Is the Speed of Light a Constant?

[Globus]

According to theory the speed of light cannot be exceeded.

But, you are on a train travelling at, say,100 mph. And you shine a torch out of a window towards the front.

What's the speed of its light?

[Sandstorm]

Is the train going to Thurrock?

[assfly]

According to theory the speed of light cannot be exceeded.

But, you are on a train travelling at, say,100 mph. And you shine a torch out of a window towards the front.

What's the speed of its light?

Of what if you're travelling at the speed of light and throw a ball forward?

[ASMO]

According to Einstein the speed of light doesn't vary with the time or place that you measure it, or how fast you yourself are moving. 186000 miles per second give or take

[Sandstorm]

According to Einstein the speed of light doesn't vary with the time or place that you measure it, or how fast you yourself are moving. 186000 miles per second give or take

Einstein? Spoofer.

[Raggs]

c is the constant. The speed of light in a vacuum.

The speed of any particular bit of light can be quite easily slowed and we see it all the time.

[Grandpa]

This is a reasonable explanation....

You've probably learned that the speed of light is a constant (c). But what if you looked at it from different points of view? For instance, if you're standing on the Earth, then the speed of light is c. But what if you're standing on the train that's moving at half the speed of light? Shouldn't it look like the light is moving at half speed? Well, for all that it certainly seems like it should, it doesn't. The light moves at the same speed whether you're standing on the Earth, or on a speeding train.
But whoa! How can the light move at the same speed from the Earth's perspective as from the train's? Because velocity is equal to distance divided by time, and it seems like everybody should agree on the distance traveled and the time elapsed. But do they? According to special relativity (Einstein's work), distance and time don't work the way that we think they do either, and that get's really important when you're talking about speeds close to the speed of light.

I know that this wasn't exactly what you were asking, but I wanted to emphasize to you how things don't always happen according to the rules that you're used to. One of the basic rules of space and time is that no object can travel faster than c. That might seem ridiculous, because if you can get a train going at 0.9999999999c , you could then walk on it at 0.0000000002 c relative to the train, and that would add up to 1.0000000001 c. However, velocities don't add that way because time and space intervals aren't the same as seen from the ground and the train. Somebody on the train says that you're walking at 0.0000000002 c. Somebody on the ground thinks that your steps are much smaller than you or your friends on the train think, and that it takes you much longer to take those steps than you think. So they still end up thinking that you're traveling at less than c.

You might think that if you could just keep applying force to something it would accelerate to greater than c. However, that assumes that F=ma, which turns out to be false. The effective 'm' in the expression mv for the momentum (v is velocity) increases for bigger v! When you exert a force, as v gets near c you end up mainly increasing that 'm', not v.

The kinetic energy grows more rapidly as v increases than classical physics would say. As v gets close to c, the energy grows toward infinity. So to get to the speed of light, it would take an infinite amount of energy.

[Insane_Homer]

According to theory the speed of light cannot be exceeded.

But, you are on a train travelling at, say,100 mph. And you shine a torch out of a window towards the front.

What's the speed of its light?

Lazy much? Look it up, ffs. There are many a youtube video that will explain it to you in very simple terms. It's pretty fundamental and has been covered extensively way before your epiphany.

A more taxing question:- Is the speed of light constant in both directions? Since we can't measure it from point A to B, we can only measure it with a return trip. A -> B ->A. So is the speed from A->B the same as B->A.

[boere wors]

Is the train on a treadmill?

[Biffer]

Relativity is awesome. It doesn't make logical sense until you really go through it, but the basics of it is that how you view things changes on where you're watching it from. You shine a torch when you're on a train, it goes away from you at the speed of light. From the point of view of an observer standing by the train, it goes at the speed of light, not the speed of light plus the speed of the train.

Time is not absolute.
Distance is not absolute.
Speed is not absolute.
All of these change depending on where you're watching from.

This isn't some hypothetical, Relativity is correct - your GPS wouldn't work without it, and we can measure relativistic effects in satellite communications etc.

[boere wors]

Think of the beginning of that light beam, when it hits your eye. The light beam starts at a specific point in space no matter how fast the source is travelling, think for example that you would flash the torch only for a fraction of a second. From there it travels with the speed of light to your eye.

[Ymx]

According to theory the speed of light cannot be exceeded.

But, you are on a train travelling at, say,100 mph. And you shine a torch out of a window towards the front.

What's the speed of its light?

Of what if you're travelling at the speed of light and throw a ball forward?

Not another forward pass thread.

[Saint]

Light has some very peculiar properties. In part it's a particle, in part a wave, and in part just a probability. When you look at the probability piece and the particle piece, it turns out that a very small % of photons exceed the speed of light, because there's an equivalent number of photons that are slightly "slow"

They think


Maybe


Possibly

[Ymx]

It’s amazing physics, loved it when learnt some of the maths on it at physics at uni.

[Saint]

To add to that, th wave part is stranger.

Imagine a wave of light that's a light year long. If you can manipulate the end correctly, the start changes instantly. So that's quantum entanglement transmitting information instantly.

(Very poor description of what happens)

[Ymx]

Time slows down. This becomes extremely apparent at speeds approaching the speed of light. Imagine a 15-year-old traveling at 99.5 percent the speed of light for five years (from the astronaut's perspective). When the 15-year-old gets back to Earth, according to NASA, he would be only 20 years old. His classmates, however, would be 65 years old.

[Kawazaki]

The speed of light cannot be measured.

[fishfoodie]

I Like that in the Gedankenexperiment, with a beam of light bouncing back and forth along the length of train carriage; as the train goes faster, & the speed of light is a constant; the train shrinks , so that the relative velocity of constant; & if the train gets close to the speed of light itself; it becomes a point.

[Grandpa]

Doesn't dark energy travel faster than the speed of light?

[Saint]

Another one. The theory is that nothing travels faster than the speed of light. But we also know that gravity acts instantly, regardless of distance. So, theoretically, if you could replace the sun with a star twice the mass, the effect on the Earth would be instant. But we wouldn't see the new star that was busy destroying the planet till the light wave caught up with us. So, some things do travel faster (effectively instantly) than light.

Stuff is weird

[Biffer]

Doesn't dark energy travel faster than the speed of light?

No.

[Biffer]

Another one. The theory is that nothing travels faster than the speed of light. But we also know that gravity acts instantly, regardless of distance. So, theoretically, if you could replace the sun with a star twice the mass, the effect on the Earth would be instant. But we wouldn't see the new star that was busy destroying the planet till the light wave caught up with us. So, some things do travel faster (effectively instantly) than light.

Stuff is weird

No, it doesn’t. Gravity doesn’t act at a distance as such, it deformed space time so that it’s the local area of space interacting with the mass, rather than the distant object. General relativity redefined gravitational fields as curvatures in space time to achieve this.

[Gumboot]

Is the train on a treadmill?

How does light behave when it encounters a roundabout?

[ASMO]

Is the train on a treadmill?

How does light behave when it encounters a roundabout?

and does it wipe back to front or the other way around?

[inactionman]

My empirical observations suggest the speed of light is different at night

[Grandpa]

Doesn't dark energy travel faster than the speed of light?

No.

Tachyons?

[Biffer]

Doesn't dark energy travel faster than the speed of light?

No.

Tachyons?

That's rather different. They're entirely hypothetical and their application in modern physics is more related to imaginary mass. Tachyons would break the law of causality, and challenge everything we know about physics if they were shown to exist afaik. They're hypothesised for all sorts of other things related to quantum fields, group velocities of / within those fields and all sorts of other things as well. Tachyons are necessary for some proposed string theories, but given string theory as a whole is broadly conjecture at this point, who knows if they're real or not.

In some theoretical presentations they're considered as travelling backwards through time. That relates to some of the most fundamental questions being asked at the moment in physics - e.g. what, actually, is time? Is it just another dimension like the spatial ones? Why do we observe and interpret it differently? Does it behave more as a spatial dimension for non-baryonic matter? Or does non-baryonic matter behave in the same way with our spatial dimensions as baryonic matter does with time? And that also relates to the fundamental question of what is mass? Again, we don't have an answer to what it is - we can measure it, observe it, quantify it, but we kind of hand wave around what it actually is by considering it a fundamental property of some particles. Same question can be asked of charge, spin, etc. And when it comes to comparisons of that nature, why can we have positive and negative charge, but we can't have positive and negative mass? Or negative energy? And is the imaginary mass implied within some of the string theory work with tachyons the same as negative mass?

[Grandpa]

No.

Tachyons?

That's rather different. They're entirely hypothetical and their application in modern physics is more related to imaginary mass. Tachyons would break the law of causality, and challenge everything we know about physics if they were shown to exist afaik. They're hypothesised for all sorts of other things related to quantum fields, group velocities of / within those fields and all sorts of other things as well. Tachyons are necessary for some proposed string theories, but given string theory as a whole is broadly conjecture at this point, who knows if they're real or not.

In some theoretical presentations they're considered as travelling backwards through time. That relates to some of the most fundamental questions being asked at the moment in physics - e.g. what, actually, is time? Is it just another dimension like the spatial ones? Why do we observe and interpret it differently? Does it behave more as a spatial dimension for non-baryonic matter? Or does non-baryonic matter behave in the same way with our spatial dimensions as baryonic matter does with time? And that also relates to the fundamental question of what is mass? Again, we don't have an answer to what it is - we can measure it, observe it, quantify it, but we kind of hand wave around what it actually is by considering it a fundamental property of some particles. Same question can be asked of charge, spin, etc. And when it comes to comparisons of that nature, why can we have positive and negative charge, but we can't have positive and negative mass? Or negative energy? And is the imaginary mass implied within some of the string theory work with tachyons the same as negative mass?

All of that is incredibly fascinating isn't it? That we have discovered so much and yet we still don't really understand what mass, gravity, light and time actually are... the "basics"!.

You feel there is another huge breakthrough still to come that will answer those questions and enable us to discover so much more.. it feels like we are on the cusp of that breakthrough.. but I have been thinking that for years...

[Plim]

I always feel humbled by those with a proper grasp of these fundamental aspects of physics.

I have a training in engineering to degree level and have studied maths to a decent standard. So I can understand the language, but the detail of this stuff is boggling.

I’m especially intrigued by (presumably, the theoretical existence of) gravitons. Also singularities. The mathematics of the infinite is another extraordinary topic.

Mind, I’ve been a lawyer for years so that might explain my stupidity.

[Tichtheid]

I always feel humbled by those with a proper grasp of these fundamental aspects of physics.

I have a training in engineering to degree level and have studied maths to a decent standard. So I can understand the language, but the detail of this stuff is boggling.

I’m especially intrigued by (presumably, the theoretical existence of) gravitons. Also singularities. The mathematics of the infinite is another extraordinary topic.

Mind, I’ve been a lawyer for years so that might explain my stupidity.

The university and I parted company by mutual consent after two years of an engineering degree.

However, I remember a quote that was on the wall of the office of the guy in charge of our course, it was by the theoretical physicist Arnold Sommerfeld

“Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two points. The third time you go through it, you know you don't understand it, but by that time you are so used to the subject, it doesn't bother you anymore..”

I just thought they were making stuff up as they went along in that class.

[Biffer]

I always feel humbled by those with a proper grasp of these fundamental aspects of physics.

I have a training in engineering to degree level and have studied maths to a decent standard. So I can understand the language, but the detail of this stuff is boggling.

I’m especially intrigued by (presumably, the theoretical existence of) gravitons. Also singularities. The mathematics of the infinite is another extraordinary topic.

Mind, I’ve been a lawyer for years so that might explain my stupidity.

The university and I parted company by mutual consent after two years of an engineering degree.

However, I remember a quote that was on the wall of the office of the guy in charge of our course, it was by the theoretical physicist Arnold Sommerfeld

“Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two points. The third time you go through it, you know you don't understand it, but by that time you are so used to the subject, it doesn't bother you anymore..”

I just thought they were making stuff up as they went along in that class.

Thermodynamics is hard. Entropy is every bit as weird as quantum mechanics sometimes. Until you start getting negative entropy introduced in quantum calculations. The the whole thing is fucking batshit.

[inactionman]

I always feel humbled by those with a proper grasp of these fundamental aspects of physics.

I have a training in engineering to degree level and have studied maths to a decent standard. So I can understand the language, but the detail of this stuff is boggling.

I’m especially intrigued by (presumably, the theoretical existence of) gravitons. Also singularities. The mathematics of the infinite is another extraordinary topic.

Mind, I’ve been a lawyer for years so that might explain my stupidity.

The university and I parted company by mutual consent after two years of an engineering degree.

However, I remember a quote that was on the wall of the office of the guy in charge of our course, it was by the theoretical physicist Arnold Sommerfeld

“Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two points. The third time you go through it, you know you don't understand it, but by that time you are so used to the subject, it doesn't bother you anymore..”

I just thought they were making stuff up as they went along in that class.

I found thermodynamics comfortably the most difficult of all the courses I sat at undergrad (I'm MEng), I've still no real conceptual idea of what entropy actually is. It's also a very dry subject for those who aren't really focused upon the pure physics aspect but just in terms of being able to quantify.

A lot of engineers are quite visual, physical people, and much of thermodynamics is generally quite hard to visualise.

[Biffer]

I always feel humbled by those with a proper grasp of these fundamental aspects of physics.

I have a training in engineering to degree level and have studied maths to a decent standard. So I can understand the language, but the detail of this stuff is boggling.

I’m especially intrigued by (presumably, the theoretical existence of) gravitons. Also singularities. The mathematics of the infinite is another extraordinary topic.

Mind, I’ve been a lawyer for years so that might explain my stupidity.

The university and I parted company by mutual consent after two years of an engineering degree.

However, I remember a quote that was on the wall of the office of the guy in charge of our course, it was by the theoretical physicist Arnold Sommerfeld

“Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two points. The third time you go through it, you know you don't understand it, but by that time you are so used to the subject, it doesn't bother you anymore..”

I just thought they were making stuff up as they went along in that class.

I found thermodynamics comfortably the most difficult of all the courses I sat at undergrad (I'm MEng), I've still no real conceptual idea of what entropy actually is. It's also a very dry subject for those who aren't really focused upon the pure physics aspect but just in terms of being able to quantify.

A lot of engineers are quite visual, physical people, and much of thermodynamics is generally quite hard to visualise.

I'm the same with entropy tbh. I just had to park it and consider it as a numerical concept.

[Globus]

Great stuff chaps.

If you shone a powerful light, would it trail around the globe, because the effect of gravity?

[Insane_Homer]

If you shone a powerful light, would it trail around the globe, because the effect of gravity?

no

[Grandpa]

Great stuff chaps.

If you shone a powerful light, would it trail around the globe, because the effect of gravity?

You mean like the sun at sunrise and sunset?

[Biffer]

Great stuff chaps.

If you shone a powerful light, would it trail around the globe, because the effect of gravity?

No. Gravity of the earth isn't strong enough. The only place you'd get light going in an orbit is at the event horizon of a black hole.

[GogLais]

I always feel humbled by those with a proper grasp of these fundamental aspects of physics.

I have a training in engineering to degree level and have studied maths to a decent standard. So I can understand the language, but the detail of this stuff is boggling.

I’m especially intrigued by (presumably, the theoretical existence of) gravitons. Also singularities. The mathematics of the infinite is another extraordinary topic.

Mind, I’ve been a lawyer for years so that might explain my stupidity.

Same here except that I was only a surveyor. I’ve also read a fair bit about without much understanding of the Bletchley Park code-breaking stuff. Amazing minds.

[troglodiet]

I always feel humbled by those with a proper grasp of these fundamental aspects of physics.

I have a training in engineering to degree level and have studied maths to a decent standard. So I can understand the language, but the detail of this stuff is boggling.

I’m especially intrigued by (presumably, the theoretical existence of) gravitons. Also singularities. The mathematics of the infinite is another extraordinary topic.

Mind, I’ve been a lawyer for years so that might explain my stupidity.

Exactly the same with me. Finished 3rd year electronic engineering, but can't grasp this stuff. Every time I think I am beginning to see some light (pardon the pun), some asshole says something that completely destroys everything I thought I knew.

For instance that video posted earlier about why c cannot be measured, which I also stumbled on a couple of months ago.

I tend to understand almost every word these quantum scientists say, just not in the sequence they say it.

[SaintK]

I always feel humbled by those with a proper grasp of these fundamental aspects of physics.

I have a training in engineering to degree level and have studied maths to a decent standard. So I can understand the language, but the detail of this stuff is boggling.

I’m especially intrigued by (presumably, the theoretical existence of) gravitons. Also singularities. The mathematics of the infinite is another extraordinary topic.

Mind, I’ve been a lawyer for years so that might explain my stupidity.

Same here except that I was only a surveyor. I’ve also read a fair bit about without much understanding of the Bletchley Park code-breaking stuff. Amazing minds.

Rancid Globus bait.