Can you see who I am?

When I am lazy I can sing
And show you worlds from your dream,
But if my heart beats fast I bring
Chaos and death under your skin.

I can't be seen running away
Unless I'm running to you straight.
If you're so quick to make me late.

I think BlackPanda's answer is closest, but I think the answer is specifically said:

Photon

And the explanation is slightly different:

When I am lazy I can sing...


Radio waves have low energy photons (and can transmit music)

...And show you worlds from your dream,...


We can use low energy photons (radio waves) to transmit video (TV)

...But if my heart beats fast I bring

Chaos and death under your skin....


Higher energy photons like X-rays and gamma rays can cause cancer (under your skin).

...I can't be seen running away...


You can't see photons traveling away from you.

...Unless I'm running to you straight...


You can only see photons that hit your sensors or eyes.

...You will return to yesterday

If you're so quick to make me late.


And (in theory) we would time travel if we traveled faster than photons (light)

Again, I think BlackPanda's answer is closest and I wanna give him props, but because of the way the explanation breaks down, I think the answer should be:

Photon

• This is exactly what I had in mind when making the riddle, and your interpretation is perfectly matching mine. However others have answered before you with "waves" and that qualifies too. – Bogdan Alexandru Sep 23 '16 at 7:49
• According to a question on meta (meta.puzzling.stackexchange.com/questions/3267/…), the best answer should be chosen as the one that best describes the answer, as it's not really about getting reputation points, but rather to show readers what was the intention. – Bogdan Alexandru Sep 23 '16 at 8:05
• I agree :) ###minimum chars### – BlackPanda Sep 23 '16 at 11:00

Edit: A better interpretation was made by Dylan Cristy here. I think his answer is better than mine.

Are you

Light

When I am lazy I can sing

And show you worlds from your dream,

We can see fantasy worlds in movies and songs in an electronic display.

But if my heart beats fast I bring

Chaos and death under your skin.

Higher frequency light like X rays and Gamma rays can kill you.

I can't be seen running away

Unless I'm running to you straight.

We can only see the light that come to our eyes.

If you're so quick to make me late.

If we travel faster than light, we can time travel (hypothetically).

• Wait, I think I got the second stanza more correctly. I'm editing it. – BlackPanda Sep 22 '16 at 12:00
• Sounds like the right answer to me! – Xenocacia Sep 22 '16 at 12:12
• Let there be light! And possibly a checkmark on this answer :) – Ivanhoe Sep 22 '16 at 12:36
• It's not light, it's radio waves. Gamma rays aren't light. – Jez Sep 22 '16 at 14:42
• @Jez Well, technically they are not radio waves either but electromagnetic waves. :) But yes, they are light. Not visible, but still light. – BlackPanda Sep 22 '16 at 17:09

My take (pretty much combines Sid's and Black Panda's, except I think I have a better interpretation of the first lines):

You are

Waves (in general)

When I am lazy I can sing

Sound waves (literally singing) OR radio waves in the frequency range for... Radio (AM or FM)

And show you worlds from your dream,

Brain waves when you are sleeping/dreaming OR radio waves in the frequency range used for TV (fictional shows can be "dream worlds")

But if my heart beats fast I bring Chaos and death under your skin.

Higher frequency waves like X-rays and gamma rays can cause physical damage

I can't be seen running away Unless I'm running to you straight.

You can't see anything if light waves are moving away from you. Even other types of waves can't be detected by sensors if they are moving away from the sensors, they need to be moving towards the sensors.

You will return to yesterday If you're so quick to make me late.

As Black Panda put it, time travel is hypothetically possible if you travel faster than light,

OR

if you look at light waves that are "late" in arriving, you are technically seeing something that happened in the past, so in a sense you could be seeing "yesterday".

• Yes, this is the better answer. – BlackPanda Sep 23 '16 at 5:19
• @BlackPanda This is the most complete answer indeed, but it was inspired by your answer, mostly a team effort :) who should be picked as winner? – Bogdan Alexandru Sep 23 '16 at 7:45
• @BogdanAlexandru I guess the one you already picked? :P I mean, it's your question, so it's really your perspective. – BlackPanda Sep 23 '16 at 11:06

You might be

Music

When I am lazy I can sing

Some peaceful, singing can make you calm and "lazy".

And show you worlds from your dream,

Listening to the music while asleep can also affect your dreams.

But if my heart beats fast I bring Chaos and death under your skin.

Aggressive music makes all your body to shake causing "chaos" felt from inside.

I can't be seen running away

Music is not visible

Unless I'm running to you straight.

Well, that doesn't fit ideally - I'll have to think more about this

Reference to a famous song of "The Beatles" - "Yesterday"

If you're so quick to make me late.

In song mentioned above the lyrics say that "Yesterday came suddenly" meaning that it came so quickly, that you seem late.

So you are

Music, but no, I cannot see who you are because you are not anything visible.

• Interesting try, but no :) – Bogdan Alexandru Sep 22 '16 at 11:51

You could be

A clock

When I am lazy I can sing And show you worlds from your dream,

An alarm clock singing in the morning, waking us from our dreams

But if my heart beats fast I bring Chaos and death under your skin.

Time goes by fast and eventually brings death

I can't be seen running away

We cannot see time go by

Unless I'm running to you straight.

Unless its on a clock straight in front of us

You will return to yesterday If you're so quick to make me late.

You can set a clock to yesterday by making it "late"

• Nice attempt, but no. You also mix "clock" with "time" when they are not completely equivalent. "I'm running to you straight" implies movement "towards you", but the clock would be, well, sitting still. – Bogdan Alexandru Sep 22 '16 at 11:49
• If the alarm awakes you, you would be removed from your dream, not be shown it. Ask any of the kids Freddy Kreuger terrorized. – Knight0fDragon Sep 22 '16 at 19:30

Here's mine

Waves

When I am lazy I can sing And show you worlds from your dream

Music, mainly the wave form of music.. the 2nd line might indicate the soothing relief of waves.

But if my heart beats fast I bring Chaos and death under your skin.

Might refer to the ECG(graph). Too high and you are in trouble, too low, you might be dead. OR second idea-EM waves.

I can't be seen running away Unless I'm running to you straight.

You only notice the height and velocity of waves near the coast not at the middle of the ocean.

You will return to yesterday If you're so quick to make me late.