In school, I was taught that the speed of light is constant, in the sense that if you shoot a laser off of a train going 200 km/h, it still just goes at a speed of c=299,792,458 m/s, not at c + 200 km/h.

What confuses me about this, is that we’re constantly on a metaphorical train:
The Earth is spinning and going around the sun. The solar system is going around the Milky Way. And the Milky Way is flying through the universe, too.

Let’s call the sum of those speeds v_train.

So, presumably if you shoot a laser into the direction that we’re traveling, it would arrive at the destination as if it was going at 299,792,458 m/s - v_train.
The light is traveling at a fixed speed of c, but its target moves away at a speed of v_train.

This seems like it would have absolutely wild implications.

Do I misunderstand something? Or is v_train so small compared to c that we generally ignore it?

  • Spzi@lemm.eeEnglish
    1·
    11 months ago

    the speed of light is constant, in the sense that if you shoot a laser off of a train going 200 km/h, it still just goes at a speed of c=299,792,458 m/s, not at c + 200 km/h.

    Yes. It is counterintuitive, but correct.

    What confuses me about this, is that we’re constantly on a metaphorical train: The Earth is spinning and going around the sun. The solar system is going around the Milky Way. And the Milky Way is flying through the universe, too.

    Do I misunderstand something? Or is v_train so small compared to c that we generally ignore it?

    The magnitude of v_train does not affect the speed of light coming in from, or going out in different directions. c simply is constant to all observers, regardless if and how they are moving.

    Light emitted or absorbed by a train will always have the speed of c, not c + v_train. Even if that train moves nearly at the speed of light itself.

    However, v_train affects wavelength, or color. Light coming in from the front will be more blue, and light coming in from the back will be more red (see ‘red shift’ in the context of distant galaxies). Geometry will warp. But light will always move at the speed of light.

    This seems like it would have absolutely wild implications.

    Sounds as if the fun begins here. Of what implications are you thinking?

    • Wutchilli@feddit.deEnglish
      0·
      11 months ago

      So is the speed of light constant because light is a particle and a wave? And when the particle moves at a constant speed the change in speed/energie is achived by a modulation of the wave?

      • Spzi@lemm.eeEnglish
        1·
        11 months ago

        the change in energy is achived by a modulation of the wave?

        Yes. Since speed is constant, all you can change is wavelength/frequency. If you try to add speed, you instead add energy, which increases frequency or shortens wavelength.

        E = hc / λ

        where

        • E is photon energy
        • λ is the photon’s wavelength
        • c is the speed of light in vacuum
        • h is the Planck constant

        So is the speed of light constant because light is a particle and a wave?

        Here, the ground is becoming shaky for me. You’re asking a good question; why. Maybe all we can find out is how. From what I understand, we have piles of solid evidence that the speed of light is constant. These observations confirmed a theoretical prediction made by special relativity, which can be summarized by …

        treating space and time as a unified structure known as spacetime (with c relating the units of space and time), and requiring that physical theories satisfy a special symmetry called Lorentz invariance, whose mathematical formulation contains the parameter c. Lorentz invariance is an almost universal assumption for modern physical theories, such as quantum electrodynamics, quantum chromodynamics, the Standard Model of particle physics, and general relativity. As such, the parameter c is ubiquitous in modern physics, appearing in many contexts that are unrelated to light. For example, general relativity predicts that c is also the speed of gravity and of gravitational waves, and observations of gravitational waves have been consistent with this prediction.

        So it seems that

        • c happens to be the speed of many things, including light
        • c does not depend on any characteristic of light waves or photons
        • c follows from spacetime mathematics

        But again, this is shaky ground for me. I’ll be happy to read corrections.