h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

wanna know abt the speed of lite.it's supposed2b constant rite?i.e. nothin goes faster rite?then how come there is a doppler 4 it?e.g. redshift?or m i mistaken?

The speed of light is only constant in a perfect vacuum - it goes more slowly through everything else, but that's not got much bearing on the Doppler effect. That's caused by the motion of the light source: if it's moving towards you then the wavelength of light shortens and tends towards red, and if it's moving away then the wavelength lengthens and tends towards blue.

so if we are in a vacuum, and I am driving towards the light source, light is slower, and if I am driving away, it's faster? isn't that kind of against what Einstein said about light being the fastest thing?

No the light does not get slower if you're driving towards it. It gets higher in frequency (bluer). It is still the same speed relative to you. If you travelling away from the light source, the light will still reach you at the same speed relative to you, but it will be shifted down in frequency (redder).

Let's say you're in a boat and waves are rolling towards you at a constant rate. If you row the boat into the waves then their frequency appears to increase, and when you row away from them their frequency appears to decrease.

Great metaphor, Mycroft!


Mikey

Sharko, go play on the freeway.

No seriously. You´ll notice that when a car is moving towards you the sound of the engine is more high-pitched than when moving away. That is the redshift thing causing that. But probably works better with an ambulance.

Also you can see that the light from a car moving towards you appears white, while when moving away from you it appears red. But that is simply the bulbs being different colors of course

On a more errr... scientific(?) note: "They" (scientists) say they slowed down light to a standstill (in some atom-fog or something) and then got it back to 'normal' speed. That was some three month ago and made headlines in the relevant mags. Check these out you if you want more info.
As for the Doppler effect: I couldn't agree more.- It's also use in medicine to check on the heart function.
Tube

yo dudes,tanks tons4the fishes.it was much appreciated,u know,i am an engineer by training,but sometimes when u tink abt tings,u jus go crazy,n then everythin becomes real strange.like the other day,i was thinking about primes,n tat lead me to tink abt how computers calculate,and binary numbers,n i tink i m goin2make a new posting abt the proof for binary number theorem,cos i simply can't find the damn thing on the web n i m damn curious2know

How is doppler used for checking heart beats? Sound or light?

Speaking of binary numbers, could someone tell me if they are actually superior to our ten-number system? If you were to deliver information using numbers, would it be faster to do it binary or with tens? Since computers use binary I would think they were the fastest, but with the other you can send off more info in a single digit.

doppler - heart beats: sound. There's a tube placed into your lung (?) to measure the difference of the heart moving in and out, as far as I know. Actually they took that from ships, where the Doppler effect was used to detrmine weak points in the structure.


Tube

To give a conclusion to the matter, I had done more work on this and have the following to report. It is taken from the online version of the dictionary of science by oxford uni press, under relativity:


Since light is a physical phenomenon, its propagation in vacuo could not be used to distinguish between uniformly moving frames of reference. Therefore in all such frames the measured speed of light in vacuo must be the same.

This conclusion has some important consequences for the nature of space and time. In his popular exposition of 1916, Einstein illustrated these consequences with thought experiments. In one such experiment, he invites the reader to imagine a very long train travelling along an embankment with a constant velocity [nu] in a given direction.

Observers on the train use it as a rigid reference body, regarding all events with reference to the train. Einstein posed a simple question: Are two events, which are simultaneous relative to the railway embankment, also simultaneous relative to the observer on the train? For example, two lightning strokes strike the embankment at points A and B simultaneously with respect to the embankment, so that an observer at M (the mid-point of the line AB) will record no time lapse between them. However, the events A and B also correspond to positions A and B on the train. M' is the mid-point of the distance AB on the moving train. When the flashes occur, from the point of view of the embankment, M' coincides withM. However, M moves with speed [nu] towards the right and therefore hastens towards the beam of light coming from B, while moving on ahead of the beam from A. The observer at M' would not agree with the observer at M on the simultaneity of the events A and B because the beam of light fromB will be seen to be emitted before the beam of light at A.

At first sight there may seem to be a problem here. If the observer at M' is `hastening towards the beam of light from B', is this not equivalent to saying that the beam of light is travelling towards M' at a combined speed of [nu] + c, where c is the speed of light in vacuo?

The resolution of this problem is the basis of special relativity. According to Einstein, the moving observer at M' must measure the speed of light in vacuo to be c, since there can be no experiment that distinguishes the train's moving frame from any other Galilean frame. It is therefore the concept of time measurement that requires revision; that is, the time required for a particular event to occur with respect to the train cannot have the same duration as the same event when judged from the embankment.

Xanatic,

the binary system is not superior to our base ten system. But it is easier to implement in hardware. The very first computer designed (although not completed at the time) was Charles Babbage's Difference Engine. It used base 10. But electronic engineers decided that base 2 was easier because it was easily implementable in transistors (1 = high voltage; 0 = low voltage or 1 = current; 0 = no current).

Base 10 is more efficient for transmission but relies on you being able to send 10 different symbols. Most transmission systems can only cope without about 4 so many modems these days use base 4 for transmission and translate it back to base 2 when it arrives.

more discussion of this binary number thing there: or search for some math proofs if the link doesn't work http://www.bbc.co.uk/h2g2/guide/F19585?thread=123623

Two things going through my mind :

first, if I remember correctly and it is not some story a teacher of mine invented to help me remember it, Doppler (the Austrian physicist we're talking about here) put musicians on a moving train and other musicians next to the rails who had to write down the apparent sound of the notes the moving musicians were playing when approaching and receding again. These experiments were used to proof the Doppler Principle. Later he did experiments with both the observer and the musicians moving. The Doppler effect is used for weather forecasts and also to proof if the Universe is expanding or not.

Second, as far as I know from my courses in electronics, the binary system is easier to use for hardware because you only need two states : 0 or 1, or for the circuits involved : low or high voltage in specific combinations of transistors (this 0 and 1 are only used to make it more understandable, it's just the most simple way of putting it)

h³

Hello,

Yes, binary has the advantage because it requires fewer states to represent with voltage (on - off is easier than on - somewhat on - more on - totally on, for example). Still, if you could guarantee that you could represent more states with wide enough error margins, you could go to higher bases.

Programmers don't just do stuff in Base 10 or 2. A lot of debugging is done in Base 16, since it's a compact way for programmers to view the contents of their binary registers (it's binary in the machine, but viewed in a different way on the program). So, 60 would be 111100 in binary, 3C in hexadecimal. And in Octal (base 8) it's 74. Which reminds me of a bad joke...

Why do Computer Programmers confuse Halloween with Christmas?
Because DEC 25 is the same as OCT 31

Yours,
Jake

It's not as much a *bad* joke, as it is one you have to think about... Or maybe I'm just an example of professional disfigurement!

Makes me think of some typical physicist- and engineers-jokes I know (the kind I tend to tell at a dinner table with only engineers and physicist who all drank too much!)...

first one is about an engineer who visits Scotland. When walking around in the hills (with his laptop) he is suddenly surrounded by sheep. Being a typical engineer (i.e. he almost never leaves office and even then wears a tie), he is quite impressed and when he sees the shepherd, he says : "I'm quite impressed, sir, with the amount of sheep you have here!" The shepherd answers: "well, mister, if you can tell me exactly how much sheep there are in my flock, you can take one with you." The engineer is delighted, opens his briefcase, takes out his laptop and starts simulating, calculating and typing. After about half an hour serious counting work, he looks up and says :"632!"
The shepherd says : "that's right! How clever! As I promised : you can pick a sheep now and take it with you". The engineer does so, thanks the old man and starts walking away. After about ten meters, the shepherd calls :"Can I just ask you one more question?" "Of course!", says the engineer. "Are you an engineer?", he asks. The engineer, obviously very proud of himself says :"yes indeed, I am! How did you know?", at which the shepherd responds : "well... you took my dog..."

:-p

h³

what really amazes me is how people come up with these things. which in a way is why doug adams is so amazing.cos he comes up with such intelligent things, like 42, and the babel fish etc. just my two pennys worth.............

Comes up with what? Jokes?

I know that it is also easier to send binary since you only need two states. I just thought maybe it was also superior to ten digits. So an alien sending the instructions to build a Machine would be using as high a number of states as possible then?

That doppler thing with the musicians sounds funny.

Hello,

Probably it would use binary too, since using more states decreases the margin of error between two states (if it's 0-1 receiving a value of 0.95 is obviously a 1, but if it's 0-10, I don't know if it's 9 or 10) and thus could increase the errors in transmission. And intergalactic space strikes me as a noisy medium to transmit through. Of course, there also isn't a lot of bandwidth to use, so increasing the number of symbols would decrease the time to send a message (eg, 59123 is a lot shorter to represent in hexadecimal than binary). But who knows, aliens may have better technology or prefer different bases (the Sumerians used Base-60 for instance. Not aliens, but an example of different cultures using different bases). How did Contact suggest it?

Yours,
Jake