Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Kedly]

Yeah ok, I guess that makes sense, thanks for answering my question 

[palsch]

You can get the speed of light = speed limit result from two basic principles;

1) The principle of relativity.

Put simply, the laws of physics remain the same in all reference frames.

For our purposes we can use the approximations used in inertial reference frames. That is, those travelling at constant velocities with no accelerations. Another way to state it is that no person moving at a constant velocity in an inertial reference frame can conduct no experiment to distinguish between their moving and being at rest.

2) Maxwell's Equations.

If you use the correct form of the equations you can describe a wave equation relating to electromagnetic wave propagation There are a few ways to do this actually, and the maths is pretty fun, but I won't go into that. Wave equations include a term describing the velocity of the wave. In most general wave equations this is a variable. In electromagnetic waves, it's a constant, related to two inherent properties of space (vacuum permittivity and vacuum permeability).

That instantly suggests that light always travels at the same speed, which causes problems with the first principle. It requires that people moving at different velocities each see light travelling at the same velocity relative to themselves. This introduces all of the relativistic weirdness we know and love. From this we can get time dilation, relativistic energy/mass, etc. Between them these are what dictate the hard speed limit and feed into everything else.

Time dilation is the easiest to explain, so I'll go over that here now.

Spoiler: "Enjoy." (click to show/hide)

A simple way to work with this is to use the universe's simplest possible clock. Imagine two perfect mirrors with a single photon bouncing between them. One bounce is one tick of the clock. For the sake of some mathematics later, let's say the distance between the mirrors is 3 of your favourite units of length.

Let's both you and I take one of these light clocks. If we are standing still next to each other we will agree that both clocks are running at the same speed.

Now we start moving relative to one another, perpendicular to the direction the light in our clocks is moving. Ignore the acceleration, let's just say we have gotten up to speed instantly and made sure our light clocks are working. Now for every tick of my light clock you are moving 2 units of length.

From my perspective, my light clock is still running at the same rate it always did. But now I try to look at yours. Now you are moving 2 units while the light tries to travel 3 units parallel to that movement. From simple Pythagoras we can see that now your light has to travel 4 units of length for each tick while mine is still only travelling three.

But light always travels at the same speed; c. My clock is ticking every 3/c units of time. Yours now appears to me to be ticking every 4/c. That's notably slower. Your clock seems to me to have slowed down.

Except it isn't just your clock. Anything else moving away at that speed will show the same slow down. Just from the light clock model it is intuitive to see that most physical events will be similarly slowed down; for one thing it is a good model of how any charged particles interact to make things happen. That all events obey the same delay is fractionally harder to see, but not that hard to accept from here.

Watch what happens as someone starts to approach the speed of light. Time slows down more and more. Wikipedia has the simple derivation which gives us an equation to describe the relative time;

The Delta-t's are the time for ticks, with the prime (') one representing the dilated time. As v²/c² approaches 1 (so as v -> c) the bottom of the equation approaches 0 and so the relative time approaches infinity.

[MonkeyHead]

Also, Michelson-Morley experiment - possibly the most famous unexpected result in Physics. Basically, a beam of light was split into 2 identical beams at right angles to each other, in order to show thier speeds were different due to the Earth drifiting through the luminiferous ether. They werent. This is more of a lead in to relativity rather than an explanation, but if you wanna build a house you have to put bricks at the bottom first.

Linky: http://en.wikipedia.org/wiki/Michelson-Morley_experiment

[Putnam]

Here's something reaally simple:

Let's call the speed of light c.

When you are not moving, you are going at c through time. As you go speed up in motion, you slow down in time. If you are at 50% c in motion, you are going at 50% c in time.

Basically, you are always going at 100% speed--that is, c. To slow down in time, you speed up in other dimensions.

[Nadaka]

Here's something reaally simple:

Let's call the speed of light c.

When you are not moving, you are going at c through time. As you go speed up in motion, you slow down in time. If you are at 50% c in motion, you are going at 50% c in time.

Basically, you are always going at 100% speed--that is, c. To slow down in time, you speed up in other dimensions.

Very simple and very wrong. It is not at all linear. You only cut speed by 50%  at >90% (or was it >85%?, i would have to look some stuff up) light speed.

[Putnam]

Here's something reaally simple:

Let's call the speed of light c.

When you are not moving, you are going at c through time. As you go speed up in motion, you slow down in time. If you are at 50% c in motion, you are going at 50% c in time.

Basically, you are always going at 100% speed--that is, c. To slow down in time, you speed up in other dimensions.

Very simple and very wrong. It is not at all linear. You only cut speed by 50%  at >90% (or was it >85%?, i would have to look some stuff up) light speed.

(shh--it's for those who are stupid )

[Starver]

(shh--it's for those who are stupid )

Hey, my reply to the question (abandoned, yesterday, due to multi-ninjaing while I was mucking about with the exact wording) was about a cliff that you had to climb that started off easy at the bottom but got harder to climb and was infinitely high[1]...  So I thought that quite reasonable.  Even if proportionally wrong.


[1] And, incidentally, it doesn't mean that there aren't places to be on the other side of the cliff, but that anything there has an equally (or equivalently) hard time getting to the essential impossibility SoL from where they are...

[10ebbor10]

Your speed through is invertly and exponentially related too your normal speed .
Basically it means that the faster your going, the slower you go though time( You won't actually notice this as your brains working slower too(exept if you look out of a window)).
The change is however only noticable when your going really fast.

[jasonwill2]

Here's something reaally simple:

Let's call the speed of light c.

When you are not moving, you are going at c through time. As you go speed up in motion, you slow down in time. If you are at 50% c in motion, you are going at 50% c in time.

Basically, you are always going at 100% speed--that is, c. To slow down in time, you speed up in other dimensions.

Very simple and very wrong. It is not at all linear. You only cut speed by 50%  at >90% (or was it >85%?, i would have to look some stuff up) light speed.

(shh--it's for those who are stupid )

The illustration is correct, it's just the math that s wrong.

[Putnam]

Here's something reaally simple:

Let's call the speed of light c.

When you are not moving, you are going at c through time. As you go speed up in motion, you slow down in time. If you are at 50% c in motion, you are going at 50% c in time.

Basically, you are always going at 100% speed--that is, c. To slow down in time, you speed up in other dimensions.

Very simple and very wrong. It is not at all linear. You only cut speed by 50%  at >90% (or was it >85%?, i would have to look some stuff up) light speed.

(shh--it's for those who are stupid )

The illustration is correct, it's just the math that s wrong.

Yes, exactly. I'm keeping the post so that the next few posts don't look slightly mental.

[Another]

There was an update about this issue. CERN press-release

Basically 2 technical faults were found. One of them may have very well been responsible for full 60 ns of previously measured excess of speed and the other may have had the opposite effect (erroneously decreasing observed speed) of unspecified magnitude. New measurements are scheduled to May and it will really be better to wait for cross-checks from other experiments further to the end of the year before any conclusions.

The overall status went from "faster than c at 6 sigma confidence" to "there may or may not be an effect, old data contained multiple errors comparable to results".

P.S. Always check that all your connectors are plugged firmly before conducting high-precision measurements.

[Loud Whispers]

So the total rewrite of physics has been postponed

[Reelya]

One thing i thought of on reviewing this thread (don't know if this has been mentioned) is the seeming confusion of frames of reference when talking about FTL things going backwards in time.

As something approaches the speed of light, it's subjective time freezes in relation to the outside world. i.e. if you traveled to another planet at light speed you'd subjectively get their in zero time. But "real" time does actually pass externally. Extending this logic, a spacecraft going "faster than light" would be subjected to such high time-dilation in that it subjectively would arrive at the destination before it embarked. Which for macroscopic objects (people etc) would obviously cause a big pile of paradox. You'd still arrive at your destination after leaving, according to external observers, just your clocks on the spacecraft would say you arrived at an earlier time to departing.

FTL does not necessarily meant "back in time" in an external frame of reference, any more than Speed Of Light means "instantaneous" in that frame.

[Starver]

Extending this logic, a spacecraft going "faster than light" would be subjected to such high time-dilation in that it subjectively would arrive at the destination before it embarked.

Worse than that.  It's not negative time that will pass, but imaginary time.  Which is different.  And almost certainly no less weird.

So the total rewrite of physics has been postponed

Darnit, and I'd just sent my thesis off to the printers and made my first downpayment for the electric wheelchair/voicebox combo...

[MonkeyHead]

Many of those of us that work in Physics never actually seriously considered a re-write anyway, and instead were pretty sure some kind of error was responsible. Note that this includes many of those working on this experiment. Its the media that kind of didnt get it and reported it badly, like they do most high level physics news.