "proper acceleration" detector.

If the universe is expanding are we accelerating, decelerating or moving at constant velocity?

astromark
2010-Mar-07, 06:34 AM
Multiple methods of measurement have confirmed independently that the Universe is eccelorating away at a ever increasing rate. I do not understand how or why you do not see this as fact.
If you know some information that might shed light on this mainstream view as incorrect would you please state it... Mark.

Ken G
2010-Mar-07, 02:02 PM
If the universe is expanding are we accelerating, decelerating or moving at constant velocity?
That's actually why I said your device was a "proper acceleration" detector, not just an "acceleration" detector, because the latter term is a lot more vague. The "acceleration of the universe" is not a proper acceleration and would not be detected by that device. The universal expansion just means distances are increasing, and is not normally framed as a "velocity" that could be "accelerated", it is just a rate of change of scale factor. Sometimes this is pictured by saying that "space itself is expanding at an accelerating rate", rather than by anything moving, or accelerating, through space.

rodin
2010-Mar-07, 03:04 PM
That's actually why I said your device was a "proper acceleration" detector, not just an "acceleration" detector, because the latter term is a lot more vague. The "acceleration of the universe" is not a proper acceleration and would not be detected by that device. The universal expansion just means distances are increasing, and is not normally framed as a "velocity" that could be "accelerated", it is just a rate of change of scale factor. Sometimes this is pictured by saying that "space itself is expanding at an accelerating rate", rather than by anything moving, or accelerating, through space.

As the earth spins on its axis as it also orbits the Sun it must accelerate and decelerate - of course the galactic rotation would also come into play and any larger scale motion.

We could at least look for shifts there? Just to test the theory that a moving source generates intrinsically Doppler shifted light rather than it simply being a function of observer-source relative movement?

Imposing a condition of invariance of vacuum lightspeed implies intrinsic frequency shifting from a moving EM source does it not?

Ken G
2010-Mar-07, 04:24 PM
As the earth spins on its axis as it also orbits the Sun it must accelerate and decelerate - of course the galactic rotation would also come into play and any larger scale motion.
The "acceleration" of an orbit is not proper acceleration and would also not show up in your device.

rodin
2010-Mar-07, 07:06 PM
The "acceleration" of an orbit is not proper acceleration and would also not show up in your device.

Ah I don't mean acceleration as per towards the centre during rotation. I mean the net displacement in space in a particular direction. Velocity can be expressed in a 3D co-ordinate system.

For example at midday orbital velocity tangential to the orbit would be at a minimum (I think - planets rotate in the same sense as they orbit - apart from Venus - this from memory maybe I have it back-to-front)

Ken G
2010-Mar-07, 07:47 PM
A rotating body does have a proper acceleration toward the center of the body. Your device would detect that, but it's still instantaneous acceleration, even when there is rotation involved.

rodin
2010-Mar-17, 10:15 PM
Regarding this idea

http://i664.photobucket.com/albums/vv9/ContrarianThinker/Absolutespeedmeasurement.png?t=1267619551

Would it be possible to construct one in the following way?

Find a suitable excitation frequency of an electron level transition in a suitable element. Tune (or choose) a coherent monochromatic light source so that maximum absorption is observed at one detector. Then check to see if the same level of absorption is observed at the other detector.

Now, if light is speed invariant, one direction must be frequency shifted relative to the other, therefore the peak should move. If it did not, and the resolution of the experiment is sufficient, this would show that lightspeed was not constant surely?

cjl
2010-Mar-17, 10:32 PM
Regarding this idea

http://i664.photobucket.com/albums/vv9/ContrarianThinker/Absolutespeedmeasurement.png?t=1267619551

Would it be possible to construct one in the following way?

Find a suitable excitation frequency of an electron level transition in a suitable element. Tune (or choose) a coherent monochromatic light source so that maximum absorption is observed at one detector. Then check to see if the same level of absorption is observed at the other detector.

Now, if light is speed invariant, one direction must be frequency shifted relative to the other, therefore the peak should move. If it did not, and the resolution of the experiment is sufficient, this would show that lightspeed was not constant surely?
I think you're misunderstanding the concept that lightspeed is invariant. The idea (which has been quite thoroughly demonstrated by now) is that light will always measure as having a constant speed relative to any inertial frame. If you measure the speed of light while you are driving down the road, it will still seem to travel at c relative to your car in both directions. If you stop and drive the other way, this still holds true. If you got in a spaceship and left relative to earth with a velocity of 0.8c, and then you shined a flashlight onboard your ship, you would still measure light's speed as c relative to the ship, even though you are moving at .8c relative to earth.

That is also why there is no single "absolute" frame. Since the speed of light always measures at C relative to any inertial observer, you cannot infer anything about the observer's motion through the measurement of the speed of light. This is unlike any wave that propagates through a medium - a submarine could determine its speed by measuring the speed of sound in several directions through the water around it, because the sound will travel at a speed that is constant relative to the water, but not relative to any other inertial frame.

rodin
2010-Mar-17, 11:03 PM
I think you're misunderstanding the concept that lightspeed is invariant. The idea (which has been quite thoroughly demonstrated by now) is that light will always measure as having a constant speed relative to any inertial frame. If you measure the speed of light while you are driving down the road, it will still seem to travel at c relative to your car in both directions. If you stop and drive the other way, this still holds true. If you got in a spaceship and left relative to earth with a velocity of 0.8c, and then you shined a flashlight onboard your ship, you would still measure light's speed as c relative to the ship, even though you are moving at .8c relative to earth.

That is also why there is no single "absolute" frame. Since the speed of light always measures at C relative to any inertial observer, you cannot infer anything about the observer's motion through the measurement of the speed of light. This is unlike any wave that propagates through a medium - a submarine could determine its speed by measuring the speed of sound in several directions through the water around it, because the sound will travel at a speed that is constant relative to the water, but not relative to any other inertial frame.

OK - that's what I would expect - but light shone forwards from this 0.8c spaceship would appear to be moving at 1.8c from a stationary planet dead ahead would it not?

Van Rijn
2010-Mar-17, 11:08 PM
OK - that's what I would expect - but light shone forwards from this 0.8c spaceship would appear to be moving at 1.8c from a stationary planet dead ahead would it not?

No, the light would appear to be moving at 1 C.

rodin
2010-Mar-17, 11:26 PM
No, the light would appear to be moving at 1 C.

How come? If the light left the vehicle at 1c relative to a vehicle travelling at 0.8c, and the planet is stationary?

Van Rijn
2010-Mar-17, 11:49 PM
How come? If the light left the vehicle at 1c relative to a vehicle travelling at 0.8c, and the planet is stationary?

Short answer: Because that's the way the universe works (or at least, from the available evidence, that's how it works). (See here (http://www.physicsmyths.org.uk/lightspeed.htm), for instance.)

By the way, "stationary" has no real meaning, though you can consider a planet arbitrarily "stationary" for calculation purposes.

pzkpfw
2010-Mar-17, 11:58 PM
rodin: this thread covers similar ground: http://www.bautforum.com/space-astronomy-questions-answers/101992-light-speed-illusion.html

(Two things both moving 0.6c away from a central thing, do not see each other as going 1.2c away from each other).

I don't want that discussion to be moved into this thread, I'm just making the link as it's essentially the same issue.

undidly
2010-Mar-18, 12:09 AM
OK - that's what I would expect - but light shone forwards from this 0.8c spaceship would appear to be moving at 1.8c from a stationary planet dead ahead would it not?

Light is NOT shot from the source like the bullet from a gun.
Light is RELEASED from the source and "swims" away at its own speed.

cjl
2010-Mar-18, 12:34 AM
OK - that's what I would expect - but light shone forwards from this 0.8c spaceship would appear to be moving at 1.8c from a stationary planet dead ahead would it not?
Nope. No matter what frame it was emitted in, and no matter what frame it was observed in (so long as the observing frame is an inertial one), light will always appear to travel at c.

rodin
2010-Mar-18, 10:29 PM
Short answer: Because that's the way the universe works (or at least, from the available evidence, that's how it works). (See here (http://www.physicsmyths.org.uk/lightspeed.htm), for instance.)

By the way, "stationary" has no real meaning, though you can consider a planet arbitrarily "stationary" for calculation purposes.

From your link


The only reason behind the existence of the Special Theory of Relativity is the experimental fact that the speed of light in a vacuum is independent of any motion of the source or receiver.


OK so from a moving source the light must be inherently doppler shifted therefore detectable by a frequency detector

Van Rijn
2010-Mar-18, 11:13 PM
OK so from a moving source the light must be inherently doppler shifted therefore detectable by a frequency detector

What is detectable?

cjl
2010-Mar-18, 11:56 PM
From your link



OK so from a moving source the light must be inherently doppler shifted therefore detectable by a frequency detector

If the light source is moving with some velocity relative to the detector, there will be a doppler shift. All it tells you is the velocity of the light source relative to the detector though - it says nothing about any sort of "absolute" velocity.

rodin
2010-Mar-19, 02:06 PM
What is detectable?

EM Frequency or wavelength

I suggested earlier using a detector based on the principle of absorption spectroscopy

rodin
2010-Mar-19, 02:15 PM
Correct me if I am wrong but the way I see it EM radiation in a vacuum either adheres to c or does not.

If it does then relative to a moving source it should behave like sound waves in air. The speed of sound is constant for a particular pressure absent wind and we observe a Doppler shift relative to a moving source.

Or the speed of light is unimpeded by the vacuum of space, and therefore emanates from a moving source at c + whatever velocity offset the moving frame has from an observer's point of view

rodin
2010-Mar-19, 02:19 PM
I will digest this

http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html

later this evening and respond further. Thanks for the link

macaw
2010-Mar-25, 11:56 PM
You can read the entire paper for yourself at

http://spiff.rit.edu/classes/phys314/lectures/mm/mm.html

and see my simplified explanation of it at

http://spiff.rit.edu/classes/phys314/lectures/mm_result/mm_result.html

At the end of that page is a figure from the MM paper
showing the precision of the measurements. The authors
estimated that they were able to measure the difference
in path length to about 0.01*lambda, where lambda was
around 550 nm.

Not too shabby.

Excellent explanation, Michael. One of the best I've seen. Congrats!