Currently I'm 17 years old, going to secondary school. So, my ideas might be totally wrong...
I know that everything is relative. In the example of speed, the earth moves, and the galaxy moves, etc.

My physics teacher told me that the speed of light is absolute, which means that the speed of the light source doesn't influence the speed of light in space. So, I was thinking that that fact could helps us to measure the absolute speed of our planet in space. Not relative to the sun, or the galaxy.

The way of measuring it was following:

A is the light emitter.
B is the light sensor, in combination with a very very precise timer.
D is the signal broadcasting point.
~~> is light, going from A to B.

 A ~~~~~>~~~~~~~>~~~~>~~~~~>~~~~~~~~>~~~~~~>~~~~~~~~~>  B

So, how it works — in my head — is that you send a signal from D to both A and B. The distances from A to D and from B to D are equal, so this should get the signal to both A and B in the same time. The distance between A and B is constant, say K.

As soon as the signal reaches B and A at the same moment, B starts the very precise timer and waits for the light coming from A, at the same time, A starts emitting light.

According to my knowledge, you should be able to calculate the speed of the whole situation along the axis A,B. Why? Because if our absolute speed is along with the light direction, it should take longer for the light to reach B, because the distance is bigger. Otherwise, we are moving in the opposite direction of the emitted light, so we are going towards the light, so, we make the distance for the light to travel shorter, because we are concede towards the light.

Compare it with a car (C) driving on the highway, next to a high speed train (T). The train goes faster than the car.

Compare both situations:

1) Train and car moving in the same direction, train starts behind the car.

T  ---------------->
                          C ------>

2) Train and car moving in opposite direction towards each other.

T  ---------------->
                <-------  C

In Situation 1, it will take longer for the car and train to meet.
In Situation 2, it will be pretty quick that they meet, because they going towards each other.

It is that difference in time that can be used to calculate our absolute speed, I think.

To define our absolute velocity vector, we can do this measurements three times, each test perpendicular to the two others, so we can apply Pythagoras to get our absolute speed as a scalar.

My teacher could hardly believe that it would work, so he thought that something should be wrong to my theory. What do you think, assuming that we have very precise measuring tools?

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    $\begingroup$ Far from being totally wrong, your idea is almost functionally identical to one of the most famous experiments in the history of physics: the Michelson-Morley experiment. So for what it's worth, you are thinking along the same lines as some very accomplished scientists. $\endgroup$
    – David Z
    Sep 20, 2012 at 5:16

5 Answers 5


Your experiment will measure the speed of light to be the same no matter what direction you do you it in. When we say the speed of light is constant we mean that every local experiment to measure the speed of light will find the same value. Even if we put our apparatus into a rocket and fire it off at 0.999c the experiment will still measure the same speed of light as it did when it was sitting on the Earth.

From a common sense perspective this seems silly. How can the measured speed of light be unaffected by the motion of the experiment? I don't know of any intuitive way to explain this, but it's a premise of special relativity, and all the weird effects like time dilation can be explained from the constancy of the speed of light.

I think the best way to explain the constancy of the speed of light is to explain it as a geometirical property of the universe. I went into some detail about this in the answer to Special Relativity Second Postulate. You might be interested to have a look at this.

  • $\begingroup$ I know this is an old question. but just to understand the answer clearly. Lets say for example, we have a source and an observer travelling at 540 million km/h in the same direction, (observer leading and source trailing) are you saying the light will travel from point source to observer at the rate of 1080 million km/h (relative to their own speeds, not the universe), which if so, sort of suggests on the face of it, that the light would have travelled at 1.5 x the speed of light right? $\endgroup$
    – Dean Si
    Nov 12, 2017 at 11:48
  • $\begingroup$ @DeanSi: correct. But velocities don't simply add when we get to relatisvitic speeds. That is we bystanders watching from earth don't observe the speed of the light to be $0.5c + c = 1.5c$. See for example this Wikipedia article for more on this, or search this site. $\endgroup$ Nov 12, 2017 at 11:56
  • $\begingroup$ The speed of light is not effected because it is measured within the reference frame of the experiment. The speed is not measured relative to absolute space (or even comoving coordinates), but relative to the experiment's setup. Kinda like how you can throw a ball between two people on a train in both ways without that making a difference for the ball's speed. $\endgroup$
    – Kevin
    Feb 16, 2018 at 19:08

Suppose two light pulses are released from A and B in opposite directions at the same time. Clock A’s timer will read 0 and clock B’s timer will read 0 at this instant. Now they both measure the time it takes until they receive a light pulse. Let’s suppose B measures 5 seconds for the pulse to get from A to B. Now I will assume the clocks of both A and B will read 5 when they receive the light pulse, and show that this doesn’t violate relativity, even if the A-D-B system is moving.

We now introduce an observer C. Let us assume the A-D-B system is moving to the right (or “B – direction”) relative to observer C. Let us assume that observer C’s clock reads 0 at the same time that observer C reads B’s clock as 0. Now when B’s clock reads 5, observer C’s clock will read a number greater than 5, say 8 because of time dilation. This can also be thought of as being due to the light pulse from A to B travelling a longer distance. (I think you understand this so far.)

Now, when observer C sees the light pulse from B to A reaching observer A, observer C sees the light pulse travel a shorter distance, and therefore C’s clock reads less than 5 seconds. So when A’s clock reads 5 seconds, C’s clock reads less than 5 seconds. But you are thinking, shouldn’t time dilation mean A’s clock reads more? No. The solution to this apparent paradox is that when C’s clock reads 0, A’s clock is actually negative. As I said earlier, C’s clock reads 0 when B’s clock reads 0. So therefore the solution to the paradox is that from C’s perspective, A and B’s clocks aren’t synchronised. This dissynchronisation allows A and B’s clocks to both read 5 seconds when they receive a light pulse, irrespective of the velocity of the A-D-B system.


Sadly I think your problem, as it is formed in your mind, and your test system, is based on the principles of classical mechanics - special relativity explains that simultaneity is just not absolute.

Look up the Michelson-Morley experiment. They couldn't believe their results either.

You have to accept that there is no actual or absolute speed of ourselves through space. Therefore there is no way to measure that speed.


It's great that you are thinking deeply about this and are asking this kind of question.

However, if I understand your experiment correctly, it is premised on two contradictory statements:

(1) "As soon as the signal reaches B and A at the same moment"

(2) "because if our absolute speed is along with the light direction, it should take longer for the light to reach B, because the distance is bigger"

These cannot both be true at the same time.

In fact, if there is motion along the direction of A to B in the lab frame of reference, the light pulses from D will not arrive at B and A simultaneously according to the lab clocks.

However, according to the clocks at A and B, the light pulses do arrive at the same time. This is because, in the lab frame, the clocks at A and B are not synchronized. This is due to the relativity of simultaneity.

There is also time dilation and length contraction that must be taken into account. Together, these "conspire" to make it impossible for your apparatus to detect an absolute motion.

EDIT (to further clarify my original answer in light of the comments): it must be understood that measurements of the one-way speed of light depend on spatially separated clocks and thus the issue of clock synchronization arises. To quote from the Wiki article One-way speed of light:

When using the term 'the speed of light' it is sometimes necessary to make the distinction between its one-way speed and its two-way speed. The "one-way" speed of light from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. What can however be experimentally measured is the round-trip speed (or "two-way" speed of light) from the source to the detector and back again. Albert Einstein chose a synchronization convention (see Einstein synchronization) that made the one-way speed equal to the two-way speed. The constancy of the one-way speed in any given inertial frame, is the basis of his special theory of relativity although all experimentally verifiable predictions of this theory do not depend on that convention.

The OP's notion of signaling from D to A & B, regardless of whether it's light or electrons in a wire, is an attempt to synchronize the clocks according to the Einstein convention. If, in fact, his apparatus manages to do this, as explained in the linked article, this guarantees that the measured one-way speed of light will be $c$.

  • $\begingroup$ Is it still impossible for the signal to reach both sides simultaneously even if the cooled so much down that we reach zero resistivity? $\endgroup$ Sep 19, 2012 at 13:46
  • $\begingroup$ I'm not sure you've grasped my point. When you say "reach both sides simultaneously", you have to realize that simultaneity is not absolute. It's not that it's impossible to reach A & B simultaneously, it's that observers moving with respect to the apparatus observe the pulses arriving at A at a different time than at B. $\endgroup$ Sep 19, 2012 at 14:00
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    $\begingroup$ I think you're still not grasping my point. Regardless of what kind of signals you use, you're treating their simultaneous arrival at A & B as a given. But the simultaneous arrival of the signals can only be judged by spatially separated clocks which must have been synchronized. But, as I've already pointed out, due to the relativity of simultaneity, clocks synchronized in one frame are not synchronized according to a relatively moving frame. $\endgroup$ Sep 19, 2012 at 14:20
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    $\begingroup$ In OPs experiment, I don't think A & B are moving relative to each other. The device essentially measures the time light takes to get from A to B, and is looking for a variation in the speed of light depending on if the earth is moving in an A->B direction or the oppisite direction, relative to some 'absolute' frame of reference. $\endgroup$
    – N Reed
    Sep 19, 2012 at 14:33
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    $\begingroup$ @NathanReed, yes, I understand completely that this experiment is to measure the one-way speed of light. And, as I have attempted to explain, this can only be done with spatially separated clocks the synchronization of which relatively moving observers don't agree on. As Wiki puts it: "The "one-way" speed of light from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector." en.wikipedia.org/wiki/One-way_speed_of_light. $\endgroup$ Sep 19, 2012 at 15:22

There is a way to measure the 'absolute velocity' WRT the space, but nothing like your experiment.
We already do that feat of measuring the abs. velocity of the Earth (in the WMAP experiment and several others), or any other spaceship.
The Cosmic Microwave Background is so uniform in spacial distribution and so sharp in the frequency that it is a perfect referential for time and space.
The only measuring apparatus in need is diretional microwave antenas, and associated electronics, to measure the intensity of radiation that is received from all directions.
Due to the doppler effect we sense a higher frequency from ahead and a lower from behind the mobile. (google for Dipole CMB) . Because the Earth rotates around the axis we have a daily large variation of the dipole (by 360º), because the Earth rotates around the Sun in 365 days, yes .. and because the Earth, and the Milky Way, is in motion in direction to Leo constelation ... yes we can draw a vector in the physical space with the absolute direction and the speed of the motion. There is no need of any other celestial body as reference, you can even swith off the stars and still we know our path.

The speed of light is indeed constant, absolute, but only in relation to the medium and it seems that you have sorted it correctly. It propagates like the waves originated by a boat in motion in the surface of a calm lake.

About your experiment:

  • problem : very very precise timer -- take two identical atomic clock when side to side then the motion from here to there affects the rate and the grav field idem
  • problem : equal distances -- there are no rigid bodies, and we measure with fields (light, wavelengts) and it is the same problem as 'precise timer'
  • problem : at the same time -- define a criterium: the usual Poincaré/Einstein one is important in the reference of the mobile, or one that do not depends on light (a super-observer that instantly knows where the photon is).

All the measures of light speed we have were made in a two-way closed path.

There are plenty of references to support this viewpoint, for instance this one from CERN: Does the motion of the solar system affect the microwave sky?

Angélica de Oliveira-Costa and colleagues studied the cosmic quadrupole and octopole and realized that both are very planar and aligned, i.e. all minima and maxima happen to fall on a great circle on the sky - another unexpected feature (de Oliveira- Costa et al. 2004)

and from this one from arxiv: Is the low-l microwave background cosmic?

The large-angle (low-l) correlations of the Cosmic Microwave Background exhibit several statistically significant anomalies compared to the standard inflationary big-bang model, ... Three of these planes are orthogonal to the ecliptic at a level inconsistent with gaussian random statistically isotropic skies at 99.8% C.L., and the normals to these planes are aligned at 99.9% C.L.

or this one Large-angle anomalies in the CMB

These apparent correlations with the solar system geometry are puzzling and currently unexplained.

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    $\begingroup$ The frame of the CMB is identifiable, but that does not make it in any way privileged. $\endgroup$ Sep 20, 2012 at 4:16
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    $\begingroup$ @dmckee It is so special that it is the only reference such that an observer at rest in relation to it, the CMB, senses the universe as isotropic. And this is a fact that make it privileged. $\endgroup$ Sep 20, 2012 at 11:01
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    $\begingroup$ The laws of physics are in no way special in the rest frame of the CMB, thus it is not a special frame. This is what is meant by the equivalence of frames in relativity. $\endgroup$ Sep 20, 2012 at 13:14
  • $\begingroup$ @dmckee I answered to the Question 'measure the absolute speed' . By relativity one's is tempted to answer 'No', here I show that the answer is 'Yes' (it is done every day). $\endgroup$ Sep 20, 2012 at 14:51
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    $\begingroup$ "Absolute motion" implies a preferred frame of reference, not a frame that people can agree upon, but one that is special. Certainly you can agree to measure relative the CMB---it is even a fairly natural choice---but that does not make it "absolute". $\endgroup$ Sep 20, 2012 at 15:12

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