Faster than the speed of light

[DaveC426913]

Photon doesn’t have any mass, but it has momentum.
Momentum is the product of the mass and velocity (p=mv)

That is the classical definition of momentum.

 

[Dale]

Photon doesn’t have any mass, but it has momentum.
Momentum is the product of the mass and velocity (p=mv)
It seems contradictory.
Is there an explanation for this, such as an interaction with something else?

Sure there is an explanation. That definition of momentum is only approximate and only applies for particles with v<<c.

In a more modern understanding momentum is the conserved quantity corresponding to spatial-translation symmetry. Thus a field can also have momentum provided that its Lagrangian is invariant wrt spatial translations. You should read up on Noether's theorem.

 

[MattRob]

Usually "your own perspective" means "in a reference frame where you are at rest", in which case your velocity is 0 by definition.

Look, I'll do it now mathematically. If observer is traveling at .9c, then he is traveling at 269,813,212 m/s.

Now to calculate time dilation using the Lorentz equation (Sorry, don't know how to use the fancy math fonts):
L = c / sqrt( c^2 - v^2 )

L = 299,792,458 / sqrt( 299,792,458^2 - 269,813,212^2 )
L = 299,792,458 / sqrt( 89,875,517,873,681,764 - 72,799,169,369,756,944 )
L = 299,792,458 / sqrt( 17,076,348,503,924,820 )
L = 299,792,458 / 130,676,503
L = ~2.294

For every 2.294 seconds of stationary observer time, 1 second of traveling observer time passes. Now, time dilation is accounted for the velocity of the observer, since speed is distance over time. I'll be using the reciprocal, since in this case time is passing at a slower rate for the traveler, while using the raw non-reciprocal would result in the opposite effect.
1/~2.294 = 0.43588989486853601900818999255813

269,813,212 meters / 1 second
Now, correcting for time dilation
269,813,212 meters / 0.43588989486853601900818999255813 second

618,993,959 meters / second

Much faster than 299,792,458 meters a second. From your own perspective, anyways. That is to say, a stationary object will fly by at 618,993,959 meters a second, and you will travel 4 Ly in 1.937 years.

No, that is something momentum does. Photons have momentum.

And momentum is proportional to mass. And yes, photons do have mass as a matter of fact. E=mc^2, the equation doesn't mean there's a relationship in-between mass and energy, it means mass is energy. "Is", being another word for "equal to".

I think you need to learn a bit more about Maxwell's equations. A light wave is not some piece of matter which undulates like a snake to go forward. It is an electromagnetic field which varies from place to place.

In it's spare time it will sometimes act like a particle, too. Just has a darned habit of not deciding which one to be.

Can you cite the experiment in question?

That's the darned thing about hearing something interesting somewhere, it's always heck trying to find it again. I think it was to do with early radar experiments, but I'll try to find this and come back to it.

 

[Oudeis Eimi]

And momentum is proportional to mass. And yes, photons do have mass as a matter of fact. E=mc^2, see the "="? It means "equals". The equation doesn't mean there's a relationship in-between mass and energy, it means mass is energy.

Most everything in the above paragraph is wrong.

 

[Dale]

Look, I'll do it now mathematically. If observer is traveling at .9c, then he is traveling at 269,813,212 m/s.

Now to calculate time dilation using the Lorentz equation (Sorry, don't know how to use the fancy math fonts):
L = c / sqrt( c^2 - v^2 )

Sure, but in what way is that the observer's own perspective? From the observer's own perspective he is at rest and his clocks are not time dilated.

And momentum is proportional to mass. And yes, photons do have mass as a matter of fact. E=mc^2, the equation doesn't mean there's a relationship in-between mass and energy, it means mass is energy. "Is", being another word for "equal to".

There are two distinct concepts of "mass" in special relativity. One is called "relativistic mass", that is the mass you are referring to, it is the "mass" that increases as a particle's speed increases. The other is called "invariant mass" or "rest mass", that is the mass that people are referring to when they say that a photon has no mass. In modern physics the unqualified term "mass" usually refers to "invariant mass" and when someone wants to refer to the relativistic mass they generally add the qualifier in order to avoid confusion. The use of relativistic mass is deprecated by most modern physicists.

 

[MattRob]

Sure, but in what way is that the observer's own perspective? From the observer's own perspective he is at rest and his clocks are not time dilated.

No, but by receiving a data stream and referencing it with his own information he can know that he is, in fact, not at rest, and that his clocks are dilated in reference to a stationary observer. (Or, at least the communications facility back on Earth.)

This is really somewhat besides the point though, I'm pretty sure I've made myself clear. It is in fact from his own perspective, as from a stationary observers' perspective, he is traveling at .9c, and not 618,993,959 m/s.

There are two distinct concepts of "mass" in special relativity. One is called "relativistic mass", that is the mass you are referring to, it is the "mass" that increases as a particle's speed increases. The other is called "invariant mass" or "rest mass", that is the mass that people are referring to when they say that a photon has no mass. In modern physics the unqualified term "mass" usually refers to "invariant mass" and when someone wants to refer to the relativistic mass they generally add the qualifier in order to avoid confusion. The use of relativistic mass is deprecated by most modern physicists.

Okay, I'll take that.

But the point of conversation is FTL travel, from a stationary observer's point of view, by finding some loophole in physics. So, to return to that point...

"Two other things, Quantum entanglement and quantum tunneling. I can sort of understand that entanglement doesn't carry information because it's randomized. But randomization is really a fancy way of saying we don't know the factors. So if they could be discovered, or the randomization somehow controlled, then couldn't entangled particles transfer information FTL?

And now Quantum tunneling. I haven't heard any reason why it wouldn't work as FTL."

 

[Dale]

he is, in fact, not at rest

What does this mean? Are you under the mistaken impression that whether or not something is moving has some sort of frame-independent meaning? If not, then what do you mean by "in fact"?

A person is at rest or moving relative to a given frame of reference. In the observer's own frame he is at rest and in any other frame he is moving. There is no coordinate independent sense in which he is "in fact" not at rest.

It is in fact from his own perspective, as from a stationary observers' perspective, he is traveling at .9c, and not 618,993,959 m/s.

He is at rest (0c) from his own perspective, by definition. He is traveling at .9c from someone else's perspective. There is no inertial frame in which he is traveling at any v>c.

randomization is really a fancy way of saying we don't know the factors. So if they could be discovered, or the randomization somehow controlled, then couldn't entangled particles transfer information FTL?

Yes, if the laws of physics that we are currently using turn out to be incorrect then it is certainly possible that they might be incorrect in such a way as to allow FTL communication. But then we would be talking about science fiction rather than science.

 

[John232]

Seems to me that the science fiction of FTL travel has already started because of cosmic background radiation showing that the universe expanded FTL. If galaxies can travel FTL with the expansion of space then any type of velocity addition done on them would be false because there is no limit of the rate of speed between them becuase a majority of their motion is defined by the expansion of space itself.

 

[MattRob]

He is at rest (0c) from his own perspective, by definition. He is traveling at .9c from someone else's perspective. There is no inertial frame in which he is traveling at any v>c.

No, but as he reaches .9c from a stationary viewpoint (stationary being an observer set on the surface of any planet, as the velocity of any nearby various body in their orbits is inconsequential relative to relativistic velocities), he will traverse 618,993,959 meters of distance in-between two stationary points every second.

I guess I was wrong to assume I was understood when I meant "at rest" to mean the velocity of any nearby common celestial bodies, the difference in-between which velocities is inconsequential in comparison to the observer's own velocity.

To get it in another time, if he is to leave Earth for Proxima Centauri, which is 4.4 Ly away, at .9c, he will arrive about 1.9 years later from his own perspective, for an average speed of 618,993,959 m/s, which is greater than the given value of c, 299,792,458 m/s. I did the math in my earlier post. However, because of this same time dilation, light will appear to move at 687,723,898.652 m/s, so though the observer strictly isn't going faster than light, he is going faster than 299,792,458 m/s.

Yes, if the laws of physics that we are currently using turn out to be incorrect then it is certainly possible that they might be incorrect in such a way as to allow FTL communication. But then we would be talking about science fiction rather than science.

I always thought the purpose of a hypothesis was to put established science to the test. Seeing as we've never achieved FTL before, at least not to my knowledge, an FTL thread is nothing but a hypothesis, which doesn't necessarily conflict existing science, as much as add to it.

With an attitude of, "if it's not something already established, it's science fiction", with an implied meaning that we can only use established facts, then no hypothesis could be made at all, and science would come to a screeching halt.

It's good to have imagination and come up with new ideas, that's the only way science can progress. The important thing is to test them adequately and correctly, not to not come up with them at all! Scientific advancement only comes through new, wild ideas, like the "strange" idea that the speed of light isn't additive to the velocity of the emitter!

EDIT: Heck, Isaac Newton's idea that gravity works by the inverse of the square was formulated then put away for years because he considered it too outlandish. We're only lucky enough to have discovered it because years later by an astounding coincidence a friend of his asked that exact question.

Anyways, not to assume that the current scientific understanding is wrong, but what is the current scientific understanding on the subject of quantum entanglement and it's use as FTL communication? Was I right that current understanding is that it can't carry information due to randomization?

Or, for that matter, I still have no response to Quantum tunneling as a method of FTL travel.

 

[John232]

We could only hope that science fiction writers do not come up with viable technology in their stories or it could lead to the biggest detourent to scientific progress.

 

[Dale]

I always thought the purpose of a hypothesis was to put established science to the test. ...

With an attitude of, "if it's not something already established, it's science fiction", with an implied meaning that we can only use established facts, then no hypothesis could be made at all, and science would come to a screeching halt.

You are not proposing a hypothesis, you are simply speculating. There is a huge difference. A hypothesis is a quantitative prediction about the measured outcome of a specific experiment. That is why what you were proposing was science fiction, not science.

It's good to have imagination and come up with new ideas, that's the only way science can progress. The important thing is to test them adequately and correctly, not to not come up with them at all!

Sure, and that is the purpose of scientific journals and scientific conferences. That is not the purpose of this site. This site is for learning mainstream physics, not for advancing the state of the art. Please review the rules on overly speculative posts which you agreed to when you signed up.

 

[MattRob]

Sure, and that is the purpose of scientific journals and scientific conferences. That is not the purpose of this site. This site is for learning mainstream physics, not for advancing the state of the art. Please review the rules on overly speculative posts which you agreed to when you signed up.

Wow. Nothing I've said so far is overly speculative. I'm only asking questions to the current state of science. Here's my current unanswered question:
"Anyways, not to assume that the current scientific understanding is wrong, but what is the current scientific understanding on the subject of quantum entanglement and it's use as FTL communication? Was I right that current understanding is that it can't carry information due to randomization?

Or, for that matter, I still have no response to Quantum tunneling as a method of FTL travel."

Breaking 299,792,458 m/s through time dilation is not overly speculative. It's current science. I'm asking the current state of science in regards to certain processes in their use of FTL travel, such as quantum entanglement and tunneling.
In current mainstream physics, is it possible to use Quantum entanglement and tunneling for FTL travel/communications?

 

[Dale]

Seems to me that the science fiction of FTL travel has already started because of cosmic background radiation showing that the universe expanded FTL. If galaxies can travel FTL with the expansion of space then any type of velocity addition done on them would be false because there is no limit of the rate of speed between them becuase a majority of their motion is defined by the expansion of space itself.

Don't take that too seriously. In curved spacetime the relative velocity of distant objects is mathematically ill-defined.

 

[Dale]

Nothing I've said so far is overly speculative.

You don't consider this overly speculative?:

But randomization is really a fancy way of saying we don't know the factors. So if they could be discovered, or the randomization somehow controlled, then couldn't entangled particles transfer information FTL?

I'm only asking questions to the current state of science

According to the current state of physics there are no tachyons yet discovered nor have any wormholes been discovered. In principle wormholes are not forbidden by GR, I don't know if tachyons are forbidden by the standard model.

 

[MattRob]

Not really, It was a way of asking what the problem is with entangled particles as FTL.

 

[Dale]

Entangled particles don't travel FTL and they don't transmit information FTL. What do you think is FTL about them?

To understand why they don't transmit information let's propose an entangled comm system and see what would happen. Observers A and B have a whole pile of entangled particles which are each entangled for their polarization state. A tries to send a message to B. First A measures a few hundred particles. B measures the entangled particles and gets a 50/50 random distribution. Then A doesn't measure the next few hundred. B measures the entangled particles and gets a 50/50 random distribution. Then A measures a subset of the next few hundred. B measures the entangled particles and gets a 50/50 random distribution.

Nothing that A does changes what B measures. When A and B later reunite they find that all of the particles that A measured are exactly anti-correlated with the particles that B measured, but B has no way of distinguishing the particles that A measured from the ones he didn't.

 

[darkhorror]

To get it in another time, if he is to leave Earth for Proxima Centauri, which is 4.4 Ly away, at .9c, he will arrive about 1.9 years later from his own perspective, for an average speed of 618,993,959 m/s, which is greater than the given value of c, 299,792,458 m/s. I did the math in my earlier post. However, because of this same time dilation, light will appear to move at 687,723,898.652 m/s, so though the observer strictly isn't going faster than light, he is going faster than 299,792,458 m/s.

Your just mixing and matching frames of reference. You use the time in the traveling frame of reference and the distance in the Earth's frame. This doesn't make any sense. How about answering these questions.

So how far did the ship travel?
How long did it take?

 

[DaveC426913]

However, because of this same time dilation, light will appear to move at 687,723,898.652 m/s

No, it will not. Light will still be measured to move at 300,000km/s.

What is true is that he will measure the distance to Centauri to be much shorter than he thought it was when he checked it on Earth.

 

[MattRob]

No, it will not. Light will still be measured to move at 300,000km/s.

What is true is that he will measure the distance to Centauri to be much shorter than he thought it was when he checked it on Earth.

Length contraction? I always thought the object accelerating experiences length contraction. So how does this work?

 

[Dale]

Length contraction? I always thought the object accelerating experiences length contraction. So how does this work?

The acceleration is not relevant, only the relative velocity and your velocity relative to yourself is always 0.

Nothing "experiences" length contraction, meaning that you never measure yourself to be length contracted. In your own frame you are never length contracted nor time dilated. It is always the moving clock or rod which is length contracted or time dilated in your frame.

 

[LBrandt]

You would never even expect to be length contracted or time-dilated in your own frame. How would that be possible? Even if you choose to consider this, what would be your standard of measurement? In your own frame, anything that you used to measure this would itself also be length contracted or time-dilated. As as example, if your own width were reduced to half its previous value, any ruler or measuring device would be reduced by the same factor. All values, including length and time are always normal in your own frame.

 

[MattRob]

That I understand. Except if you're contracted and you brought a ruler along, then wouldn't it make the rest of the universe seem to expand? Since because your ruler is shorter, it would take more copies of your ruler to reach from point A to point B, so wouldn't they seem farther apart instead of closer? That's what I meant.

Except now that you mention it, according to length contraction you should measure the distance in-between point A and point B to be contracted. But your own contracted length will make it seem just as much longer as it's contraction makes it seem shorter.

But, if you don't measure your own contraction at all not because of impossibility of measuring but because you don't actually contract from your own standpoint, and you contract from other viewpoints, then wouldn't you measure your own ship to be larger than a stationary observer would measure it? (Stationary being the velocity of adjacent celestial bodies),
i.e. your measurement says distance A is 2.2e50 shiplengths and a stationary observer sees distance A as being 6.6e50 shiplengths?

I'm sorry if the tone is coming off wrong, I'm not saying you're wrong, I'm just trying to understand how this works...

 

[LBrandt]

If you are in uniform motion relative to an observer, he will appear length contracted to you, and you will appear length contracted to him, but neither of you will consider yourselves to be length contracted.

 

[DaveC426913]

Your ruler, as with anything inside your spaceship, will be exactly as it has always been.

But the distance to Centauri will be dramatically shortened. Also, everything along your axis of movement will be flattened. i.e Centauri will appear as a flattish disc, rather than a sphere.

 

[MattRob]

So... Hypothetical question, then. Say there's a railroad crossing in North Carolina, and the train is somehow moving at relativistic speed along the rails. There's a car in N. Carolina about to cross the track. From the car's point of view, only going 25 mph, the train is contracted to a length of only a few feet, and is somewhere back in New Jersey, meaning the track ahead of them is clear and they can safely cross. (I know that's not far enough away, but for this point let's pretend New Jersey is a light-minute away).
However, from the train's point of view, because the distance from New Jersey to North Carolina is contracted, the front of the train is on the railroad crossing in N. Carolina when the car crosses.
But from the Car's point of view, because the train is contracted, the front of the train is somewhere in New Jersey.
So what happens to the car when it crosses?
From the train's point of view, it will smash into the train engine.
From the car's point of view, it will simply cross safely.

That's why I have such difficulty understanding length contraction as it is...

 

[Dale]

From what you describe here I think that your confusion is with the relativity of simultaneity, not length contraction.

 

[DaveC426913]

From what you describe here I think that your confusion is with the relativity of simultaneity, not length contraction.

Yeah, I started to describe why the length-contracted train was not still in New Jersey but realized my answer would only make it more confusing. I thought I'd leave it to someone who could be more eloquent and succinct.

MattRob, suffice to say, at least for now, that the front of the train is exactly where it is expected to be for the car river and the train engineer. So, car go boom.

The confusion comes when the car driver attempts to time when the caboose reaches the crossing. And timing events is where relativity gets tricky. You cannot assume that you simply "see" everything (such as the entire train). You must take into account distances and times when you examine properties about an object moving relative to you.

 

[vish_al210]

I don't think tha tlight travels but rather it propogates... by exchange of energy. If an object needs to move it needs to overcome a lot of factors, but if the light energy emitted by it or sound energy or heat energy emitted by it needs to propagate then it only needs to exchange the energy, but then the question is what about vacuum, well heat transfers through vacuum and so does light. but still it is transfer of energy... Hence they are much faster..
All equations, postulates and proofs are only extensions of what we can see, observe and understand. As we understand, observe, see more and more, equations change, exceptional cases are putforth and so on... so wait.. and the equations would change... someday when we understand and pick a different reference than C, things would change... equations would change, then that new reference would be the limit... remember at one point in time 'sky' was the limit...

 

[acarpenter]

the only thing that can travel at the speed of light, is something massless, eg a photon.

however what i fail to understand is that photons have a rest mass of zero. yet photons have to be in motion and cannon be at rest. photons not at rest have mass. so therefore how can a photon be traveling at the speed of light when surely it cannot be Massless??

 

[Dale]

It sounds like you may be confused by the various definitions of mass:
invariant mass: sqrt(E²/c⁴ - p²/c²)
rest mass: a less precise name for invariant mass
relativistic mass: E/c²

 

[Antymattar]

Funny thing is that the speed of light has been observed to be slowing down.
The speed of light is calculated using atomic clocks. Scientists have observed that, in fact, either the atomic orbits are slowing down, or the speed of light is slowing down because every once in a while you have to change the calculations to accurately fit the atomic clocks. Einstein himself said that he would be surprised if the speed of light was an actual constant.

Also, neutrino is slower than the speed of light and light has a small amount of bass itself.

 

[Mr.GaGa]

An example to exceed c has been proposed.

A recent paper is to prove the velocity of motion V in point mechanics should be the energy flow velocity S/w (S_Poynting vector,w_energy density) of the wave theory instead of the group velocity and so on. This velocity(V or S/w) of surface electromagnetic waves in vacuum can be faster than c. It does not lead to any negative and imaginary mass(energy).

 

[Dale]

Hi Mr.GaGa, welcome to PF.

Yes, there are in fact many quantities with units of speed and values > c. None of them can be used for superluminal travel or superluminal communication.

 

[epenguin]

Funny thing is that the speed of light has been observed to be slowing down.
The speed of light is calculated using atomic clocks. Scientists have observed that, in fact, either the atomic orbits are slowing down, or the speed of light is slowing down because every once in a while you have to change the calculations to accurately fit the atomic clocks.

Is that true and accepted? Sounds like a big thing for a throwaway comment.

Anyone else?

Also it seems to me that nothing can be faster than the speed of light, although I know there was a book of that name which I have even read. Also that nothing can be slower than the speed of light. Not for physical reasons but for grammatical ones.

 

[JesseM]

Is that true and accepted? Sounds like a big thing for a throwaway comment.

No, it's a completely silly claim that Antymattar seems to have picked up from a creationist source:

With respect to the fact that measurements made after 1960 do not show any decrease in the speed of light, Walt Brown has concocted his own misinformed "explanation" based on the assumption of two different systems of time:

By way of background, scientists found that it was necessary to revise the length of a "standard" second. The standard second is equal to the number of vibrations of a cesium atom that correspond to a second based on the time required (in seconds) for the Earth to orbit the sun.

The cesium atom vibration frequency is extremely constant. Scientists have constructed instruments which can count these vibrations. By assigning a specific number of vibrations to a standard second, a super-accurate clock can be constructed. However, the cesium clock must be calibrated in order to correspond to the average period of revolution of the Earth around the sun. In order to make the standard second (as defined by the cesium clock) precisely equal to the length of a second based on new and more accurate astronomical measurements, it was necessary to revise the previously selected number of vibrations corresponding to the standard second. The change was extremely minute.

The CSC website speaks of "orbital" time versus "atomic" time as if they were two different systems of time measurement. Because of the necessity to re-calibrate the cesium clock, Brown mistakenly concludes that "atomic" time is "slowing." He states: "If atomic frequencies are decreasing, then both the measured quantity (the speed of light) and the measuring tool (atomic clocks) are changing at the same rate. Naturally, no relative change would be detected, and the speed of light would be constant in atomic time-but not orbital time." Of course, this is complete nonsense.

(many creationists like the idea that light is slowing down because they want to believe the universe was created only a few thousand years ago, but this leads to the obvious problem of explaining how we can see galaxies and supernovas and such that are millions or billions of light-years away)