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

Here's an interesting effect...

You know how if you travel faster than the speed of sound you create a sonic boom? Well, apparently if you travel faster than the speed of light you creat a little flash of light - A "Photonic boom".

I don't mean travelling faster than the speed of light in a vacuum - which for all intents and purposes is impossible - but in another substance.

Scientists have actually done this. They sped a neutron up to near-light speeds in a particle accelerator and shot it into a piece of glass at the same time as some light. The light of course slowed down, while the neutron did not. Thus, in the glass, the neutron was travelling faster than the light and it created little flashes of light.

Yes this is called Cherenkov Radiation (presumably after the chap who first observed or calculated the effect.

I am told, (no idea whether true!) that it is this effect that occours as nuclear material is stored in huge water vats before or after being used in a reactor, the particles emmitted from the sample exceed the local light speed and emmit a faint glow (that which is associated with bad films!).

Have you heard of the Twin Paradox? That's all to do with time dillation as well.

Im not entirely sure, please go on...

If you travel near the speed of light then time stands virually still. The Twin Paradox is that if two twins, A and B were born and Earth and A was sent away at nealry the speed of light for what he percieved was one year then because of time dilation 40 years may have passed on Earth so twin A is 39 years younger than B!

Mad eh?

It may be of some interest to people interested in the nature of the universe, or just plain madmen, that it is not only travelling near the speed of light that affects the flow of time, but also the presence of mass, or gravitation.

I have heard that mentioned, how does that work? Does the presence of a gravitational field slow time relative to an external observer?

Relativity is quite interesting, shame I avoided it as much as possible for fear of failing exams!

But do you know why it is a paradox?

The paradox is that for the majority of the journey, the twin in the spaceship doesn't feel he's moving but he can see his brother (or sister or thinger) receeding away from him into the distance at some speed and then, after the turnaround, he sees him returning to him. So why isn't the guy on the back on earth older than the guy in the space ship?

The answer is that the two twins' situations are not identical. Only the guy that moves off in the spaceship experiences the accelerations of take-off, landing and turnaround.

it is true that gravity can distort space-time. If you imagine space-time as a flat sheet of paper, far away from any large objects the space-time will remain flat. Now consider what happens with a planet in the middle of the sheet, the gravity of the planet curves space-time so you end up with a hemi-spherical crater in the middle of the sheet of paper. Gravity reaches to infinity but reduces by
1/(distance squared) so the effect of curved space-time will only be significant close to the planet in the form of a few concentric circles around the crater, like ripples on a pond.

this effect was one of the first proofs for einsteins relativity. light from a distant star was observed and then observed again as it passed close to a planet (eg. venus) between us and the star. it was noticed that the light had been shifted to the red end of the EM spectrum. this fitted in with the equations of relativity which explained that the light has passed through the region of curved space-time reducing it's velocity, it's wavelength and thus it's spectral colour.

gravity can also destroy time. as time gets closer to the singularity of a black hole it gets slower and slower to a point where it becomes quantised. This means that the normally steady flow of time is chopped up into little chunks and can be shuffled about into a different order. This can only happen near the extreme gravitation of a singularity where gravity and mass are infinite.

But say both twins travelled away from a "stationary" point at equal and opposite velocites. If they were both travelling at near the speed of light then their relativistic velocities to each other would be approximately the same to each other as to the "stationary" point.(another quirk of Lorentz transormations) So each one would then see the other as being older even though they have both been subjected to the same situation.
Or am I missing something out somewhere?

Am fascinated by "time" but am not sure I fully undrstand - can anyone suggest an " idiot's guide " for me to study please
thanx

It should be noted that, if I remember correctly, the mass of a singularity is NOT, in fact, infinite. The curvature of space-time is what becomes infinite at a singularity. Black holes and other such known singularities have measurable finite mass. And that's a good thing, too, or else the entire universe would be one big singularity.

maybe, the maths around this is still very hazy!

since gravity is a function of mass you would surely expect the mass to tend to infinity with the gravity, otherwise the whole
g =(G.m)/d^2 is wrong and Sir Isaac was not as clever as we all thought. Also, if the mass were finite then the singularity would not exist, you would have a lump of ultra-dense matter rather than a point source.

if you are looking for an 'idiots' guide Marilla the best one I can suggest would be:

The River of Time - Igor Novikov (an excellent russian theoretical physist who has worked with all the heavyweights in the field)

or the old favourite:

Brief History of Time - Stephen Hawking

I'm not sure about that..

I thought that a singularity had a finite mass, zero size, infinite density and infinite gravity.

In answer to earlier posts:

The glow of neutron emitting materials is actually blue not green.

I don't understand relativity, but an experiment has actually been carried out similar to that of the twins paradox. Two nuclear clocks (super-accurate) had exactly the same time. One was sent around the world on a jet while the other stayed stationary (on the earth's surface anyway). When the first clock returned it did have a slighlty earlier time than that that hadn't moved. Odd!

(Oops! The aricle mentions that last one.)

Gravitation *is* a distortion of space-time. Also, the depression is more likely parabolic than hemispherical.

Try the "Tao of Physics "- by Fritof Capra
This is a most excellent book which includes a history of modern physics and its relationship to Eastern mystical findings from thousands of years ago.
A search for truth and two parties are sent in either direction.
One group, say some Zen monks are sent into the burning intensity of the sun.
The other group, Galileo and his children seek to chart and graph the shadows cast by that sun.
Ultimately , they circumnavigate the whole shabang, and end up right dab in the middle of Capra's book!
Amazing! at your library today!
Ya Basta!

Time dilation does not actually occur as a result of travelling at high speeds, but as a result of being *accellerated* to high speeds. In fact, it occurs whenever the potential energy of a body is altered in some way.
As a mass falls into a black hole (or my pen falls from my desk to the floor) it loses potential energy (which is converted to kinetic energy) and its own personal rate of time is reduced. That is one reason why a light emitted from a body falling into a black hole is drastically red-shifted.
It is also why a body can not be accellerated to light-speed. As you pump energy in to speed it up, the energy is stored as mass (relative to the observer). The more you pump in, the heavier the body becomes, needing even *more* to be accellerated!

By the way, a black hole is *not* the same as a singularity as it would theoretically be possible to have a black hole without a singularity in the centre. This could happen if the solar system was filled with water (I think). Basically, as soon as a body's escape velocity becomes infinite it becomes a black hole.

This is absurd, between which points is speed compared against? and would the exact opposite effect happen if you were to travel towards the origin of the big bang at the same speed you were moving away from it accelerating towards a speed of zero "km/h" and thus creating "anti-light".