# Tag Info

### Measuring the one-way speed of light?

I think we can measure one way speed of light. we need to redefine simultaneity. My proposition is as follows: if a rigid body AB of length l is moving without any acceleration parallel to X axis and ...

### Invariant upper bound for velocity of a particle

Alright, so I was strugling with this exact page and I think I can give an explanation. So, the essential point to be shown is that there must always exist some sort of speed limit (whether that be ...
• 5,381
1 vote

### How do we know that the gluon travels at $c$?

Gluons are massless in the Standard Model, and all massless particles travel at $c$. This is pretty much by definition, but a hand-wavy way to explain it is that a massless particle will be ...
• 5,818
Accepted

### Time differential between two signals sent at two different near-relativistic speeds

This is a very interesting question. Indeed, as a one-way-speed-of-light (OWSOL) experiment the answer is a duplicate of all of the other myriad OWSOL questions: there is no possible experiment which ...
• 64.4k

### Time differential between two signals sent at two different near-relativistic speeds

Photons are bound by relativity. Even if a really fast vessel. It is o ly when an object approaches 1/10th c that dilation becomes heavily noticeable. One aspect of Special Relativity speed of light ...
• 186

### Can this experiment successfully be used to find out one-way speed of light?

Edited to add: The first four paragraphs below constitute my original answer. Below that is a new and better answer. Original Answer: Suppose the distance from A to C (and from B to C) is $s$. ...
• 12.4k

### Can this experiment successfully be used to find out one-way speed of light?

No. "identical apparatus which can shoot a ball at the same speed [in opposite directions]" The same what now? The same speed in two directions? Sure, the apparatus looks like it works the ...
• 8,528

### A question about relativity and time dilation from a lay-person

The best way to see what's going on is with a special Minkowski Diagram called a Lodel Diagram. Let's say the unprimed frame is on Earth, and the primed frame is the returning rocket ship. Since you ...
• 23k

### A question about relativity and time dilation from a lay-person

Time dilation can be confusing until you view it in the right way, and then it becomes obvious. Time dilation is caused because clocks in two moving frames are out of synch relative to each other- it ...
• 18.2k

### A question about relativity and time dilation from a lay-person

Alice is on earth. Bob travels from Far Away. The story according to Alice: At 2PM, Bob started his journey, with his clock correctly set to 2PM (as was mine). The journey took him four hours, but ...
• 12.4k

### A question about relativity and time dilation from a lay-person

When you say "he will reach the Earth in one week" you have to also specify whether this is as measured by A or as measured by B. They have different coordinate systems. They're both ...
• 5,818

### A question about relativity and time dilation from a lay-person

It is better to think of a row of ships travelling to earth with the same velocity of A (Alice) and with synchronized clocks. And a row of stationary clocks synchronized with the B (Bob) earth's clock....
• 11.8k
1 vote

### Self-coupling of gravity and gravitation escaping a black hole - contradiction?

"Self-coupling" means that yes, gravity gravitates -- that for example gravitational waves would be affected by the gravitational field of objects they move past. Gravitons, if they exist, ...
• 5,818
1 vote

### Can a spaceship accelerate at $1g$ forever?

Acceleration is a change in velocity, and velocity describes both speed and direction. What that means is that an object (a space ship) can be accelerated in such a way that the direction is ...
• 111
Accepted

### What happens to the time period when refraction occurs?

The frequency is the inverse of the period. So since the frequency is the same the period is also the same
• 64.4k

### Could the speed of causality be (significantly) faster than $c$?

There is no such thing as an unambiguous "speed of causality", because causality itself is a very vague notion when you actually try to nail it down. What is true is that the speed that we ...
• 105k
Accepted

### Could the speed of causality be (significantly) faster than $c$?

The $c$ that appears in the equations of relativity (including the famous $E = m c^2$ is the speed of causality. This is the special, unique speed that is the same for all observers regardless of ...
• 5,818

### Could the speed of causality be (significantly) faster than $c$?

Firstly note that the speed of light is now defined as an exact value. Light will travel at this speed in a vacuum simply because we defined it that way. We used to define a meter and measure the ...

### Recordings of journey traveling near speed of light

Each recorder shares proper time with its corresponding clock, so both sets record and show the same amount of time during playback. The clocks themselves though, after luminal travel, would show ...

### Can a spaceship accelerate at $1g$ forever?

Constant rotational speed (angular acceleration) can provide 1G simulated gravity. There is no need for infinite (linear) acceleration to meet that requirement. The ship would get closer to the speed ...

### Is light's momentum $0$?

For me it is simple to think in terms of classical light as the ensemble of photons, quantum mechanical particles obeying the Lorentz transformations. This means that a photon, which has zero mass, ...
• 221k

### Is light's momentum $0$?

Your original equation is only correct when you mean m to be rest mass, which is actually denoted $m_{0}$. m is actually $\gamma m_{0}$ and is called relativistic mass. You are finding conditions on ...
• 3,705
1 vote

### Is light's momentum $0$?

If you start with the energy momentum relation $$E^2 = \left(m_{0} c^2\right)^2 + \left(pc\right)^2$$ where $m_{0}$ is the rest/invariant mass, and apply it in the rest frame (where $p = 0$), you ...
• 2,718
1 vote

### How to understand $E=mc^2$?

It's very easy to understand $E=mc^2$. All Einstein's very short paper on the subject, with the English translated title "Does The Inertia of a Body Depend on Its Energy Content?", sais is ...
1 vote

### In an expanding universe, can two people communicating to each other about their cosmological horizons get around their horizon limit?

I think the right way to think about it is to rescale space into comoving coordinates $\chi$ and time into conformal time $\eta$. That way the expansion effect goes away (hidden in the coordinates). ...
• 26.8k

### Has the Michelson-Morley experiment been done in space?

I mean that exact experiment Well, ”exact” is a little excessive. The exact experimental apparatus is on the ground at Case Western Reserve University, and nobody will be burning an argand lamp in ...
• 64.4k

### Explanation for invariance of $c$ / Lorentz transformations?

Explanation for invariance of c There is a semi-explanation in Maxwell theory. Basically, this theory says that the interaction between charged particles travels at certain speed. Of course, speed is ...
• 5,471

### Explanation for invariance of $c$ / Lorentz transformations?

Your example of the oscillator moving along a line normal to its axis is an exception, and your game would not show the correct relativistic effect in other cases. In particular, if you had two ...
• 18.2k

### Explanation for invariance of $c$ / Lorentz transformations?

The modern view of special relativity is as a geometric theory, with time dilation and similar effects explained as a consequence of different observers having different directions in 4D spacetime as ...
• 5,818

### Finiteness of speed of light

The speed of C (which is also the speed of anything that is massless (including light)). Is just a 45° angle in a spacetime diagram. It reveals and illustrates the relation between space and time. Now ...
• 35

### The origin of the value of speed of light in vacuum

There is a term that I like to use that is called "effectively" If you are massless in the Universe (like light) you are traveling with what will look to you (and you only) as effectively ...
• 35

### What is the complete proof that the speed of light in vacuum is constant in relativistic mechanics?

There is no proof, because the constancy and invariance of the speed of light in space is an axiom. We took for granted that C is a constant and invariant speed (and also the speed limit in the ...
• 35
1 vote
Accepted

### Energy-momentum tensor of a perfect fluid flowing at the speed of light?

Once the radiation fluid is traveling at the speed of light in a particular spatial direction, the situation effectively becomes equivalent to a mixture of plane waves of light traveling in the same ...
• 1,449
Accepted

### How would the following image look like, if we didn't use $ct$ for time?

What will happen depends on your units. If you are using SI units, and use $t$ instead of $ct$, then you will effectively stretch the graph by an enormous factor such that the light cone will be ...
• 33.3k
1 vote

### How would the following image look like, if we didn't use $ct$ for time?

That is the same as setting c equal to 1, so the scale should not change at all. In general, yes, changing the units of c or t would just visually expand or contract the horizontal axis, equivalent to ...
• 2,359

### Special Relativity: If speed of light is constant, then why measured lengths and times aren't changing in the same way?

Other answers are right but here is the quickest way to see where you went wrong: The speed of light (or anything) is distance-traveled/time, not length-of-some-rod/time. Of course if the rod is ...
• 12.4k

### Special Relativity: If speed of light is constant, then why measured lengths and times aren't changing in the same way?

For a purely qualitative answer, the way that I often think about relativistic rods is to imagine that the rod changes color uniformly in its own rest frame. That is, at any given moment in its rest ...
• 131
Accepted

### Special Relativity: If speed of light is constant, then why measured lengths and times aren't changing in the same way?

Dale's answer is correct, but this might illuminate it with an example: Say Gary is on the ground beside a pretty long train carriage - 100 metres long. He measures that the photons from a flash of ...

### Special Relativity: If speed of light is constant, then why measured lengths and times aren't changing in the same way?

One of the big pitfalls of physics equations is using them in situations where they do not apply. In this case, the time dilation formula only applies when the two events occur in the same location in ...
• 64.4k

### Speed of light and the equivalence principle

The quote isn't strictly wrong, but it may convey a false impression about what constant or inconstant speed of light would mean. It suggests to me that when it was written, Einstein had identified ...
• 8,528

### Speed of light and the equivalence principle

In 1913 Einstein was still working on general relativity and it was not complete. Furthermore, it would be decades before the community, including Einstein, really began to understand the important ...
• 64.4k
1 vote

### Is our understanding of relativity / the speed of light / anthropocentric due to our limited 5 senses?

There is actually nothing special about light in special relativity. The parameter $c$ that enters formulas for length contraction, time dilation, mass-energy equivalence, etc, is called the speed of ...
• 33.3k

### Is our understanding of relativity / the speed of light / anthropocentric due to our limited 5 senses?

Suppose a series of bullets, all traveling at different supersonic speeds, hit a bunch of different bats. Is the bat detective bureau (using its sonar, its sense of touch, etc) not going to notice ...
• 12.4k

### Is our understanding of relativity / the speed of light / anthropocentric due to our limited 5 senses?

Neutrinos are not superluminal. If there were some superluminal field excitations we would need some way to detect those quanta that were traveling faster than c. This would imply something other than ...
• 72.7k
Accepted

### When looking through a telescope at the moon, do I see it at a more recent time point than with my naked eye?

When you look at the Moon, be it with your naked eyes or through a telescope, you receive the light that was scattered from the lunar surface about one second ago, and then propagated to you (this ...
• 26.3k
1 vote

### Perception of light speed when traveling between two light sources

The explanation you have quoted is quite misleading- time dilation cannot account for the constancy of the speed of light in all directions. As @lpz mentioned, the effect responsible is the relativity ...
• 18.2k
1 vote

### Perception of light speed when traveling between two light sources

The problem with this question is it imbues special meaning to the light's source. That is important when doing astronomy and spectroscopy, but this is not that. The problem is that it implies there ...
• 23k
1 vote

### Lorentz vs Einstein

Einstein came up with the correct set of postulates and framework (all inertial reference frames are indistinguishable, speed of light is invariant, two-way speed of light convention for measuring ...
• 2,359