Discussion in 'Astronomy, Exobiology, & Cosmology' started by Magical Realist, Jun 7, 2011.

1. Magical RealistValued Senior Member

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12,497
Yes..but only a slight one.

"Physicists today claim that they reached 300 times the speed of light. But don't write off Einstein, and don't hold your breath for a time-travelling Star Trek universe, warns Paul Davies

On the face of it, today's announcement in Nature that a team of Princeton physicists have broken the light barrier demolishes what is arguably science's most cherished principle.

Ever since Albert Einstein formulated his theory of relativity nearly a century ago, it has been a central tenet of physics that nothing can travel faster than light. Now it is claimed that in certain circumstances, light itself can be accelerated up to 300 times its usual speed. But it's too soon to consign the textbooks to the dustbin. As always, the devil is in the detail.

Moving through a vacuum, light travels at 300,000 km per second. According to the theory of relativity, it is the ultimate speed limit for the propagation of any physical influence. That includes spacecraft, subatomic particles, radio signals, or anything that might convey information or cause an effect.

When light passes through a medium such as air, it is slowed. The effect is best explained by analogy with water waves. Try throwing a stone in a pond to make ripples. Focus on a particular wave crest, and it will appear to move fairly fast, but then take a wider perspective to view the group of waves as a whole, and it travels outwards from the point of disturbance noticeably more slowly. It is almost as if the waves are rushing to get nowhere fast. You can watch as new ripples rise up at the back of the wave group, whiz forwards, and fade away at the front.

The same thing happens to light in a medium. It comes about because atoms in the medium create outgoing ripples of light as the primary light wave sweeps by them. When these ripples overlap and combine with the primary wave, they obliterate the parts racing on ahead, suppressing the fast-moving wave front and serving to slow down the group. So light passing through a medium has two associated velocities: that of the group as a whole, and that of the individual wave crests, known as the phase velocity.

A normal medium always reduces the group velocity of light to below its phase velocity, leading to the familiar phenomenon of refraction - the effect that causes a stick to look bent when it is stuck in water. The special feature of the Princeton experiment was the creation of a peculiar state of matter in which this situation is reversed: the secondary ripples of light actually make the wave group travel faster than the phase velocity.

To achieve this odd state of affairs, the scientists used a gas of cold caesium, and then excited the caesium atoms with a laser. So energised, the atoms do more than cause secondary ripples of light, they amplify the light too. It is this amplification that is the key to boosting the speed of the wave group, reportedly to 300 times the speed of light in a vacuum. Bizarrely, the wave distortion achieved is so large, it causes the group velocity to become negative, which means the peak of the wave pulse appears to exit the gas before it enters. In other words, the light waves seem to run backwards.

What makes this result so sensational is the relationship between light speed and causality. The theory of relativity predicts that speed slows time. For example, time passes a bit slower in an aircraft than on the ground, an effect that has been verified using atomic clocks. The time warp is small for everyday motion, but grows without limit as the speed of light is approached. Cosmic rays, for example, travel exceedingly close to the speed of light, and their internal clocks are slowed millions of times.

Relativity theory predicts that if a particle could exceed the speed of light, the time warp would become negative, and the particle could then travel backwards in time.

As Dr Who fans are aware, travel into the past opens up a nest of paradoxes. For example, suppose a faster-than-light particle is used as a signal to explode a bomb in the very lab that the particle itself is created. If the bomb explodes yesterday, the particle cannot be made today. So the bomb won't explode, and the particle will be made.

Either way, you get contradictory nonsense. At stake, then, is the very rationality and causal order of the universe. Allow faster-than-light travel, and the physical world turns into a madhouse .

Timing the speed of a pulse of light is fraught with complications, not least because the shape of the pulse changes when it passes through a medium. To make a pulse of a short duration, it is necessary to mix together waves of many different frequencies, and in a medium each wave will propagate differently.

As for transmitting information, opinions differ about how to associate it with a pulse that has a complicated, changing shape. The inherent fuzziness in a light pulse made up of many different waves superimposed precludes a clean definition of how fast actual information travels.

The problem is closely related to the quantum nature of light, where each frequency component can be thought of as made up of pho tons that behave in some ways like particles. But photons are subject to Heisenberg's principle, according to which there is an inherent uncertainty in their whereabouts. In the pulses of light used in the experiment, it isn't possible to pick out a given component photon and observe it travelling at superluminal velocity.

The Princeton physicists believe this fundamental fuzziness associated with a finite pulse of waves prevents information from exceeding the speed of light, so in an operational sense the light barrier remains unbroken and the causal order of the cosmos is still safe. It is intriguing to see how the wave nature of light rescues the theory of relativity from paradox."===

Light goes backwards in time | Science | The Guardian
http://www.guardian.co.uk/science/2000/jul/20/technology2

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Again, where is the paradox in my scenario? I gave the distances and times of travel, and there is no causality violation traveling at 2c. According to my method, it is impossible to travel back in time to cause a causality violation. Traveling takes time, period!

5. Magical RealistValued Senior Member

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What can I say? The evidence contradicts you. Sorry.. it was only physicists at Princeton confirming relativity theory. Hey..maybe they are biased because Einstein taught there!

7. DywyddyrPenguinaciously duckalicious.Valued Senior Member

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So, John99, still no substantiation for your claims?

8. John99BannedBanned

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My claims stand on their own. Einstein was wrong, even say that in the link. And any "claims" i made here are copyright (by me).

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Bwahahahahahahaha

Did I make a math mistake? Is 2c not traveling at twice the speed of light? Did traveling take time? You claim that traveling faster than light causes you to go back in time. Prove it!

10. DywyddyrPenguinaciously duckalicious.Valued Senior Member

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No they don't. Please show how they stand, especially given that you have already admitted that emitted light (and anything else) takes time to travel.

I gathered that. It's easy to spot your ideas: they're so obviously wrong.

11. Magical RealistValued Senior Member

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Awww..don't get all excited now. It's not good for your heart. Take heart--EVERYone makes a mistake every now and then.

12. NietzscheHimselfBannedBanned

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Adding is the only way. Which is inherently unequal. Adding 1 to 0. Where did the one come from? eh?

13. billvonValued Senior Member

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OK. Sounds like you've changed your mind. If you can see an object with aids (i.e. a video screen) the object might not be in real time, and might not be reality. Progress!

14. James RJust this guy, you know?Staff Member

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Magical Realist:

Sure. It's not too hard to show that light has a finite speed. And once you have that, the result follows automatically as long as you understand how we see things (unlike, for example, John99, who obviously has no clue).

There's nothing special about light, by the way. Consider sound. When you hear a sound, does that sound tell you what IS there or what WAS there? Obviously, sound takes time to travel (have you ever heard an echo?). So, sounds reaching our ears are made by things in the past.

Similarly, light reaching our eyes was emitted by things in the past.

There's no way to prove that anything exists if light has not had enough time to travel from the thing to our eyes. Because if light hasn't had enough time, nothing has had enough time.

That's because stars emit light continuously.

Nobody mentioned any "dark stars" existing separately from their light, except you.

No, because things can move away after they emit their light. Obvious, isn't it?

All you're saying, in a convoluted way, is that it takes light more time to travel a greater distance through space. That's common sense, nothing profound.

Big word there: ontic. Impressive. But what do you mean by it?

You're confusing the star and the light it emits. They are two separate things.

We see the star as it was when it emitted the light. That doesn't mean that it doesn't exist in our present. We just can't know about what it is doing right now.

Not in this context. The same applies to any method for transmitting information from one place to another, whether it be by light, sound or carrier pigeon.

If a carrier pigeon brings you a letter from a friend, then when you read the letter you read about your friend in the past. That doesn't mean your friend doesn't exist in the present, too. And it doesn't mean that in the absence of the letter your friend lives in a world of perpetual darkness.

15. John99BannedBanned

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aided means with a telescope or microscope etc. why bother to even ask this question? i could never say what is real time from looking at a monitor that is being fed through a productions studio.

16. billvonValued Senior Member

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There was nothing in the report to describe causality violation (i.e. time travel.) Phase velocities have been known to exceed the speed of light for decades. This is merely a special case of that.

17. James RJust this guy, you know?Staff Member

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John99:

Have you only got your opinion, or do you have an actual argument or reason to back it up?

I'm really not interested in hearing from you that you imagine that I'm wrong about something. If you have a reason for thinking I'm wrong, then tell me what it is. Otherwise, don't bother replying to my posts.

18. John99BannedBanned

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22,046
From what i can gather from your statement:

You are saying that it takes the light from the door a few "nanoseconds" to reach you. Is that right? or are you saying it is due to something physiological?

Think i would need to know that.

19. billvonValued Senior Member

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Exactly. Because everything is delayed somewhat. Even if that monitor were connected to a camera that's in "real time" you'd have frame delays on top of the lightspeed delays from the object to the camera, and from the screen to your eyes.

Nothing is truly "real time." You just have to decide on a delay (say, 10 nanoseconds from the object to your eyes, or 8.33 milliseconds for a video frame, or 500 milliseconds for a typical HD delay) and say "that's close enough."

20. James RJust this guy, you know?Staff Member

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Light travels at a speed of $3 \times 10^8$ m/s. If your door is 3 metres away from where you are right now, then the light will take 10 nanoseconds to reach you from the door. Therefore, your view of the door is always 10 nanoseconds out of date.

Do you agree?

21. John99BannedBanned

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yes, i agree in as far as that is how it is measured and accept it as what is known.

Except that the light is not coming from the door (of course light will be reflected but that is true of all surfaces to varying degrees throughout the room). Light is all around the door, if we are talking about in a normal room or even outdoors with natural lighting BUT best to stick with one scenario for now.

The example below is made to represent an everage, everyday, real world example of a room with a door. We will remove all objects from the room except fro the walls, the ceiling, the floor and the lighting. No windows.

We have a large well lit room. The lighting is from normal, evenly distributed, overhead
lighting.

Take around 10 people place them 3 metres from each other each going further from the door
and the first is of course 3 meters from the door.

You are saying the first person is seeing the door as it was 10 ns ago and then each additional person add another 10 ns ?

Last edited: Jun 10, 2011
22. John99BannedBanned

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22,046
also maybe we should use something other than a door. something that moves linearly from left to right. a door whilst going fro open to close is closer and further away. best way to gauge this is if the door is moving slowly. i would imagine.

23. billvonValued Senior Member

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Yes. As you get further back, each person sees an earlier version of the door. (Of course, since we cannot perceive differences of 10ns, each person sees an almost identical door.)

Now take a more interesting example. Person 1 is on Mercury, person 2 is on the Earth and person 3 is on Mars. They all look at the Sun.

Then, suddenly, for reasons unknown, the Sun suddenly dims; its light output goes down greatly. For the next two minutes no one notices anything different. They are still seeing the "earlier" Sun when it was putting out its normal light.

Then, 2 minutes and 30 seconds later, the guy on Mercury notices the change. In 8 minutes the guy on Earth notices the change. At 8:10, both people are seeing a dimmer Sun, but the guy on Mars still is seeing the earlier bright sun. It's not until 14 minutes later that all people see the new, dimmer Sun.