You always hear this phrase if you watch something about astrophysics ‘Nothing can move faster than light’. But speed is only relative. How can this be true if speed can only be experienced/measured relative to something else?
In: Physics
This is a brilliant question. It is the question the led to special relativity!
Motion is relative: the velocity of an object depends on the velocity of the thing measuring it.
Speed of light is not relative: everything measures speed of light the same.
That is the paradox. The universe tells us that is the way it is when we measure it! …and we try to explain why. But I believe understanding should start there, not with explanations of space time.
The speed of light is constant relative to everything. What Newton – and later, Einstein – showed was that there is no underlying reference frame; all motion is relative. Light differs only in that everyone perceives light to have the same relative speed; 299,792,458m/s in a vacuum.
The speed of light is also constant, in that it doesn’t accelerate by adding velocity but instead instantly starts traveling at it’s maximum speed.
Speed of objects is relative when measured from different viewpoints, but not in the case of light.
Light is measured at the same speed regardless of the viewpoint of the observer. That’s part of the principle of relativity.
I’m not a physicist, but I think it works thus:
2 spaceships, one stationary and one travelling at 10000kph, both turn on their spotlights at exactly the same moment, pointing to a stationary observer placed mid way between them.
In theory, the light from the moving ship should arrive at the observer sightly earlier, because it has a 10000kph running start.
However, the observer will measure the speed of the arriving light as exactly the same from both ships.
As the distance between the objects is objectively known, then the only way that physics can accommodate the consistent speed of light is to allow time to distort.
Time moves slower for the speeding spaceship to allow the light to arrive at the same time as that from the stationary ship.
Edit to answer the actual question!
Light has no mass. Everything else has a mass which requires energy to accelerate it.
As an objects speed increases, so does it’s mass. Increased mass requires increased energy to accelerate it. This becomes exponential as the object approaches light speed, meaning that the object requires an ever increasing amount of energy to accelerate it. This becomes an impossible achievement just short of the speed of light.
E=mc2 is the equation that states this principle.
The speed of light always appears to be the same relative to you regardless of your speed. It’s the exception to the rule. Space-time compensates to make this true to every observer.
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