# How to know if a vehicle is moving without any external source of information?

The situation is the following:

I'm inside a vehicle (plane or a car, it doesn't matter) and I need to know if the vehicle is moving at a constant speed BUT I cannot perceive any external change like visual changes, vibration, etc.

How can I know if the vehicle is moving? Do I really can know?

Additional question Can I know my speed?

• You can know your acceleration, but not your speed. Nov 7, 2014 at 20:21
• Well...if you wanna know the acceleration, try a helium balloon. If it's angle of tilt is $\theta$, then $g \tan \theta$ is the acceleration of the car. Nov 8, 2014 at 17:57
• Can you use a magnetic field sensor or radio receiver? These deal with external effects, but they can most likely be sensed internally. Nov 8, 2014 at 18:04
• @Rohinb97: That's big and complicated and lags a lot and wont' be that precise. A mass attached with pressure-sensitive elements is all you need and it can be made pretty small, cheap and quite accurate (every mobile phone has accelerometer these days). Nov 8, 2014 at 22:27
• Do you know some initial condition? That is do you know the vehicle was stationary when you boarded it and can you keep some (purely internal) sensor active since then? Nov 8, 2014 at 22:45

You cannot tell moving with constant speed apart from standing still. This is the principle of Galilean relativity.

• It's more general than Galilean relativity, really; it applies to special relativity too. (or any other sort of relativity you might come up with) Nov 8, 2014 at 8:10
• @DavidZ: Moral relativity?! ;) Nov 8, 2014 at 11:47
• Yeah, I mean think about it. When you're sat inside a car that's stationary, you're still moving (very, very, quickly) and cannot tell at all unless you can see the stars, sun, etc. And even then you're going to need precise instruments or wait for a while ... and even then you do not know that it is not just the celestial bodies moving, until you do a bunch of mathematical proofs and re-invent astronomy. Nov 8, 2014 at 21:51
• @ACuriousMind you are not really speeding if everyone else is going at the same speed? Mar 20, 2015 at 16:37
• In fact Galileo answered the OP about 500 years ago - en.wikipedia.org/wiki/Galileo%27s_ship#The_proposal. Dec 11, 2021 at 12:35

As an abstract spherical-chickens-in-a-vacuum-type question, then no, basic relativity says not.

But that isn't much fun. ..

First up, what do you mean by speed? If you mean speed along surface of earth then you have a chance. Since the earth is curved you are always accelerating, and you could measure the drop in g as you speed up. So long as you don't go fast enough to go into orbit... Of course the is some degeneracy between speed and altitude for this.

Of course. If we are moving on a true straight line it is easier still as you will move through the earth's gravity as you pull away from the surface. Or burrow beneath it!

A more practical method (that is actually used) would be dead reckoning. If we assume (or assert!) that the vehicle was stationary when you got in then you can in principle detect every acceleration the vehicle makes from then on. Integrating these will give you your current velocity. Of course the accuracy of this method decreases with time as the errors simply accumulate.

Finally, if our vehicle is ground based, then an oddity of fiction could at least tell us if we are probably moving or stationary. Friction between surfaces is typically very different for static and dynamic friction, with static being stronger. This means that when something slides to a stop (be it a sled or break discs) it almost always jolts slightly at then end as the stronger static fiction kicks in. You will feel this on buses and trains frequently. Although careful control can minimise the jolt, looking out for it with a sensitive accelerometer would give you a strong hint you had stopped.

• You meant oddity of fRiction, unfortunately. I thought you'd show how we could use some common fiction trope to figure out a way, like Pratchett's "one in a million chances come up nine times out of ten" :-) Nov 8, 2014 at 10:03
• General relativity might have caught on less well if it had asserted that gravity is a frictional force. Nov 8, 2014 at 20:27
• Nice answer man, that was fun :) Nov 10, 2014 at 14:56

Actually special relativity states that all inertial reference frames are equivalent, so there is no such thing as absolute speed.

You can't ask "What is my speed", this question is just not well formed. You can ask: "What is my speed in reference to this object", and in fact all "real life" examples of asking "What is my speed" actually have some implied reference frame. Traffic sign specifying that maximal speed is 50 km/h, reall specifies that: "Maximal speed of car in reference to road surface is 50 km/h".

In case of moving on earth you allways have a handy reference frame (that is: earth) and you can measure:

• Your speed in reference to air.
• How fast wheels of your car are moving.
• You could measure speed using GPS

Without Earth as reference frame (or some othere reference frame) this question has no answer. This is because special relativity postulated (and we have experiments for that!) that there is not "special" reference frame that "is not moving".

Before Einstein it was assumed that there is a special medium named ether, that would be that special reference frame, and if this was the case you could always measure your speed in reference to ether using simple physical experiments: https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment .

If you had some type of gravimeter you could check whether local $g$ were changing. If so, you might reject the idea that you're stationary and infer that the variation in $g$ is due to you moving over different parts of Earth. This would require your hypothetical vehicle to be pretty darn smooth, though.

It's important to state that the reverse situation is not true: A constant $g$ would not necessarily imply you're stationary.

Note that this does not violate Galilean relativity since the laws of motion are still the same; you're just detecting the spatially-varying gravitational field by moving through it.

• Unfortunately, I don't know of an implementation of this which directly senses the field strength. They all seem to sense force of G, which would be offset by motion if in free fall/orbit. (And gyros won't work to detect orbit if craft rotates exactly opposite its orbital speed.) Nov 8, 2014 at 14:39
• @keshlam I think passing through an uneven gravitational field would appear like gravity waves... so in principle you'd be looking for a very sensitive, portable, gravity wave detector. Amazon doesn't have any in stock currently. Nov 10, 2014 at 15:12
• Given the scale at which "gravity waves" have been detected, you'd have to be moving extremely fast... I'm not sure lightspeed would be fast enough. Nov 10, 2014 at 15:34

Assuming that you know the gravitational field in the area you're passing through, you can determine whether or not you're moving at a constant speed (which I take to mean "stationary in some inertial frame") by whether or not you're feeling gravitational forces other than what you'd expect from the field. For example, if there is no gravity and your scale says you weigh 10 pounds, then you are not moving at a constant speed.

Does humidity and similar cues count in the answer or not?

For example if moving continously after a while certain environmental parameters will change. Which cannot be screened in the vehicle.

For example humidity, temperature etc..

Assuming the vehicle can actually screen all those parameters. Then one can use non-inertial cues: For example rotations, turns, acceleratios etc..

Assuming no non-inertial cues are present, then as already stated, the principle of relativity (for inertial frames) makes the uniform motion and no-motion equivalent.

Are you traveling in a vacuum? If not, use a pitot tube

• Use of sensors seems to violate the assumption "no external changes can be perceived." Nov 8, 2014 at 12:32

## Acceleration

If your exact need is, as you say, to determine "if the vehicle is moving at a constant speed" then there is a wide range of accelometers available on the market with various degrees of accuracy.

If the accelometer shows 9.8 m/s2 downwards (assuming you install it in a fixed orientation relative to the vehicle), then it is moving at a constant speed (possibly with a constant speed of 0 relative to the ground), if it shows anything else, then it's speed of movement is not constant.