# Is it possible to know if you are travelling close to the speed of light if you have nothing to compare your speed with?

This is a question that has been bugging me for some time. Let's see if I am able to make me understood!

Imagine you are travelling in a spaceship at 99.9999999999% the speed of light and you are somehow within a spaceship without windows.

My question is, is there ANY kind of experiment that would show you any time/space dilation effect as compared to being stationary? Is there ANY lab or thought experiment you could make or think that would tell you there are weird things happening with time and space outside your spaceship? I mean, would physics work differently than they would at home? Would radioactive atoms take longer to decay in a way you could measure and compare with your earth-based decay tables? Anything?

Or the only way to find out would be simply to step out of the spaceship and find out that life on Earth has gone extinct in the last couple of hours?

And another question related to the first. If you were travelling in the spaceship at that speed in the X-axis direction and you handthrew a baseball in that same direction, would that ball's mass or inertia explode asyntotically to practically infinity since you are increasing a fantastically high speed a tiny winy bit more? Or perhaps you could not threw the ball at all because of the huge mass increase being that close already to the speed of light? Or you wouldn't notice nothing special and a regular sized with normal weight baseball would impact the other side of the room at a relative speed to you of 60 km/h?

• Velocity is relative. If you can't detect anything outside the ship, you can't tell what your speed is relative to Earth. Jun 14, 2019 at 3:38
• Yes, it's simply not possible. "How fast am I travelling?" isn't answerable even if I can see outside the ship. Velocity and speed only make sense in relation to some other thing, eg "how fast am I travelling relative to Earth", or "how fast am I travelling relative to the centre of the galaxy?". You have to specify some frame of reference because velocity is not absolute. The exception to this is light, which always travels at c (in a vacuum) in every reference frame. Jun 14, 2019 at 6:13
• In fact I'd take it a step further-- it's not that you need a reference to know how fast you're moving, it's that you are never moving at all. Other things are always what is moving, never you. So the question is not how would you know your speed from inside the ship, it's how would you know the speed of what is outside when you are inside. Pretty much you'd only know when something from outside came blasting through your shielding. Jun 14, 2019 at 7:52
• "I mean, would physics work differently than they would at home?" -- No. That's a basic part of Special Relativity: the laws of physics are identical in different inertial reference frames. Jun 14, 2019 at 13:43
• @Ignacio There's no transition like that. You can never boost a sublight speed to c, so such a transition can't occur. At such high speeds, it's often more convenient to measure motion with rapidity rather than speed. The speed of light corresponds to infinite rapidity. Jun 14, 2019 at 17:49

Velocity is relative. From your perspective, your velocity is zero. Without windows (or sensors) on your spaceship, you cannot know the difference. To measure your velocity, you have to define a reference point, eg a star, the average velocity of surounding stars or the galaxy center. If you have such a huge velocity relative to them, from you perspective, they are this fast and not you. So you can observe the time and space dilation of these objects, while your time appears to be normal. You ship is an inertial reference frame.

As for throwing the baseball: From your perspective, you have the speed zero, and the baseball eg. the speed 0.5c. A "fixed" observer, which observes you at 0.99c will measure the balls velocity at 0.997c. With those relative effects, speeds don't add up, only kinetic energy does. http://curious.astro.cornell.edu/about-us/139-physics/the-theory-of-relativity/special-relativity/1016-why-can-t-relative-velocities-add-up-to-more-than-the-speed-of-light-intermediate The increased mass is only a relativistic observation, resulting from the kinetic energy (relative to the observer).

If you have nothing to compare yourself with then there is no speed.

Speed is like marriage - you can't be "married" if you're alone. To define speed, you need an external reference. Then you say "my speed is XYZ km/s relative to object ABC".

This is what most people get wrong about speed. They think it's something you have in yourself, like the number of atoms, or the net electric charge. That is not true. Speed is always relative - you always, ALWAYS, measure your speed relative to an external object.

In a completely empty universe, where only you would exist, speed would be meaningless. You could not define your speed in any way, because how would you measure it? You need an external reference, always.

And to prevent another question that people usually ask: you cannot measure your speed relative to "space". Speed can only be measured relative to other things, and space is not a thing. Space is just the background where the relationship called "distance" takes place. You cannot grab the blue marker pen and put a big X on "space". But you can grab the blue marker pen and put a big blue X on an asteroid, and then say "measure speed relative to this".

So, if you move at 0.999c relative to object A, then space contraction and time dilation apply to you (and to A), as calculated from relativity. But if at the very same time your speed is only 0.5c relative to object B, then space contraction / time dilation are different as seen by B (or seen by you with regards to B), again as calculated from relativity. Object A will see a certain amount of space contraction being applied to you; object B will see a different amount of space contraction applied to you. Both are right.

This is why it's called "relativity" - because nothing is absolute, everything is relative, and it all depends on the relative speeds between objects.

You don't "contract" in an absolute way when you move - because motion (speed) is always relative. The contraction is just something that happens between you and the external object you use to measure your speed. Again, see the comparison with marriage - it's something between you and the other person, and applies only to the two of you.

By the way, that doesn't mean that space contraction is an "illusion". It is very much real. If you move at 0.999c relative to object A, you're shrinking length-wise from the p.o.v. of object A. But if at the very same time you're not moving at all relative to object B, then B will say your length remains the same. Both are right. Both are for real.

We grow up learning that length and duration are absolute and fixed, but that's just an illusion - that, actually, is the illusion. They're not fixed, they're not absolute. They're just relative attributes, that depend on your motion relative to other things. Relativity gives you the exact math to calculate the amount of length/time change, depending on relative speed (well, "relative speed" is like saying "wet water" - speed is always relative, by definition).

The only thing absolute in this universe is the speed of light - it's always c in your local frame of reference, no matter what. Everything else sort of shifts around and gets adjusted as needed.

First of all, common sense should tell you you’re not travelling at that speed; only cosmic rays and accelerated particles can reach such speeds. The incredible amount of energy needed to boost a spaceship to such a speed, far more than is produced by the largest multi-megaton hydrogen bomb, is one of the reasons it couldn’t happen. But let’s assume for the sake of argument that your spaceship has managed to reach this speed, there are ways and means to see what is ahead without having windows. All the photons coming toward you would be dramatically blue-shifted, while those coming from behind would be dramatically redshifted.

Space is not a perfect vacuum, even in the most transparent regions there is a proton every few metres. At speeds close to the speed of light your spaceship would be subject to a headwind of fast protons, and you would feel the walls of the spaceship heating up. If you had the misfortune to run into a cloud of gas or dust, your spaceship would rapidly burn up, and your problems would be over.

Assuming there was no gas or dust in your path and space was apparently perfectly transparent, I still wouldn’t advise stepping out of your spaceship; the proton storm and other radiations would kill you. Only objects which didn’t share your motion would have relativistic mass increase, so the ball you threw would behave as though you had thrown it in the international space station. Inside the spaceship, you would not notice any change in the decay rates of radioactive elements. The spaceship itself, when viewed by an outside observer who saw himself as at rest, would have acquired a colossal amount of relativistic mass increase because both you and the spaceship are now made of relativistic particles. This mass increase couldn’t come from nowhere, it would have to come from the energy provided by the fuel used, and no such fuel exists.

• Sure, it would take an insane amount of fuel to reach that speed, and your shields need to be miraculous to deal with the high kinetic energy that the space gas & dust has in your frame. And the photons coming towards the front of your ship are blue-shifted to extremely high energy X-rays that will cause secondary radiation via antimatter as they collide with your shields. But for the purpose of this question we're ignoring those things and assuming that the shielding is perfect. Jun 14, 2019 at 10:57
• You seem to be stating the obvious.I'd assumed that his hypothetical but impossible speed would be relative to his point of departure ie the Earth,& that the external observer would either be back on Earth or in the same frame of reference.. Jun 14, 2019 at 15:39