Why do photons travel at the speed of light without having mass?

In summary, Einstein's theory of relativity states that the faster an object travels and the more mass it has, the more it will curve space-time. This explains why a spaceship cannot travel at the speed of light. While photons have no mass, they still possess energy according to E=hf. However, the equation E=mc2 only applies to rest mass, which does not apply to photons since they never "rest". This is why they are excluded from this rule. For more information, please refer to the FAQ section in PF.
  • #1
madmike159
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Einstein's theory of relativity means that the faster an object travels and the more mass it has the more it will curve space-time. This means something like a spaceship could never travel at the speed of light because it would take an infinite amount of energy to reach that speed. So obviously photons have no mass or else they wouldn’t travel at the speed of light and there would be no speed of light.

However we know from E=[tex]h[/tex]f that photons have a certain amount of energy.

So it should have a mass = E/c[tex]^{2}[/tex]

but that would mean that a photon couldn't travel at the speed of light. Does anyone know why they either don’t have mass or why they are excluded from this rule?
 
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  • #2
Please read the FAQ in this section of PF.

Zz.
 
  • #3
No, E= mc2 refers to "rest" mass: mass measured in a reference frame where the velocity is 0. Since photons never "rest" it does not apply to them.
 

1) What is the speed of light?

The speed of light, denoted by the symbol c, is a constant value in the theory of relativity. In a vacuum, it is approximately 299,792,458 meters per second (m/s). This means that light can travel roughly 300,000 kilometers (km) in one second.

2) How was the speed of light determined?

The speed of light was first measured in a scientific experiment by Ole Rømer in 1676. He observed the eclipses of the moons of Jupiter and noticed that the time between eclipses varied depending on the position of Earth in its orbit. This led him to conclude that light has a finite speed, and he calculated it to be about 225,000 km/s.

3) Does the speed of light ever change?

According to Einstein's theory of relativity, the speed of light is a constant and does not change. However, it can appear to be slower when passing through different mediums, such as water or glass. This is due to the light being absorbed and re-emitted by atoms in the medium, causing a delay in its overall speed.

4) How does the speed of light relate to mass?

In Einstein's famous equation E=mc^2, the speed of light plays a crucial role. This equation shows the relationship between energy (E), mass (m), and the speed of light (c). It states that mass and energy are interchangeable, and the speed of light is the conversion factor between the two. This means that as an object's speed approaches the speed of light, its mass also increases.

5) Is it possible for anything to travel faster than the speed of light?

According to the theory of relativity, it is not possible for anything to travel faster than the speed of light. As an object's speed approaches the speed of light, its mass would become infinite, making it impossible to accelerate any further. This is known as the "cosmic speed limit."

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