# Light energy, just like time, is relative?

1. Feb 19, 2013

### Aspchizo

Gravity and speed through space has an effect on time.

Increased gravity = slower time
Increased speed = slower time

When we look out at the stars and measure their spectrum, we see a red shift. This red shift is the result of the increase in the distance between the source of the light and us, otherwise known as the expanding of space.

So if we were to fly towards a star, as we increase our speed, the light should shift more and more. Eventually it would pass down through the spectrum to gamma rays, and our space shuttle would be torn apart by the immensly high energy gamma rays.

So the speed of light remains constant, but as you increase your speed towards the source of the light, your perception of time is slowed and the energy of the light shifts.

Anything wrong with this?

2. Feb 19, 2013

### Staff: Mentor

This is not incorrect, but it is relative; when you say someone's time is slower, you have to specify what it is slower relative to. No one perceives their own time to be slower; everyone experiences their own time flowing at the same rate.

That's one way of looking at it--the light gets stretched out as it travels through expanding space. But it's not the only way of looking at it; another is that it's just a Doppler redshift, caused by the fact that the source of the light is moving away from us--it's due to relative velocity, not light stretching out. These are not incompatible explanations; they're just different ways of looking at the same process.

Ned Wright's cosmology tutorial has a good discussion of this:

http://www.astro.ucla.edu/~wright/cosmo_02.htm

Yes, this is true, because the Doppler blueshift in frequency (blueshift because you're now moving towards the light source, rather than away) corresponds to a shift in energy, since energy is just Planck's constant divided by frequency.

Yes.

No; see above. Another observer moving relative to you might say that your time is slowed, but you yourself perceive no difference in your own flow of time.

Yes.

3. Feb 20, 2013

### Aspchizo

Thanks for clarification. Also, I never meant to imply that we perceive a change in time, it is relative to someone else.

4. Feb 27, 2013

### BruceW

also, energy in all its forms is relative (whether it is contained in the electric field, or some other way). google four-momentum, or also, the stress-energy tensor.

5. Feb 28, 2013

### Saw

Hi Peter. Surely you mean Planck's constant *times" frequency.

6. Feb 28, 2013

### Staff: Mentor

Yes, you're right.