# Topics about speed of light - 215 Discussions, Page 1

1. ### B Measuring the speed of light in a straight line

So recently I watched a video detailing how it is impossible to measure the speed of light in a straight line because it's not possible to synchronize two-time measuring devices without first knowing the speed of light. But I was thinking if light can orbit a black hole in the photon sphere...
2. ### B Achieving the speed of light at the end of a whip

Is there a equation for the speed at the end of the whip. is there a consistent way to calculate how fast the tip of the whip will be including constant velocity and trajectory or is it chaotic. Im new to physics and had a thought that with enough legnth and power a wiphs end colud reach the...
3. ### I Speed of light not an invariant in GR

Hi all, I need help understanding the light ray bending in the original GR 1916 paper, Die Grundlagen.... First of all, Einstein states the ##c## is not an invariant in GR. In fact, from (70) and (73), it stems that $$\gamma = \sqrt{ -\frac {g_{44}}{g_{22}} },$$ where ##\gamma## is ##|c| <= 1##...
4. ### Optical Compact Device For Measuring Speed of Light

Hi. This is an idea which I just happened to think of, and I was curious if it would be at all feasible. Here's a quick sketch I drew: The two curved mirrors should have a laser attached on one end and a video camera attached on the other. The laser would be tilted very slightly above...
5. ### B Can the relative speed of an object meet/exceed the speed of light?

I was curious if the relative speed of an object can exceed the speed of light. Specifically, I am curious about the following thought experiment. I am not a physicists (and if I were asking the following would make me a poor one) and it has been 20 years since college physics. If a vessel is...
6. ### B Decay in c caused by Universe expansion

I'm currently writing a research paper about the speed of light. I have researched universe expansion, specifically, the quantised redshift spectral index fluctuations of distant galaxies and other structures over time, however, I need to suggest why universe expansion possibly causes a recorded...
7. ### B Is there a thought experiment to show that the speed of light is constant?

I know the amazing thought experiment by Albert Einstein with the two light clocks. (The observer at the train station has a light clock and the person in the train.) It's amazing because you can even deduce the formula to calculate how fast the clock in the train goes. But this experiment...
8. ### B How much in the past do we see these planets?

The HR 8799 star is 129 light-years away from us, in this image we can see thermal emissions coming from these planets. Do these emissions move at the speed of light? If so does it mean that what we are seeing in this image is how the planets looked like in infrared 129 years in the past?
9. ### I The vacuum speed of light (invariant speed) in a non-inertial frame

Hi, I read various threads in PF about the concept of invariant speed and the speed of light in vacuum that in our universe happens to be the same as the 'invariant speed'. My doubt is about the speed of the light in vacuum as measured from a non-inertial frame (basically in the context of SR...
10. ### B Energy, mass, and the speed of light

Hello everyone! Let's say that you were to attempt to go as fast as possible on a spaceship with the mass of an average car in an absolute perfect vacuum. What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship...
11. ### I Why is the speed of light absolute?

To describe the movement of the planets, Newton assumed that there was such a thing as gravity. But he didn't know what gravity was. To derive the Lorentz transformation, Einstein assumed that the speed of light was absolute (not relative), but is it also known why the speed of light is absolute?
12. ### B Is red-shifted light traveling at a speed less than c?

The wavelength of light from a moving source is red shifted which means that the wavelength has increased and the quantity of energy arriving per second at a relatively static destination is less than the quantity of energy emitted per second at the source. If so then the original quantity of...
13. ### B Why does the speed of light change in different media?

In relativity, the speed of light in vacuum is a universal constant. Also, it has wave-particle duality. So if the speed of light slows down in a different media other than vacuum, what exactly is slowing down? Macroscopically speaking, the speed of light does slow down. What about in the...
14. ### I Another mirror question...

If you’re travelling at the speed of light TOWARDS a mirror, will you be able to see yourself?
15. ### B An experiment to measure the Speed of Light

So, pretty much I want to make an experiment in order to get the speed of light. What I plan to do is to have a lantern in the dark(initially off) perpendicular to a wall, two sensors(one closest to the lantern and the other closest to the wall), then turn on the light making sensor 1 go off as...
16. ### I Can an object fall into a black hole at faster that the speed of light?

I am wondering if an object can fall into a blackhole at faster that the speed of light. I have heard that the expansion of the universe can make distant galaxies appear to recede from one another at velocities faster than the speed of light.   Intuitively, this makes sense to me. I am...
17. ### I Speed of Light when not in a vacuum?

In a vacuum the speed of light is a constant. What if its not in a vacuum? Okay, as always I'm confused. I'm very interested in physics but am a layman. Alright, so we know that time is relative to one's distance from a mass, so that as we go further from the Earth, let's say, time moves...
18. ### B What does the constancy of speed of light mean?

The second postulate says the speed of light is constant c independent of all inertial observers. Does it mean the speed of the wave front relative to the observer , that is, the relative speed between the wave front and the observer?
19. ### I Maxwell's calculations on electromagnetism's speed = light speed

I'm studying history of electromagnetism. Here is my question regarding Maxwell's classical brilliant work. I understand these constants (for now) And I understand this: And I understand this: But I need to understand how this math below gets us to the speed of light. I don't understand...
20. ### B Speed of light during acceleration

I'm a bit confused as to why the speed of light changes in an accelerating ship relative to an onboard observer. In other words, on a ship with a clock at the nose and a clock and observer at the tail, in an accelerating ship, the clock at the nose will tic faster. The reason (according to a...
21. ### B Can an observer perceive he is traveling faster than light?

I have seen thought problems with an observer on a train or in a station, etc., but I have not seen ones with the observer travelling at relativistic speeds. It seems to me that at sufficient speed he would observe himself exceeding the speed of light due to the slowing of time. This seems...
22. ### B Why is the speed of light a constant?

Why is the speed of light a constant?
23. ### B How can any physical body truly be at rest?

Can we truly have a rest frame or should it be a close to rest frame? Even if I'm stationary and sitting on my porch and the observer in the car passing is moving, I'm still not at 0 velocity. The earth is moving at 67,000 mph and the galaxy is moving at 250,000 mph. I'm never in a single...
24. ### B Does the size of the Universe change with motion?

This is a question I was looking at based on Relativity and John Wheeler's one-electron universe theory. https://en.wikipedia.org/wiki/One-electron_universe My question is this. The faster you move towards the speed of light, wouldn't everything in the universe contract to a single particle...
25. ### B What would happen if the speed of light were different?

if speed of light were not 3*10 ^8 m/s and something else would it affect the reality ?
26. ### B Speed of light circling around a black hole

Let us imagine a photon circling around a black hole, as the picture shows. The gravity of the black hole curves the movement path of the photon into the shape of a circle. From point 0, geometric points A and B appear simultaneously with the photon, each in its own direction. The points travel...
27. ### I Consequences of light moving in a medium

I'm reading Special Relativity by TM Helliwell and in it he describes the second postulate and the fact that moving with respect to air changes the speed of sound, and that because light doesn't need a medium it's speed is constant. I remember my physics teacher saying that light itself(EM...
28. ### A General relativity: time dilation and speed of light

Hello everyone, I'll go straight to the question. The gravitational time dilation is equal to tearth = tspace*sqrt(1 - rs/r), with rs = 2GM/c2. However, the formula for speed of light in gravitational field is equal to v = c(1 - rs/r). My intuition tells me that these two formulas must be the...
29. ### B How can the speed of light be constant for all observers?

A rocket is in constant velocity. The velocity of the rocket is 150Mm/s (or 0.5 of the speed light, or 150 million meters per second) relative to us (we as observer). We observe two lights, one moving in parallell with the rocket, another is moving in the opposite direction. Below I have made...
30. ### B Mass of photon and FTL travel

Really silly question, but if we assume that our current science is correct, is it plausible that we can move faster than light in a vacuum? Say, for example, can we make the mass of something less than a photon so it then can it move faster than light in a vacuum. I know this sounds like a...