# Tag Info

1

You seem to be asking two things, could a black hole's magnetic fields cool nearby matter, and could this cooling produce cold fusion. But maybe we should first ask whether "cold fusion" is a real thing. Nuclei contain protons and neutrons held together by pions. Fusion is when two nuclei become one. The barrier to this happening, is the positive electric ...

3

velocities don't add up like A + B = C. This is only approximately true in the limit of every day perception. You have to consider your coordinate frames relative to each other if you approach relativistic (meaning a fraction of the speed of light) velocities. $s = \frac{u + v}{1 + \frac{u v}{c^2}}$ would be the added velocity s of two objects moving ...

0

Well, light is a cosmological limit of speed in the whole multi-verse.We can't reach the speed of light but even if we, our mass would increase and increase and at a point, we would die. So when it comes to time, it is just a virtual way of looking at the world moving forward or arrow of time. Well think, if light speed really stops time, so why doesn't the ...

2

In physics, frames moving at the speed of light are not valid. What "list verse" means, more accurately, is that as you approach the speed of light, your time, as seen by a "stationary" observer, ticks slower. This is a well-documented effect that needs to be accounted for in all manner of applications, ranging from particle accelerators to GPS satellites ...

0

So, I'm not sure how much relativity you know, but usually we write the 'Proper time' for a particle moving through the x direction and time as $$(c \Delta \tau)^2 = (c \Delta t)^2 - (\Delta x)^2$$ Where proper time $\tau$ is just the time that the particle would measure in it's own internal frame. So if you are travelling fast on a rocket ship, someone ...

0

I think that there's two things which you're leaving out: First, slanty lines. This was adequately covered by @Jaywalker but if you need a further synopsis: suppose in reference frame $R_1$ both $A$ and $B$ are at rest and $A$ emits a laser pulse at $B$, describing the trajectory $x = 0, y = c \tau.$ We transform to a reference frame $R_2$ moving in the ...

0

This is a very good question. For your second example, the light does actually reach B, but the paths it takes to get there are different for each frame of reference. If you are in A and B's reference frame, you could argue to be at rest and the light travels a direct path. Looking from the labs perspective however, the light moves along with the frame AB. ...

1

let's assume we have 2 rays moving at 180°. If the container ( and the background ) move instead of the photons, the container would have to move in the direction of both rays. But, it's impossible since we assumed 2 antiparallel rays. How can container move forward and backward simultaneously.

2

Your question is the one that Einstein pondered for long time and from which Special Theory of Relativity was born. He wondered what could happen if you travel at the speed of light how would you see a ray light. The problem was that according to Maxwell's Electrodynamics, explained light as oscillating $E$ and $B$ vectors along space and time, so as a ...

1

Elementary particles do not have consciousness, individuality or volition. They follow the rules of the boundary value solutions of the quantum mechanical equations they obey. The relativistic quantum mechanical mathematics have zero mass particles moving at velocity c, and in all valid frames massive particles move at velocities less than c. It is the ...

2

The above picture I drew to expand on Kostya's wonderful answer. Basically, imagine people who measure height of buildings in degrees of angle of the buildings' visibility from the certain fixed distance. This is not at all unreasonable if you fix the distance C large enough compared to the building heights'. However, for taller buildings you'd notice that ...

0

If it's inside the event horizon, can this light escape it? no , it cannot escape. Anything falling cannot see the ground, nor send message back, meaning in the sphere surface direction. can you escape it by the speed of light itself? no, it not sufficient the light is red-shifted means that an object near the BH may emit or reflect this light ...

1

We won't be able to observe the light beam because photons don't interact with each other -- there is no way for light to bounce off the other photon so we won't be able to see it! Even if we imagine a particle travelling parallel to our path, both at the speed of light, if a photon were to bounce off of the particle, it could never reach our position ...

0

Some constant c with dimensions of a velocity is necessary because boosts do not commute, and therefore boosts must be done by dimensionless (mathematical) radians. The constant c converts velocities to radians. The constant c can not be infinite because that would make boosts commute. Giving an object a velocity (boosting) in the x-direction does not ...

0

It is not particularly unusual for physical systems to have speed limits. Consider the classic vibrating elastic string, defined by the equation $\frac{d^2}{dt^2}y(x, t) ~=~ -a\frac{d^2}{dx^2}y(x, t)$ Using that equation, you can see that a small disturbance in one part of the string will propagate outwards at a particular speed. In fact you will see that ...

-3

The mere fact that you can even read this posting is proof that the Speed of Light does in fact vary. The speed of light is related to the permittivity and permeability of free space. The index of refraction is the ratio of the speed of light in a medium and that of a vacuum. IceCubeLab in Antarctica also showed that neutrinos are faster than light over a ...

3

Special relativity is the spacetime geometry described by the Minkowksi metric: $$ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2$$ where $c$ is a constant. The Minkowksi metric is the solution to the equations of general relativity when no mass or energy is around to curve spacetime$^1$. All the symmetries you alluded to are encapsulated in the Minkowski metric - ...

2

There is a fairly general discussion of relativistic aberration on John Baez's Physics FAQ site, and a more mathematical treatment on Wikipedia. The formula telling how the original angle is changed for the moving observer is: $$\cos\theta_O = \frac{\cos\theta_S - v/c}{1 - \cos\theta_S\,v/c}$$ I knocked up a quick graph in Excel to see what happens with ...

1

While some galaxies are billions of light years away, there are hundreds of galaxies withing just a few dozen million light years away. Astronomically speaking, a few dozen million years is pretty brief. We can be fairly certain that the galaxy hasn't changed much in this time.

2

We wouldn't be able to see it if a star went supernova right now because the light wouldn't have reached us yet. If a star 100 light years away from us went supernova 100 years ago we would see it now because the light would have had enough time to travel to us.

28

can we get a false positive of a theory being right just because the instruments doing the measuring have that theory built in? This sounds dangerously close to a contradiction-in-terms, so let me carefully read you as saying that the instruments doing the measuring "are interpreted according to that theory," possibly by calculations that sit between ...

2

Can we tell when an established theory is wrong? Not always. And not everybody. Sometimes one or more people can, and they explain why. But other people won't entertain it, then the "established theory" gets even more established. I was reading the following answer from this question: In physics, you cannot ask / answer why without ambiguity. ...

15

You have to give a concrete example. Experiments are designed so as not to depend on what they are trying to measure. Your speed of light example is not good. Was not the whole scientific community in a dither because superluminal neutrinos were supposed to have been measured? Until it was found that there was a malfunction in an instrument? In any case ...

4

1) In view of the fact that we know how to measure the speed of light, it follows that a change in the speed of light would be detectable. 2) Any change in the speed of light would have to be accompanied by either a change in $\mu_0$, a change in $\epsilon_0$, or (far more drastically) a failure of Maxwell's equations, any of which would be easy to ...

7

This question has sparked some interesting answers, and i'd like to throw contribution in as well. It should be perfectly clear that we are living in a world with a finite upper speed, and many answers have touched upon the consequences of and reasons for this. I would however like to point out an aspect that seems to have been forgotten altogether in the ...

2

There are two things here that aren't quite right. The speed of light is the same for all observers. The speed that light travels is invariant for any observers in any reference frames observing the same beam of light. I can move at 100 meters per second (as measured in one reference frame) in one direction, while you move 256 meters per second (as ...

0

People did pick a round number when they defined the meter. It was defined so the distance from the equator, through Paris, to the north pole was exactly 10000 meters. They also defined the kilometer, centimeter, and millimeter as round number multiples and fractions of the meter. Roundness was a theme. The gram is the mass of a cubic centimeter of water. ...

1

Although the metre is now defined as the length travelled by light in a certain time interval, the unit has a historic origin (one ten millionth of a quarter of the earth's circumference) and more importantly, historic usage. We can't suddenly say that a metre is the length travelled by light in 1/300,000,000s of a second, because then the new metre would ...

0

The reasons are compatibility with older definition based on the prototype bar. The meter is a widely used unit of measurement and it's better if the new definition would reproduce with enough precision the old one or it's will be huge pain in the neck for everyone to adapt new unit. It's ok if the difference is very small. However the "round speed of light ...

2

Well, it's possible to prove (theoretically, and I advice you for Feigenbaum, 2008) that the homogeneity and isotropy of the space and the homogeneity of time lead necessarily to the existence of a speed limit. Let's do that: imagine about taking the Universe and delete (remove) every kind od object. You remain only with the spacetime itself. In this ...

3

This question has a very short answer but uses the assumption that all relativity uses. i.e. The speed of light is constant for all observers. Based on this assumption it is trivial to show that an event horrison is observed at a velocity of c when trying to accelerate infinitely. To answer why this assumption is valid you have to look at the derivation of ...

0

There are two things: The light and the cable. Light moves at speed 18600 miles/second to the north, cable moves at speed 98000 miles/second to the south. The light has nothing to do with the speed of cable. If the observer stands at the end of north, he will read the speed equals to speed of light only. It is to be understood that the light is a ...

0

Note that when you travel by any mean, and even when you stay in a chair, you do travel towards the future, right ? At 99.99% of c you would arrive very fast in your frame, because for everybody else looking you, you were almost frozen during the 4.22 times. But it's not different to the first case above concerning the "pre-existence" of the future. PS: ...

2

Without being able to manipulate gravity. We are manipulating gravity all the time, except on earth, labs and constructions do not allow timing gravitational effects, which is why newtonian gravitational theory which has instantaneous effects is so successful. How do we know that gravity is restricted to the speed of light? or gravitational ...

3

A lot of the answers here are focusing on the wrong half of the problem, I think. They're telling you how we know there's a limit, rather than explaining why it has to be that way. For the most part, there's nothing preventing the creation of a universe with infinite light speed that's otherwise similar to ours1. However, there is one important property ...

3

In contrast to the other answers I'll try to give a simple answer. First, be aware that "Why" is a poor question for modern science as modern science prefers to predict "what" will happen as accurately as possible using "models" of what they guess reality is doing. Speed and Time are heavily inter-related and are effectively under the same "speed limit". ...

0

The answers to this problem can be more easily obtained by having the light source "attached" to the optic cable. This way the frame of reference for all cases, is the optic cable, and its motion will have no effect on the light pulse propagation speed in the cable. Under this condition, it is easy to see that the pulse will take 1 second to travel the ...

1

One can say that this is just an experimental result. That light (and other signals / interactions) do not travel with infinite velocity / speed. One can leave it there and say that is how it is. One can also say, look if you take that variable and do that transformation (e.g rapidity) it can be added ad-infinitum, so the question is around the correct ...

0

There is no mystery about this behavior. A presumed photonic physicist staying on a photon has rules to map space but not any to map time. We can consider that his time unit vector is null. Without a light time unit vector , you cannot build a frame which ie in a Minkowski space, is supposed to associate a 4d origin and a 4d vector space basis generator. ...

6

Do we know WHY there is a speed limit in our universe? Your question is similar to : "Do we know WHY there is a length limit?" The same way we need finite lengths to measure size or interval between two points in 3D - Euclidean space, we need finite speed of light to measure interval between events in 4D - Minkowski space. Minkowski developed his ...

36

The best answer I can come up with is "because the Universe would be fundamentally unpredictable otherwise." We can imagine spacetime as a four-dimensional manifold $\mathcal{M}$; the laws of physics then dictate how matter and energy behave on this manifold. (For the sake of argument, you can view this as plain old flat Minkowski space, though the ...

3

As you have read from other answers, it's not an easy thing to explain. It seems so counter-intuitive. "If I want to go faster, why not just accelerate more?" or "If my speed is very near the speed of light, then I shoot a bullet, won't it be going faster than light? Why doesn't it?" Relativity Let's start by refining what we mean by "speed limit". Let's ...

18

Maxwell made a rigorous, mathematical study of the properties of electricity and magnetism, and he proved that there must be a phenomenon that he called electromagnetic waves. According to Maxwell's theory, an electromagnetic wave must propagate at a constant speed that he called $c$, and which could be calculated from other physical constants that were ...

10

The existence of the speed limit is related to the existence of time [UPDATE: time is a measurement which is only available when $c$ is limited. If you do not agree, provide a way to measure time when $c$ is infinite before down voting]. If there'd be no speed limit, everything would happen instantly. Also, any waves in any matter would not be affected and ...

6

What you describe is the phenomenon called optical dispersion. For example in most optical media the refractive index and therefore the velocity of light depend on frequency. The question is whether empty space shows the same effect. This has been studied as part of attempts to detect any granularity in spacetime due to quantum gravity effects - short ...

31

Physics is a scientific discipline where observations and measurements are fitted with mathematical models which describe existing data and successfully predict new values for new boundary conditions. When this happens one says that the model has been validated. If new experiments and observations should falsify the model, one will have to re-examine the ...

71

There is a wonderful paper you can search for called Nothing but Relativity. That is one of several which use basic algebra only to determine the most general form of the formula to add velocities, based only on general principles of symmetry (what works here also works there, etc.). In the end, it shows that the familiar special relativity is the ...

20

In physics, you cannot ask / answer why without ambiguity. Now, we observe that the speed of light is finite and that it seems to be the highest speed for the energy. Effective theories have been built around this limitation and they are consistent since they depend of measuring devices which are based on technology / sciences that all have c built in. In ...

2

There is a fundamental reason why it might be very, very difficult (not even mentioning the engineering). As you start approaching the speed of light, it becomes harder and harder to accelerate. At 0.5c, this would definitely become a factor. Accelerating from 1%c to 2%c is much easier than accelerating from 50%c to 51%.

138

Imagine that there is a person who prefers to measure the amount of money in his bank account with the value $V$. The equation is $V = C\tanh N$, where $N$ is the actual amount of money in dollars. This person will also be confused: Why is there a limit ($C$) on the amount of money that I can have? Is there any law that says the value of my money, $V$, ...

Top 50 recent answers are included