# Light speed

#### drjohsmith

Joined Dec 13, 2021
383
A thought experiment,
could be I typed without thinking to much ,
but,,

Two people , traveling directly away from each other
each traveling faster than half speed of light

Assuming optics can cope,
would they be able to see each other ?

#### BobTPH

Joined Jun 5, 2013
4,769
Yes. The speeds do not add the way you think, and they will not be moving away from each other at the speed if light.

Bob

#### BobTPH

Joined Jun 5, 2013
4,769
Also, there will be a huge red shift, so the illumination needed for them to see each other will be at higher frequencies than visible light.

Bob

#### Wendy

Joined Mar 24, 2008
22,788
I once did a thought experiment with someone who knew the math two objects moving to a third object both in opposite directions. Each going 50% of l light speed as referenced to the third object. The speed each sees of the other is 7/8 the speed of light. It all depends on your point of reference. There is no outside point of reference to measure speeds only the one you pick. Depending on your point of reference you cannot exceed the speed of light between them.

#### drjohsmith

Joined Dec 13, 2021
383

#### nsaspook

Joined Aug 27, 2009
9,466
Thank you

never heard of relativistic speeds

interesting
Einstein's Relativity and Everyday Life

https://physicscentral.com/explore/writers/will.cfm

But in a relativistic world, things are not simple. The satellite clocks are moving at 14,000 km/hr in orbits that circle the Earth twice per day, much faster than clocks on the surface of the Earth, and Einstein's theory of special relativity says that rapidly moving clocks tick more slowly, by about seven microseconds (millionths of a second) per day.

Also, the orbiting clocks are 20,000 km above the Earth, and experience gravity that is four times weaker than that on the ground. Einstein's general relativity theory says that gravity curves space and time, resulting in a tendency for the orbiting clocks to tick slightly faster, by about 45 microseconds per day. The net result is that time on a GPS satellite clock advances faster than a clock on the ground by about 38 microseconds per day.

#### ApacheKid

Joined Jan 12, 2015
617
The "speed of light" can be thought of as a law, a law of physics. It pops out of what are called Maxwell's field equations, the speed of light is a characteristic of those equations.

So because it represents a law, it does not change, it cannot change, this is the reason it is constant. That constancy is often a source of puzzlement until one realizes that it is inherently a law, or a consequence of a law.

#### drjohsmith

Joined Dec 13, 2021
383
The "speed of light" can be thought of as a law, a law of physics. It pops out of what are called Maxwell's field equations, the speed of light is a characteristic of those equations.

So because it represents a law, it does not change, it cannot change, this is the reason it is constant. That constancy is often a source of puzzlement until one realizes that it is inherently a law, or a consequence of a law.
Its interesting

Always "known" that speed of light is fixed,
BUT

this came up as I was reading some flim flam article about red and blue shift
all of which I also "know"

But the flim flam was very missing on why the question I gave did not violate the faster than light

Seems that the velocity vectors dont just add up,

Thanks all ,
given me a new set of stuff to read up on

wonder what happens if a "bullet" could be sent near speed of light, etc,
fun stuff

#### nsaspook

Joined Aug 27, 2009
9,466
thank you @nsaspook

interesting article,
but was that meant to be pointing to something about relativistic speeds ?
Einstein's theory of special relativity -> relativistic speed.

Yes because the relativistic speed can be a factor when things are not near light speed too if the needed precision of time, length or mass can be affected by special relativity. The force we normally say is magnetic in a electromagnet is actually a consequence of very tiny relativistic speed adjustments causing fixed light of speed frame of reference shifts.

Energy: A .308 "bullet' @ 99.999% of the speed of light
Newtonian model:
KE= ~585,000,000,000,000 J or about 134,000 metric tons of TNT

Relativistic model:
KE = ~2.5 x 10^20 joules
4.2 x 10^12 joules per megaton of TNT

https://courses.lumenlearning.com/physics/chapter/28-6-relativistic-energy/

We all die from one .308 bullet.

#### Wendy

Joined Mar 24, 2008
22,788
Part of the reason nothing can go at the speed of light is the energy required is infinite. The nearest thing we can hit to the infinite that we know of in the universe is black holes (infinite density). There might even be some relationship there.

#### nsaspook

Joined Aug 27, 2009
9,466
Part of the reason nothing can go at the speed of light is the energy required is infinite. The nearest thing we can hit to the infinite that we know of in the universe is black holes (infinite density). There might even be some relationship there.
Maybe:
https://www.forbes.com/sites/starts...an-how-dense-is-a-black-hole/?sh=6fd830525b92
If your black hole is non-rotating, the singularity is nothing but a mere point. If all the mass is compressed into a single, zero-dimensional point, then when you ask about density, you are asking “what happens when you divide a finite value (mass) by zero?”

If you need a reminder, dividing by zero is mathematically bad; you get an undefined answer. Thankfully, perhaps, non-rotating black holes aren’t what we have in our physical Universe. Our realistic black holes rotate, and that means that the interior structure is much more complicated. Instead of a perfectly spherical event horizon, we get a spheroidal one that’s elongated along its plane of rotation. Instead of a point-like (zero-dimensional) singularity, we get a ring-like (one-dimensional) one, which is proportional to the angular momentum (and the angular momentum-to-mass) ratio.

Unfortunately for us, there’s no way we know of to test this experimentally or observationally. We might be able to calculate – to help us visualize – what we theoretically expect to happen inside of a black hole, but there’s no way to get the observational evidence.

The closest we’ll be able to come is to look to gravitational wave detectors like LIGO, Virgo and KAGRA, and to measure the ringdowns (i.e., the physics in the immediate aftermath) of two merging black holes. It can help confirm certain details that will either validate or refute our current best picture of black hole interiors. So far, everything lines up exactly as Einstein predicted, and exactly as theorists expected.

#### ApacheKid

Joined Jan 12, 2015
617
Its interesting

Always "known" that speed of light is fixed,
BUT

this came up as I was reading some flim flam article about red and blue shift
all of which I also "know"

But the flim flam was very missing on why the question I gave did not violate the faster than light

Seems that the velocity vectors dont just add up,

Thanks all ,
given me a new set of stuff to read up on

wonder what happens if a "bullet" could be sent near speed of light, etc,
fun stuff
Einstein was a big "thought experiment" person, he did that a lot. One of the questions he pondered when young, perhaps a few years before his 1905 paper, was "what would a light wave look like if I ran along side it" (so to speak).

Maxwell's equations lead to a speed of wave propagation that is constant:

It was therefore immediately assumed that the meaning of this speed must be relative to something, hence the "aether" concept was embraced, not for the first time in physics, this made sense theoretically because a) it made the meaning of the speed clear and b) it defined the "medium" within which the waves are able to propagate.

It looked to many back then that pretty much physics had everything all sewn up, looked like they were on the home stretch...

#### Wendy

Joined Mar 24, 2008
22,788
Except little facts kept popping up to poke holes in their balloon.

#### nsaspook

Joined Aug 27, 2009
9,466
By the Magic of Goole always listening to what I type
this popped into mail box

https://spacepsychiatrist.com/nasa-...-the-speed-of-light-engine-of-speed-of-light/

Seems some one has idea of how to go fast, even if it takes time to get to speed, using the things change mass as speed changes of special relativity

Amazing stuff,
Total Bull-hockey. Another without propellant and go beyond the principle of action-reaction opium dream like the EMDrive. https://forum.allaboutcircuits.com/threads/reactionless-drives.104359/

http://www.projectrho.com/public_html/rocket/reactionlessdrive.php
Friends Don't Let Friends Use Reactionless Drives In Their Universes.
https://arstechnica.com/science/201...e-would-provide-thrust-in-different-universe/

Conservation of momentum should not be ignored
Now, the author argues that because the inertial mass grows nonlinearly with speed, there is an average acceleration in one direction. Even better, this difference increases as the peak particle speed gets closer to the speed of light. Unfortunately, the Universe just doesn't quite work like that.

So, let's just state up front: this drive won't work. The problem is that, even though the author does a very nice simulation, he has left out the fields that do the accelerating. When we accelerate ions using a magnetic or electric field, the ions push back on the field. There is an equal and opposite force exerted on the electrodes and coils that produce the fields, and those just happen to be in the spaceship, too.

In the first step, where we accelerate the mass to a high relativistic speed, we also accelerate the cylinder in the opposite direction. Now, in special relativity, we don't conserve energy and momentum separately. Instead, they are conserved together. If you only consider momentum (and not energy), then you will find net forces everywhere due to inertial mass changes—things get heavier as they approach the speed of light. This is exactly what the author has found. If you consider energy and momentum simultaneously, those forces will suddenly disappear.

This is where the increase in inertial mass comes from in the first place: energy is sucked out of the field and turned into mass. When the particles are slowed, that mass is given up as photons in the field, which slow the cylinder as they are absorbed. What is the net force? Zero, 0N of force.
Helical Engine
https://ntrs.nasa.gov/citations/20190029294

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#### dcbingaman

Joined Jun 30, 2021
559
The speed of light is not constant for an accelerating frame of reference. And Newtons laws are not valid either for an accelerating frame of reference. If the accelerating frame of reference considers itself not moving or moving with constant speed. (perfectly valid viewpoint as any other) the speed of light will be found to vary at least over measured distances considered significant enough. But it will continue to have speed c when measured locally that is when delta x is small. The x coordinate being the direction of acceleration. So there are some exceptions. But again another frame of reference not accelerating will see the speed of light for the accelerating frame of reference is still c. Both are correct.

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#### crutschow

Joined Mar 14, 2008
29,508
I read that if you have a rocket that can accelerate constantly at 1G (to give a nice, normal gravity environment in the ship) then, due to time dilation, you would be able to get the edge of the know universe in a lifetime (as measured inside the ship).