Question about Speed of Light and Relativity

[funkey100]

My question is, what is the speed of light relative to? I did some Googling and the answer is everything. But I don't quite understand. If I'm on Earth and the Sun is directly above me, and I have a laser and shoot a beam of light to Earth's pro- and retrograde, it would be moving c for me since it's relative to everything, included me. That will mean that relative for the sun its going at c+ and c- Earth's speed respectively. I'm slightly (really) confused. Can some one please help me understand this?

Also, if two objects were moving at each other at (relative to their point of impact) 0.60c, and I were on one of them, then it would be moving 1.2c relative to me which is impossible.

P.S., You can see I'm new to all this relativity thing but I want to understand.

EDIT: I understand the bus moving and one km/h less than c and I walked in it two km/h I couldn't go faster than c because special relativity makes the bus and everything in it contract relative to the outside (and everything outside to contract relative to the inside).

EDIT: I'm trying to read the Wikipedia article on special relativity.

Edited September 29, 2013 by funkey100

[TheDarkStar]

The main idea of special relativity is that light goes at the same speed (about 300 million m/s in a vacuum) no matter HOW fast you are moving. In the first case, the sun would see two light beams going in opposite directions at exactly the speed of light.

For the second, you're right in saying 1.2c (even relative) is impossible. Instead, they each see the other moving at a slower speed than c (this can be calculated easily, but I'm too lazy to look up the formula) and with slowed time, each with the same factor (like 0.8 times slower each).

Edited September 29, 2013 by TheDarkStar

[Kulebron]

Is the speed of light constant related to anything in physics, in a sense that why is it exactly 300 Mm/s, not 100?

[Bunsen]

The critical thing here is that our normal idea of how velocities add (e.g. if I'm running at 10mph and I throw a baseball at 30 mph, the baseball whizzes past you at 40 mph) is wrong (you see that baseball traveling a tiny, tiny, tiny bit slower than 40 mph), and the faster things go the more wrong it is. It's really close to right if the velocities are small enough (39.999999999999973 mph, in fact), but it breaks down horribly when the numbers approach c. That happens because distance and time depend on who's asking about them. I see you've already caught the distance part -- Lorentz contraction squishes things -- but there's also time dilation involved. Between the two of those, the apparent contradictions from the counterintuitive velocity addition disappear.

[GregoryNeal]

The critical thing here is that our normal idea of how velocities add (e.g. if I'm running at 10mph and I throw a baseball at 30 mph, the baseball whizzes past you at 40 mph) is wrong (you see that baseball traveling a tiny, tiny, tiny bit slower than 40 mph), and the faster things go the more wrong it is. It's really close to right if the velocities are small enough (39.999999999999973 mph, in fact), but it breaks down horribly when the numbers approach c. That happens because distance and time depend on who's asking about them. I see you've already caught the distance part -- Lorentz contraction squishes things -- but there's also time dilation involved. Between the two of those, the apparent contradictions from the counterintuitive velocity addition disappear.

Well said, you beat me to it

[Kerbart]

The disappointing answer is: It's not relative to anything. No matter where you are, no matter how fast you go -- light will always have a speed of 300,000 km/s.

In your example, if you were moving at . 6c towards another object moving at .6c, wouldn't it appear to approach at 1.2c?

The answer is no, because of your velocity time is slowing down. Thus, when that object has come 360,000 km closer to you, not one second has passed in your frame of reference, but more than one second. Making the object still move slower than the speed of light towards you.

[funkey100]

So I think I get it now. Since time slows down in the object (let's say by half), since c is distance per time, it would appear as the other object would be going half that speed, or 0.6c. But, in the point of impact they would still appear to be going 0.6c each. But then what is the contraction for? The slower time makes it so the person inside the bus couldn't achieve that speed already, what difference does the contraction make?

[K^2]

It's a package deal. Picture a third object moving relative to Earth and the Sun. If you are using time dilation to explain speed of light staying constant, it will result in speed of that 3rd object changing too. And if you don't introduce space contraction, that change is inconsistent. (After all, relative to that object you still have constant speed of light.)

Of course, the universe doesn't work this way to keep speed of light constant. Rather, because it has this structure, the speed of light is constant. And the reason you get both time dilation and space contraction is in how distances work in space-time. Two events separated by space and time are closer together than two events separated by space alone.

[funkey100]

Sorry, but I'm new to all of this. The 3rd object will change speed relative to the Earth and Sun because of the time dilation (I get this, as anything that's going at a relative speed greater than 0 will be slower than the other object). But why would that change not last without contraction?

[badgers_in_space]

So, if it were possible for one to travel at the speed of light, and one fired a laser whilst travelling at the speed of light - would the laser beam be travelling at the same speed as you? (the speed of light)

[Klingon Admiral]

Is the speed of light constant related to anything in physics, in a sense that why is it exactly 300 Mm/s, not 100?

It isn't exactly 300,000 km/s, it is 299,792,458 m/s. And exactly this length, as the metre is defined by the speed of light in vacuum and the second.

[-Velocity-]

Picture a cartesian (x,y,z) coordinate system that moves with you, so that the "origin" point of the coordiante system is always YOU. This is your "inertial reference frame". You ALWAYS measure a beam of light travelling through your coordinate system- your inertial reference frame- at the speed of light.

Since all frames of reference frames observe the same thing- that light always travels at the speed of light in their reference frame- and all reference frames must see the same EVENTS take place, that can only mean that time and length mean different things to different reference frames.

It sounds crazy only because humans evolved to live in a non-relativistic world, where velocities are low enough, and gravity is weak enough, that all these effects can be ignored. We think that such things as simultaneity are real, when they are a fiction... there is NO SUCH THING as simultaneous events if those events are separated in space. Different reference frames will NEVER ALL agree on the order of all events if those events do not occur in the same spot in space.

Very often, when reading an astronomy article, I will come across a phrase like this and cringe:

"The light we are seeing from M31 left it 2.4 million years ago; thus we are actually seeing what the Andromeda galaxy looked like 2.4 million years ago, not now."

This is actually an INCORRECT STATEMENT. Einstein PROVED that the whole idea of there being some sort of absolute, cosmological time was wrong. The idea that 2.4 million years ago in the Andromeda galaxy was somehow simultaneous with 2.4 million years ago on Earth is totally wrong, as there is no such thing as simultaneity between two different points in space! There is no such thing as "what is happening now?" in the Andromeda galaxy. That question is unphysical.

To say that "Betelguese might have already exploded and the light just hasn't reached us yet" is similarly incorrect. Betelgeuse has not exploded UNTIL the light reaches us; lightspeed is the fastest speed that distant events can truly exist from our frame of reference!

From a reference frame roughly at rest with the Andromeda galaxy and with the Milky Way, sure, it will look like 2.4 million years will pass for light from M31 to reach the Milky Way, but that is only for reference frames roughly at rest with the two galaxies. To the light, the trip was instantaneous; to any particle launched at extremely high velocities near the speed of light, the trip was nearly instantaneous. Einstein showed us that there is nothing special about any particular reference frame, and the whole notion of simultaneity is nothing but a fantasy.

Edited September 30, 2013 by |Velocity|

[Vilheim]

Is the speed of light constant related to anything in physics, in a sense that why is it exactly 300 Mm/s, not 100?

The speed of light was measured, and it was also calculated from Maxwell's electromagnetic equations using a few well established constants in that field (Vacuum permittivity and vacuum permeability). After that, we redefined the meter in relation to the speed of light as some one above me has explained.

[Camacha]

This is actually an INCORRECT STATEMENT. Einstein PROVED that the whole idea of there being some sort of absolute, cosmological time was wrong. The idea that 2.4 million years ago in the Andromeda galaxy was somehow simultaneous with 2.4 million years ago on Earth is totally wrong, as there is no such thing as simultaneity between two different points in space! There is no such thing as "what is happening now?" in the Andromeda galaxy. That question is unphysical.

To say that "Betelguese might have already exploded and the light just hasn't reached us yet" is similarly incorrect. Betelgeuse has not exploded UNTIL the light reaches us; lightspeed is the fastest speed that distant events can truly exist from our frame of reference!

Isn't the point of entanglement that two observers see the same event (or the other side of the same event) at the same time?

[WestAir]

Question for the science professionals here:

What is the difference between an object with mass traveling at the speed of light (299,792,458 m/s), and an object traveling at almost the same speed, but slower by 1 Plank Length x a time larger than the age of the Universe? One is unachievable, and the other is achievable, yet both would have the object move the exact same distance over the exact same time if clocked today. How is that explained?

[ArmchairGravy]

"To the light, the trip was instantaneous; to any particle launched at extremely high velocities near the speed of light, the trip was nearly instantaneous."

This is where I go fetal. From a photon's point of view, space/time is irrelevant as they get to their destination instantaneously no matter the distance involved. I was watching Brian Greene's The Fabric of the Cosmos, and it did a nice job of blowing my mind on this subject.

[Geckgo]

*Grabs the popcorn*

I really thought this thread was going to end up more heated for some reason.

Armchair, no reason to go fetal, think about it in terms of sublight and you sitting on a spacecraft traveling near c. Everything you observe moving relative to you at almost c will be squished in the direction of motion, thus the closer you get to c, the shorter the distance becomes that you need to traverse. At least from your point of view.

When I was studying physics in school I remember a LOT of word problems posed about klingons and the Enterprise. Made it fun. I'm just glad to see that someone pointed out Maxwell, everyone forgets about him, and most people (himself included) thought there was something wrong with his equations when it was first realized that c could be represented as a constant until relativity came along. I don't remember the equation exactly but it's something like c = sqrt([mu][epsilon]). In admitting that relativity doesn't make sense to you, you are already smarter than most people who claim they understand it and really don't. I'm not talking about anyone on this thread, just most of the sci-fi people that I'll run into sometimes on the street.

[ArmchairGravy]

"Armchair, no reason to go fetal, think about it in terms of sublight and you sitting on a spacecraft traveling near c. Everything you observe moving relative to you at almost c will be squished in the direction of motion, thus the closer you get to c, the shorter the distance becomes that you need to traverse. At least from your point of view."

Yes, that really helps pull me from the fetal position.

I've been an armchair cosmologist since I could read, and while my math skills suck, I am totally fascinated by the implications. To think that when I look into the night sky, those photons from other stars that strike my eye have, from their point of view, instantaneously made that journey. Those photons are absorbed by my retina. I am participating in a quantum event that is both instantaneous and took 1000's of years depending on what frame of reference is used. If this doesn't bake your noodle a bit I'm not sure what would.

[-Velocity-]

Isn't the point of entanglement that two observers see the same event (or the other side of the same event) at the same time?

NO.

From my understanding of quantum entanglement, here is how it works:

Imagine two electrons entangled so that one is spin up and one is spin down. They are separated by cosmological distances. Now, you locally measure one of the electrons to be spin down. Before you measure the electron, both electrons are fundamentally in both states, but after you measure it, this means that the other electron must be spin up, even if it is in another galaxy.

Say if distant aliens are measuring the other electron; they measure it to be spin up, and know that yours must be spin down. However, they don't KNOW what state their electron is in UNTIL they measure it- your observation of your electron makes no observable change to their electron.

This is not communication, it's just knowing that some other particle in a distant location must be in some state. The "spooky" part is merely that quantum properties of particles appear to be fundamentally random. It's NOT that the electron is in one state or the other, it is truly in both states at the same time, and thus, when we measure our electron, we determine the state of another particle in a distant location "instantaneously" (from our frame of reference, I guess). But that does not transmit any information, we only KNOW, here in this location, what the particle is in another location. That particle in the other location, from that other location's point of view, does nothing special.

[Dispatcher]

Thank goodness we are discussing the speed of light, as opposed to the speed of the expansion of space. :-) As for Entanglement, what a web we weave.

[Camacha]

This is not communication, it's just knowing that some other particle in a distant location must be in some state.

I never spoke of communication, I said that two observers would see the same thing at the same time, as you argued that at the same time does not exist. If you measured the spin of the same electron at different times, both could see the same spin, I guess.

[K^2]

I never spoke of communication, I said that two observers would see the same thing at the same time, as you argued that at the same time does not exist. If you measured the spin of the same electron at different times, both could see the same spin, I guess.

The time ordering in measurement of entangled spins makes no difference. And, in fact, depending on coordinate system choice, the order in which they are measured can change. The important fact is that prior to measurement both spins are in superposition. After the measurement the state of one can be determined from the state of the other.

[Geckgo]

Now it's getting good, where'd I put that popcorn? Nice to see people politely discussing this stuff instead of arguing and bashing eachother over typos. Kudos to everyone on this thread.

[WestAir]

Anyone know the answer here?

Question for the science professionals here:

What is the difference between an object with mass traveling at the speed of light (299,792,458 m/s), and an object traveling at almost the same speed, but slower by 1 Plank Length x a time larger than the age of the Universe? One is unachievable, and the other is achievable, yet both would have the object move the exact same distance over the exact same time if clocked today. How is that explained?

[Geckgo]

Sorry westair, got my degree nearly 10 years ago and don't remember anything about that. To be honest I'm not sure what you are asking, can you reference an article or be more specific?