Radical Edward

ok that's great, thanks. I thought it would be down to something like that.

I thought that was just an experimentally observed fact rather than an axiom

well a mathematician wouldn't use the word axiom

to put it another way, Special Relativity is simply Galilean relativity (or invariance) reformulated to include Maxwell's Equations, and General Relativity includes the equivalence principle.

The idea of Strange, but a bit simplified:

 

The speed of light is the same for all observers. A rocket flies by, and sends a lightbeam in the direction of its flight. So the astronaut measures a speed of c in the direction of his flight. I see the lightbeam also with a speed of c. If the rocket would be faster than c, I would see the astronaut measuring the light beam behind him, but for the astronaut it would be in front of him. That is logically impossible, so I will see the rocket always with a velocity lower than c.

on thing worth mentioning here is that you see a red beam of light moving away from your rocket, whereas someone on the ground sees a blue beam of light (or something blue shifted anyway, depending on how fast you're going and what you emit)

I have held a long time (50 year!)inclination to believe that there should be no universal speed limit.I feel that I have seen the error of this (unconscious) disposition.

 

Let's hope this sticks☺

essentially the speed of light is 1/sqrt(mu*epsilon) which are invariant of the speed of the observer. everything else just falls out of that.

Try to explain why nothing can go faster than speed of light without using the term relativistic mass.

Can it be done?

it's not hard. You do need to start off with the axiom that the speed of light in a vacuum is the same for all observers though. Some people struggle with that bit.

I'm really not working in this area at all, so my thoughts might be right off, and if there is good reason, I'm quite happy to abandon this line of enquiry... anyway, on to my issue with gravitons.

Firstly, by the equivalence principle if we imagine ourselves in a box which is being accelerated, we cannot tell whether it is being accelerated at 1g towards a planet which is below us, or accelerated at 1g by a rocket moving upwards (relative to the box) in free space - both cases in an otherwise empty universe. in my box I have a hypothetical 'graviton detector' which is good enough to detect gravitons from a planet (in reality no such thing could exist, because you'd need a detector the size of jupiter orbiting a neutron star for 10 years just to make a single measurement)

now if we consider gravitons being emitted from the planet towards the box, we have lots of sources - a whole planet's worth. but if we consider the empty universe with a box on a rocket, then there is no source.

so what is the source? when we consider other force carriers, like photons, they are emitted from a source (accelerating electron/proton or something like that), but with the graviton it is not so trivial. Both of these cases should result in an ideal detector as mentioned above detecting the same number of gravitons, but where are they coming from? First thoughts are something related to the curvature of space itself but it's not clear where to go with that.

yep. The antiknot springs to mind. It is impossible to tie an antiknot.

Definitely a different beast. Which is why I've been arguing that we operationally define time, for scientific purposes, in terms of clocks. There doesn't seem to be a more fundamental definition than that.

Rather we quantify time based on oscillations because it is convenient. Another, perfectly valid oscillationless and far more fundamental definition of a unit of time is that of the planck time; the amount of time required for light to travel a planck length. We can use that as the unit length on a time ruler.

Why challenge it and check it? Because that's what scientists do. Asking "is this really constant" or "what is the range of validity of this concept" are some basic things a physicist would ask. It's a challenge to measure something very small, so new measurements are always pushing some part of an experiment to new levels of precision.

 

That's not one I've ever heard. There are a bunch of other physical interpretations

http://en.wikipedia.org/wiki/Fine-structure_constant#Physical_interpretations

He's referring to the interpretation put forth by Feynman.

Drag.

I'd say No ,the goldfish is not in heaven unless it accepted Jesus Christ. If it didn't It, it will be in hell, burning, forever. Best to traumatize kids early on about how horrible Christianity is.

My personal favourite is Leibniz

 

Despite Netwon's brilliance in understanding the domain of the universe that was available to him, Leibniz's vision was more creative and stretched further. His relational views were diametrically opposed to the notions of absolutism held by Netwon and provided the philosophical foundation stones for the theories of relativity and quantum mechanics. The central tenets of Leibniz's philosophies are particular valid today in theoretical physics such as the 'identify of indiscernibles', the 'identify/contradiction' and the 'Principle of Sufficient Reason'.

Leibniz is also a delicious biscuit.

Wow, it's almost a decade since I started this thread. It's nice to see it is still going.

Hey everyone,

 

I have a few questions about physics, engineering and so on...

 

The thing is I'm currently 18 years old... Studying IT and i know quite a lot about computer.. well compared to the most people i know... but that's not important.

 

Since my school didn't really work out the way i wanted... I wasn't able to get that much into engineering, physics and robotics as much as i wished...

 

So the question is if i want to start learning about all this stuff, where do i begin? there is just so much knowledge i'd like to learn but i'm lost and have no idea where to begin :/

 

So if you could help me and point me of what should i start with, it would be great.

 

I'm almost asleep and i'm not native English speaker, so i hope this post is at least a bit understandable

Hi there. To be honest I wouldn't worry too much about science education prior to 18 years old. Being blunt, most high school curricula are crap. If there are any particular areas of science that you are interested in, then look up university books, or increasingly commonly, online university courses like those that can be found on iTunes. The Khan academy is also an excellent resource for quite a few things. Don't forget of course getting involved in places like here!

Well it is a bit unusual - as we're using OLEDs, devices from different areas of the glass substrate have slightly different thicknesses and operate at different voltages, however the current which flows at a given brightness is always the same. Hence if we have a fixed voltage supply of say 3V, then all the devices will be a different brightness, but if we have a fixed current supply of 0.2mA, then they will all have the same brightness. That's the reason that I am using the circuit above. The problem is that sometimes I want to run a load of devices at 0.2mA and other times 0.3mA and so on, and trimming every device manually would be an utter pain.

You have to use General Relativity.

I'm currently using this simple circuit to power an organic LED that I want driving at constant current, using a 330 ohm resistor (so I supply 0.2mA)

putting the OLED in the position of R_L

Now if I want to be able to change the current, I suppose I can just put a variable resistor in there, but what I really want to do is to have a single control that allows me to change the current for a whole load of these circuits with OLEDs in them. I guess I could just put the OLEDs in series, but as I am testing devices, if they're in series and one goes, then they all go and my experiment is ruined. Does anyone know how I might construct such a circuit?

The goal of the Planck Telescope is primarily the analysis of the Cosmic Microwave Background, however some recent images from Planck show the huge amounts of interstellar gas that exist.

We typically see interstellar space as pretty empty, with occasional nebulae and stars, but as we see from these images of the Orion Constellation, that's pretty far from the truth

Just as interestingly, the light that we see there is actually stuff that has to be discarded in order for Planck to do it's prime job, though of course there will still be researchers who are very interested in these images as a part of their work.

http://news.bbc.co.uk/2/hi/science/nature/8645156.stm

nice. what hack is it you use?

The Vatican's apology was a pretty good april fool.

light follows geodesics. Space is not Euclidean.

What I don't understand; the fusion of higher elements, up to iron, is exothermic, or gives off a lot of energy since this heads matter to lowest energy. This means it is energetically favored by nature, to make higher atoms. Why isn't the burning of hydrogen and helium, which give the most exothermic output of all the atom building steps, not enough energy to induce all the exothermic elements to fuse?

 

The modern theory, "this not be favorable in the sun", seems inconsistent with laboratory observation. For example, a modern fission reactor is able to make higher atoms. Uranium is atomic number 92 and we have synthetically made atoms up to atomic number 103, without having to collapse a star. Is the modern theory saying, these scientists have exceeded the sun in the lab and have even rivaled the best supernova?

 

All these synthetic atoms, higher than uranium, are endothermic during formation or need energy to form. It turns out the hardest atoms to make in the universe, are not all that hard to make. We didn't even need the extreme continuous pressures within the solar fusion core. We did it at earth atmospheric pressure. Pressure almost always helps reactions by keeping things closer.

 

Good thing the old timers who made all these higher atoms didn't fall for, "this is impossible we need a supernova", or else they would have had to cover up the ease of higher atom formation, so they don't hurt anyone feelings or be censored.

There are a few issues here.

Yes we can make elements heavier than uranium using accelerators and such, however we only tend to make the very short lived elements, since the options for combinations that we have at hand are very small. Indeed most of the heavier elements are very unstable anyway, as you can work out using the semi-empirical mass formula.

Additionally, To say we did it at atmospheric pressure however is really wrong; we have to use particle accelerators which generate extremely high energies, just like those experienced in the sun.

take Copernicium for example (atomic number 112), it was manufactured by ramming zinc70 into a lead208 target using an ion accelerator. That is not atmospheric pressure.

Finally we can only make these things in very small quantities.

Could you explain how negative refractive indices are achieved?

you do it with photonic crystals, particularly three dimensional ones. The basic principle is that there are lots of tiny holes and chambers etched within a block of material, so that when you find the refractive index, it is negative. You can even do it for larger structures. I think the first negative refractive index materials were made for microwaves (much simpler as the wavelength of light is longer and the structures larger.

Why do Red Giants output more energy than a main sequence star when the reason they have become red giants in the first place is because their nuclear fuel is running low?

main sequence stars burn pretty much purely hydrogen. When they start to run low, then the radiation pressure in the star decreses. Gravity of course still remains (roughly) the same, and so the star collapses some more and the core becomes hotter. Now the core is hotter, fusion of heavier elements can occur releasing more energy and increased radiation pressure, which pushed out more on the outer regions of the star. This fusion is also much faster*. So it is only the hydrogen fuel that has run low, there are still more fuels in there. This goes right up to iron for the largest stars.

*it's an interesting fact, but fusion in stars is actually a very weak and slow process. Per kilo, the sun radiates less heat than you do.

What.. wait, really? Uh.. I was always under the impression that human eye has the nerves on the back of the eye, which is the source of our "blind spot".... I.. guess I'm wrong?

 

(I *knew* I should've taken bio instead of chemistry as my science non-physics requirement..)

the rods and cones point into the skull, with the nerves passing over the top to a bundle which then punctures the retina at the blind spot. with cephalopods, the receptor cells point outwards, the nerves run underneath and there is no blind spot.