# Tag Info

### Moving in the same direction as a photon

The speed of the photon is the speed of light, and that remains constant regardless of what speed you are moving at. So the answer is the latter - the photon moves at the speed of light.
• 18.1k

### Does it actually make sense to talk about "An inertial frame" instead of a collection of frames that are inertial with respect to another?

Newton's first law of motion says that an object that starts moving at a constant velocity, and isn't acted on by an external force, will continue moving at that velocity forever. Let me define two ...
• 41.5k

### How does SR explain constant light speed where the distance between observer and light source is increasing?

You are neglecting the relativity of simultaneity and length contraction. All three are needed to explain the invariance of $c$. Specifically, the usual derivation is to assume the invariance of $c$ ...
• 78k

### Is there a Lorentz contraction of the radius of the rotating disk, after all?

I suspect the problem is that we are missing context, and Einstein is referring to some of the specific variables used in Petzoldt's derivation. Looking at the whole letter (this is Einstein to Joseph ...
1 vote

• 26.9k
1 vote

### How do emitted photons in a moving light clock keep moving sideways with the clock when photons have no mass and therefore no inertia?

Once emitted, light travels in a straight line. In the moving light clock thought experiment, the light is emitted (from the point of view of an observer who sees the clock under motion) in a diagonal ...
1 vote

### How meaningful is the notion of 'now' here on Earth?

scale of relativistic effects To get a sense of scale, the Lorentz factor appears in many S.R. equations and gives the general scale of relativistic effects. \gamma = \frac{1}{\sqrt{1-\left(\frac{v}{...
• 6,143
1 vote
Accepted

### Measurement of an Inertial frame according to two other inertial frames at the same position

This can be solved in a straightforward (but not particularly simple) manner using the Lorentz transforms. Here there are 3 reference frames of interest, the reference frame of the drawings $S$, the ...
• 78k

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