stupid physics questions

[disciple]

How come nothing can travel faster than light? Like how come?

And why's the limit 186,000 miles per second, why not 187,000?

Anyone know this?

[RedOktober]

Things are always changing. At one time it was illegal to go faster than the speed of sound.

[gravy]

The plane never takes off from the treadmill

[Zen-Chan]

From what I remember at school (which was not yesterday) the speed of light happens to be the speed of any massless particle in a vacuum - and some sort of formula related to e=mc2 dictates that any particle with mass would require an amount of energy tending towards infinity in order to reach the same speed - and thus nothing can ever actually do it.

My understanding of this may be blown to pieces when cleverer people post

[disciple]

Yeah that's my understanding that as speed increases so does mass, this is imperceptible at normal speeds but as we near light speeds mass increases exponentially so that at the speed of light the mass would be theoretically infinite.

But why 186,000 mps? And why does it happen? It probably comes down to mass is energy but I don't really understand what that actually means.

I suppose if the above is the limiting factor then like you suggest, a massless particle(?) could possibly go faster than lights?

[F-ZeroOne]

I think the book "Why does E=mc2?" might help you. I've read it, and would give you the answer, and would do if I had understood more than about three words a page. IIRC, the short answer is "there is a clause".

[gilesgoat]

I think the 'standard reply' goes around 'because it takes an INFINITE amount of energy to accelerate something with mass
to the speed of light, therefore nothing having a mass can run at the speed of light'.

I think ultimately it's all down to some equations where something approaches 0 or 1/0 or stuff like that.

I think it would also mean that something with mass accelerated at the speed of light would have infinite mass or
something like that.

I am sure that people like Mr Dom can give you a much more detailed explanation.

Now .. one thing I never understood is what the hell is the electron for example.

Ah photons do not have any mass.

So I still don't understand "if they don't have any mass I suppose it means they are made of nothing so how comes light
do exist ?".

[Absolute_Zero]

^ If you could catch a photon just lounging around, then you could say it had zero mass <-- rest mass. Otherwise, in flight: m = hf/c^2

OP, what is speed? Velocity = distance over time. You might want to focus on spacetime.

I suppose the answer you're looking for is that it relates to the permittivity/permeability of space.

c = 1/sqrt(permittivity*permeability of free space)

[Kelloggs]

I had an answer sorted, but Absolute_Zero has covered the gist of it - it might still be useful for someone, though. There's two questions here that I can see; "why can't anything exceed 'c'?", and "why is 'c' 186,000 miles per second?"

The 'c' limit is a mathematical one (and "limit" is more correctly used it its mathematical sense of an asymptote, i.e. something you can't reach), and it comes from a handful of different places, and the energy needed is the "can't be reached" bit. It's a concept at which maths goes odd, and divisions by zero start to take place, so we just sort of handwave round it and call it the symbol 'c' so people can talk about it. Photons can do it (almost by definition; they are, after all, light) so it's not that nothing can travel at it. Things might go faster than it, but we can't know about them, because we don't know how to look for them (the general name for this class of theoretical objects would be tachyons). The only thing we can meaningfully ask questions about is the slower-than-light particles which we can interact with (tardyons, if anyone's taking notes.)

As to why it's 186,000 miles per second: why not? It's an arbitrary number of arbitrary units (divided by other arbitrary units) and it's not even 186,000 miles per second, anyway (but I can't remember more exact than that). If you did it as metres per second instead, it's 298 point mumble million. It's all arbitrary, but it's not a made up value; it's also to do with the permittivity of free space (which we can measure, but we actually just define as a particular number) and the permeability of free space (which we can also measure) and the more accurately you can measure that, the more accurately you can derive c. (Again, for the note-takers: permittivity is how easily an electric field can be set up in something; permeability is how easily a magnetic field can. It's electromagnetic waves after all, these photon things...)

[pogo]

Why 186,000 mi/s, or about 300,000km/s, is because that's what it is - it's just a physical constant and if it were different by e.g. 10% we would probably not notice. On the other hand the 1km is 1000m and by definition the metre is the distance travelled by light in 1/299,792,458 of a second. So it has the value it has (299,792,458 m/s) because we've defined out metre like that (to match the previous definition). In relativistic maths it's usual to think of space and time as the same thing, different dimensions of four dimensional space, and the speed of light is just a scale factor between the dimensions. With this in mind in natural units the metre and second are usually chosen to coincide, so the speed of light is just 1.

[Kelloggs]

Ta, pogo; 299 point mumble million, not 298. Serves me right going from memory, and mixing it up with gravity at 9.8 metres per second squared, not 10, and getting lost in thought (unfamiliar territory as it is).

[Dio]

Well, that's one of the biggest questions in physics: where exactly do these fundamental constants come from? It may be that some or all of them fall out of some future more mathematical theory, but right now they're just fundamental constants.

Of course, if you get to this point, you also have to look at the constants where if they weren't just right, we wouldn't be here, like the fine structure constant. Of course, if we weren't here, we wouldn't notice. Hello Mr. Weak Anthropic Principle...

[Absolute_Zero]

IMHO, ultimately and fundamentally, it's *all* maths. Clue here...

http://www.maths.bris.ac.uk/research/hi ... /random-m/

More specifically, in the fundamental particles of maths: prime numbers

[disciple]

A lot for the layman to take in here already.

With the limit question, for example -273 degrees kelvin is the lower limit for temperature because it's the point where (showing my limited knowledge here) all molecular vibration ceases (or as good as). But I suppose there's no equivalent easy explanation of C, or in fact any upper limit?

it's also to do with the permittivity of free space (which we can measure, but we actually just define as a particular number) and the permeability of free space (which we can also measure) and the more accurately you can measure that, the more accurately you can derive c.

So space is not completely permeable? Isn't it a vacuum? It's almost sounding like ether.

So are we saying that if space was more permeable than it is then C would be higher?

[gilesgoat]

According to wikipedis ..

According to special relativity, the energy of an object with rest mass m and speed v is given by γmc2, where γ is the Lorentz factor defined above. When v is zero, γ is equal to one, giving rise to the famous E = mc2 formula for mass-energy equivalence. Since the γ factor approaches infinity as v approaches c, it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with positive rest mass.[31]