# I have 2 questions about relativity

• hrishikesh
In summary: Tidal forces are not detected in special relativity. This has to do with the principle of relativity which states that the laws of physics are the same in all inertial frames of reference.
hrishikesh
First of all, velocity is a relative phenomenon. When we say velocity of an object is s m/s, it is relative to some other object or a frame of reference (fixed to an object).

1)According to the Theory of relativity, the speed of light is always equal to 3*10^8 m/s. But this velocity is considered relative to what?

2) When two photons are moving in opposite directions, what is the velocity of one photon with respect to the other?

1) c is an universal constant. That's the basic ingredient of special relativity ! So, c is constant with respect to "EVERYTHING" !

2) The photon speed is always c. One cannot exceed this speed, which is another consequence of the basic theorem of special relativity. Just try this for instance : Use the Lorentz transformations to calculate how fast one observer is moving with respect to 2 others. Of these 2 other observers, one is not moving and the other has some speed v. So the third one is moving with some speed v' with respect to the second one, which is moving with speed v compared to the third one.

In classical physics, you will find out that one can just add up the velocities linearly. In special relativity this will not work : so speed does not add up linearly ! This is one of the biggest differences between Newton and special relativity.

Enjoy...

marlon

hrishikesh said:
2) When two photons are moving in opposite directions, what is the velocity of one photon with respect to the other?
Photons don't have their own rest frame, so all we can say is that in any inertial reference frame the speed of each photon will be c. Since their direction is opposite then the distance between the two photons will be increasing at 2c. Note that this is not the same as saying what is their velocity wrt each other.

I'm going to jump in here with a question too.

Is only light at the constant c or the whole electromagnetic spectrum?
and if space is warped, is it warped in the frame too or does it remain the same?

hrishikesh said:
First of all, velocity is a relative phenomenon. When we say velocity of an object is s m/s, it is relative to some other object or a frame of reference (fixed to an object).

1)According to the Theory of relativity, the speed of light is always equal to 3*10^8 m/s. But this velocity is considered relative to what?

2) When two photons are moving in opposite directions, what is the velocity of one photon with respect to the other?

1) Relative to any inertial observer.

2) The relatavistic velocity addition formula give the the answer as 0/0. In other words indeterminate. This is not a contradiction to (1) because no physical observer can move at c relative to any other observer.

maxwilli06 said:
I'm going to jump in here with a question too.

Is only light at the constant c or the whole electromagnetic spectrum?
and if space is warped, is it warped in the frame too or does it remain the same?

Yes, the constant speed c applies to the whole electromagnetic spectrum.

As far as space warping is concerned the term is vague and often used in hand waving arguments. It is better to be more specific by what you mean by warping. If you mean the curvature of space due to gravity then we are talking about General Relativity and not Special Relativity. An observer that is stationary in a gravitational field would notice the curvature unless they are restricted to a infinitessimally small volume. Technically the frame of that observer is not an inertail frame as they are experiencing accleration. You would have to clear if you are talking about an observer that is free falling or stationary in the gravitational field.

kev said:
An observer that is stationary in a gravitational field would notice the curvature unless they are restricted to a infinitessimally small volume.

With the exception of an uniform gravitational field, wouldn't tidal forces still be detected?

## 1. What is the theory of relativity?

The theory of relativity, developed by Albert Einstein, is a fundamental concept in physics that explains how objects in motion behave in relation to each other. It consists of two parts: the special theory of relativity, which deals with objects moving at a constant speed, and the general theory of relativity, which takes into account the effects of gravity on the motion of objects.

## 2. How does the theory of relativity impact our understanding of space and time?

The theory of relativity states that space and time are not absolute, but rather are relative to the observer's frame of reference. This means that the passage of time and the measurement of distance can vary depending on the observer's relative motion. This concept has revolutionized our understanding of the universe and has led to advancements in fields such as astronomy and GPS technology.

## 3. What evidence supports the theory of relativity?

There is a wealth of evidence that supports the theory of relativity, including astronomical observations, experiments, and technological applications. For example, the bending of light by massive objects, the equivalence of mass and energy, and the predictions of gravitational time dilation have all been confirmed through various experiments and observations.

## 4. Can the theory of relativity be proven?

The theory of relativity is a scientific theory, meaning it is supported by evidence and has been extensively tested and verified. However, it cannot be proven in the same way that a mathematical theorem can be proven. Instead, it is continually refined and tested through experiments and observations, and so far, it has withstood all challenges.

## 5. How does the theory of relativity relate to everyday life?

The theory of relativity has many practical applications in our everyday lives. For example, GPS technology would not be possible without taking into account the effects of relativity on the signals being sent and received. Additionally, the theory has also led to advancements in fields such as nuclear energy and particle physics, which have numerous real-world applications.

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