# Tag Info

141

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, $V$, ...

73

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 ...

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 ...

32

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 ...

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 ...

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 ...

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 ...

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 ...

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 ...

8

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 ...

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 ...

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 ...

6

She will feel motionless, seeing the other car approach with the combined velocity of the two cars: $$v_\mathrm{tot} = \frac{v_\mathrm{\scriptsize{1}} + v_\mathrm{\scriptsize{2}}}{1 + v_\mathrm{\scriptsize{1}} v_\mathrm{\scriptsize{2}}/c^2} = 0.8c,$$ with all the usual implications of observing something moving at $0.8c^\dagger$. $^\dagger$E.g. an epic ...

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 ...

4

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 ...

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 ...

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 - ...

3

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 ...

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". ...

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

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 ...

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 ...

2

Mass doesn't depend on time. As in your equation, if the difference in time increases or decreases, the difference in velocity will increase or decrease, so the mass will be constant. As velocity is a variable you cannot come to a conclusion like that. To get a proportional relationship all the other factors should be constant on the equation.

2

Let's replace the fiber optic cable with one laser source and one photodetector a distance $L=186,000$ mi apart in vacuum and at rest relative to each other. The laser source is pointed straight at the photodetector. Alice observes the laser source and the detector moving at constant velocity $v = 93,000$ mi/s $= c/2$ with respect to her inertial frame, in ...

2

This link summarizes the measurements of the speed of light. The first measurement of c that didn't make use of the heavens was by Armand Fizeau in 1849. He used a beam of light reflected from a mirror 8 km away. The beam was aimed at the teeth of a rapidly spinning wheel. The speed of the wheel was increased until its motion was such that the light's ...

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%.

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 ...

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 ...

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 ...

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 ...

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