# Laser optic velocity detector. **URL:** \ **Category:** Factual Questions\ **Created:** [April 5, 2018, 6:19am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856 "2018-04-05T06:19:10Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [April 5, 2018, 6:19am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/1 "2018-04-05T06:19:10Z")
I know there are laser gyroscopes to detect rotation. Are there similar devices to detect linear movement? ---
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [April 5, 2018, 6:23am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/2 "2018-04-05T06:23:15Z")
Pretty sure cops have laser speed detectors. ---
**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [April 5, 2018, 6:53am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/3 "2018-04-05T06:53:31Z")
Similar devices. A laser gyroscope is a self contained solid state device. It detects the movement of itself. Outputs that movement. I am asking if there is a device with no moving parts, that detects itself moving in a linear direction and outputs that information. It does not need to detect another external thing moving relative to itself to detect that it is moving. That would at least fit in a breadbox. Have no moving parts. ---
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [April 5, 2018, 7:28am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/4 "2018-04-05T07:28:49Z")
I don’t thing a laser speed detector would have moving parts. But I don’t see why it couldn’t be put in a breadbox and determine its relative speed to something else. That’s what it is designed to do. Am I driving 100mph towards the detector, or is it coming 100 mph towards me? Same thing, right? ---
**Author:** ![voltaire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voltaire/32/313_2.png) [@voltaire](https://boards.straightdope.com/u/voltaire)\ **Post date:** [April 5, 2018, 7:30am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/5 "2018-04-05T07:30:00Z")
Does a cordless laser mouse fit your needs? ---
**Author:** ![smithsb](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smithsb/32/12107_2.png) [@smithsb](https://boards.straightdope.com/u/smithsb)\ **Post date:** [April 5, 2018, 7:48am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/6 "2018-04-05T07:48:58Z")
Are we talking an infrared beam switch like for intrusion/motion detection or stopping a garage door? ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [April 5, 2018, 12:41pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/7 "2018-04-05T12:41:29Z")
Rotation is a form of acceleration, so it can be measured even without an outside reference. Linear motion at a constant velocity features no acceleration and can’t be measured without reference to something else. If the velocity wasn’t constant, i.e. some acceleration, an accelerometer can measure it. By the way, this is a basic premise of relativity, i.e. all inertial frames of reference are equivalent. Acceleration of any sort (linear or rotation) would be non-inertial and measurable. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 5, 2018, 1:30pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/8 "2018-04-05T13:30:34Z")
> [@snfaulkner](#): > > I don’t thing a laser speed detector would have moving parts. But I don’t see why it couldn’t be put in a breadbox and determine its relative speed to something else. That’s what it is designed to do. Am I driving 100mph towards the detector, or is it coming 100 mph towards me? Same thing, right? You’re thinking of a device that measures its velocity relative to something else. OP is thinking of a device that measures its own velocity relative to, I guess, its velocity a short time ago. The [ring laser gyro](https://en.wikipedia.org/wiki/Ring_laser_gyroscope) the OP is referring to doesn’t reference any outside device. Laser light propagates around a circuit (composed of a sequence of mirrors) in both directions. If the framework holding the mirrors is stationary, the two beams of light produce a stationary interference fringe pattern, and a detector watching for those fringes measures a frequency of zero. If the framework holding the mirrors is rotating, then the time of flight in one direction around the circuit is longer than the time of flight in the other direction; the interference fringe pattern moves, and the detector measures a frequency that’s proportional to the rate of rotation. So could you do something like this with a linear array of mirrors? Laser light goes out along the detector’s tube, hits a mirror and bounces back along its original path, creating an interference fringe pattern. If mirror/emitter array is in motion, then the outbound path length is longer than the return path length, and the interference fringe pattern should move, creating a measurable frequency at the detector that’s proportional to velocity. It’s a Newtonian frame of reference, so this should be impossible. > [@Great Antibob](#): > > Rotation is a form of acceleration, so it can be measured even without an outside reference. The thing with the ring laser gyro is that the measurement is possible because of a change in path length. It seems like this should be just as possible as the linear velocity detector. 😕 ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [April 5, 2018, 1:35pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/9 "2018-04-05T13:35:19Z")
There are lots of laser devices for measuring linear speed. The most obvious is the Laser Doppler Velocitometer, which measures the Doppler shift of rapidly-moving things. They use this for flow of particles in water or gas > **[Laser Doppler velocimetry](https://en.wikipedia.org/wiki/Laser_Doppler_velocimetry)** > > Laser Doppler velocimetry, also known as laser Doppler anemometry, is the technique of using the Doppler shift in a laser beam to measure the velocity in transparent or semi-transparent fluid flows or the linear or vibratory motion of opaque, reflecting surfaces. The measurement with laser Doppler anemometry is absolute and linear with velocity and requires no pre-calibration. > The development of the helium–neon laser (He-Ne) in 1962 at the Bell Telephone Laboratories provided the optics communit... For more substantial items, they use Laser Surface Velocitometry (which also uses Doppler shifting): > **[Laser surface velocimeter](https://en.wikipedia.org/wiki/Laser_surface_velocimeter)** > > A laser surface velocimeter (LSV) is a non-contact optical speed sensor measuring velocity and length on moving surfaces. Laser surface velocimeters use the laser Doppler principle to evaluate the laser light scattered back from a moving object. They are widely used for process and quality control in industrial production processes. > The Doppler effect (or Doppler shift) is the change in frequency of a wave for an observer moving relative to the source of the wave. The wave has a frequency f and ... Or LIDAR/LADAR used to measure positions at different times, or you can use various position sensors over time to determine velocity. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 5, 2018, 1:43pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/10 "2018-04-05T13:43:27Z")
It’s easy to make a box that shoots a laser out at something else, and measures the speed of the box relative to that something else. That’s LIDAR. It’s also easy to make a sealed box that measures changes in its own velocity. That’s an accelerometer. But it sounds like the OP is asking about a sealed box that measures its own absolute velocity. That’s impossible, because there is no such thing as absolute velocity. ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [April 5, 2018, 4:17pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/11 "2018-04-05T16:17:45Z")
> [@Machine\_Elf](#): > > The thing with the ring laser gyro is that the measurement is possible because of a change in path length. Yes, but it’s a path length change induced by the rotation (and technically it’s measuring the observed frequency difference rather than measuring the path length). It’s the rotation, i.e. acceleration, itself which induces this difference in path length/frequency. For linear motion, this simply isn’t possible. Without acceleration, i.e. in an inertial frame of reference, there’s no preferred frame against which to measure a velocity. Your proposed system of two mirrors can’t work if relativity is correct. It can work in a Newtonian system but we only see a difference if there’s a preferred absolute frame of reference, which doesn’t exist. What will happen is you yourself won’t see a difference in the interference pattern, whether you are at rest or travelling relative to some inertial frame of reference. But from the perspective of an observer in that travelling frame of reference, a difference in the interference pattern is observed. But that doesn’t really help, since that’s not self-contained. If you are sitting in a train going 60 mph and pass another train also going 60 mph, it is equally valid to say each of the following (ignoring the fact the earth itself rotates, and that Lorentz contraction is a negligible effect): A) Each train is going 60 mph relative to the earth B) Your own train is standing still, the earth moving backwards at 60 mph, and the other train backwards at 120 mph C) Your train is going forward at 120 mph, the earth at 60 mph, and the other train is standing still. With such a proposed closed box sitting on your lap and in the absence of acceleration, there are infinitely many “correct” answers to “what linear velocity do I possess?” It is simply not possible to pick a single correct one for the box to measure. > [@Great\_Antibob](#): > > By the way, this is a basic premise of relativity, i.e. all inertial frames of reference are equivalent. Ack! This is wrong. All inertial frames of reference are NOT equivalent but are equally valid. Typing too fast. ---
**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [April 6, 2018, 6:04am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/12 "2018-04-06T06:04:28Z")
Machine Elf is correct in what I am looking for. The device does not interact with an outside object. Like the laser gyro. The path through the fiber does increase with rotation. The coil rotates as the light travels through it. To the light, it seems the length is changing. Think about walking on a moving sidewalk. Going against it takes you more time to traverse the distance. But linear seems a lot harder if not impossible. It doesn’t have to use laser and fiber. But have no moving parts. Except electric current or light. It also need not detect a sustained steady motion. But detect acceleration and the change of acceleration. Thanks for the replies folks. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [April 6, 2018, 6:59am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/13 "2018-04-06T06:59:34Z")
I will note that navigation systems that are closed boxes that can only measure acceleration are quite real. You just have to integrate the accelerations to get the velocity components. Of course this means your velocities are relative to your starting conditions - which is fine. However continual integration invariably leads to accumulating errors, so the system is only useful for a limited amount of time before it needs a reset or calibration against another known good navigation system. The well known early examples of such nav systems are the Apollo spacecraft and the 747 (100, 200, 300) series. But there are others. Systems that are combined with GPS are around, and can navigate when GPS signal is lost for a reasonable amount of time. When it comes to linear acceleration, pure relativistic optical measurement isn’t what you usually see - rather optical methods (such as fringe counting in an interferometer) are used to measure the movement of a proof mass. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [April 6, 2018, 10:23am UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/14 "2018-04-06T10:23:41Z")
> [@Kedikat](#): > > It doesn’t have to use laser and fiber. But have no moving parts. Except electric current or light. It also need not detect a sustained steady motion. But detect acceleration and the change of acceleration. Wow, that’s a completely different question. You don’t want something that detects linear velocity, you want an accelerometer. Solid(ish) state accelerometers are [a dime](https://www.siliconsensing.com/technology/mems-accelerometers/) [a dozen](http://www.st.com/en/mems-and-sensors/accelerometers.html?querycriteria=productId=SC444)\*. (Assuming that you aren’t so much of a stickler that you won’t accept moving parts that you can’t see without an electron microscope.) \*(Note: bulk pricing not literally 0.83 cents each.) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 6, 2018, 12:54pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/15 "2018-04-06T12:54:54Z")
Maybe not literally 0.83 cents, but they are rather impressively cheap and accurate, nowadays. Most phones have them built in, as do modern game controllers. I think you’ll also find them in fitness trackers. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [April 6, 2018, 1:11pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/16 "2018-04-06T13:11:31Z")
> [@Chronos](#): > > Maybe not literally 0.83 cents, but they are rather impressively cheap and accurate, nowadays. I thought so, too, but apparently some are still pretty expensive, such as at the site I linked that had the neat gif animation of how their models work. [This](https://www.siliconsensing.com/products/accelerometers/gemini-in-plane/) model, for instance, is $191. > [@](#): > > Most phones have them built in, as do modern game controllers. I think you’ll also find them in fitness trackers. I like playing with [this](https://play.google.com/store/apps/details?id=com.chrystianvieyra.physicstoolboxsuite&hl=en) app, which among other things lets you view a graph of the x, y, and z acceleration on the phone as you are handling it looking at the screen (also graphs for the angular velocity on x, y, and z as determined by the gyroscopes, can give numerical output of pitch, yaw, and roll, x, y, and z strengths of magnetic fields with the magnometers, light level using the ambient light sensor, sound volume, tone detector, and other fun if not particularly important statistics from the suite of sensors on a modern device.) ---
**Author:** ![PoppaSan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/poppasan/32/189_2.png) [@PoppaSan](https://boards.straightdope.com/u/PoppaSan)\ **Post date:** [April 6, 2018, 1:32pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/17 "2018-04-06T13:32:43Z")
I think the OPs problem is velocity is relative to some other point. Not to itself. I mean right now I am moving roughly [1/5 of 1% of light speed](https://astrosociety.org/edu/publications/tnl/71/howfast.html) but you’d never know it because you are too. So a detector of motion without an outside point of reference is not moving ever. \*I’m sure I missed the boat somewheres. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 6, 2018, 1:56pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/18 "2018-04-06T13:56:25Z")
Yeah, someone on this board (who might have been you) recommended that app to me, and it’s got a lot of fun things in it. And in any product line, there are always going to be the high end ones available with 10% better performance for 10 times the price (and also low-end ones with 1/10 the performance for 10% lower price, of course). But even the genuinely cheap ones are still pretty good. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 6, 2018, 2:33pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/19 "2018-04-06T14:33:38Z")
> [@Francis\_Vaughan](#): > > When it comes to linear acceleration, pure relativistic optical measurement isn’t what you usually see - rather optical methods (such as fringe counting in an interferometer) are used to measure the movement of a proof mass. Is it even possible to measure linear acceleration with a purely optical method, without a proof mass? ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [April 6, 2018, 2:45pm UTC](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856/20 "2018-04-06T14:45:11Z")
> [@scr4](#): > > Is it even possible to measure linear acceleration with a purely optical method, without a proof mass? I’m imagining a way to do it in a controlled, artificial setting–set up the object with pairs of cameras facing various directions. Place it a setting with reference objects or patterns of known dimensions at known distances. Make note of changes in the position, size, and parallax of those objects, making the assumption that those changes come from movement of the sensor, not the objects. Calculate linear acceleration based on those changes. [Next page](https://boards.straightdope.com/t/laser-optic-velocity-detector/811856.md?page=2)