Are hikers behind the lights seen near Mount San Jacinto's Skyline Trail?

By Jonathan Horwitz |

PALM SPRINGS DESERT SUN - A few weeks ago, David Foxen asked why he occasionally sees lights high up in the San Jacinto Mountains near the Palm Springs Aerial Tramway’s Mountain Station and the Skyline Trail.

As we reported at the start of the pandemic and answered earlier in our Ask The Desert Sun series, the tramway station’s main light turned blue as part of the national "Light It Blue" campaign, a tribute to health care workers, first responders and other essential staff, and it will stay that way until the end of the pandemic.

But Foxen specifically said he was not referring to the blue light atop the tramway.

Greg Purdy, vice president of the aerial tramway, said he wasn’t aware of the lights Mr. Foxen might be seeing and he didn't want to speculate.

What else could be alit only occasionally and at various points on the slope of Mount San Jacinto?

The short answer after attempting to report on this question: I’m not sure.

As an avid hiker, I thought maybe our reader could be seeing hikers’ headlamps.

I summited Mount Whitney overnight last summer, and from my base camp miles away and thousands of feet below the trail, I could see hikers’ headlamps like bright dots on higher elevation switchbacks.

Perhaps, in this case, headlamps could appear to be blinking lights as hikers move along the winding trail toward the tramway station.

University of California Riverside physics and astronomy professor Harry Tom explained that even though headlamps are relatively dim, their lights can be seen from far distances.

“In a clear night with clean air, light will travel, really, for miles,” Tom said. He added, “Our eyes are very sensitive, so we can see less than a microwatt of power.”

How small is a microwatt? It’s the power of an average lightbulb divided by one-hundred-million. So, why might we see headlamps from miles away even if they only help hikers see the trail a few dozen yards ahead?

To answer that question, Tom compared headlamps to headlights.

Headlights illuminate only a small stretch of road in front of a car, but drivers coming from the opposite direction can see headlights from much further away.

In general, Tom explained, light sources can be seen from further away than the light, itself, illuminates. That’s because when the human eye sees an illuminated object, it’s seeing light diffracted off an object.

“Whereas what we normally think about in terms of seeing with a flashlight, is we're starting with the light source, it has to go to the object, then it has to come back to us. And by the time it's reflected from the other object, it's so weak, that's why we can't see it,” Tom said.

For a hiker to see a tree 50 yards away, for instance, light from a headlamp or flashlight would have to travel to the tree, reflect off the tree and then find its way back to the human eye. Along the way, the light is projected in a cone, and it scatters off the tree — and other objects — in random directions. So, only a tiny amount of the original light reaches the eye, and that’s what helps the hiker see the dimly-lit tree.

But when the eye sees a light source, it’s seeing light traveling directly from that object.

Tom says direct light can be a million times stronger than diffracted light.

Although the laws of physics suggest that headlamps could be the sources of the “blinking” lights, there is no way to prove that is what our reader has seen all along.

Nevertheless, Tom confidently ruled out another hypothesis floated around. “It’s probably not aliens.”

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