The speed of light is not the fastest.

[Guest]

Don't black holes have gravity stronger than the speed of light? Which is why light can't escape their gravity?

[t00fri]

Don't black holes have gravity stronger than the speed of light? Which is why light can't escape their gravity?

Whoever you are, but you cannot compare the strength of a force (gravity) with a speed (light).
Both are dimensionful quantities and thus their dimensions should match if you want to compare them.

Someone should display a Penrose diagram for illustration of the capture of light by a black hole...You find it in every textbook on the matter.

or here:
http://origins.colorado.edu/~ajsh/schw.shtml


Bye Fridger

[cpotting]

Don't black holes have gravity stronger than the speed of light? Which is why light can't escape their gravity?

The problem there is a little grammatical sematics. It is not that 'gravity is stronger than the speed of light' - a force can't be stronger than a speed - it is that the escape velocity for a black hole is greater than the speed of light. Nothing, not even light, can move fast enough to not be pulled back to the black hole's "surface".

[thearbiter]

if ur saying that microwave light is faster than the speed of light isnt that contradicting itself because thats saying light is faster than light itself. its like saying that blue light is faster than yellow light. their are just different forms

[eburacum]

These experiments allow the group velocity of the waves within the lightbeam to travel faster than light; but it is just a trick, acheived by manipulating the waveforms to add together in a particular way- it is just a matter of a progressive waveform travelling down a lightbeam.
The wave form is created by successive waves of various frequencies which add together to make a group waveform with a velocity faster than light; but this wave is created in a light beam which is itself travelling only at light speed.
You can't send any information using this method faster than light; it is just clever use of constructive and destructive interference.

[dirkpitt]

Boy, every time I visit the Celestia forum someone has posted to this thread. IANA physicist (but I've read some courses) but I do hear that until someone brilliant either truly understands quantum physics or unifies quantum and relativity then we're stuck on the issue of FTL (Faster-Than-Light). Wormholes seem to be the best bet, but it looks like they need negative energy or antigravity and whenever someone brings that up then the quantum physicists start jumping up and down saying it's all wrong... the situation in the physics field is hopeless as it is now.

[Drake]

the situation in the physics field is hopeless as it is now.

Erm, well, I don't know how you get to that statement. The two theories that humans have developed that have been proven with countless experiments from elementary school kids to physics powerhouses and have been shown to be accurate down to insanely small errors are General and Special Relativity and the current incarnation of the Standard Model - both in physics. To describe that as hopeless tosses away a magnificant achievement of human intellect in understanding reality.

To rephrase your statement, these are fascinating times for physics, where we are beginning to have the ability to probe even these theories to their most extreme ramifications. Are they complete descriptions of reality? Nope. That is what is so cool. These two theories govern the macroscopic universe and the very smallest scale, work perfectly in those areas, but have yet to meet. We live in a time where humans have for the first time begun to understand how the universe happened and how it works today by using verifiable data rather than subjective fairy tales.

What is unutterably beautiful is that even with what we have learned, we can still have competing hypotheses for these grand ideas that fit all available data, but it may be within our lifetimes that data is able to winnow these down. Or, even more exciting, open up even more amazing conjectures.

Hopeless? No, the situation in physics is hopeful.

[dirkpitt]

Ha ha, I guess you're right. I was being pessimistic. I was trying to say that physics as it is is still very partisan. The two parties of Classical (Relativity) and QM are polarized and only in recent years has at least an interdisciplinary minority emerged to challenge these two theories.

Just four years ago my E&M prof was saying, "Membranes, M-Theory? What a load of bull" while iconoclasts at other schools were trying to get people to listen to how dark matter could be explained by gravity waves transmitted between folded branes. There are still a lot of conservatives in the field, and as long as they retain control of how physics is educated, we may have to wait a while before we see any results. It's great that we've finally got around to doing all these gravity experiments recently, but we have to do much, much more.

[Spaceman Spiff]

I think dirkpitt raises an interesting point about the state of physics today, which Drake does a nice job about explaining why the apparent gap between Relativity and Quantum Mechanics is something to appreciate.

I've tried in an earlier post to this thread to get people querying this 'can't go faster than the speed of light thing' to understand where it came from by pointing them back to its origin: the Michelson-Morley experiment. I haven't seen any feedback that anyone's doing this.

The 'incompatibility' of Relativity and Quantum Mechanics in modern physics is a bit like the state physics was in end the end of the 19th Century. There were several strange problems that posed real difficulties.

One was the problem of light traveling through an 'ether'. This is what the Michelson-Morley experiment challenged: if light travels through an ether, and so does the Earth, why do we always see light travelling at the same speed anywhere, no matter which direction the Earth travels? This was 'solved' by Einstein's Special Theory of Relativity.

Another was the 'Principle of Equivalence'. Initially noted by Galileo, it was: why are mass and inertia always equal, so that all masses fall towards other masses at the same rate, no matter how heavy they are? This was 'solved' by Einstein's General Theory of Relativity.

And another was the 'Ultraviolet Catastrophe'. This was based on the theory that light was (only) made of waves, and the theory predicted that any hot object should always radiate more light with shorter wavelengths (like UV) than longer (like IR). Yet, the experiments showed that there was a certain wavelength in the middle which had the most light radiated. There was also the photoelectric effect: why are electrons freed from semiconductors in response to short wavelength light, no matter how dim or bright the light is?

The solution was proposed yet again by Einstein, and this is what he really got his nobel pize for (not Relativity!). He proposed the light was being emitted as particles with a particular 'quantum' of energy - the beginnings of Quantum Mechanics.

Of course, other famous scientists were also responsible for great work in these problems.

The point is, if you want to understand why we have the theories we have now, you should find out what problems they were originally meant to solve.

You might note that we seem to have come back to a similar position: great mysteries that are nagging us. You might be able to guess what happens next. However, it should be realised that the problems of explanatory power by Relativity and Quantum Mechanics are partly because:

1. Relativity describes the behaviour of very fast or very heavy objects, while Quantum Mechanics describes the behaviour of the very small. So, the two theories don't overlap in application very well.

2. The theories (especially General Relativity) have some extremely difficult maths behind them, which means part of the problem is making the calculations to find out what exactly the theories are predicting...

Spiff.

[Cham]

2. The theories (especially General Relativity) have some extremely difficult maths behind them, which means part of the problem is making the calculations to find out what exactly the theories are predicting...

Spiff.

Actually, Quantum Mechanics is more difficult than General Relativity. The myth about GR comes from the fact that it was discovered before QM (1915 vs 1925).

The problem with both theories and their incompatibility is the stage in which physical events are taking place : space-time.

In GR, space-time is a physical "object". For the theory, future already exist. It's "there", in space-time. In some mathematical sense, it "coexist" with the past. All events are already there, in space-time. Therefore, there is no place for freewill, which becomes only an illusion.

In QM, space-time does not really exist. Events are created continuously from the past to the future, and the past is erased continuously. There is no future "coexisting" with the past. And observers have access to the "free will". They can choose freely what to observe and how they'll make experiments.

I think that, fundamentaly, this is why both theories aren't compatible. Both are only approximate "maps" of nature. Do space-time exist, or not ? Is there any freewill, really ? I can't tell.

[fsgregs]

As a high school teacher with only a modest and somewhat timeworn understanding of Special Relativity (its been many years since I studied the topic), and no understanding of the complexities of General Relativity, I have suggested to my students that I would assemble a detailed, easy to understand, graphics-laden lesson on Special Relativity. Alas ... the more I try to put one together, the more time-impossible it looks. With the many things I have to do every day (try grading the work of 160 students every day), I simply can't get it together in a way a 17 year old teenager will understand.

Does anyone know of a great website anywhere that has assembled a detailed ... easy-to-understand ... graphics-laden lesson on Special Relativity, including all the pertinent topics (time dilation, mass expansion, space contraction, etc.)??? .... Yes ..... maybe .... hopefully .....

Frank

[dirkpitt]

This site appeared several years ago and impressed me:

http://www.anu.edu.au/Physics/Searle/

Basically it talks about a 3d renderer that was used to generate scale-accurate pictures of what things would look like at relativistic speeds. I'd imagine this could be a pretty fun way to teach many (but not all) of the tricky special relativity phenomena such as doppler shifting, contraction, rotation etc.

Now what if Celestia could render accurately--and at real-time--the same kinds of optical effects when traveling at relativistic speeds...

[Drake]

I would add to Cham's very interesting observation that part of the reason GR and QM are hard to resolve is the huge difference in magnitude of the forces involved. Gravity/accelleration is way by far the weakest of the forces, so QM can easily ignore gravity at the scale of the other forces. My favorite example of this is a paper clip. I tell my daughter (5 and precocious) that it takes the whole mass of the Earth to make the paper clip weigh what it does, but a tiny magnet can pick it up against that. If the strong force is "1" unit strong, the electromagnetic is 1/137, the weak is 10^-5 and gravity is 6x10^-39! Yowsa! See http://hyperphysics.phy-astr.gsu.edu/hb ... unfor.html for more. But gravity and electromagnetic go on forever, the weak only to 10^-7 and the strong to 10^-5 meters, so we bigguns feel gravity as primary since we live on a big chunk 'o mass, electromagnetic second, since we like to play with electricity nowadays, and not the others since they affect only across tiny distances.

And Cham, the free-will/time paradox is resolved if you postulate all possible choices being made in so-called "alternate universes." So far this is the only way out of the discussion of an observer affecting outcomes. So all possible pasts, presents, and futures co-exist, yes. But there is free will in that the choices "we" make choose the reality we experience. So saying that "quantum mechanics made me do it" is no guarantee of a happy life, or of a successful criminal defence!

[julesstoop]

@fsgregs:

http://www.aliceinphysics.com/

[fsgregs]

Thanks folks. I've gone to the sites you recommend and they collectively are excellent. I will now be able to put something together,

Thanks again.

Frank