Faster than the speed of light

[LBrandt]

You would never even expect to be length contracted or time-dilated in your own frame. How would that be possible? Even if you choose to consider this, what would be your standard of measurement? In your own frame, anything that you used to measure this would itself also be length contracted or time-dilated. As as example, if your own width were reduced to half its previous value, any ruler or measuring device would be reduced by the same factor. All values, including length and time are always normal in your own frame.

 

[MattRob]

That I understand. Except if you're contracted and you brought a ruler along, then wouldn't it make the rest of the universe seem to expand? Since because your ruler is shorter, it would take more copies of your ruler to reach from point A to point B, so wouldn't they seem farther apart instead of closer? That's what I meant.

Except now that you mention it, according to length contraction you should measure the distance in-between point A and point B to be contracted. But your own contracted length will make it seem just as much longer as it's contraction makes it seem shorter.

But, if you don't measure your own contraction at all not because of impossibility of measuring but because you don't actually contract from your own standpoint, and you contract from other viewpoints, then wouldn't you measure your own ship to be larger than a stationary observer would measure it? (Stationary being the velocity of adjacent celestial bodies),
i.e. your measurement says distance A is 2.2e50 shiplengths and a stationary observer sees distance A as being 6.6e50 shiplengths?

I'm sorry if the tone is coming off wrong, I'm not saying you're wrong, I'm just trying to understand how this works...

 

[LBrandt]

If you are in uniform motion relative to an observer, he will appear length contracted to you, and you will appear length contracted to him, but neither of you will consider yourselves to be length contracted.

 

[DaveC426913]

Your ruler, as with anything inside your spaceship, will be exactly as it has always been.

But the distance to Centauri will be dramatically shortened. Also, everything along your axis of movement will be flattened. i.e Centauri will appear as a flattish disc, rather than a sphere.

 

[MattRob]

So... Hypothetical question, then. Say there's a railroad crossing in North Carolina, and the train is somehow moving at relativistic speed along the rails. There's a car in N. Carolina about to cross the track. From the car's point of view, only going 25 mph, the train is contracted to a length of only a few feet, and is somewhere back in New Jersey, meaning the track ahead of them is clear and they can safely cross. (I know that's not far enough away, but for this point let's pretend New Jersey is a light-minute away).
However, from the train's point of view, because the distance from New Jersey to North Carolina is contracted, the front of the train is on the railroad crossing in N. Carolina when the car crosses.
But from the Car's point of view, because the train is contracted, the front of the train is somewhere in New Jersey.
So what happens to the car when it crosses?
From the train's point of view, it will smash into the train engine.
From the car's point of view, it will simply cross safely.

That's why I have such difficulty understanding length contraction as it is...

 

[Dale]

From what you describe here I think that your confusion is with the relativity of simultaneity, not length contraction.

 

[DaveC426913]

From what you describe here I think that your confusion is with the relativity of simultaneity, not length contraction.

Yeah, I started to describe why the length-contracted train was not still in New Jersey but realized my answer would only make it more confusing. I thought I'd leave it to someone who could be more eloquent and succinct.

MattRob, suffice to say, at least for now, that the front of the train is exactly where it is expected to be for the car river and the train engineer. So, car go boom.

The confusion comes when the car driver attempts to time when the caboose reaches the crossing. And timing events is where relativity gets tricky. You cannot assume that you simply "see" everything (such as the entire train). You must take into account distances and times when you examine properties about an object moving relative to you.

 

[vish_al210]

I don't think tha tlight travels but rather it propogates... by exchange of energy. If an object needs to move it needs to overcome a lot of factors, but if the light energy emitted by it or sound energy or heat energy emitted by it needs to propagate then it only needs to exchange the energy, but then the question is what about vacuum, well heat transfers through vacuum and so does light. but still it is transfer of energy... Hence they are much faster..
All equations, postulates and proofs are only extensions of what we can see, observe and understand. As we understand, observe, see more and more, equations change, exceptional cases are putforth and so on... so wait.. and the equations would change... someday when we understand and pick a different reference than C, things would change... equations would change, then that new reference would be the limit... remember at one point in time 'sky' was the limit...

 

[acarpenter]

the only thing that can travel at the speed of light, is something massless, eg a photon.

however what i fail to understand is that photons have a rest mass of zero. yet photons have to be in motion and cannon be at rest. photons not at rest have mass. so therefore how can a photon be traveling at the speed of light when surely it cannot be Massless??

 

[Dale]

It sounds like you may be confused by the various definitions of mass:
invariant mass: sqrt(E²/c⁴ - p²/c²)
rest mass: a less precise name for invariant mass
relativistic mass: E/c²

 

[Antymattar]

Funny thing is that the speed of light has been observed to be slowing down.
The speed of light is calculated using atomic clocks. Scientists have observed that, in fact, either the atomic orbits are slowing down, or the speed of light is slowing down because every once in a while you have to change the calculations to accurately fit the atomic clocks. Einstein himself said that he would be surprised if the speed of light was an actual constant.

Also, neutrino is slower than the speed of light and light has a small amount of bass itself.

 

[Mr.GaGa]

An example to exceed c has been proposed.

A recent paper is to prove the velocity of motion V in point mechanics should be the energy flow velocity S/w (S_Poynting vector,w_energy density) of the wave theory instead of the group velocity and so on. This velocity(V or S/w) of surface electromagnetic waves in vacuum can be faster than c. It does not lead to any negative and imaginary mass(energy).

 

[Dale]

Hi Mr.GaGa, welcome to PF.

Yes, there are in fact many quantities with units of speed and values > c. None of them can be used for superluminal travel or superluminal communication.

 

[epenguin]

Funny thing is that the speed of light has been observed to be slowing down.
The speed of light is calculated using atomic clocks. Scientists have observed that, in fact, either the atomic orbits are slowing down, or the speed of light is slowing down because every once in a while you have to change the calculations to accurately fit the atomic clocks.

Is that true and accepted? Sounds like a big thing for a throwaway comment.

Anyone else?

Also it seems to me that nothing can be faster than the speed of light, although I know there was a book of that name which I have even read. Also that nothing can be slower than the speed of light. Not for physical reasons but for grammatical ones.

 

[JesseM]

Is that true and accepted? Sounds like a big thing for a throwaway comment.

No, it's a completely silly claim that Antymattar seems to have picked up from a creationist source:

With respect to the fact that measurements made after 1960 do not show any decrease in the speed of light, Walt Brown has concocted his own misinformed "explanation" based on the assumption of two different systems of time:

By way of background, scientists found that it was necessary to revise the length of a "standard" second. The standard second is equal to the number of vibrations of a cesium atom that correspond to a second based on the time required (in seconds) for the Earth to orbit the sun.

The cesium atom vibration frequency is extremely constant. Scientists have constructed instruments which can count these vibrations. By assigning a specific number of vibrations to a standard second, a super-accurate clock can be constructed. However, the cesium clock must be calibrated in order to correspond to the average period of revolution of the Earth around the sun. In order to make the standard second (as defined by the cesium clock) precisely equal to the length of a second based on new and more accurate astronomical measurements, it was necessary to revise the previously selected number of vibrations corresponding to the standard second. The change was extremely minute.

The CSC website speaks of "orbital" time versus "atomic" time as if they were two different systems of time measurement. Because of the necessity to re-calibrate the cesium clock, Brown mistakenly concludes that "atomic" time is "slowing." He states: "If atomic frequencies are decreasing, then both the measured quantity (the speed of light) and the measuring tool (atomic clocks) are changing at the same rate. Naturally, no relative change would be detected, and the speed of light would be constant in atomic time-but not orbital time." Of course, this is complete nonsense.

(many creationists like the idea that light is slowing down because they want to believe the universe was created only a few thousand years ago, but this leads to the obvious problem of explaining how we can see galaxies and supernovas and such that are millions or billions of light-years away)

 

[DrGreg]

Funny thing is that the speed of light has been observed to be slowing down.
The speed of light is calculated using atomic clocks. Scientists have observed that, in fact, either the atomic orbits are slowing down, or the speed of light is slowing down because every once in a while you have to change the calculations to accurately fit the atomic clocks.

Is that true and accepted? Sounds like a big thing for a throwaway comment.

I'm guessing that Antymattar is getting confused with the leap second which we need to occasionally add to compensate for the fact that the Earth's rotation is very gradually slowing down. The second is defined using atomic clocks, not the Earth's rotation which is no longer reliable enough for accurate timing, which is why we have to add leap seconds to compensate.

Technically it's impossible for the speed of light to slow down since 1983, when the new definition of the metre fixed the speed of light at 299,792,458 m/s precisely.