Can the Speed of Light Be Changed and What Does It Mean for Space Exploration?

[h8ter]

What is the barrier when you start considering speeds relativistic?

Correct, nothing is physically happening to it, there is no force making it contract. It simply IS shorter at a certain velocity.

How is it simply shorter and at the same time simply the same size as it is when considered at rest to itself?

 

[quantumdude]

What is the barrier when you start considering speeds relativistic?

There isn't one. All speeds are relativistic. Now as to the related question, "When do you have to use relativity, and when can you get away with not using it?" the answer is another question: "How accurate do your calculations need to be?"

How is it simply shorter and at the same time simply the same size as it is when considered at rest to itself?

It is shorter than its proper length according to an observer watching it move by.
It is exactly its proper length in its rest frame.

You have to qualify observational statements with the frame in which the observation is made to make any sense.

 

[h8ter]

How can it have two lengths at the same time? Isn't that impossible. I know this is in regards to different inertial reference frames, but reference frames doesn't make something heavier or lighter depending on the speed of one related to another. That would make something have two different masses at the same time.

A good explanation would help me understand.

 

[quantumdude]

How can it have two lengths at the same time?

Because it happens to be a feature of the universe we live in that the length of an object is a function of its speed.

Isn't that impossible.

No, it isn't. In fact, it's quite impossible for it not to be that way!

I know this is in regards to different inertial reference frames, but reference frames doesn't make something heavier or lighter depending on the speed of one related to another. That would make something have two different masses at the same time.

The concept of relativistic mass can be accepted or abandoned at will, but we are stuck with length contraction and time dilation. So, I'll be confining my remarks to the latter two phenomena.

A good explanation would help me understand.

It all starts with Maxwell's equations of electrodynamics. This is how Einstein derived Special Relativity from two postulates:

1. The laws of physics must be the same in every inertial frame.
2. The speed of light must be the same in every inertial frame.

The first postulate means that you should not be able to tell whether you are moving or at rest merely by performing an experiment in a closed laboratory. What it really amounts to is that there is no such thing as a state of absolute rest. Equivalently, it means that there is no preferred inertial frame of reference.

The second postulate means that, for any light pulse, its speed will be measured to be 'c', no matter what the relative motion between the source and the observer. So if a source is stationary in your frame and you measure the speed of a pulse, it is 'c'. And if that same source comes at you at 0.5c and you measure the speed of another pulse, you still measure the speed to be 'c' (not 1.5c!).

That second postulate gives an inkling of length contraction and time dilation: Space and time cannot possibly be reckoned the same for all observers, if the speed of light is reckoned the same for all observers.

Now back to Maxwell. What Einstein did was pose the question, "What sort of coordinate transformation would leave Maxwell's equations in the same form for all inertial observers?" This question is relevant because of the first postulate. If the laws of physics have to be the same for everybody, then the equations have to be the same for everybody. Einstein didn't just pose the question, he also answered it: The coordinate transformation is the Lorentz transformation.

And derivable from the LT are the formulas for length contraction and time dilation. So the best way I can think of the "explain" these phenomena is to say that they are deduced from the postulates of SR, which in light of all the experimental data are eminently reasonable, and the covariance of Maxwell's equations, which are well-confirmed experimentally.

 

[h8ter]

The concept of relativistic mass can be accepted or abandoned at will, but we are stuck with length contraction and time dilation.

We are not necessarily stuck with length contraction or time dilation. I've seriously been a firm believer in Einstein's theories, but as of lately, I've kind of trailed off thinking less of what he has theorized.

Time dilation has been tested, and scientist would even say it is proven to exist. I would have to argue that statement. Length contraction has never been observed, nor has mass increase been observed (Not sure about the mass increase, but I'm thinking it hasn't been measured, so don't go too hard on that one).

Would you like to explain to me exactly how light keeps it's constant velocity? I am in accordance with Maxwell saying the frequency is inversly proportional to the wavelength only when the source and detector are stationary. What I do not believe is that this is true when velocity of the source of detector is thrown in. I have no belief that the Lorents Transform did a good job in explaining this phenomenon. So, saying what you just said, has not influenced my thought. Can you go more indepth? I'm going astray; I need to get back on path with physics.

 

[quantumdude]

We are not necessarily stuck with length contraction or time dilation.

Yes we are.

Time dilation has been tested, and scientist would even say it is proven to exist. I would have to argue that statement.

There's nothing to argue. Time dilation has been observed, and in the precise quantities predicted by relativity.

Length contraction has never been observed,

That's true. But the invariance of the speed of light has been observed, and so has time dilation. It is not logically possible for space to be absolute, when the speed of light is absolute and time is not.

nor has mass increase been observed (Not sure about the mass increase, but I'm thinking it hasn't been measured, so don't go too hard on that one).

As I already said, "mass increase" is a matter of convention, not physics. If you define mass as m=γm₀, then you will observe that that quantity increases with speed. It's done all the time in particle accelerators. But most particle physicists define mass as the norm of the 4-momentum, and as such it is a Lorentz invariant.

Would you like to explain to me exactly how light keeps it's constant velocity?

No one knows why. We only know that it is true.

I am in accordance with Maxwell saying the frequency is inversly proportional to the wavelength only when the source and detector are stationary. What I do not believe is that this is true when velocity of the source of detector is thrown in.

It doesn't matter if you believe it or not. The speed of light has been measured from moving sources. The most direct test has been performed by T. Alvager, et al, Physical Letters 12, 260 (1964).

Look it up.

I have no belief that the Lorents Transform did a good job in explaining this phenomenon.

The Lorentz transform doesn't explain the phenomena, it predicts them. I said that the explanation is in terms of the postulates: Time dilation and length contraction are necessary, deductive consequences of the postulates.

But since the postulates and the transforms have survived every experimental test with flying colors, I'll say it again: it doesn't matter in the slightest what you believe.

 

[bino]

say there is a ship going .90c we figured out the length of the ship from the viewpoint of something going .30c. if we were standing still and shot two knives that go .30c, the same distance apart from each other as the ship's length we figured earlier, would we miss the ship or would the ship get cut into three portions?

 

[quantumdude]

say there is a ship going .90c we figured out the length of the ship from the viewpoint of something going .30c. if we were standing still and shot two knives that go .30c, the same distance apart from each other as the ship's length we figured earlier,

You need to be more specific. First, are the knives moving perpendicular to the path of the ship, or antiparallel to it? This matters because length contraction is only in the direction of relative motion. So, if the knives are moving perpendicular to the path of the ship, then a person in the frame of the knives would agree with an observer in the lab frame as to the length of the ship.

Second, you should assign spacetime coordinates to each event, and state where the ship is when the knives are launched.

would we miss the ship or would the ship get cut into three portions?

There's a false dilemma here, because those aren't the only 2 options. Depending on when you shoot the knives, it's possible that only 1 knife will hit the ship.

 

[bino]

the ship's and the knives' path are to intersect at the exact same time.

 

[bino]

why doesn't time move slower at the equator than at the poles?

 

[quantumdude]

the ship's and the knives' path are to intersect at the exact same time.

That doesn't help, because you have to say which knife is to intersect the ships path when the ship gets there. As I already said, in the frame of the knives, the ship is too short to be hit by both of them.

 

[quantumdude]

why doesn't time move slower at the equator than at the poles?

Why would you think that time dilation is inapplicable here?

 

[bino]

so if i lived on the equator i would live longer than if i lived on the northpole? granted i don't die of any other reason other than old age.

 

[jcsd]

the ship's and the knives' path are to intersect at the exact same time.

You haven't given enough information but if we assume that the knifes are fired simulataneously in the gun's refernce frame in such a manner that the front of the ship is 'clipped' by the knife on the right.

What happens in the gun's frame is simply that the knife 'clips' the front of the ship, but the knife on the left misses the ship completely due to the fact that the length of the ship is contarcted in this frame.

In the shi's frame the length of the gun is contracted, howver the knifes are not fired simulatanoeusly; the knife on the right fires first and the ships front is 'clipped', then the ship moves on and the knife on the left fires missing the ship completely.

The lengths of the knifes are the same in both frames.

 

[bino]

the knives are the same length apart from each other so that one would slide past right in front of the ship and one would slide right behind the ship. they both cross the ships path at the same time.

 

[quantumdude]

the knives are the same length apart from each other so that one would slide past right in front of the ship and one would slide right behind the ship. they both cross the ships path at the same time.

It's not possible to tell for certain from your language, but you seem to think that the ship should fit snugly between the two knives, which is wrong. As I've said twice already, the ship has the same length in the frame of the knives as it has in the frame of the knife launcher.

 

[quantumdude]

so if i lived on the equator i would live longer than if i lived on the northpole? granted i don't die of any other reason other than old age.

Longer as measured by whom? You would not notice your life being any longer, of course, because relativistic effects always turn up in the other guy's frame.

It seems like you have not looked at that relativity textbook I linked you to. If you learn the material in that link, you can answer all these questions for yourself.

 

[jcsd]

the knives are the same length apart from each other so that one would slide past right in front of the ship and one would slide right behind the ship. they both cross the ships path at the same time.

As Tom Mattson says length is frame dependent and the length of the ship is contracted (i.e. smaller) in the gun's and the knife's frame, only the proper lengths are the same.

 

[bino]

why would one be shot off sooner than the other, from the view of the ship, if they are both shot at the same time from the view of the gun?

 

[jcsd]

Of course you could mean that the length of the ship is L in the gun's frame (which means that the proper length of the ship would actually be greater than 2L) in which case both front and back will be clipped under the previous assumptions (simultaneously in the gun's frame, non-simultaneously in the ship's frame) .

 

[jcsd]

why would one be shot off sooner than the other, from the view of the ship, if they are both shot at the same time from the view of the gun?

Because of the failure of simultaneity at distance in relativity - i.e. events that are simultaneous and separated by distance in one frame are not simultaneous in another frame.

 

[quantumdude]

why would one be shot off sooner than the other, from the view of the ship, if they are both shot at the same time from the view of the gun?

Because simultaneity is relative. Events that are spatially separated and are simultaneous in one inertial frame, are not simultaneous in any other inertial frame.

It's all spelled out quite clearly in that book I linked you to.

edit: jcsd already got it, I see.

Here's that link again, in case you change your mind about reading it:

Special Relativity

 

[bino]

that does not make sense. say I am traveling in a ship that its width is longer than its length and another ship flies perpendicular to me at a speed of .90c then my ship will look like its slanted to the right? both of my side are moving at the same time in the view of my ship.

 

[quantumdude]

that does not make sense. say I am traveling in a ship that its width is longer than its length and another ship flies perpendicular to me at a speed of .90c then my ship will look like its slanted to the right?

In general extended objects are rotated in a frame in frames that are moving relative to those objects.

both of my side are moving at the same time in the view of my ship.

And as we keep repeating, simultaneity is relative. Two events that are spatially separated and simultaneous in one inertial frame are not simultaneous in any other inertial frame.

 

[bino]

so then things that are farther in front of the ship are then closer together from the ship view?

 

[quantumdude]

so then things that are farther in front of the ship are then closer together from the ship view?

I can't answer that, because this question is ill formulated. You haven't specified the state of motion of the "things" in front of the ship. If they are stationary with respect to the ship, then the distance between the ship and the things is a proper length, and it would be shorter from the point of view of someone watching the ship pass by. If the things are stationary with respect to the onlooker, than the distance will be shorter from the point of view of the ship.

Bino, please read the link I gave you. You keep asking the same type of questions over and over and over and over and over and over..., and this rapid fire questioning is getting tiresome. Please take some of the responsibility of your own education. You'll be the better for it.

 

[russ_watters]

so if i lived on the equator i would live longer than if i lived on the northpole? granted i don't die of any other reason other than old age.

Well, you also have to consider gravitational time dilation since the Earth isn't a perfect sphere...

 

[pervect]

Just to amplify a few things about this last point...

One of the very interesting facts of life is that all clocks on the Earth's surface at sea-level (the geoid) run at the same rate. The reduced gravitational potential at the Earth's equator due to it's equatorial bulge exactly cancels out the time dilation due to the rotational motion.

Atomic clocks are precise enough nowadays that these issues are actually important - one of the primary clocks at Denver, Colorado, has to have it's contribution to atomic time (TAI time) adjusted because of it's altitude above sea level.

 

[bino]

good to know pervect

 

[bino]

i don't understand how the michelson-morley experiment could have worked in the first place. everything is fixed to the Earth the speed of light could not have changed anyway. how does that have to do with the rotation of the earth?

 

[kuenmao]

First of all, I really agree that you should do some reading on special relativity and general relativity before you come back to ask another set of questions.

In reply to your question, first of all, not everything is fixed to the Earth. For example, winds experience forces and move in interesting patterns due to Earth's rotation. That is because the Earth is strictly an inertial reference frame. Since people thought that light was supposed to move at c only when measured at an inertial reference frame, c measured in different directions with respect to the motion of the planet were supposed to have different speeds. That was the whole idea of the Michelson-Morley experiment.
It's like measuring the time taken for a boat to go and come back across a river of width k units, and then comparing the time to the time measured when the boat moves upstream for k units and then downstream. There would be some difference in the times if the water in the river were running.
It doesn't only depends on the rotation of the Earth...it also depends on Earth's orbiting around the sun, and all other accelerating movements of the Earth.
Anyway, relativity explained the results very beautifully.

 

[bino]

"Since people thought that light was supposed to move at c only when measured at an inertial reference frame, c measured in different directions with respect to the motion of the planet were supposed to have different speeds. That was the whole idea of the Michelson-Morley experiment."

their experiment could not do what they thought it would since the whole experiment was moving along at the same speed as the earth. (it would be more like if someone was throwing a ball in a box the ball will be moving at the same speed no matter what direction i throw it in front to back, side to side even if the box was moving.) from the frame of the person in the box.

 

[bino]

It's like measuring the time taken for a boat to go and come back across a river of width k units, and then comparing the time to the time measured when the boat moves upstream for k units and then downstream. There would be some difference in the times if the water in the river were running.

i think i read somewhere that flowing water has an effect on the speed of light also. light moves faster going downstream than it would if it was going upstream or if the water was not flowing. ill try to find out where i read that.

 

[Nenad]

i don't understand how the michelson-morley experiment could have worked in the first place. everything is fixed to the Earth the speed of light could not have changed anyway. how does that have to do with the rotation of the earth?

the reason for the Michelson experiment is to test for an 'ether' that light was supposed to travel through, since it was thought to be a 'wave'. And since the Earth torates around the sun and rotates on itrs axis, it was thought that this ether was raming into it, since it was supposed to be stationary. The point of their experiment was king for like sending a boat upstream and downstream, the ether was like water, so the light should go faster in one direction than in the other.

 

[bino]

ok so then its important because it debunked the ether theory.

 

[bino]

but how does that equate into light being the same speed for all frames?

 

[kuenmao]

It all starts with Maxwell's equations, which predicted light to move at a certain speed c. However, there wasn't any implication of which reference frame we would measure speed at. It seemed as if there was one unique reference frame at absolute rest. That would be the frame which "ether" would exist at.
If ether existed, they would have measured a difference in the speed of light going "upstream, downstream" and "across the river", which is actually just the difference between Earth's reference frame and the "frame at absolute rest". However, they didn't measure any difference.
Einstein solved this problem by suggesting that light perhaps was at the same speed c for all frames, not just one. Nobody know why it is like that, but nowadays it seems that it just explains the phenomena beautifully. However, consequences of SR changes our views and makes classical physics no longer applicable at great speeds.

 

[russ_watters]

but how does that equate into light being the same speed for all frames?

Because ether theooy is what says that light speed is not the same in all frames. If ether theory were correct, then measuring the speed of light traveling east, you'd get C+1000mph, and measuring the speed of light traveling west, you'd get C-1000mph. That's (basically) what the M&M experiment was trying to find.

 

[bino]

right right. but their test was not tested in different frames the whole thing was only in one frame. or is that they thought that the ether was moving around the surface of the earth? because the way I am thinking, they were thinking, is that the ether is going around the outside of Earth's atmosphere.

 

[pervect]

People thought about ether dragging theories for a while, but couldn't reconcile the results with stellar aberration results. If the Earth was dragging the ether around it as it moved, this should have some optical effects on observing stars (depending on the exact variety of theory being used) - no such effects were found.

Of course, nowadays we don't have to rely on the limited experimental results that were available when relativity was first being formulated, we have a whole host of experimental confirmations of relativity. See for instance

http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.htmlhttp://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html

For instance, we have measured the speed of "light" (gamma rays) emitted from the decay of fast (.99975) pi mesons with a resolution of 400ppm, and found no change. That's just one of a huge number of experiments.

Modern high particle accelerators wouldn't work at all without relativity - the early cyclotron designs gave way to the synchrotons. GPS wouldn't work the way it does without relativity. These are examples of the large number of "informal" tests of relativity - things that simply wouldn't work if relativity were not true, things that are used every day. With the advent of atomic clocks, some of the predictions of general relativity, formerly of only theoretical interest, are of practical importance. As I mentioned before, the accuracy of the Boulder clock is such that the small relativistic shift in frequency due to it's altitude is about 40x as large as the frequency stability of the clock itself.

 

[Figgy04]

seems like i read somewhere that in some 1999 issue of "Nature" there was an article that said that scientists managed to slow down light by a large factor by using a high density clump of Bose-Einstein condensate...
if this is true, what applications can it have?

 

[quantumdude]

right right. but their test was not tested in different frames the whole thing was only in one frame.

Look at the MMX apparatus. There is a beam of light going in two different directions. So relative to the ether, the two beams should have had two different velocities.

 

[bino]

was the ether sapossed to moving around the surface of Earth or around the atmosphere of earth?

 

[quantumdude]

was the ether sapossed to moving around the surface of Earth or around the atmosphere of earth?

Read Chapter 1 of the book I linked you to. It explains everything.

 

[bino]

i did that's where i got my question from. alls it said was that it was believed that Earth moved through the ether. but it didnt say if it went around the surface or atmosphere of earth? becuase if it went around the surface then they should have figured that they would be able to feel it but if it went around the atmosphere then their experement could not have measured anything. either way it was doomed. and i still don't see how it would prove the speed of light is constant? pervect i looked over the website you posted and it had a lot of experiments that where for and against SR. do you any other sites maybe one that has more discriptions of the experiments.

 

[quantumdude]

alls it said was that it was believed that Earth moved through the ether.

You didn't read it very carefully then. It also said that the two beams were moving perpendicularly to each other. That means that the light beams should have had two different speeds, if the Earth were truly moving through the aether. The book is both specific and quantitative on this point. I honestly don't understand how you could have missed it.

but it didnt say if it went around the surface or atmosphere of earth?

It says that, if the aether exists, then the Earth should be moving through it. It says so explicitly on page 3, middle left column and on page 4, upper right column.

becuase if it went around the surface then they should have figured that they would be able to feel it but if it went around the atmosphere then their experement could not have measured anything. either way it was doomed.

The aether doesn't move around anything. It's an inertial frame!

 

[bino]

if the Earth moves through it then the ether has to either move around the Earth or move through the earth.

 

[bino]

i understand what the apparatus was suppose to do and what it looked like. i don't see how it could be considered a valid test to prove anything.

 

[Nenad]

I suggest somebody end this thread right now. LOCK DOWN.

 

[pervect]

nenad, but don't you like the way bino always replies in one at most two sentences, never uses capital leters, and always asks a question at the end?