Physics Question

Podge_irl

Decerto wrote: »

It was all hypothetical anyways like i said, because calrac asked the question + i know the basics of special rel or at least im supposed to after doing the course in college, ive got another question aswell about inertial ref frames, does the fact that since the earth is rotating and in orbit around the sun and the solar system is in orbit around the black hole at the centre and also since the universe is expanding not completely invalidate the idea of an inertial reference frame since everything is constantly accelerating or are these effects countered or neligible

It doesn't invalidate the idea of an inertial reference frame, it just means we're not in one. As far as I understand (and it wouldn't be a first if I was wrong) there is no actual inertial reference frame in the universe. But then you could equally say that SR is always incorrect since gravity is a long range force and you thus can't discount its effects. It provides a good approximation under the right conditions.

Decerto

Podge_irl wrote: »

It doesn't invalidate the idea of an inertial reference frame, it just means we're not in one. As far as I understand (and it wouldn't be a first if I was wrong) there is no actual inertial reference frame in the universe. But then you could equally say that SR is always incorrect since gravity is a long range force and you thus can't discount its effects. It provides a good approximation under the right conditions.

the amount of times ive heard that in physics

Professor_Fink

Decerto wrote: »

ive got another question aswell about inertial ref frames, does the fact that since the earth is rotating and in orbit around the sun and the solar system is in orbit around the black hole at the centre and also since the universe is expanding not completely invalidate the idea of an inertial reference frame since everything is constantly accelerating or are these effects countered or neligible

Well, the acceleration cancels the effect of gravity, so freely falling frames are inertial. Since the planet is in free fall, it constitutes an inertial frame. The local effect of gravity from the mass of the earth means we don't experience a truely intertial frame unless we do something like parabolic flight or go into orbit. On the other hand, the earth's gravity is so week it doesn't really change things.

General relativity is required to deal with regimes with strong gravitational fields.

Decerto

Decerto wrote: »

the amount of times ive heard that in physics

An intertial reference frame is basically just a frame where the laws of physics work in their simplest form. They don't hold in their simplest form around gravity because, for example, you'd have to take the force of gravity into account in your equations of motion etc.

But, as Professor Fink said, a free falling frame cancels out the force of gravity. So, that's inertial. Likewise, the shuttle orbiting earth "doesn't" feel the force of gravity, as it's in freefall, so that's an intertial frame of reference. Or at least they're as close as we can get!:pac:

carlrac

If nothing is faster than light, how come it can't escape a black hole?

Also, wasn't it scientifically accepted years and years ago that it was impossible to travel faster than the speed of sound!?

carlrac

carlrac wrote: »

If nothing is faster than light, how come it can't escape a black hole?

Without getting into too much detail, a blackholes gravity is so strong that light can't escape. It doesn't have anything to do with its speed.

Also, wasn't it scientifically accepted years and years ago that it was impossible to travel faster than the speed of sound!?

I'm not sure about that, perhaps it was. But, it's now accepted and proven mathematically that nothing with mass can travel at, or faster than, the speed of light. If something with mass travels at the speed of light it becomes infinitely massive, infinitely thin in the direction it's travelling, and time stops for the object.

pljudge321

carlrac wrote: »

If nothing is faster than light, how come it can't escape a black hole?

Also, wasn't it scientifically accepted years and years ago that it was impossible to travel faster than the speed of sound!?

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html

This page will answer pretty much any questions you could conceive.

Professor_Fink

carlrac wrote: »

If nothing is faster than light, how come it can't escape a black hole?

Well, every massive body has an escape velocity associated with it. For a black hole, the density is sufficient for the escape velocity to be c at the event horizon. Anything within it cannot escape, since nothing travels faster than c.

carlrac wrote: »

Also, wasn't it scientifically accepted years and years ago that it was impossible to travel faster than the speed of sound!?

No. We've had things that travel faster than the speed of sound for hundreds of years. The tip of a bull whip breaks the speed of sound (hence the crack). Some people thought it would be essentially impossible to engineer a plane to fly faster that the speed of sound, but that was a concern about engineering limitations, not what was fundamentally possible.

lurrrvs2sp00ge

eeemmmmmm, when did this stop being about the whole train and bullet thing

lurrrvs2sp00ge

lurrrvs2sp00ge wrote: »

eeemmmmmm, when did this stop being about the whole train and bullet thing

A long time ago!

I just thought of something interesting there. If a photon moving at c could be used as a frame of reference, would the entire universe be "destroyed"? Because now, relative to the photon, the entire universe is moving at c, so it'd have infinite mass and it'd be lorentz contracted to be infinitely thin in the direction the photon is moving, relative to the photon of course.

I know that this question is completely and absolutely theoretical. But, I suppose that's why a photon can't be used as a frame of reference, yah?

Professor_Fink

-JammyDodger- wrote: »

I just thought of something interesting there. If a photon moving at c could be used as a frame of reference, would the entire universe be "destroyed"? Because now, relative to the photon, the entire universe is moving at c, so it'd have infinite mass and it'd be lorentz contracted to be infinitely thin in the direction the photon is moving, relative to the photon of course.

Well, things aren't quite so simple, since a photon doesn't experience time or space (because of time dilation/length contraction), so it is meaningless to ask about energy, inertial mass, etc. You see the problem? You can't ask about dynamic properties if you don't have time.

Professor_Fink

Professor_Fink wrote: »

Well, things aren't quite so simple, since a photon doesn't experience time or space (because of time dilation/length contraction), so it is meaningless to ask about energy, inertial mass, etc. You see the problem? You can't ask about dynamic properties if you don't have time.

Ya, I get you. But, I always thought length contraction/time dilation just applied to massive objects? Does it apply to the photon as well?

Professor_Fink

-JammyDodger- wrote: »

Ya, I get you. But, I always thought length contraction/time dilation just applied to massive objects? Does it apply to the photon as well?

Length contraction is space contracting, not the specific object contracting, so in that context5 everything contracts.

Professor_Fink

Professor_Fink wrote: »

Length contraction is space contracting, not the specific object contracting, so in that context5 everything contracts.

I know these are pretty basic questions! But, does the space contract locally (I don't think that's it); or does the space everywhere contract relative to the photon (I'm afraid of saying relative to the photon, because it can't be used as a reference frame)?

Professor_Fink

-JammyDodger- wrote: »

I know these are pretty basic questions! But, does the space contract locally (I don't think that's it); or does the space everywhere contract relative to the photon (I'm afraid of saying relative to the photon, because it can't be used as a reference frame)?

I may have phrased my las response poorly, so let me try to explain what I mean. Imagine you have a collection of massive objects at rest relative to one another. As you approach the speed of light, the distance between the objects contracts in the direction of motion. At c, all these objects lie in the same plane since there can be no distance between them (in the direction of motion). This is true for all massive objects, so the photon basically is everywhere along the geodesic at once.

Professor_Fink

Professor_Fink wrote: »

I may have phrased my las response poorly, so let me try to explain what I mean. Imagine you have a collection of massive objects at rest relative to one another. As you approach the speed of light, the distance between the objects contracts in the direction of motion. At c, all these objects lie in the same plane since there can be no distance between them (in the direction of motion). This is true for all massive objects, so the photon basically is everywhere along the geodesic at once.

Ah I see, that makes it pretty clear. Any book recomendations about the consequences of SR, or indeed GR? If you know what I mean. I've attempted to read Gravitation by MTW, but, I'm stumped ~200 pages in; guess I'll have to wait until college to gain the relevant knowledge in maths.

timbrophy

-JammyDodger- wrote: »

Any book recomendations about the consequences of SR, or indeed GR?

I can highly recommend "Spacetime Physics" by Taylor and Wheeler as an introduction to SR. It does not shy away from Maths but anyone with Leaving Cert Higher Maths will have no trouble following it. GR does demand a much higher level of Maths but "Black Holes and Time Warps" by Kip S. Thorne does provide a very good introduction to GR without the Maths. You will understand GR after reading this book, you just will not have the mathematical equipment to analyse various scenarios. Remember that Einstein could not do the Maths himself, he had the ideas but needed help with the Maths.

Professor_Fink

There is a really nice really short book by Dirac on general relativity. Only costs a few quid, and is excellent, although not as gentle an introduction as it could be.

Thanks guys, I'll take a look into both.