# Frame of Refrence on Rotating Earth

#### MacAran

I have no idea where to start for looking for an answer to this question. At essence, I am confused how a photon knows it's frame of reference in laboratory experiments conducted on the earths surface.

If a photon travels at the universal speed limit regardless of the velocity of the object emitting it, how does an experiment, say done on the equator of the Earth, impart horizontal momentum? Say a photon shot in a vacuum tube north to south. Shouldn't the Earth rotate out from under it? In a vacuum there is nothing to push it side to side.

It seems to me as if spacetime itself must be dragging the frame of reference on the surface of the earth. If that is so, doesn't it mean there are a ton of odd ways in which the movement of Earth in our galaxy would mess up light viewed from an external source?

Any help pointing me at an answer is highly welcomed.

#### oz93666

All good questions Mac....

I take General Relativity with a pinch of salt . I'm sure there are many believers here who can answer such things ...

Let me specify one precise question ....

A laser is positioned at the equator pointing due North . It fires light which hits a mirror a few Km way and bounces back ... will the light come back to the point of origin if the mirror is at 90 deg to the light ?

#### Woody

Like a circle in a spiral

In theory yes the motion of the Earth through space is following all sorts of orbits in various gravity fields.
Beyond our galaxy there is the gravity of the "local group" of galaxies,
and beyond that there is the...

However, in practice, the accelerations imposed by these gravitational tugs on our local reference frame are negligibly small.

The accelerations due to the Earths rotation and the orbit round the sun can be important in precise measurements,
Obviously the tidal effect of the moon,
If you are really being ultra precise perhaps Jupiter...

#### benit13

I have no idea where to start for looking for an answer to this question. At essence, I am confused how a photon knows it's frame of reference in laboratory experiments conducted on the earths surface.

If a photon travels at the universal speed limit regardless of the velocity of the object emitting it, how does an experiment, say done on the equator of the Earth, impart horizontal momentum? Say a photon shot in a vacuum tube north to south. Shouldn't the Earth rotate out from under it? In a vacuum there is nothing to push it side to side.
In GR, the presence of a gravitational field affects the trajectory travelled by a photon without the need for an explicit interaction. The idea of having to impart momentum to a photon in order for it to travel in its curved route is classical thinking (and incorrect). I don't understood GR or its nuances, but loosely speaking the path travelled by a photon is the "path of least resistance" for a space-time which is not flat. The curved nature of space-time due to gravity is really what GR is all about.

As for the effect of the rotation of the Earth? Yes, the Earth rotates under it. It will have an effect similar to the Coriolis effect, but that effect is negligibly small for photons unless you care about kilometre-long interferometers (e.g. LIGO). The mathematics of ballistics for rotating reference frames is well understood and is employed by targeting systems for artillery and missiles. The Coriolis effect has a very small (but measurable) impact on some sports like cricket, golf and tennis.

Knowledge of the "local group" (i.e. the effects of nearby gravitating objects, like the Sun and the plants) has an impact as well as the rotation of the Earth. It's important for astronomy and critically important for pulsar timing experiments.

It seems to me as if spacetime itself must be dragging the frame of reference on the surface of the earth. If that is so, doesn't it mean there are a ton of odd ways in which the movement of Earth in our galaxy would mess up light viewed from an external source?

Any help pointing me at an answer is highly welcomed.
Rotating frames are a specific case of non-inertial frame. Yes, there are many ways that the Earth's rotation affects observations of external sources. If you're interested in this phenomenon, you should grab yourself an astronomy reference since it's a routine phenomenon in astronomy. Also, try looking up the Coriolis effect.

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