Just read that the speed of light is the same for all freely moving observers. First question – does that mean if i was travelling at half the speed of light and i flashed my lights, if i was sufficiently observant woyld i observe light zipping away from me at one and a half times the speed of light?

There will almost certainly more questions shortly.

No because the speed of light is a limit, nothing can exceed it.

I believe the measured speed is constant for any observer, regardless of their own movement

… but that the nature of time will differ for each of them 😯

If you were going close to the speed of light, then you’d still see your headlight flash moving away from you at the speed of light. But that’s because your personal time slows down, so it’d look like it was going away from you at the speed of light but to others it wouldn’t.

It’s because the speed of light cannot be exceeded that time has to slow down… Einstein worked this all out just by thinking about it.

No – you’d see the light travelling away from you at the speed of light.

Or, if you were driving at half the speed of light and another car was approaching you at half the speed of light, if he flashed his lights at you, you’d see the light approaching at the speed of light.

Speed of light in a vacuum is a constant, no matter the speed of the observer.

Buuuuuut… If there was someone stood off to one side at 90 degrees to my direction of travel, would THEY not see the light from the flash of my lights heading away from me at one and a half times the speed of light?

pondo, see that cupboard over there in the corner ?

you need to put your common sense in there – it ain’t no good round here

there’s some crazy, crazy shit going on

Ooo – to clarify my original question, I understand that I would perceive the light leaving me at the speed of light, but if I was doing half the speed of light, would that not mean that the light would actually be doing 1.5xC?

They’d see the light moving away from you at the speed of light. Why do you think the light from your headlights would be moving away at 1.5 times the speed of light? (what is your reasoning behind this thought?)

Amusingly, outside of a vacuum, the speed of light can be exceeded.

Which calls into question the definition of a vacuum of course.

Edit : typo removed

over there, the cupboard with the cat in it

Amusingly, outside of a vacuum, the speed of light can bad exceeded.

Sure about that? It goes slower in other materials…

You have to think about what you mean by “speed” – speed is distance in a period of time. But time isn’t the same everywhere in the universe, and it isn’t the same for everyone – and I don’t mean different time zones, I mean that time runs at different rates for different people.

Some things can exceed the speed of light. A blackbird flying over a garden or a pair of scissors.

Amusingly, outside of a vacuum, the speed of light can bad exceeded.

Slowing light down enough so you can cycle faster than it?

Even at that speed, I’m not going to overtake it. Not at my size and fitness.

I think it’s my inability to comprehend… I guess the scale of things moving at that level of speediness. I try to take things to extremes of simplicity to try and understand them, so I’m thinking of, say, watching a car drive past at 30 miles per hour and someone throws a ball out of the window at 15mph in the direction of the car in front – from my point of view, the ball would be travelling from left to right in front of me at 45mph, the thrower’s car would perceive it as travelling at 15mph, the car in front would see the ball approaching at 15mph too, if they were keeping an eye on their mirrors. To apply a massive and erroneous simplification, if Einstein had stated that C=35mph, then would I have observed the ball moving at C+10mph?

I think my poor laboured mind is just unable to comprehend the scale of things moving at or close to C. 🙁

molgrips – Member
“Amusingly, outside of a vacuum, the speed of light can bad exceeded.”
Sure about that? It goes slower in other materials…

I don’t think we are disagreeing. The speed of light in a vacuum can not be exceeded (probably, the maths means the particles need some very special characteristics and the CERN experiment was apparently flawed), but because in other materials light is slowed then in some of those materials other things can outpace light in while both travel through that material.

There used to be some good photos from the NPL.

To apply a massive and erroneous simplification, if Einstein had stated that C=35mph, then would I have observed the ball moving at C+10mph?

No, you would have observed it moving at a bit less than C=35mph. Because what you perceive as an “hour” would be longer than what an outside observer would perceive.

Don’t try and relate it to the everyday. It’s not like that. The speed of light isn’t just fast, it bends reality in ways that are nothing like what you’ve experienced.

Unless you’ve done acid maybe. Just think of it in those terms.

Cheers folks – I don’t think the simplification approach is the best way to try and wrap your head around relativity. 🙂

Once you can get your head around the idea that time and space are not constant – an hour for you isn’t the same as an hour for me* – then it begins to make sense. The only thing that’s constant is c. Time and space warp themselves to keep c constant for everyone.

*by an undetectably tiny amount, because we don’t move very fast.

does that mean if i was travelling at half the speed of light and i flashed my lights

Now hang on a minute – lets phrase this in terms that STW can understand.

“if i was travelling at half the speed of light making progress…..”

Ride off a 1000ft cliff on your bike and then jump off your bike 1ft off the ground. See if you survive as you weren’t moving after all were you and you only fell 1ft.

Here’s some stuff to ponder (get it?), if a particle (which has to be of 0 mass, so a photon (or boson/gluon, but they don’t do fun stuff like this)) is travelling at the speed of light, it’s relativistic factor will be 1/0, so effectively infinity.
This means that from the perspective of a particle travelling at the speed of light, an infinitely long period of time to an outside observer, would appear to be an infinitely small amount of time to the high speed particle.
So at the speed of light, time, and hence distances (to the thing travelling at that speed, NOT the outside observer), become a bit meaningless – it can travel an infinitely long distance in what seems to be an infinitely small amount of time.

😯

Surely it’s all relative, though??

DrP (Boom boom)

Drac – Moderator
Ride off a 1000ft cliff on your bike and then jump off your bike 1ft off the ground. See if you survive as you weren’t moving after all were you and you only fell 1ft.

If you push hard enough with your legs it will work.*

*Even Chris Hoy’s legs can’t push hard enough.

What bit of road are we traveling on? As you will be traveling around corners you’ll need some really good tyres. “What tyres for traveling at the speed of light” OR is this all on a treadmill, because that changes thing.

Ride off a 1000ft cliff on your bike and then jump off your bike 1ft off the ground. See if you survive as you weren’t moving after all were you and you only fell 1ft.

Perfectly valid strategy for your Road Runner. Coyotes? Not so much. 😀

I think of it like this-

We’re all moving through 4D spacetime at the speed of light. When you’re sat perfectly still you’re moving through time at the speed of light because you’re not ‘using up’ any of your allocated speed to move through the three physical dimensions.

If/when you go charging down a hill, or grinding up it, or do some sweet jumps, you have to use up some of your ‘speed allowance’ so you in fact end up moving through time a little more slowly (relative to an observer).

If we’re moving through 4D spacetime at the speed of light (300,000 m/s I think?), if we use some of that speed to move through the three physical dimensions at 100 km/h or 28 m/s (in a car for example) then we’re still moving through spacetime at 300,000 m/s, but moving through time at 300,000 – 28 m/s = 299,972 m/s because we’re ‘using’ 28 m/s to move through 3D space.

I’m probably way off the mark and missing a square of something somewhere, but that’s how I think about it.