Ask a physicist anything.

Wiccan_Child · Nov 15, 2009

Cabal said:
Was wondering if anyone knew what the shortest pangram is?

Wikipedia has a nice list. I like:

Cwm fjord bank glyphs vext quiz.

Wiccan_Child · Nov 15, 2009

Wiccan_Child said:
We had this one about 20 pages back I think .

James while John had had had had had had had had had had had a better effect on the teacher.

I can work out most of these funky sentences, but not this one! Anyone give me a hand?

Bushido216 · Nov 15, 2009

I've got a question.

This is something I posed to my 7th grade science teacher, and didn't get a response.

If nothing can go as fast as the speed of light, how come light can? And how come it is limited at its speed? Why not some other speed? Why not some forever increasing speed?

Also, is it possible that the medium through which light travels, since we've so far not found any, is the structure of space itself?

Thanks.

Cabal · Nov 15, 2009

Bushido216 said:
If nothing can go as fast as the speed of light, how come light can?

Zero rest mass, afaik.

And how come it is limited at its speed? Why not some other speed? Why not some forever increasing speed?

Also, is it possible that the medium through which light travels, since we've so far not found any, is the structure of space itself?

I'll answer these questions in reverse order.

Yes, spacetime affects the flow of light depending on its shape, it's why light can't escape a black hole, and why light from distant stars is "lensed" around stars that are effectively "in front of" them relative to our observing position.

To that end, the speed of light is seen in wave equations describing electromagnetic wave propagation, and it is defined in terms of the product of the....tendency, let's say, of spacetime to propagate electric and magnetic fields.*

Of course, I realise this isn't a wholly satisfactory answer, as one can just relegate the question to "so why does spacetime propagate EM with those parameters", but one can at least attempt to tie your questions together.

*c=1/Sqrt(electric permittivity of the vacuum * magnetic permeability of the vacuum), to be precise.

Bushido216 · Nov 15, 2009

Okay.

Why does spacetime propagate EM like that? =)

Thanks, glad to know at least my idea about light moving through the medium was somewhere near the mark. I'm by no means a physicist and I thought that up a while ago while thinking about something else.

Thanks for the answers mate.

Cabal · Nov 15, 2009

Bushido216 said:
Okay.

Why does spacetime propagate EM like that? =)

I think the inference I epic-failed to infer there was, I have no blinking idea . At a guess I'd say there's some cosmological argument for why the constants are at such values, but what they are, I have no idea.

Thanks, glad to know at least my idea about light moving through the medium was somewhere near the mark. I'm by no means a physicist and I thought that up a while ago while thinking about something else.

Thanks for the answers mate.

No worrehs. One can think of spacetime as a medium in some respects, but I don't think one can say it behaves in the same way as a typical medium for wave propagation (usually a classical fluid, which I suspect was what the aether was expected to be). And it's hard to know whether or not one could have light exist independent of spacetime, but since that would involve leaving the universe....

Chesterton · Nov 15, 2009

In E=MC2, why do we multiply by the speed of light squared? What's the significance of squaring the speed, especially considering that no speed can exceed c ?

Cabal · Nov 15, 2009

Chesterton said:
In E=MC2, why do we multiply by the speed of light squared? What's the significance of squaring the speed, especially considering that no speed can exceed c ?

I'm actually amazed I haven't come across this derivation before....maybe I did waaaay back in sophomore year....hmmm..

Anyway: Special Relativity: Derivation of E=mc^2

C^2 in this case is merely a scaling factor by the way, it doesn't actually mean that something is travelling faster than c, it just means that energy - mass equivalence is scaled with respect to a factor of c*c.

AV1611VET · Nov 15, 2009

Cabal said:
Was wondering if anyone knew what the shortest pangram is?

Mr. Jock, TV quiz Ph.D., bags few lynx.

Cabal · Nov 15, 2009

AV1611VET said:
Mr. Jock, TV quiz Ph.D., bags few lynx.

Proper nouns and abbreviations though!

Wiccan_Child · Nov 15, 2009

Bushido216 said:
I've got a question.

This is something I posed to my 7th grade science teacher, and didn't get a response.

If nothing can go as fast as the speed of light, how come light can? And how come it is limited at its speed? Why not some other speed? Why not some forever increasing speed?

It travels as fast as it can, and that just so happens to be c. We don't know anything else that travels that fast, so we call c 'lightspeed'.

You can actually derive how fast it travels using Maxwell's equations, and it's related to the permittivity and permeability of space. So, c is dictated by the electromagnetic properties of photons themselves and how they interact with space.

Bushido216 said:
Also, is it possible that the medium through which light travels, since we've so far not found any, is the structure of space itself?

Thanks.

Well, you've hit a right niggle there. According to some, everything travels 'through' space. According to others, space is just the distance between points.

But the 'luminiferous aether' idea has been empirically disproven.

Wiccan_Child · Nov 15, 2009

Chesterton said:
In E=MC2, why do we multiply by the speed of light squared? What's the significance of squaring the speed, especially considering that no speed can exceed c ?

It's a constant of proportionality. Dimensionally, energy (measure in joules) can only be equal to something that has the same units (and a 'joule' boils down to kg m[sup]2[/sup] s[sup]2[/sup]).

The equation itself is derived directly from Einstein's postulates about special relativity.

AV1611VET · Nov 15, 2009

When asked about the science party:

Volt thought it had potential.
Einstein said he had a relatively good time.
Coulomb got a charge out of it.
Pascal was under too much pressure to enjoy himself.
Heisenberg may or may not have been there.
Watt was a powerful speaker.
AV1611VET went to the wrong house.
Murphy was afraid something would go wrong.

Cabal · Nov 15, 2009

AV1611VET said:
When asked about the science party:

Volt thought it had potential.

Einstein said he had a relatively good time.

Coulomb got a charge out of it.

Pascal was under too much pressure to enjoy himself.

Heisenberg may or may not have been there.

Watt was a powerful speaker.

AV1611VET went to the wrong house.

Murphy was afraid something would go wrong.

And Schrodinger was cooped up in the basement murdering cats

OR WAS HE

brinny · Nov 15, 2009

Wiccan_Child said:
I'm enjoying these physics-related threads so...

Ask a physicist anything .

i don't know much about physics. What is it?

Chesterton · Nov 15, 2009

AV1611VET said:
When asked about the science party:

Alfred Nobel had a blast.

kangitanka · Nov 15, 2009

AV1611VET said:
When asked about the science party:

Bohr behaved discreetly.
Copernicus was convinced that the party revolved around him.
Marie Curie radiated charisma.
Tesla alternated between drinks.
Planck felt very small.
Newton kept falling down ("It's the booze" he was rumored to complain).
Goodall just went ape.
Arrhenius felt warm all over.
Archimedes found it (although fellow party goers felt that he was all wet).
Carver was just nutty.
And of course, Pauli was just excluded from the party since he was already there.

brinny · Nov 15, 2009

AV1611VET said:
When asked about the science party:

Volt thought it had potential.

Einstein said he had a relatively good time.

Coulomb got a charge out of it.

Pascal was under too much pressure to enjoy himself.

Heisenberg may or may not have been there.

Watt was a powerful speaker.

AV1611VET went to the wrong house.

Murphy was afraid something would go wrong.

hee hee

brinny · Nov 15, 2009

Chesterton said:
Alfred Nobel had a blast.

kangitanka · Nov 15, 2009

On a more serious note, what is electricity?
I know about the various ways to measure it (amps, volts, current, charge, etc, none of which *are* electricity), but what IS electricity?
I've heard from many (including teachers) that it is the flow of electrons through a conductive medium, but apparently that is not necessarily correct in all cases.
So, asking a physicist....what *is* electricity?

Thanks in advance, since I've found nobody (not a single person online or IRL) who can successfully answer this question to date.

Ask a physicist anything.

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