We know that the speed of light is always constant in space no matter the speed of the object emitting light, right?

Now consider this. An object is moving at half the speed of light and shined 2 beams of light; one in the direction it is moving and one in the opposite direction. If it was possible to see the beam of light, it would appear from the perspective of the object, that the beam of light moving in the same direction as its moving would appear to be moving at half the speed of light, and the beam of light opposite of the direction of the object would appear to be moving at 3/2 the speed of light.

Now, to apply this concept to something more possible. In theory, a device consisting of 3 laser beams, and 3 sensors could be able to calculate the absolute speed and direction it is moving in the universe. Say we made a device with 3 lasers set up so that they are all perpendicular to one another and sensors with known distances from the lasers, and all of this is occurring in a vacuum, then all at the same time the lasers would shoot at their respective sensors, and the times it took for the light to reach the sensors were recorded. The velocity of the lasers from the perspective of the device could then be calculated by using distance over time. Using the apparent velocity of the laser, the velocity of the object in the direction of the laser could be calculated by subtracting the apparent velocity from the speed of light. Then using these 3 velocity measurements, you could measure the speed that the device is moving through space itself. Using this device, it would be possible to measure the velocity of Earth at any point in time.

After researching the speed the Earth is moving, I saw that the method of figuring this out involves background radiation in the universe. Why would my method be discarded and the background radiation method be used instead. Surely I am not the first to think of this. Why would my method not be used?

Edit: I am aware that the speed of light is constant no matter what. Just it can seem to be going at 3/2 or 1/2 the speed of light from the perspective of something also in motion in the same way that a car going 60mph views a car going 90mph as going 30mph faster than it, or just 30mph in the perspective of the 60mph car.

  • $\begingroup$ Isn't the 2nd sentence of your 2nd paragraph inconsistent with both theory and experiment? $\endgroup$ – Alfred Centauri Dec 21 '19 at 3:29
  • $\begingroup$ Trying to understand relativity using concepts such as "perspective" and "apparent velocity/size/time-rate" is a major pitfall for many. There is no apparent velocity, Lorentz contraction is not a matter of perspective and so on. Try to reformulate your scenario in terms of any number of arbitrarily accurate observers in each frame, who can reconstruct their 3+1 reality as needed. $\endgroup$ – JEB Dec 21 '19 at 4:08
  • $\begingroup$ In a galaxy far far away, to them Earth seems to be going at ludicrous speed! What gives? $\endgroup$ – user6760 Dec 21 '19 at 6:22

"...that the beam of light moving in the same direction as its moving would be moving at half the speed of light, and the beam of light opposite of the direction of the object would seem to be moving at 3/2 the speed of light."

That part right there is why your method would be discarded. The first rule of special relativity, "Thou shalt not add thy speed to the speed of light," to quote Carl Sagan, who does a REALLY good job of explaining this concept better here:


The reason the background radiation is used is due to a phenomenon called redshift. See, since the speed of light is constant in a vacuum, and space is a vacuum, for light to gain or lose energy, it actually has to shift frequency up or down accordingly. Another good article on this can be found here:


So, by telling something about the redshift or blueshift of the light surrounding us, we can tell whether it's moving away from us, meaning it's redshifting, or coming towards us, meaning it's blueshifting, something about the speed of objects, or the light, relative to us. From the surrounding speeds, we can tell something about the speed the earth, our observation platform, is travelling at.

Note: The redshift or blueshift can tell us something about our speed because the rate of frequency shift can tell us how quickly the universe is expanding away, or the speed of objects relative to us, and from general relativistic calculations and geometry, we can then infer our absolute speed. I hope this helps.

  • $\begingroup$ I edited the question to make it more clear. I am aware that the speed of light is constant. but it could appear to be faster or slower than it actually is by an observer also in motion in the same way that a car going 60mph views a car going 90mph as going 30mph faster than it, or just 30mph in the perspective of the 60mph car. $\endgroup$ – Erik Low Dec 21 '19 at 3:54
  • $\begingroup$ No. That's the point of special relativity. The light would not appear to change speed. Rather, it would change frequency up or down depending on whether it was moving towards you or away from you, just like I referenced. The relative speed of light using laser experiment was the Michelson-Morley interferometer experiment: en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment This actually debunked the varying speed of light and the aether it supposedly propagated through, even as appearing to travel different speeds. $\endgroup$ – Guthrie Douglas Prentice Dec 21 '19 at 3:58
  • $\begingroup$ I want to add the red/blue-shift has nothing to do with light propagation, rather, that is a property of the source motion relative to the observer motion (and the physics producing the light). Light by itself doesn't have a frequency or wavelength, since those are frame dependent properties. $\endgroup$ – JEB Dec 21 '19 at 4:03
  • $\begingroup$ @ JEB Then how come photons that are either travelling down or escaping a gravitational field blueshift or redshift respectively? (en.wikipedia.org/wiki/Blueshift#Gravitational_blueshift) That's not due to the relative motion of the object but due to the motion of the light itself in the presence of a static gravitaitonal field at that point, isn't it? $\endgroup$ – Guthrie Douglas Prentice Dec 21 '19 at 4:07
  • $\begingroup$ I mean, isn't that the light gaining or losing energy depending on what field it is travelling through? E=hf after all. $\endgroup$ – Guthrie Douglas Prentice Dec 21 '19 at 4:08

Your first sentence contradicts the rest of your question: the speed of light is constant for all observers, no matter their relative velocity. If an object moving at $c/2$ emits light, it is not observed as moving at $3c/2$ or $c/2$, it is observed as moving at $c$.

The experiment you describe would not work for precisely this reason.

  • $\begingroup$ It is observed moving at 3c/2 or c/2 in the perspective of the object in the same way a car going 60mph views a car going 90mph as going 30mph faster than it, or just 30mph in the perspective of the 60mph car. $\endgroup$ – Erik Low Dec 21 '19 at 3:47
  • $\begingroup$ @ErikLow, are you not familiar with the relativistic velocity addition formula? According to which both the forward and backward propagating beams of light have speed c from the perspective of the object? $\endgroup$ – Alfred Centauri Dec 21 '19 at 3:53
  • $\begingroup$ No, this is not true. The laws of special relativity are not intuitive to us when we are used to working with Newtonian physics. The speed of light is the same in all reference frames. Try reading an introduction to special relativity such as this; this concept is one of the first things explained $\endgroup$ – DavidH Dec 21 '19 at 3:53
  • $\begingroup$ @DavidH Oh I see. I am not familiar with special relativity. My question then is why is do objects seem to be thinner as you pass them at higher speeds. I recall seeing a Vsauce video about it. Is this just outright wrong because that would mean moving photons are something you can catch up to. $\endgroup$ – Erik Low Dec 21 '19 at 4:08
  • $\begingroup$ It is true that objects appear thinner if they are moving relative to you — this is called Lorentz contraction. But it’s not because you’re catching up to photons, it’s a real physical process. You can actually use the fact that light speed is constant to prove that this happens. If you’re interested there are lots of introductory explanations available — I personally found Brian Cox and Jeff Forshaw’s book “Why does $E=mc^2$ ?” very useful when I read it with only high school knowledge of maths and physics $\endgroup$ – DavidH Dec 21 '19 at 5:06

Your edit is also incorrect, relativistic speeds and car speed are very different. Saying that you can add speeds of light like you can for cars "60mph views a car going 90mph as going 30mph faster than it, or just 30mph in the perspective of the 60mph car" is incorrect. Light ALWAYS travels at the speed of light for any observer regardless of the speed the observer is traveling. If cars behaved like light then, regardless of if your car is traveling in the same direction or opposite direction as another car, that other car would be traveling the speed of light just like your own car and every other car traveling along the space highway.

EDIT: https://van.physics.illinois.edu/qa/listing.php?id=1355&t=car-traveling-at-the-speed-of-light

EDIT 2: Silly video but it explains what you want to know: https://www.youtube.com/watch?v=ACUuFg9Y9dY

Essentially, distances change when an object is traveling at significant fractions of the speed of light which allows all observers, moving or stationary, to observe light traveling at light speed relative to themselves. This is explained in greater detail in the links, this is the abridged version.


Not the answer you're looking for? Browse other questions tagged or ask your own question.