faster-than-light newbie question

[Mephedr0ne]

i'm on a ship travelling 1mph less than light speed, i decide to run the length of the ship in the direction we're travelling, lets say 10mph running speed.

can i do this? or will the light-speed barrier prevent me from moving faster than 1mph?

i know relative to the ship i'm moving at 10mph, but relative to any point in space behind the ship its now lightspeed + 9mph.

[cragar]

you haft to use loretnz transformations , you cant just add the speeds like that , And yes you cant break the light barrier .

[jtbell]

i know relative to the ship i'm moving at 10mph, but relative to any point in space behind the ship its now lightspeed + 9mph.

No, it's not. Look up relativistic "velocity addition." For example:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html

[LostConjugate]

i know relative to the ship i'm moving at 10mph, but relative to any point in space behind the ship its now lightspeed + 9mph.

But in order for the point behind the ship to be relative, your speed would not be invariantly 10mph

[jamesa00789]

Someone standing still on your ship will observe you travelling at 10mph. To you, you will observe yourself moving at 10mph because the floor will be moving at 10mph.

To someone stationary outside the ship, who sees the ship travelling at 0.99c, will see you travel at 0.99c + a little bit, but not greater than c (due to lorentz transformation).

While you are running on your ship (which is travelling at 0.99c) you shine a light in the direction of motion, you will see the light come out of the torch at c, not 1.99c. The person who is outside of your ship will also observe light to come out of the torch at c, not 1.99c.

[kj30]

To the original poster, the "inconsistency" you describe is exactly what Einstein pondered too. It's why he came up with his theory. It's without his theory that you have the apparent contradiction you ask about.

[thread mark]

I think a good questions here is, can we measure speeds higher than the speed of light?

[yuiop]

I think a good questions here is, can we measure speeds higher than the speed of light?Yes you can. You could for example sweep a laser pointer across the surface of the Moon from the Earth and the spot of light on the moon could be travelling faster than the speed of light. The important thing to note is that no information or energy is transported faster than light by doing this. No one has ever observed a material object or energy or information being transmitted faster than the speed of light, but in principle if an object did travel faster than light we could easily measure it. Tachyons are hypothetical particles that can travel faster than light and they are constrained to always move faster than the speed of light which is their minimum speed. Tachyons have never been observed and it seems that if they do exist, that nature prevents them from interacting with normal particles in our universe and are undetectable. If tachyons were detectable they would violate causality.

[DaveC426913]

i'm on a ship travelling 1mph less than light speed, i decide to run the length of the ship in the direction we're travelling, lets say 10mph running speed.

can i do this? or will the light-speed barrier prevent me from moving faster than 1mph?

i know relative to the ship i'm moving at 10mph, but relative to any point in space behind the ship its now lightspeed + 9mph.

There are many ways to crack this nut. One may work for you. Here is another way to look at it:

You recall that relativistic velocities result in time dilation, right? Moving at 1mph below lightspeed you are travelling at (let's call it) .9999999c. At that speed, you are dilated by a factor of more than 2000. You are aging 2000 times slower in your spaceship than anyone on Earth. It means that Earth, while watching you whiz by at .9999999c, looks in a window at sees you running in very slow motion; i.e. it will take you days to reach the front of your spaceshjip. Measuring your progress, they see you moving at .9999999c + 10/2000ths mph, which is still less than c.

You can use whatever numbers you want, but the closer your spaceship gets to c, the more your frame of reference within it is dilated, slowing you, such that your total progress will never add up to c.

[Hawkwind]

i'm on a ship travelling 1mph less than light speed, i decide to run the length of the ship in the direction we're travelling, lets say 10mph running speed.

can i do this? or will the light-speed barrier prevent me from moving faster than 1mph?



According to the principle of relativity, you can of course do this.

The rest system of a ship travelling with a constant speed is an inertial coordinate system which can by no experiment be distinguished from any other inertial system. So, you can do your walk there just the same way as in the case of your ship resting.

However, relativistic speed addition laws will make sure, that there is no inertial system where your relative speed will be measured as equal or larger c (speed of light in vacuum).

[thread mark]

Yes you can. You could for example sweep a laser pointer across the surface of the Moon from the Earth and the spot of light on the moon could be travelling faster than the speed of light. The important thing to note is that no information or energy is transported faster than light by doing this. No one has ever observed a material object or energy or information being transmitted faster than the speed of light, but in principle if an object did travel faster than light we could easily measure it. Tachyons are hypothetical particles that can travel faster than light and they are constrained to always move faster than the speed of light which is their minimum speed. Tachyons have never been observed and it seems that if they do exist, that nature prevents them from interacting with normal particles in our universe and are undetectable. If tachyons were detectable they would violate causality.

I was referring to a type of technology that can detect speeds past the light barrier. But tools can include moons I guise. Velocity of the moon away from the beem plus the speed of light.

[DaveC426913]

I was referring to a type of technology that can detect speeds past the light barrier. But tools can include moons I guise. Velocity of the moon away from the beem plus the speed of light.

This doesn't make sense.

Are you talking about a massive body receding at relativistic velocities, and a beam of light being sent ahead of it? That beam of light will be observed to be moving at c from all reference frames.

[harrylin]

i'm on a ship travelling 1mph less than light speed, i decide to run the length of the ship in the direction we're travelling, lets say 10mph running speed.

can i do this? or will the light-speed barrier prevent me from moving faster than 1mph?

i know relative to the ship i'm moving at 10mph, but relative to any point in space behind the ship its now lightspeed + 9mph.

Yes and no. :smile:

According to relativity you cannot go as fast as light.

1. Suppose that you define the speed of your ship relative to the Earth and you track your speed with a very precise GPS tracker. Then you'll find that you're by far not walking 1mph faster than the ship; instead you may be shocked to see that you hardly advance in the ship. You'll not achieve lightspeed at all, let alone lightspeed + 9 mph.

2. Suppose that you set up an independent measurement system in your ship. Then you may be able to run 10mph in your ship, as measured with that system. However, your system measures the speed of your ship to be 0mph, and it measures light to move at the usual speed of light with respect to it. You will again not find yourself going at lightspeed + 9mph, instead you will find yourself to go at 10mph.

Harald

[DaveC426913]

...instead you may be shocked to see that you hardly advance in the ship. You'll not achieve lightspeed at all, let alone lightspeed + 9 mph.
Yes. But you have not explained how this could be.

[harrylin]

Yes. But you have not explained how this could be.

Indeed I did not, Mephedr0ne didn't ask for that.

[russ_watters]

I was referring to a type of technology that can detect speeds past the light barrier. But tools can include moons I guise. Velocity of the moon away from the beem plus the speed of light. There is no reason why we would not be able to detect an object moving at greater than C. If an object with a beacon on it quickly accelerated away from you to a speed much greater than C, you'd be able to watch it leave - just as you can hear an object moving away from you at greater than the speed of sound. Heck, that's how we measure the expansion of the universe at high redshifts!

You didn't really specify, but we would also not have any problem watching an object going around a circular track at greater than C (for example, in a particle accelerator), simply measuring the time from emission to hitting a detector.